AU2003271342A1

AU2003271342A1 - A method of continuous cardiac output measurement from the airway

Info

Publication number: AU2003271342A1
Application number: AU2003271342A
Authority: AU
Inventors: William Geoffrey Parkin
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-12-30
Filing date: 2003-12-24
Publication date: 2004-07-15

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD

PATENT

A METHOD OF CONTINUOUS

CARDIAC

OUTPUT MEASUREMENT FROM THE AIRWAY The following statement is a full description of this invention, including the best method of performing it known to me: A METHOD OF CONTINUOUS CARDIAC OUTPUT MEASUREMENT FROM THE

AIRWAY

This invention relates to improvements in devices to measure the cardiac output in mammals including humans by fast processing of airway carbon dioxide signals.

The cardiac output is the flow of blood from the heart in litres per minute and is a very important signal in the control of the circulation.

Since the pulmonary blood flow is almost identical to the systemic blood flow from the heart,measurement of the pulmonary blood flow effectively provides knowledge of the cardiac output.

A number of methods using the subject's own carbon dioxide production are described for measuring pulmonary blood flow.

The Fick Method In the Fick method the arterial (C.CO 2 and venous (CCO 2 carbon dioxide contents of blood

(LCO

2 /Lblood) are obtained by blood sampling and the carbon dioxide evolution (V.CO 2 from the lung is measured. The cardiac output, CO, is obtained from CO VxC02 CvC02 C.C02 Equation I This method is discontinuous and invasive.

The Derivative Fick Method In the derivative Fick method, the difficulty is sampling the venous CO 2 content. (CvCO 2 is circumvented by producing transient change in VXCO 2 and CaCO 2 by causing the subject to rebreathe exhaled gas. The rebreathing interval (30 40 seconds) is insufficient to cause change in the venous CO 2 content, (ie ACCO 2 0) thus, differentiating Equation 1 co=

AVCO

AC CO2 AC.CO2 but ACCO 2 0 whence CO AVC02

AC.CO

2 Equation 2 Change in the arterial content of CO 2

(AC-CO

2 relates to change in the arterial partial pressure of CO 2 (Ap.CO 2 via the local slope, S, of the CO 2 dissociation curve, ie

AC.CO

2 SAp 0 C0 2 Substituting in Equation 2 co A VCO 2 S.PoCO 2 Equation 3 Change in paCO2 approximates closely to change in alveolar pCO 2 PACO2 and PACO2 approximates to peICO 2 the end tidal pCO 2 of expired breath, ie ApC.

0 2 APA C 2 -,vAPet CO 2 Substituting in Equation 3 CO= VC02 S.ApeCO 2 Equation 4 Equation 4 is the basis of commercially available instruments for the measurement of cardiac output (CO) from the airway. In such measurement the subject rebreathes for 30 40 seconds and the new steady state VxCO 2 and petCO 2 allow calculation of AVCO 2 Ap 0 1CO 2 from the non rebreathing state. Particularly, in the rebreathing interval pvCO 2 and CC 2 have not changed (ACICO 2 In practice, the derivative Fick method is intermittent for after each rebreathing cardiac output estimation interval the CO 2 economy of the circulation must be allowed to re-equilibrate for approximately 3 minutes.

Second, since the method involves snapshot 'before' and 'after' measurements of VCO 2 and petCO2 irregular breathing, in locally perturbing these measurements, leads to inaccuracy.

The Invention (i)Theoretical Background In this invention we propose that information about the cardiac output is continuously available from the ratio Co- AVIC02 S.Apet CO 2 (see Equation 4) as the magnitude of rebreathing is continuously varied either regularly or irregularly or when

VXCO

2 and pXtCO 2 vary in response to the subject's respiratory efforts.

The problem with such an arrangement would appear to be the corruption of the ratio

AVCO

2 by the venous recirculation of the applied CO 2 perturbation. Normally V,C0 2 (S.Ap,C0 2 and p~tCO 2 are reciprocally related, that is, if the subject's CO 2 evolution (VxCO 2 is falling, the alveolar and end tidal pCO 2 (PeCO 2 is rising. If venous recirculation of CO 2 is rising both VCO 2 and ptCO 2 will rise, thus corrupting the ratio (AVxCO 2 /ApeCO 2 The rising venous recirculation will soon be followed however by an equivalent fall so that if the ratio is averaged over medium time a stable ratio will be obtained.

