WO2013152415A1

WO2013152415A1 - Accurate tidal volume measurement

Info

Publication number: WO2013152415A1
Application number: PCT/CA2012/000876
Authority: WO
Inventors: Michael Klein; Joseph Fisher; James Duffin
Original assignee: Michael Klein; Joseph Fisher; James Duffin
Priority date: 2012-04-12
Filing date: 2012-09-05
Publication date: 2013-10-17

Abstract

The inventors have found a simple modification of the standard ventilator circuit that allows a flow sensor in the inspiratory limb of the ventilator circuit to directly measure the inspired tidal volume received by the patient. This modification of the ventilator circuit passively redirects the compressible volume around the flow sensor, and yet does not interfere with the function of the ventilator or ventilator breathing circuit.

Description

ACCURATE TIDAL VOLUME MEASUREMENT

FIELD OF THE INVENTION

The present invention relates to modifications of a standard ventilator, ventilator breathing circuit and components thereof that enable inspiratory and expiratory tidal volumes to be determined without measuring compressed volumes of gas in the circuit.

BACKGROUND OF THE INVENTION

Tidal volume is a key parameter to monitor in critically ill patients requiring mechanical ventilator support. Large tidal volumes may cause hypocapnia, and over-distention of the alveoli can result in mechanical and biochemical injury to the lung (1) and distant organs (2, 3). Tidal volumes smaller than intended may cause hypercapnia, hypoxia, and damage the lung by repeated recruitment and derecruitment of alveoli (1, 4, 5).

Modern ventilators measure the volume of gas delivered to the patient from flow sensors within the machine. However, the volumes measured by these flow sensors include the compressible volume - the gas that is compressed in, and causes expansion of, the ventilator tubing in addition to the tidal volume received by the patient. Although it is theoretically possible to calculate an effective tidal volume by correcting the volumes measured by a flow sensor in the ventilator for compressible volume, these corrections require accurate determination of the circuit compliance and airway pressures (6). The circuit compliance can change with time, temperature, humidity, replacement of the circuit, addition of circuit extensions, and the interposition of in-line devices such as humidifiers, spirometers, nebulizers, and filters. Circuit compliance may even vary with the peak inspiratory flow rate (7). As such, correlations between corrected effective tidal volume and the tidal volume received by the patient remain poor (8-10). Castle and colleagues, after observing errors up to 91% between corrected effective tidal volume and the tidal volume received by the patient, concluded that the errors that occur when the volume delivered by a ventilator is not measured at the airway opening are of such magnitude and variability that no clinical credence should be given to either displayed or calculated effective tidal volumes from ventilators (11).

Measuring tidal volume at the patient airway is not a trivial matter. Flow sensors at the patient airway add dead space, which may significantly impair carbon dioxide elimination, particularly in children and small adults (12, 13). They further add bulk and weight to the airway interface, increasing the risk of accidental extubation with patient movement, and interfering with ready access to the patient for mouth care and pulmonary toilet. Flow sensors in the limbs of the ventilator circuit do not add dead space, bulk, or weight to the patient airway, but, like the flow sensors in the ventilator, measure the compressible volume in addition to the tidal volume received by the patient.

SUMMARY OF INVENTION

With a standard circuit, during inspiration, the gas compressed in the expiratory limb passes through the Y-piece and would be measured by a flow sensor in the inspiratory limb. Similarly, the gas compressed in the inspiratory limb passes through the flow sensor as it is released through the ventilator exhalation port during expiration (Standard circuit, Figure 1).

Using a ventilator breathing circuit according to the invention (H-bridge circuit), during inspiration, the gas compressed in the expiratory limb is routed through a cross-bridge between the inspiratory limb and expiratory limb, and does not pass through the flow sensor. During expiration, the gas compressed in the inspiratory limb passes out the expiratory port of the ventilator by flowing retrograde through the cross-bridge and by-passing the flow sensor (Figure 4).

If the ventilator produces a bias flow during the expiratory phase, it passes through the cross-bridge in a circuit of the invention bypassing the flow sensor. This rerouting of the bias flow does not interfere with the detection of inspiratory efforts by the patient. With both a standard circuit and the H-bridge circuit, the bias flow exits through the ventilator expiratory port when there is no inspiratory effort and is redirected to the patient if an effort is made.

Therefore, according to one aspect, the invention is directed to ventilator breathing circuit comprising:

(a.) an inspiratory limb including a proximal first conduit portion (optionally comprising a one way inspiratory valve), the first conduit portion conducting flow downstream to a subject airway interface;

(b.) an expiratory limb including a second conduit portion (optionally comprising a one way expiratory valve), the second conduit portion fluidly connectable to or connected to the first conduit portion proximal to the subject via a proximal connecting portion, the proximal connecting portion integral to or adapted to be connected to a subject airway interface; (c.) at least one flow sensor portion fluidly connected to the inspiratory limb for measuring flow to the subject via the proximal connecting portion; and

(d.) a distal connecting portion fluidly connecting the inspiratory limb and the expiratory limb; wherein the one way inspiratory valve is positioned in the proximal connecting portion (e.g. where the proximal connecting portion is a Wye connector) or first conduit portion (optionally substantially adjacent to the proximal connection portion, optionally immediately adjacent to where the first and second conduit portions join) and wherein the one way expiratory valve is positioned in the proximal connecting portion (e.g. where the proximal connecting portion is a Wye connector) or second conduit portion (optionally substantially adjacent to the proximal connection portion, optionally immediately adjacent to where the first and second conduit portions join) and wherein the flow sensor portion is adapted to be operatively connected to a flow sensor system, the flow sensor system including one or more flow sensors positioned to determine net flow through the one way inspiratory valve, the ventilator breathing circuit configured so that, when the ventilator breathing circuit is in use, air flow resistance through the distal connecting portion is less than air flow resistance through an alternate flow path through the one way inspiratory valve such that the flow sensor measures a volume of gas inspired by a subject to the substantial exclusion of a compressed volume of gas in the ventilator breathing circuit that is not inspired by the subject.

Optionally, the one way inspiratory valve is positioned substantially adjacent to the proximal connecting portion. This is important for greatest accuracy as any compressed gas volume in the circuit downstream from the one way inspiratory valve cannot be discriminated from gas inspired by the patient. Analogous mirror image configurations are described hereafter for measuring exhaled tidal volume. Similarly, the one way expiratory valve is positioned substantially adjacent to the proximal connecting portion so that any compressed gas volume in the circuit downstream from the one way expiratory valve cannot be discriminated from gas expired by the patient.

Optionally, the flow sensor is bi-directional.

Optionally, the flow sensor adapted to be connected to a processor which integrates the flow to measure tidal volume.

Optionally, the processor is a processor which controls the ventilator.

