WO2013137797A1

WO2013137797A1 - Apparatus and method for monitoring of mechanical ventilation

Info

Publication number: WO2013137797A1
Application number: PCT/SE2013/000035
Authority: WO
Inventors: Björn Jonson
Original assignee: Innotek Ab
Priority date: 2012-03-13
Filing date: 2013-03-11
Publication date: 2013-09-19
Also published as: SE1200155A1

Abstract

The invention relates to an apparatus for monitoring of physiological parameters at mechanical ventilation. The objective is to identify beneficial and injurious effects of ventilator resetting within few breaths while the operator remains at bedside. Monitored parameters include e.g. tidal volume, airway pressures, end tidal CO₂, haemodynamics and volume of CO₂ eliminated per minute, V_MINCO₂. Data sets of each signal before and after resetting are statistically analyzed to reduce noise and to allow accurate presentation of changes in conjunction with resetting. The quotient between V_MINCO₂ after and before resetting is used to illustrate the effect on alveolar ventilation and thereby effects on arterial partial pressure of CO₂. The course of V_MINCO₂ after resetting is analyzed to get an accurate value of V_MINCO₂after resetting. The invention can be used at different modes of ventilation because of noise reducing strategies, which are of particular importance at supported and irregular ventilation.

Description

APPARATUS AND METHOD FOR MONITORING OF MECHANICAL VENTILATION

Description of the invention

Field of the invention

The present invention relates to an apparatus used at mechanical ventilation of man or animal, hereafter referred to as "the patient", for follow up of physiological effects of a change of ventilator setting, allowing the operator to nearly immediately evaluate effects of ventilator resetting according to the preamble of claim 1. Thereby the operator timely receives information allowing judgement of physiological effects of the new setting with regards to therapeutic and physiological goals.

Description of the Prior Art

The properties of the respiratory system comprising airways, lung parenchyma, alveoli, pulmonary blood vessels, heart and thoracic cage are complex, particularly so in disease. The operator of a ventilator, usually a physician or a respiratory therapist, frequently changes the setting of a ventilator. The purpose behind resetting is to reach desired goals of mechanical ventilation. Due to the complexity of physiology, it is in general not possible to foresee which effects a certain resetting will have on respiratory mechanics, gas exchange and circulation. Therefore, the effects need to be evaluated by measuring the physiological status before and after resetting. A particularly important parameter is arterial partial pressure of carbon dioxide, P_aC0₂, that reflects alveolar ventilation and influences arterial pH. The body contains large amounts of exchangeable carbon dioxide, C0₂. This implies that a change of alveolar ventilation leads to a slow change in P_aC0₂. It takes more than 20 minutes to reach a new steady state with regards to P_aC0₂ and arterial pH after resetting. Accordingly, an arterial sample does not properly indicate the effect on P_aC0₂ until long after resetting. During that interval other physiological events may obscure the effect of resetting. Therefore, there is a need for faster feedback to the operator than that obtained with conventional monitoring methods.

C0₂ exchange is only one phenomenon that needs to be monitored during mechanical ventilation, particularly in conjunction with ventilator resetting. High tidal volumes and high airway pressures are injurious to the lung and must be controlled to avoid ventilation induced lung injury, VI LI. In the acute respiratory distress syndrome repetitive lung collapse and re- expansion of lung units, RECOREX, is a particularly injurious process that must be avoided. To get an overview of the ventilation process with respect to goals which should be reached and risks for VILI a number of parameters need to be monitored. These parameters refer to oxygen and carbon dioxide exchange, volumes and pressures associated with the ventilation process. High intrapulmonary pressures may perturb circulation. Therefore also parameters relating to circulation need to be monitored. A large variety of physiological parameters are indeed monitored with several systems on the market, but not in a comprehensive way, easy to survey with relation to goals and dangers of the ventilation process. Delays of the physiological response to resetting and noise in the recorded signals are factors which make it difficult to survey physiological effects of resetting in prior art systems.

A method that may alleviate problems to foresee effects of resetting a ventilator is computer simulation of potential effects of ventilator resetting. Such simulation is based upon a physiological profile of the respiratory system and a mathematical description of the function of a ventilator at different settings. Such a method is described in U.S. patent 6,578,575 B1. The accuracy of the prediction based upon computer simulation is limited by the non- exactness of the physiological profile and by such changes in physiology, which may be the results of the resetting. Accordingly, there is always a need to follow up effects of ventilator resetting.