(ii) Mechanical elements The mechanical elements are typically incorporated in a standard respirator circuit. The invention requires means of continuous measurement of the end tidal CO 2 (petCO 2 with each breath and the continuous measurement of the subject's CO 2 evolution, VCO 2 Additionally, means are provided to continuously vary the magnitude of rebreathing in the respirator circuit in order to change ptCO 2 and V.CO 2 The rebreathing may be servo controlled so that the VC0 2 acquires a particular time profile (eg a sine wave) which may be regular or irregular. A corresponding reciprocal profile will occur in the peICO2 signal.

Reference is made to the accompany drawings.

Figure 1 shows the general arrangement. Ventilator is provided with inspire and expire tubes which conduct gas cyclically to and from the subject The expire tube (3) normally contains CO 2 rich gas (dotted; there is normally no CO 2 in the inspired gas).

The expire port of the respirator conducts the expired gas to a sterilisable bag which is aligned with the expire tubing to absorb the kinetic 'shock' of expiration. The expired gas is conducted to a sterilisable mixing box past a side tube which is open to atmosphere.

Mixing box is aspirated by a constant flow suction which is provided by a constant flow fan or the wall suction available in clinical areas. By aspirating room air through the side tube the suction converts the intermittent flow of expiration to a constant flow. The partial pressure of CO 2 in the mixing box exit is measured by the capnograph Because the flow is constant this partial pressure is directly proportional to the subject's CO 2 evolution, VCO 2 The bag, mixing box, suction arrangement acts as a device to scavenge infected expired air from the clinical environment. The end tidal pCO 2 of each breath of the subject is measured immediately proximal to the subject This is a standard clinical measurement often obtainable from a 'clip on' monitor or incorporated with the respirator A microprocessor (10) acquires the two CO 2 signals and because the suction flow is constant, calculates VCO 2 3 and the cardiac output. The microprocessor regulates (and profiles) the rebreathing device, (12).

The novel rebreathing device consists of a cross tube (11) which may be placed at different stations between inspire and expire tubes and provided with a device to regulate the cross flow. This may be performed in a binary (eg clamp) or scalar (eg variable orifice) manner with the binary approach capable of pulse width modulation.

Figure 2 shows the principle of operation of the cross tube. With the cross tube open, during expiration, CO 2 is added to the inspire limb of the tubing. With the cross tube open, during inspiration, CO 2 is inspired from both the inspired and expired limb of the tubing. In one form of the cross tube device, the cross tube (11) consists of a length of sterilisable compliant rubber tubing fixed at each end by the T-tube join to the inspired and expired tubes The clamp (12) has normally closed jaws which are electrically, pneumatically or hydraulically opened by the microprocessor (10) to initiate rebreathing. The operation of the cross tube may be such as to achieve a particular time profile in the VxCO 2 and peCO 2 signals.

Claims

2. The method of claim 1 where the carbon dioxide evolution is measured by measuring the partial pressure of carbon dioxide in a constant flow of gas obtained by mixing the subject's expiration with entrained air in a tube or mixing box.
3. The method of claim 2 where the constant flow of gas is generated by a wall suction system and the apparatus operates as a scavenge for expired gas.
4. The method of claim 1 where the carbon dioxide evolution and end tidal carbon dioxide are obtained by placing a directional flow meter and inline capnograph at a point immediately proximal to the patient and cross multiplying the signed flow and PCO 2 The method of claim 1 where the carbon dioxide evolution is obtained by placing a flow meter and in line capuograph at a point in the expired line downstream of the cross tube (11) and cross multiplying the outputs.
6. The method of claim 1 where the means of changing the rebreathing is by varying the resistance of a cross tube between the inspired and expired line of a breathing apparatus. Such an arrangement when open allows inspired gas to traverse the expired line during inspiration and expired gas to traverse the inspired line during expiration. (This is different from standard valve arrangements which direct inspired gas into the expired line to produce rebreathing.)
7. The means of changing rebreathing claimed in 6 where the cross tube is comprised of a normally open sterilisable pliable tube compressible between the jaws of a controllable clamp.
8. The methods of claims 6 or the method of claim 1 (where other methods of producing rebreatbing are used) where the rebreatbing is servo controlled to produce a designed time profile in the VCO 2 and a reciprocal profile in thle pdCO2.
9. The method of claim I where a cycled solenoid valve is placed on the capnograpbic lines so that one capnograph may be used to sample the two separate CO 2 signals. WILLIAM GEOFFREY PARKIN 22/12/2003 (Name of Applicant) (Date)