Optionally, the flow sensor portion is positioned substantially adjacent to the first portion and upstream therefrom (the second portion in analogous embodiments for measuring exhaled tidal volume). Optionally, the flow sensor is positioned in the inspiratory limb at a distance from the first portion such that a compressed volume of gas is contained between the one way inspiratory valve and the flow sensor, and wherein a processor is operatively associated with the flow sensor to compute tidal volume based on output from the flow sensor by accounting for both the flow of gas in a direction away from the proximal connecting portion and the flow of gas in a direction towards the proximal connecting portion.

Optionally, the proximal connecting portion is the common limb of a Wye connector and the one way inspiratory and expiratory valves are positioned within respective alternate branches of the Wye connector. With some loss of measurement accuracy, it is to be understood that the proximal end of the first conduit portion of the inspiratory limb may optionally house the one way inspiratory provided that it positioned close to the proximal end. Analogously, is to be understood that the proximal end of the first conduit portion of the inspiratory limb may optionally house the one way expiratory valve provided that it positioned close to the proximal end.

It will be appreciated that the flow sensor system for measuring exhaled tidal volume is analogous to the flow sensor system for measuring inhaled tidal volume. The invention contemplates novel ventilator breathing circuit configurations that accomplish one or both of these measurements as well as their varied component parts (for example H-shaped breathing circuit component with appropriate connecting ends for integration with one or more flow sensor positions and other components of the ventilator breathing circuit, for example a Wye connector outfitted with a one way inspiratory valve and a one way expiratory valve). Optionally, such an H-Bridge component comprises inspiratory and expiratory female ends for connection to a ventilator side of the breathing circuit and inspiratory and expiratory male ends for connection to the patient proximal side of the breathing circuit. The invention also contemplates novel ventilators outfitted with a flow sensor system and a flow-sensor by-pass portion that measures one or both of inhaled and exhaled tidal volumes to the exclusion of a compressed volume of gas in the ventilator breathing circuit.

According to another aspect, the invention is directed to a ventilator breathing circuit kit comprising:

(a) one or more components of an inspiratory limb including a first conduit portion comprising a one way inspiratory valve, the first conduit portion comprising a connecting portion for fluidly connecting the first conduit portion to a flow sensor portion, the flow sensor portion adapted to be connected to the first conduit portion at a position remote from its connection to a proximal connecting portion, the proximal connecting portion of type integral to or adapted to be connected to a subject airway interface;

(b) one or more components of an expiratory limb including a second conduit portion comprising a one way expiratory valve;

(c) a distal connecting portion for connecting the inspiratory limb and the expiratory limb, the distal connecting portion adaptable or adapted to be fluidly connected to the flow sensor portion at a position remote from its connection to the first conduit portion, the distal connecting portion configured to provide less air flow resistance, when the ventilator breathing circuit is in use, than air flow resistance along an alternative flow, such that when parts of the kit are assembled and connected to a ventilator, the flow sensor portion is positioned for measuring a volume of gas inspired by a subject to the substantial exclusion of a volume of gas in the breathing circuit that is not inspired by the subject.

Optionally, the distal connecting portion comprises a portal for connection to the expiratory limb and a separate portal for connecting to the inspiratory limb, the expiratory limb comprising a portal for connecting to the distal connecting portion positioned remotely from the one way expiratory valve, the inspiratory limb comprising a portal for connecting to the distal connecting portion positioned more remotely than a portal for connection to the flow sensor portion.

The kit is optionally adapted for operative connection to a flow sensor that is unidirectional or bidirectional.

The kit optionally comprises a proximal connecting portion in the form of a Wye connector.

According to another aspect the invention is directed to the use of the kit in conjunction with a flow sensor portion, to assemble a ventilator breathing circuit, the flow sensor portion including a unidirectional or bi-directional flow sensor.

According to another aspect, the invention is directed to method of measuring the tidal volume per breath of a subject breathing on a ventilator comprising connecting a ventilator breathing circuit to a ventilator and obtaining output from the flow sensor. The ventilator breathing circuit may or may not be assembled from a kit.

According to another aspect, the invention is directed to a kit for assembling a flow sensor by pass device comprising at least the following parts: A. an inspiratory conduit portion; B. an expiratory conduit portion; and C. a flow sensor by-pass portion. These parts are optionally characterized by the features ascribed to these parts described above.

According to yet another aspect, the invention is directed to a ventilator of the type having a flow sensor, a processor for receiving input from the flow sensor and integrated conduit portions configured for connection to an external ventilator breathing circuit of the type comprising an inspiratory limb including a one way inspiratory valve and a distally positioned first ventilator connection portion, and an expiratory limb including a one way expiratory valve and a distally positioned second ventilator connection portion, and a proximal connecting portion for connecting the inspiratory limb and the expiratory limb, the proximal connecting integral to or adapted to be connected to a subject airway interface, the ventilator characterized in that the integrated conduit portions include an inspiratory conduit portion operatively connected to an inspiratory limb port, an expiratory conduit portion fluidly connected to an expiratory limb port, at least one flow sensor portion including a bi-directional flow sensor and a distal connecting portion connecting the inspiratory conduit portion and the expiratory conduit portion at a position distal to the proximal connecting portion, the flow sensor portion positioned between the inspiratory conduit portion and the flow sensor by-pass portion, and wherein, when the ventilator breathing circuit in use, air flow resistance through the distal connecting portion is less than air flow resistance through an alternate flow path through the breathing circuit such that the flow sensor is positioned to measure a volume of gas inspired by a subject to the substantial exclusion of a volume of gas that is not inspired by the subject, the processor adapted to compute a tidal volume based on output from the flow sensor by accounting for gas flow in both directions through the flow sensor.

According to yet another aspect, the invention is directed to a method of measuring a tidal volume per breath of a subject breathing on a ventilator comprising:

(1) connecting a flow sensor by-pass limb to a ventilator breathing circuit of the type comprising an inspiratory limb including a one way inspiratory valve, a distally positioned ventilator connection portion and an expiratory limb including a one way expiratory valve and a distally positioned ventilator connection portion, the inspiratory limb adapted to be connected to a flow sensor portion including a flow sensor at a position between the one-way inspiratory valve and the distally positioned ventilator connection portion, the flow sensor by-pass limb placed to interconnect the inspiratory limb and expiratory limb at a position within the inspiratory limb that is distal to the flow sensor portion and at a position within the expiratory limb that is between the one way expiratory valve and the distally positioned ventilator connection portion of the inspiratory limb, the flow sensor by-pass limb configured such that the flow sensor bypass limb offer less resistance to flow than through an alternate path within the breathing circuit;

(2) obtaining output from the flow sensor; and

(3) mathematically integrating flow sensor output corresponding to a breath to obtain a tidal volume for the breath.

Optionally, the flow sensor is a bi-directional flow sensor positioned in the inspiratory limb at a distance from the one way inspiratory valve which causes accumulation of a volume of compressed gas between the one way inspiratory valve and the flow sensor in each respective inspiratory cycle, this respective volume of compressed gas flowing back through the flow sensor via the flow sensor by-pass limb in each respective ensuing expiratory cycle, and wherein a tidal volume for a breath is computed by accounting for gas flow in both directions through the flow sensor.