The invention

The objective of the present invention is to make possible evaluation of physiological effects following ventilator resetting within a limited number of breaths after resetting, preferably while the operator performing the resetting is present at bedside. The physiological effects in question include effects on airway pressure, for example mean airway pressure, peak airway pressure, postinspiratory plateau pressure, and positive end-expiratory pressure, PEEP. Other physiological effects are end tidal concentration of C0₂, volume of C0₂ eliminated by ventilation per minute, V_M|_NC0₂, and change of P_aC0₂ and pH in blood imposed by resetting at a new steady state. Oxygen saturation in blood studied in the periphery, S_P0₂, is an important parameter. Physiological effects are measured with sensors providing signals, which are analyzed using a computer. The sensors and the computer may be integrated with the ventilator into one single apparatus. An alternative is that the sensors are the same as those of the ventilator and that a separate computer receives the signals from those sensors and performs the analysis. Signals representing circulation, such as arterial pressure, are commonly recorded by special monitoring equipment, the information from which may be entered to the system according to a preferred embodiment of the invention. Another alternative according to an embodiment of the invention is that both sensors and the computer are separate from the ventilator. This form of embodiment can be used in combination with different kinds of ventilators. Recording and analysis of signals are started before resetting is performed and continue through the resetting and during a period after resetting. Within a number of breaths, or within few minutes after resetting, the computer reports how selected physiological parameters changed allowing the operator to judge if the changes are in line with expectations and accord with the objectives behind resetting. According to a preferred embodiment of the invention the signals are analyzed with respect to noise and to trends caused by physiological phenomena related to non-instant establishment of a steady state. Thereby effects on for example V_MINC0₂ and PaC0₂ caused by resetting are exposed.

Description of the drawings

Figure 1

Figure 1 illustrates an apparatus that accords with a preferred embodiment of the invention. The apparatus is only schematically depicted, since configuration options are virtually unlimited with modern technology.

A pneumatic inspiratory system of the ventilator comprises inlets for gases like air and oxygen 2, a blender for the gases 3 and a flow controller in the inspiratory line 4. In an alternative embodiment of the invention the blender 3 and the controller 4 are integrated into a single unit. The inspiratory line is equipped with a flow meter 5. Outside the ventilator or integrated into the ventilator the inspiratory line is often equipped with a humidifier 6 and continues in the form of a flexible inspiratory tube 7 that leads to the Y-piece 8. The ventilator is connected to the patient 10 with a tracheal tube 9 but can be connected by other means. Expiration occurs through a pneumatic expiratory system of the ventilator starting at the Y-piece 8 and further through a flexible expiratory tube 11, an expiratory valve 12 and an expiratory flow meter 13. The order of 12 and 13 may be the opposite. A C0₂ analyzer 14 measures fraction of C0₂ at the Y-piece. A pressure transducer 15 measures airway pressure. It can alternatively be connected to the expiratory line 11 or be duplicated in both inspiratory line and in expiratory line.

The function of the ventilator is controlled by an electronic control unit 17 that may be an analogue or digital device. In a preferred embodiment of the invention the control unit comprises at least one computer that records and analyzes the signals from flow, pressure and C0₂ transducers 5, 13, 15 and 14. This computer can also receive signals from systems for monitoring of circulation such as arterial pressure. The control unit is able to communicate with the user through a keyboard, by touch controls or by other means. Communication is also possible from distance, e.g. from a central system in a critical care unit. All the stipulated parts can be integrated into a single apparatus or functionally distributed among different apparatuses. The latter option could mean that e.g. the function serving to control the pneumatic systems is located within the ventilator, whereas e.g. calculation and monitoring functions are physically located in another apparatus such as an external computer.

The control unit receives analogue or digital signals representing flow rate, pressure and C0₂ and sends signals to the inspiratory and expiratory valves 4 and 12 through means for electronic communication 16. The control unit may apart from components within the ventilator itself comprise components and systems outside the ventilator. The technique of today offers virtually limitless possibilities to embody the invention with respect to technical solutions of electronic components and their communication with each other by wired or wireless means. Monitoring of the ventilation process may be achieved by a system incorporated in the ventilator or by a system outside the ventilator but communicating with the control unit inside the ventilator. The control unit 17 is in a preferred embodiment of the invention equipped with a visual screen for monitoring of flow and pressure signals and for display of other information.