Brief Description of the Drawings

Figure 1 is a schematic representation a ventilator and ventilator breathing circuit according to the prior art.

Figure 2 is a schematic representation of a ventilator breathing circuit according to one embodiment of the invention which shows at what positions (demarcated by either broken line A or B and/or C or D) the circuit may be outfitted with a flow sensor according to the invention.

Figure 3a is a schematic representation of a ventilator breathing circuit according to one embodiment of the invention which is outfitted with a uni-directional flow sensor at a position demarcated by broken line A in Figure 2.

Figure 3b is a schematic representation of a ventilator breathing circuit according to one embodiment of the invention which is outfitted with a bi-directional flow sensor at a position demarcated by broken line B in Figure 2.

Figure 4 is a schematic representation a ventilator including an internal ventilator breathing circuit according to the invention. Figure 5a is a schematic representation of a ventilator breathing circuit according to one embodiment of the invention which is outfitted with a uni-directional flow sensor at a position demarcated by broken line C in Figure 2.

Figure 5b is a schematic representation of a ventilator breathing circuit according to one embodiment of the invention which is outfitted with a bi-directional flow sensor at a position demarcated by broken line D in Figure 2.

Figure 6 comprises a graphic (FIG. 6a) and related schematic representations (FIGS. 6b-6e) of flows measured by a flow sensor in the inspiratory limb of a standard ventilator circuit during inspiration (FIG. 6b) and expiration(FIG. 6c) and with an embodiment of a ventilator breathing circuit of the invention (during inspiration FIG. 6d and expiration 6e).

Figure 7 is a graph showing meaniSD percent difference between inspired volumes measured by a flow sensor positioned in the inspiratory limb (Vi) and at the airway (V_A) with the standard circuit and the H- bridge circuit. Values summarize the percent difference at four test volumes. Full data set is shown in Figure 9.

Figure 8 is a graph showing mean + SD percent difference between inspired volumes measured at the airway with the standard circuit (V_A,s) and the H-bridge circuit (V_A,_H) for the same ventilator setting. Values summarize the percent difference at four test volumes. Full data set is shown in Figure 10.

Figure 9, is a table (Table 1) showing inspired volumes measured by a flow sensor positioned at the airway (V_A) and in the inspiratory limb (V|) with the standard circuit and the H-bridge circuit at four test volumes. With the standard circuit, V, significantly over-estimated V_A at all test volumes. With the H- bridge circuit, V, and V_A did not differ at any test volume. These data are summarized graphically in Figure 7.

Figure 10 is a table (Table 2) showing inspired volumes measured at the airway with the standard circuit (V_A,s) and the H-Bridge circuit (V_{A H}) for the same ventilator setting. Ventilation received by the test lung was not affected by replacement of the standard circuit with the H-bridge circuit. These data are summarized graphically in Figure 8.

Figure 11a and lib are schematic representations of other embodiments of a ventilator breathing circuit according to the invention showing the use of two flow sensors acting in concert to determine net tidal inhaled volume and net tidal exhaled volume respectively. Detailed Description of Invention

As described above, a ventilator breathing circuit of the invention is optionally generically referred to (i.e. without limitation as to a specific embodiment) as an "H-Bridge circuit" owing to the cross-bridge connecting the inspiratory limb and expiratory limb of the circuit. The cross-bridge may simply be a interconnecting aperture and is generically referred to, depending on the context, as a flow sensor bypass conduit (or by-pass means) to reference its function or as a distal connecting portion of the circuit to describe its structure and position within the circuit relative to a proximal connecting portion which describes, at least in one embodiment, a standard Wye connector that joins the inspiratory and expiratory limbs proximal to the patient airway interface, as opposed the more distally positioned cross- bridge.

The term "bi-directional flow sensor" includes two unidirectional flow sensors positioned near one another in opposite directions.

The term "patient airway interface" describes any interface between the ventilator breathing circuit and the patient's airway, commonly a mask or endotracheal tube.

The terms "distal" and "upstream" are used to describe a relative position and direction of flow, respectively, with reference to a patient airway interface at/from a position which is relatively remote from the patient airway interface. The terms "proximal" and "downstream" are similarly used having regard to the patient airway interface as a position of reference.

The term "portal" is used to mean an opening of suitable diameter that defines a means of fluidic connection and includes openings between functionally defined portions of conduit or tubing that are integrally formed and well as ports that interconnect parts of a breathing circuit. Ports are apertures configured for connecting separate parts. These parts may be fluidly connectable in any manner known the in the art for connecting parts such as conduits or tubing directly or via intermediate connector, a manifold etc. in a fluidly efficient manner.

The term "ventilator processor" means a processor built into a ventilator for controlling at least one or more key functions of the ventilator.

The term "ventilator connection portion" means a portion of a gas channelling means e.g. a conduit that is of the type suitable and normally used for fluidly connecting an inspiratory limb or expiratory limb of an externa! breathing circuit to a ventilator i.e. it has a port (a ventilator connection port) at its end adapted for making a fluidly efficient connection, which port may also be used or adapted for connecting to other portions of the breathing circuit that may interposed between the distal end of either limb and the ventilator.

A "flow sensor" means a device that may be used directly or indirectly to measure gas flow e.g. a flow meter that may be operatively connected to at least a minimal length of gas conduit, which conduit is the "flow sensor portion" of a breathing circuit. A preferred embodiment of a flow sensor is one which is adapted for connection to a processor to enable continuous flow measurement for computation of tidal volume or exhaled volume, depending on its placement within a breathing circuit.

The term "proximal connecting portion" means a component of a breathing circuit wherein flow to the patient via the inspiratory limb and flow away from the patient via the expiratory limbs is not separated i.e. downstream from the one way inspiratory and expiratory valves. A proximal connecting portion typically takes the form of a Wye connector that physically interconnects the two limbs but in the absence of such a connector or equivalent manifold may also be a portion of a patient airway interface wherein these two flows join in a common portion.

It will be understood that the distal connecting portion (alternatively termed a flow sensor by-pass conduit) may be a conduit passage of any length defined by a connector or an actual length of tubing, or may simply take the form a discontinuity between otherwise separately walled inspiratory and expiratory flow channels that may or may not otherwise share a common wall along their respective lengths.

The term "gas conduit" includes any minimal length of a gas channel of any functional cross-section (e.g. circular 22 mm diam. (adult) or 15 mm diam. (pediatric) )and may be constituted by a visible length of conduit or the masked internal configuration of any part of a breathing circuit.

When using a uni-directional flow sensor to measure inspired tidal volume, the flow sensor should be as close as possible to the inspiratory one-way valve in the inspiratory limb. If the flow sensor is more proximal to the ventilator, it will over-read the tidal volume received by the patient by the compressible volume of gas between it and the patient airway. However, with the ventilator breathing circuit of the present invention, as long as the flow sensor is between the H-Bridge and the inspiratory one-way valve, any compressible volume that passes through the flow sensor on inspiration will pass retrograde through the sensor on expiration. Therefore, inspired tidal volume may be measured by integrating the output of a bi-directional flow sensor anywhere in the inspiratory limb between the cross-bridge and one-way valve. The importance of having the flow sensor by-pass conduit be of less resistance to flow than flow via an alternate flow path through the ventilator breathing circuit may be understood as providing a path of least resistance when the ventilator is in use so that flow does not cause incorrect flow or net flow readings during inspiration or expiration. For example, when a uni-directional flow sensor is positioned immediately upstream from the one way inspiratory valve, the alternate flow path during inspiration is a flow path through the flow sensor. If the path through the flow sensor by pass portion was not the path of least resistance, upon inspiration, at least a portion of the gas compressed in, and causing expansion of, the expiratory limb would instead enter the expiratory limb through the one way inspiratory and expiratory valve and flow sensor. This gas would not return through the flow sensor on expiration. Similarly, on expiration, at least a portion of the gas compressed in, and causing expansion of, the inspiratory limb would instead flow out the exhalation port of the ventilator through the one way inspiratory and expiratory valve and flow sensor.

Similarly, it will be appreciated that a bi-directional sensor may be used so that any flow toward the subject through the flow sensor that does not constitute part of the tidal volume inspired by the subject may be netted out by a volume computed from flow in the reverse direction. Thus if the flow meter is placed at a distance from the one way inspiratory valve, the path of least resistance through the flow sensor by-pass conduit or distal connecting portion during expiration ensures that compressed gas residing between the one way inspiratory valve and the flow sensor passes back through the flow sensor and out through the ventilator exhalation valve via the distal portion of the expiratory limb.

Analogously, it will be appreciated that when the by-pass limb is back in the ventilator portions of the circuit, per se, a path of least resistance to flow through the by-pass conduit will cause the inspiratory bidirectional inspiratory flow meter to measure tidal volume with good precision.

Analogously it will also be appreciated that a flow sensor by-pass means {conduit or portal) will enable a flow sensor connection portion positioned in the expiratory limb of an external ventilator breathing circuit between the by-pass means and the one way expiratory valve to accurately measure expired tidal volume. Analogously the flow sensor connection portion (using a bi-directional sensor) may be placed on the expiratory side of ventilator portions of the breathing circuit e.g. within the ventilator itself assuming such portions are internal (distal to the inspiratory and expiratory ports leading out of the ventilator for attachment to the external parts of the circuit). Analogously, as this flow sensor is placed a distance from the one-way expiratory valve in the second conduit portion, a bi-directional flow- sensor is used to account for the volume of gas between the one-way expiratory valve and the by-pass means or distal connecting conduit. The invention is broadly directed to measuring inspired tidal volume or expired tidal volume or both in the manner herein disclosed.

As shown in Figure 2, the invention contemplates four primary general positions for placing a single flow sensor in accordance with 4 principal embodiments of the invention. These are demarcated by lines A, B, C and D in Figure 2 and represent, respectively: A=placement of uni-directional (or bi-directional) flow sensor at position A proximal to the one way inspiratory valve. B=placement of bi-directional flow sensor at position B at a position removed from the one-way inspiratory valve to keep the flow sensor {even if reusable flow sensor and protected by a filter) away from the inspiratory valve to avoid excess circuit bulk and weight proximal to the patient. C=placement of uni-directional (or bi-directional) flow sensor at position C proximal to the one way expiratory valve. D=placement of a bi-directional flow sensor at position D at a position removed from the one-way expiratory valve to keep the flow sensor (even if reusable flow sensor and protected by a filter) away from the expiratory valve to avoid excess circuit bulk and weight proximal to the patient.

Figure 11a and lib show examples of the use of two sensors in a manner which results in functional alternative placements. A flow sensor may be positioned in the inspiratory limb between the distal connecting portion and the ventilator, for example, as shown in Figure 11a, proximal to the distal connecting portion and a second portion may be placed in the distal connecting portion itself. Tidal volume may thus be measured with two flow sensors. It will be appreciated that the first flow sensor will measure all the gas output by the ventilator. The second flow sensor will measure only compressible volume. The tidal volume received by the patient may be calculated by subtracting the integral of the output of the second sensor from the integral of the output of the first sensor over a breath cycle. In an alternative placement adapted to measure exhaled tidal volume, as shown in Figure lib, the second flow sensor is in the same position, and the first flow sensor is placed in the expiratory limb of the circuit between the distal connecting portion and the ventilator. The compressible volume in both limbs, as measured by the second sensor, may thereby be subtracted from the total volume flowing through first flow sensor.

Other alternative placements using two sensors will be apparent to those skilled in the art using well known adaptations and controls. For example (not shown), inspired tidal volume can sub-optimally be measured by placing a flow sensor in the distal connecting portion and a second flow sensor in the inspiratory limb between the distal connecting portion and the proximal connecting portion, preferably as close as possible to the distal connecting portion. In this case, it is useful to provide breath detection and make the tubing before the distal connecting portion as short as possible. Analgously, to measure exhaled tidal volume one sensor is placed in the distal connecting portion to account for the compressed volume already in the expiratory limb and another sensor may be placed between the distal connecting portion and the proximal connecting portion (preferably as close as possible to the distal connecting portion) to measure volume leaving the expiratory limb (from which the compressed volume already in the expiratory limb from the inspiratory cycle would be subtracted because this placement results in this volume being measured first).

As seen in Figure 1, in a standard ventilator 80, a bellows or blower 8 is operatively connected to an inlet valve 9 and a one way valve 7. A uni-directional inspiratory flow meter 2 is positioned upstream from inspiratory limb connection port 32 and a uni-directional expiratory flow meter 4 is located upstream from an expiratory limb connection port 30 and downstream from an exhalation valve 6. A ventilator breathing circuit comprising an inspiratory limb 12, an expiratory limb 14 and a proximal connecting portion 36 is attached to the ventilator 80 to make-up a standard ventilator breathing system 10.

Figure 2 shows an overview of a ventilator breathing circuit according to the invention, comprising an inspiratory limb 12, an expiratory limb 14, a one way inspiratory valve 16, and a one way expiratory valve 20 located within a proximal connecting portion 36. In each case, the inspiratory limb and expiratory limb are connected by a distal connecting portion 18.

Figure 3a shows one configuration of a ventilator breathing circuit according to the invention 102 in which a uni-directional flow sensor 50a is positioned adjacent to the one-way inspiratory valve 16. An alternative embodiment is presented in figure 3b wherein a bi-directional flow sensor 50b is positioned more proximal to the distal connecting portion 18. In both figure 3a and 3b, the flow sensor 50 is positioned between the one way inspiratory valve 16 and the distal connecting portion 18.