Figure 2

Figure 2 illustrates an alternative preferred embodiment of the invention in which the numbers 1-17 indicate the same structures as in Figure 1. The system used for monitoring according to the present invention is embodied with an apparatus that is separate from the ventilator 1. The monitoring apparatus comprises a computer 20 and transducers for C0₂ 14 flow rate 18 and airway pressure 19, which through wired or wireless means of communication 21 send signals to the computer 20. According to the embodiment illustrated in Figure 2 the invention can be applied to any type of ventilator.

According to a further embodiment of the invention the computer 20 may receive signals for one or more of the parameters flow rate, airway pressure and C0₂ from transducers integrated in the ventilator thus avoiding duplication of transducer equipment. The computer 20 may also have access to other information from the ventilator 1 such as ventilator setting, respiratory rate and information about timing of partitions of the respiratory cycle through a digital or analogue, wired or wireless communication link. Likewise, the computer 20 may receive information from other sources such as those used for monitoring of circulation like arterial pressure. Figure 3

Figure 3 illustrates a monitoring screen in conjunction with ventilator resetting in a patient with ARDS that was performed at the time 2 minutes. The upper panel A shows Respiratory rate, tidal volume, V_T, and Positive End Expiratory Pressure, PEEP. V_T is obtained by integration of the flow signal over each breath. PEEP is measured by the transducer 15 or 19. Panel B shows post inspiratory plateau pressure, P_PLAT, and the difference between P_PLAT and PEEP denoted delta-P. Ideally, delta-P should represent the true difference of the elastic recoil pressure of the respiratory system after and before inspiration. To measure these values exactly, the patient needs to be passive and airway flow rate zero at these moments. Zero flow rate can be achieved with some modern ventilators by closing the valves 4 and 12. Under circumstances this is not achievable or desirable. Then, P_PLAT can be determined at minimal inspiratory flow rate towards the end of inspiration. PEEP can be determined at minimal expiratory flow rate towards the end of expiration. These PEEP values can be corrected for flow and resistance dependent pressure gradients in the airways according to principles described by Jonson et al. (Bull. Physiopath. Resp. 1975, v 11 , pp 729-743) Panel C illustrates the volume of C0₂ eliminated per minute through ventilation, V_M|_NC0₂. V_MINC0₂ is calculated from the volume of C0₂ eliminated during each breath divided by the duration of the breath. V_M|_NC0₂ is derived from integration of the product between flow rate and fraction of C0₂ measured at airway opening. Panel D is a recording of peripheral saturation of oxygen, S_P0₂, measured by conventional means and Panel E shows end tidal C0₂, E_TC0₂.

In the example.at the initial ventilator setting, monitoring showed that V_T, PPLAT and delta-P were higher than conventionally recommended. PaC0₂ was also high, 62 mmHg. In order to comply with recommendations, at 2 minutes RR was increased from 25 to 33 breaths per minute, V_T was reduced from 6.9 to 5.8 ml per kg body weight and PEEP reduced from 12 to 8 cmH₂0. Immediately after resetting, pressures fell to within recommended values. The immediate increase in V_M!NC0₂ was followed by a slow decline towards initial level. S_P0₂ and E_TC0₂ fell slightly with some delay. Figure 4

Figure 4 Panel A illustrates how different kind of noise can affect the signals, e.g. the one representing V_MINC0₂. Such noise will negatively affect the precision in determination of the change in V_MINC0₂ that occurs after ventilator resetting, in this example at 2 minutes. A slow drift is common in V_UINC0₂ that may reflect changing metabolic rate due to variation in body temperature or varying inflammatory activity in the body as well as other reasons for changing homeostasis. In the example in Figure 4 there is a trend towards increasing V_MINC0₂. A second type of noise is caused by breath by breath variation in volume of C0₂ eliminated. This variation has several causes such as measurement errors, breath by breath variation in tidal volume and varying pulmonary perfusion at heart arrhythmia.

The slow drift was in the example in Figure 4 mathematically characterized during the 2 minutes before resetting. The heavy straight line in Panel A shows this trend extrapolated to 11 minutes. After setting the trend value at 2 minutes to zero the trend was subtracted from the signal as shown in Panel B. After ventilator resetting, determination of the immediate change in V_MINC0₂, AV_MINC0₂, is an important aspect of the invention. It is therefore important to minimize the influence of breath by breath noise on AV_M|_NC0₂. This must be done without applying conventional filtering of the signal over the resetting period.