In Figures 5a and 5b, a mirror image of the positions of the flow sensors relative to the positions shown in 3a and 3b are presented. The configurations shown in figure 5a and 5b adapt the ventilator breathing circuit to measure an expired tidal volume without measuring a compressed volume of gas in the ventilator breathing circuit.

Figure 4 presents the H-Bridge circuit, including distal connecting portion 18, integrated into a ventilator. Bi-directional flow sensors 2 and 4 positioned between distal connecting portion 18 and the one way valves in the external breathing circuit 100 enable the flow sensors to measure inspired and expired tidal volumes without measuring a compressed volume of gas in the breathing circuit 100.

It will be appreciated that the bi-directional sensors 2 and 4 may be used individually to measure just one of inspired and expired tidal volume and serve the purpose of enabling the compressed volume to be subtracted from the total volume measured by the flow sensors, owing to their bi-directional capability.

Figures 6a - 6e contrast what is measured using a standard ventilator breathing circuit and a breathing circuit according to embodiments of the invention.

In Figure 11a, an alternate placement of flow sensors is shown by contrast to those shown in Figure 2 for measuring inspired tidal volume. In Figure lib, an alternate placement of flow sensors is shown by contrast to those shown in Figure 2 for measuring expired tidal volume.

Examples

Apparatus

A standard ventilator circuit was constructed from 22 mm diameter tubing (Plastic corrugated tubing; GlobalMed Inc.; Trenton, ON) and a Y-piece (Part no. 51089; Qosina Corp.; Edgewood, NY). An H-bridge circuit was constructed from the same tubing and Y-piece by adding inspiratory and expiratory one-way valves (Part no. 1664; Hudson RCI; Temecula, CA) to the Y-piece, and a cross-bridge proximal to the ventilator. The total volume of the one-way valves and cross-bridge was 40 mL. This additional volume was removed from the H-bridge circuit by removing 5.2 cm of tubing from both the inspiratory and expiratory limbs. The individual circuit volume of both circuits, including the Y-piece, was 1.42 L.

The circuits were used with a ventilator (MA-1; Puritan Bennett Inc.; Los Angeles, CA) to ventilate a variable compliance test lung (QuickLung; IngMar Medical Ltd.; Pittsburgh, PA). Volume measurements were made by integrating the output of a mass flow sensor (Model AWM720P1; Honeywell International Inc.; Morristown, NJ) sampled at 1 kHz.

Protocol

We investigated the difference between inspired volumes measured by the flow sensor when placed at the airway (V_A) and in the inspiratory limb (V,) with the standard circuit and the H-bridge circuit. For each circuit, the tidal volume control of the ventilator was adjusted to vary VA to within 5 mL of 200, 300, 400, and 500 mL. At each test volume, after measurements were made at the airway, the flow meter was positioned in the inspiratory limb to record VI.

We also compared inspired volumes measured at the airway with the standard circuit (V_{A S}) and the H- bridge circuit (V^H) for the same ventilator setting to determine if the H-bridge circuit would affect ventilation received by the test lung. The tidal volume control of the ventilator was adjusted to vary V_{A S} to within 5 mL of 200, 300, 400, and 500 mL. At each test volume, after measurements were made with the standard circuit, the H-bridge circuit was connected to record V_A,_H.

Both of the above protocols were executed with the test lung in the normal compliance (20 mL/cmH20) and low compliance (10 mL/cmH20) configuration. Respiratory rate was set at 12 breaths/min, l:E ratio at 1:2, and PEEP at 5 cmH20 for the entire experiment. Reported volumes are the mean of five consecutive breaths.

Statistical analysis

We calculated percent difference between V, and V_A with respect to V_A at each test volume and performed a paired t-test over all test volumes for both the standard circuit and the H-bridge circuit. We also calculated percent difference between V_AjH and V_{A S} with respect to V_{A S} at each test volume and performed a paired t-test over all test volumes.

Results

With the standard circuit, there was a significant difference between V, and V_A at all test volumes with both the normal compliance (p = 0.02) and low compliance (p = 0.01) lung models. The mean±SD percent difference over all test volumes between V, and V_A was 12.31+0.69% with the normal compliance lung model and 26.49+0.78% with the low compliance lung model.

In contrast, with the H-bridge circuit, V| and V_A did not differ at any test volume with both the normal compliance (p - 0.54) and low compliance (p = 0.87) lung models. The mean+SD percent difference over all test volumes between V| and V_A was 0.13+0.23% with the normal compliance lung model and - 0.08±0.43% with the low compliance lung model. Figure 3 summarizes these data. The complete data set is given in Table 1 (figure 9).

Replacement of the standard circuit with the H-bridge circuit did not affect ventilation received by the test lung. There was no difference between V_A/H and V_{A S} at any test volume with both the normal compliance (p = 0.72) and low compliance (p = 0.84) lung models. The meantSD percent difference over all test volumes between V_{A H} and V_AS was 0.03+0.37% with the normal compliance lung model and 0.0l±0.13% with the low compliance lung model. Figure 4 summarizes these data. The complete data set is given in Table 2 (Figure 10). Discussion

Evidence suggesting that tidal volume, rather than pulmonary pressures, are primarily responsible for ventilator-associated lung injury (14) emphasizes the need for accurate tidal volume monitoring.

Our study shows that the H-bridge modification of a standard ventilator circuit enables measures of inspired tidal volume from a flow sensor in the inspiratory limb of the ventilator circuit which are equivalent to measures made at the patient airway. We further show that replacement of the standard ventilator circuit with the H-bridge circuit does not interfere with the function of the ventilator.

When a flow sensor is interposed at the patient airway, consideration must be given to the size and weight of the sensor. Traditionally, differential pressure flow sensors are used at the patient airway since these flows sensors are relatively small and resistant to moisture (15). Differentia! pressure flow sensors, however, provide poor resolution at low flows, and lack accuracy if the output is not precisely compensated for variations in the physical properties of the gas (16-18) or tubing geometry (15) from the conditions under which they were calibrated.

Since, with the H-bridge circuit, the flow sensor is removed from the patient airway, the size and weight of the sensor is less critical. Here, mass flow sensors, which are larger and heavier than differential pressure flow sensors, but provide improved resolution at low flows, include integrated pressure and temperature corrections resulting in improved accuracy, and can compensate for variations in gas composition, may be used for tidal volume monitoring. Therefore, in addition to eliminating the added dead space, weight, and bulk of a flow sensor at the patient airway, the H-bridge circuit enables more accurate and robust measures of the tidal volume than normally obtained at the airway.

References

1. Slutsky AS. Lung Injury Caused by Mechanical Ventilation Chest. 1999 Jul l;116:9S-a-15.

2. Plotz FB, Slutsky AS, van Vught AJ, Heijnen G. Ventilator-induced lung injury and multiple system organ failure: a critical review of facts and hypotheses. Intensive Care Med. 2004

Oct;30(10):1865-72.

3. Dreyfuss D, Saumon G. From ventilator-induced lung injury to multiple organ dysfunction?

Intensive Care Med. 1998 Feb;24(2):l02-4.

4. Muscedere J, Mullen J, Can K, Slutsky A. Tidal ventilation at low airway pressures can augment lung injury. Am. J. Respir. Crit. Care Med. 1994 May l;149(5):1327-34. 5. Chu EK, Whitehead T, Slutsky AS. Effects of cyclic opening and closing at low- and high-volume ventilation on bronchoalveolar lavage cytokines. Crit. Care Med. 2004 Jan;32(l):168-74.

6. Forbat AF, Her C. Correction for gas compression in mechanical ventilators. Anesth.Anaig. 1980 Jul;59(7):488-93.

7. Silvestri S. The influence of flow rate on breathing circuit compliance and tidal volume delivered to patients in mechanical ventilation. Physiol Meas. 2006 Jan;27(l):23-33.

8. Cannon ML, Cornell J, Tripp-Hamel DS, Gentile MA, Hubble CL, Meliones JN, et al. Tidal

volumes for ventilated infants should be determined with a pneumotachometer placed at the endotracheal tube. Am. J. Respir. Crit. Care Med. 2000 Dec;162(6):2109-12.

9. Neve V, Leclerc F, Noizet O, Vernoux S, Leteurtre S, Forget P, et al. Influence of respiratory system impedance on volume and pressure delivered at the Y piece in ventilated infants. PediatrCrit Care Med. 2003 Oct;4(4):418-25.

10. Heulitt MJ, Thurman TL, Holt SJ, Jo CH, Simpson P. Reliability of displayed tidal volume in infants and children during dual-controlled ventilation. PediatrCrit Care Med. 2009

Nov;10(6):661-7.

11. Castle RA, Dunne G, Mok Q, Wade AM, Stocks J. Accuracy of displayed values of tidal volume in the pediatric intensive care unit. Crit. Care Med. 2002 Nov;30(ll):2566-74.

12. Figueras J, Rodriguez-Miguelez JM, Botet F, Thio M, Jimenez R. Changes in TcPC02 regarding pulmonary mechanics due to pneumotachometer dead space in ventilated newborns. J Perinat Med. 1997; 25(4):333-9.

13. Claure N, D'Ugard C, Bancalari E. Elimination of ventilator dead space during synchronized ventilation in premature infants. J. Pediatr. 2003 Sep; 143(3):315-20.

14. Dreyfuss D, Soler P, Basset G, Saumon G. High inflation pressure pulmonary edema. Respective effects of high airway pressure, high tidal volume, and positive end-expiratory pressure. Am. Rev. Respir. Dis. 1988 May;137(5):1159-64.

15. Kreit JW, Sciurba FC. The accuracy of pneumotachograph measurements during mechanical ventilation. Am. J. Respir. Crit. Care Med. 1996 Oct; 154(4 Pt l):913-7.

16. Muller NL, Zamel IM. Pneumotachograph calibration for inspiratory and expiratory flows during He02 breathing. J Appl Physiol. 1981 Oct;51(4): 1038^1.

17. Turney SZ, Blumenfeld W. Heated Fleischpneumotachometer: a calibration procedure. J Appl Physiol. 1973 Jan;34(l):117-21.

18. Grenvik A, Hedstrand U, Sjogren H. Problems in pneumotachograph^. Acta Anaesthesiol Scand. 1966;10(3):147-55.

Claims

1. A ventilator breathing circuit comprising:

(a.) an inspiratory limb including a proximal first conduit portion, the first conduit portion conducting flow downstream to a subject airway interface;

(b.) an expiratory limb including a second conduit portion, the second conduit portion fluidly connectable to or connected to the first conduit portion proximal to the subject via a proximal connecting portion, the proximal connecting portion integral to or adapted to be connected to a subject airway interface;

(c.) at least one flow sensor portion fluidly connected to the inspiratory limb for measuring flow to the subject via the proximal connecting portion; and

(d.) a distal connecting portion fluidly connecting the inspiratory limb and the expiratory limb; wherein the one way inspiratory valve is positioned in the proximal connecting portion or first conduit portion and wherein the one way expiratory valve is positioned in the proximal connecting portion or second conduit portion and wherein the flow sensor portion is adapted to be operatively connected to a flow sensor system, the flow sensor system including one or more flow sensors positioned to determine net flow through the one way inspiratory valve, the ventilator breathing circuit configured so that, when the ventilator breathing circuit is in use, air flow resistance through the distal connecting portion is less than air flow resistance through an alternate flow path through the one way inspiratory valve such that the flow sensor measures a volume of gas inspired by a subject to the substantial exclusion of a compressed volume of gas in the ventilator breathing circuit that is not inspired by the subject.

2. A ventilator breathing circuit according to claim 1, wherein a flow sensor is positioned between the first conduit portion and the distal connecting portion.

3. A ventilator breathing circuit according to claim 2, wherein the flow sensor is bi-directional.

4. A ventilator breathing circuit according to claim 1 or 2, wherein the flow sensor adapted to be connected to a processor which integrates the flow to measure tidal volume, the processor optionally one which controls the ventilator.

5. A ventilator breathing circuit according to any of the preceding claims, wherein the flow sensor is positioned substantially adjacent to the one way inspiratory valve (upstream therefrom).

6. A ventilator breathing circuit according to claim 2, wherein the flow sensor is positioned in the inspiratory limb at a distance from the first portion such that a compressed volume of gas is contained between the one way inspiratory valve and the flow sensor, and wherein a processor is operatively associated with the flow sensor to compute tidal volume based on output from the flow sensor by accounting for both the flow of gas in a direction away from the proximal connecting portion and the flow of gas in a direction towards the proximal connecting portion.

7. A ventilator breathing circuit according to claim l, wherein a first flow sensor is positioned in the inspiratory limb upstream from the distal connecting portion and wherein a second flow sensor is positioned to measure flow through the distal interconnecting portion.

8. A ventilator breathing circuit kit comprising:

(a) one or more components of an inspiratory limb including a first conduit portion optionally comprising a one way inspiratory valve, the first conduit portion comprising a connecting portion for optionally fluidly connecting the first conduit portion to a flow sensor portion, the flow sensor portion adapted to be connected to the first conduit portion at a position remote from its connection to a proximal connecting portion (the proximal connecting portion optionally comprising the one way inspiratory valve), the proximal connecting portion of type integral to or adapted to be connected to a subject airway interface;

(b) one or more components of an expiratory limb including a second conduit portion optionally comprising a one way expiratory valve (the one way expiratory valve is optionally in the one proximal connecting portion);

(c) a distal connecting portion for connecting the inspiratory limb and the expiratory limb, the distal connecting portion adaptable or adapted to be fluidly connected to the flow sensor portion at a position remote from its connection to the first conduit portion, the distal connecting portion configured to provide less air flow resistance, when the ventilator breathing circuit is in use, than air flow resistance along an alternative flow path, such that when parts of the kit are assembled and connected to a ventilator, the flow sensor portion is positioned for measuring a volume of gas inspired by a subject to the substantial exclusion of a volume of gas in the breathing circuit that is not inspired by the subject.

9. A kit as claimed in claim 8, wherein the distal connecting portion comprises a portal for connection to the expiratory limb and a separate portal for connecting to the inspiratory limb , the expiratory limb comprising a portal for connecting to the distal connecting portion positioned remotely from the one way expiratory valve, the inspiratory limb comprising a portal for connecting to the distal connecting portion positioned more remotely than a portal for connection to the flow sensor portion.

10. A kit according to claim 8 or 9, adapted for operative connection to a flow sensor that is unidirectional or bi-directional.

11. A kit according to claims 8, 9 or 10, comprising a proximal connecting portion in the form of a Wye connector.

12. The use of a kit according to any of claims 8 to 11, in conjunction with a flow sensor portion, to assemble a ventilator breathing circuit, the flow sensor portion including a unidirectional or bidirectional flow sensor.

13. The use of a kit according to claim 12, in conjunction with a ventilator, for measuring, on a breath by breath basis, volumes of gas inspired by a subject breathing on the ventilator, the flow sensor operatively connected to a processor to integrate the flow through the flow sensor.

14. The use of a kit according to claim 13 wherein the processor is a ventilator processor.

15. A ventilator of the type having a flow sensor, a processor for receiving input from the flow sensor and integrated conduit portions configured for connection to an external ventilator breathing circuit of the type comprising an inspiratory limb optionally including a one way inspiratory valve and a distally positioned first ventilator connection portion, and an expiratory limb optionally including a one way expiratory valve and a distally positioned second ventilator connection portion, and a proximal connecting portion for connecting the inspiratory limb and the expiratory limb, the proximal connecting portion integral to or adapted to be connected to a subject airway interface, the ventilator characterized in that the integrated conduit portions include an inspiratory conduit portion operatively connected to an inspiratory limb port, an expiratory conduit portion fluidly connected to an expiratory limb port, at least one flow sensor portion including a bi-directional flow sensor and a distal connecting portion connecting the inspiratory conduit portion and the expiratory conduit portion at a position distal to the proximal connecting portion, the flow sensor portion optionally positioned between the inspiratory conduit portion and the flow sensor by-pass portion (optionally a second flow sensor may be positioned in the by-pass portion), and wherein, when the ventilator breathing circuit in use, air flow resistance through the distal connecting portion is less than air flow resistance through an alternate flow path through the breathing circuit such that the flow sensor is positioned to measure a volume of gas inspired by a subject to the substantial exclusion of a volume of gas that is not inspired by the subject, the processor adapted to compute a tidal volume based on output from the flow sensor by accounting for gas flow in both directions through the flow sensor.

16. A method of measuring the tidal volume per breath of a subject breathing on a ventilator comprising connecting a ventilator breathing circuit according to any of claims 1 to 7 to a ventilator and obtaining output from the flow sensor.

17. A method of measuring a tidal volume per breath of a subject breathing on a ventilator comprising:

(1) connecting a flow sensor by-pass limb to a ventilator breathing circuit of the type comprising an inspiratory limb optionally including a one way inspiratory valve, a distally positioned ventilator connection portion and an expiratory limb optionally including a one way expiratory valve, and a distally positioned ventilator connection portion, the inspiratory limb adapted to be connected to a flow sensor portion including a flow sensor at a position optionally between the one-way inspiratory valve and the distally positioned ventilator connection portion, the flow sensor by-pass limb placed to interconnect the inspiratory limb and expiratory limb at a position within the inspiratory limb that is distal to the flow sensor portion and at a position within the expiratory limb that is between the one way expiratory valve and the distally positioned ventilator connection portion of the expiratory limb, the flow sensor by-pass limb configured such that the flow sensor by-pass limb offer less resistance to flow than through an alternate path within the breathing circuit;

(2) obtaining output from the flow sensor; and

IS. A method according to claim 17, wherein the flow sensor is a bi-directional flow sensor positioned in the inspiratory limb at a distance from the one way inspiratory valve resulting in accumulation of a volume of compressed gas between the one way inspiratory valve and the flow sensor in each respective inspiratory cycle, this respective volume of compressed gas flowing back through the flow sensor via the flow sensor by-pass limb in each respective ensuring expiratory cycle, and wherein a tidal volume for a breath is computed by accounting for gas flow in both directions through the flow sensor.

19. A flow sensor by-pass device adapted for use with a flow sensor and a ventilator breathing circuit of a type generally comprising an inspiratory limb and an expiratory limb at least one of which is adapted for connection to a flow sensor, the inspiratory limb including a port configured for connection to a proximal connecting portion e.g. Wye connector (the one way valves are optionally in the Wye connector), a distally positioned first inspiratory conduit portion optionally comprising a one way inspiratory valve and a more distally positioned second inspiratory conduit portion comprising a ventilator connection port, the expiratory limb including a port configured for connection a proximal connecting portion, a distally positioned first expiratory conduit portion optionally comprising a one way expiratory valve, and a more distally positioned second expiratory conduit portion comprising a port for connection to a ventilator, the flow sensor by-pass device comprising:

(1) a flow sensor by-pass portion;

(2) an inspiratory conduit portion including a first port for connection to a flow sensor portion and a second port for connection to the second inspiratory conduit portion in a position remote from the ventilator connection port and a portal to the flow sensor by pass portion positioned between the first port and second port, the flow sensor portion comprising a port for connection to the first inspiratory conduit portion via a port positioned remotely from the port configured for connection to a Wye connector;

(3) an expiratory conduit portion including a third port for connection to the first expiratory conduit portion and a fourth port for connection to a second expiratory conduit portion in a position remote from the ventilator connection port and a portal to the flow sensor by pass portion positioned between the third port and fourth port; wherein the flow sensor by pass portion is configured to interconnect the expiratory limb and inspiratory limb such that when the flow sensor by-pass device is in use in a ventilator breathing circuit, air flow resistance through the flow sensor by-pass portion is less than air flow resistance through an alternative flow path within the ventilator breathing circuit.

20. A kit for assembling a flow sensor by pass device comprising at least the following parts: A. an inspiratory conduit portion; B. an expiratory conduit portion; and C. a flow sensor by-pass portion.

21. A ventilator breathing circuit according to any of claims 1 to 7, further comprising a second flow sensor portion between the one way expiratory valve and distal connecting portion.

22. A ventilator breathing circuit comprising:

(a.) an inspiratory limb including a first conduit portion (optionally comprising a one way inspiratory valve), the first conduit portion conducting flow downstream to a subject airway interface;

(b.) an expiratory limb including a second conduit portion (optionally comprising a one way expiratory valve), the second conduit portion fluidly connectable to or connected to the first conduit portion proximal to the subject via a proximal connecting portion, the proximal connecting integral to or adapted to be connected to a subject airway interface;

(c.) at least one flow sensor portion fluidly connected to the expiratory limb for measuring flow away from the subject via the proximal connecting portion; and

(d.) a distal connecting portion fluidly connecting the inspiratory limb and the expiratory limb; wherein the one way inspiratory valve is positioned in the proximal connecting portion or first conduit portion and wherein the one way expiratory valve is positioned in the proximal connecting portion or second conduit portion and wherein the flow sensor portion is adapted to be operatively connected to a flow sensor system (optionally substantially adjacent to where the first and second conduit portions join, optionally immediately adjacent to where the first and second conduit portions join) and wherein the flow sensor portion is adapted to be operatively connected to a flow sensor system, the flow sensor system including one or more flow sensors positioned to determine net flow through the one way expiratory valve, the ventilator breathing circuit configured (e.g. the diameter of the distal connecting portion) so that, when the ventilator breathing circuit is in use, air flow resistance through the distal connecting portion is less than air flow resistance through an alternate flow path through the one way expiratory valve such that the flow sensor measures a volume of gas expired by a subject to the substantial exclusion of a compressed volume of gas in the ventilator breathing circuit that is not expired by the subject.

23. A ventilator of the type having a flow sensor, a processor for receiving input from the flow sensor sufficient to compute an expired tidal volume and integrated conduit portions configured for connection to an external ventilator breathing circuit of the type comprising an inspiratory limb optionally including a one way inspiratory valve and a distally positioned first ventilator connection portion, and an expiratory limb optionally including a one way expiratory valve and a distally positioned second ventilator connection portion, and a proximal connecting portion for connecting the inspiratory limb and the expiratory limb (the one way valves may optionally be positioned in the proximal connecting portion, optionally immediately adjacent (upstream) to where the inspiratory and expiratory limbs join), the proximal connecting integral to or adapted to be connected to a subject airway interface, the ventilator characterized in that the integrated conduit portions include an inspiratory conduit portion fluidly connected to an inspiratory limb port, an expiratory conduit portion fluidly connected to an expiratory limb port, at least one flow sensor portion including a bi-directional flow sensor and a distal connecting portion connecting the inspiratory conduit portion and the expiratory conduit portion at a position distal to the position of the proximal connecting portion, the flow sensor portion optionally positioned between the expiratory conduit portion and the distal connecting portion, and wherein, when the ventilator breathing circuit in use, air flow resistance through the distal connecting portion is less than air flow resistance through an alternate flow path through the breathing circuit such that the flow sensor is positioned to measure a volume of gas expired by a subject to the substantial exclusion of a volume of gas that is not expired by the subject, the processor adapted to compute an expired tidal volume based on output from the flow sensor by accounting for gas flow in both directions through the flow sensor.

24. A method of measuring an expired tidal volume per breath of a subject breathing on a ventilator comprising:

(1) connecting a flow sensor by-pass limb to a ventilator breathing circuit of the type comprising an inspiratory limb including a one way inspiratory valve, a distally positioned ventilator connection portion and an expiratory limb including a one way expiratory valve and a distally positioned ventilator connection portion, the expiratory limb adapted to be connected to a flow sensor portion including a flow sensor at a position between the one-way expiratory valve and the distally positioned ventilator connection portion, the flow sensor by-pass limb placed to interconnect the inspiratory limb and expiratory timb at a position within the expiratory limb that is distal to the flow sensor portion and at a position within the inspiratory limb that is between the one way inspiratory valve and the distally positioned ventilator connection portion of the inspiratory limb, the flow sensor by-pass limb configured such that the flow sensor bypass limb offer less resistance to flow than through an alternate path within the breathing circuit; (2) obtaining output from the flow sensor; and

(3) mathematically integrating flow sensor output corresponding to a breath to obtain an expired tidal volume for the breath.

25. A flow sensor by-pass device adapted for use with a flow sensor and a ventilator breathing circuit of a type generally comprising an inspiratory limb and expiratory limb at least one of which is adapted for connection to a flow sensor, the inspiratory including a port configured for connection to a proximal connecting portion e.g. Wye connector, a distally positioned first inspiratory conduit portion comprising a one way inspiratory valve and a more distally positioned second inspiratory conduit portion comprising a ventilator connection port, the expiratory limb including a port configured for connection a proximal connecting portion, a distally positioned first expiratory conduit portion comprising a one way expiratory valve, and a more distally positioned second expiratory conduit portion comprising a port for connection to ventilator, the flow sensor by-pass device comprising:

(1) a flow sensor by-pass portion;

(2) an inspiratory conduit portion including a first port for connection to the proximal connecting portion and a second port for connection to the second inspiratory conduit portion in a position remote from the ventilator connection port and a portal to the flow sensor by pass portion positioned between the first port and second port;

(3) an expiratory conduit portion including a third port for connection to a flow sensor portion and a fourth port for connection to a second expiratory conduit portion in a position remote from the ventilator connection port and a portal to the flow sensor by pass portion positioned between the third port and fourth port, the flow sensor portion comprising a port for connection to the first expiratory conduit portion via a port positioned remotely from the port configured for connection to the proximal connecting portion; wherein the flow sensor by pass portion is configured to interconnect the expiratory limb and inspiratory limb such that when the flow sensor by-pass device is in use in a ventilator breathing circuit, air flow resistance through the flow sensor by-pass portion is less than air flow resistance through an alternative flow path within the ventilator breathing circuit.

26. A ventilator breathing circuit according to claim 22, wherein a flow sensor is positioned between the second conduit portion and the distal connecting portion.

27. A ventilator breathing circuit according to claim 22, wherein the flow sensor is bi-directional.

28. A ventilator breathing circuit according to claim 26 or 27, wherein the flow sensor adapted to be connected to a processor which integrates the flow to measure tidal volume, the processor optionally one which controls the ventilator.

29. A ventilator breathing circuit according to any of claim 22, 26 or 27, wherein the flow sensor is positioned substantially adjacent to the one way expiratory valve (upstream therefrom).

30. A ventilator breathing circuit according to claim 22, wherein the flow sensor is positioned in the expiratory limb at a distance from the second portion such that a compressed volume of gas is contained between the one way expiratory valve and the flow sensor, and wherein a processor is operatively associated with the flow sensor to compute tidal volume based on output from the flow sensor by accounting for both the flow of gas in a direction away from the proximal connecting portion and the flow of gas in a direction towards the proximal connecting portion.

31. A ventilator breathing circuit according to claim 22, wherein a first flow sensor is positioned in the expiratory limb upstream from the distal connecting portion and wherein a second flow sensor is positioned to measure flow through the distal interconnecting portion.