Before resetting, breath by breath noise in V_M)NC0₂ is reduced by statistical analysis of values before resetting, as illustrated by the solid line during the initial 2 minutes. The value of V_MINC0₂ before resetting is denoted V_M|_NC0₂baseline. At resetting, V_M|_NC0₂ will immediately increase or decrease to a new value V_M|_NC0₂reset. The change will be indirectly proportional to the change in effective alveolar ventilation. During the following period of 15-30 minutes V_M|_NC0₂ will slowly return towards the value that represents current metabolic C0₂ production following an exponential course. To reduce breath by breath noise in V_M|_NC0₂ after resetting, V_M|_NC0₂ data after resetting are used to analyze this course by statistical means as further described below and in Figure 4 shown by a heavy line representing decline towards baseline. The immediate change of V_M|_NC0₂ after resetting, AV_M|_NC0₂, is determined from the difference between the V_Mi_NC0₂reset and V_Mi_NC0₂baseline as illustrated by interrupted vertical line in Figure 4. Figure 5

Figure 5 shows a numerical display of the same parameters as depicted in figure 3 based on data during 2 minutes before ventilator resetting and 15 s to 2 minutes after resetting. A change is according to a preferred embodiment of the invention accompanied by information about its statistical significance. In this example significant changes (p<0.05) are highlighted in bold characters. SE is standard error of the estimation. Description of preferred embodiments

The system is based upon sensors for airway flow rate, airway pressure and C0₂ as illustrated in Figure 1 and 2. Flow rate and airway pressure may be measured within the ventilator 5, 13 and 15 in Figure 1 or at the airway opening of the patient 18, 19 in Figure 2. According to preferred embodiments, C0₂ is measured at the airway opening 14. In an alternative embodiment of the invention, C0₂ is measured somewhere along the expiratory pneumatic system. Then, the small volume of C0₂ re-inspired at the start of inspiration from the Y-piece 8 and the inspiratory and expiratory lines 7 and 11 will not be measured. This limitation may be balanced by technical advantages of this alternative embodiment. For all embodiments of the invention, signals for flow rate, airway pressure and C0₂ should have an adequate frequency response and be adequately in synchrony with each other so that events during breaths representing each signal or combinations of signals can be accurately recorded and monitored. Optional transducers for S_P0₂, arterial pressure and other signals are foreseen to be incorporated in alternative preferred embodiments of the invention.

A computer that may be integrated into the ventilator 17 or be a separate computer 20 samples the signals for C0₂, airway pressure and flow at an adequate rate. These signals, together with data calculated from the signals and other information may be displayed and stored by the computer in accordance with conventional monitoring systems. Accordingly, volumes are calculated by integration of flow rate over time. Respiratory rate in breaths per minute may be derived from signals controlling the valves of the ventilator 4, 12 or from analysis of pressure and flow signals by the computer 17 or 20.

The objective of the present invention is to monitor parameters providing the most essential information with respect to goal achievement of mechanical ventilation and risks associated with mechanical ventilation and particularly how ventilator resetting affects these parameters. Figure 3 illustrates an example of combinations of such parameters. The combination of parameters may be varied for example with the nature of the disease of the patient and availability of optional transducer signals. The traced parameters are accompanied by numeric information. An example is given in Figure 5. Figure 3 Panel A shows respiratory rate, PEEP and V_T, all of which at controlled ventilation are parameters directly related to ventilator setting. Panel B shows the end expiratory plateau pressure, PPLAT, that is the pressure at zero of very low flow rate at the end of an inspiration. P_PLA_T is the commonly used parameter that indicates the degree of lung distension that when too high will cause lung damage related to hyperdistension. delta-P is the difference between P_PLAT and PEEP and is a recommended parameter to estimate risks for VI LI. Panel C describes the elimination of C0₂ eliminated per minute, V_M|_NC0₂. The value is in a preferred embodiment of the invention calculated breath by breath from the integral of the product of flow rate and C0₂ determined at airway opening with the transducers 5, 13, 14 and 18. Panels D and E show tracings of peripheral oxygen saturation and end tidal C0₂ concentration.

The primary objective of ventilation, mechanical or spontaneous, is exchange of 0₂ and C0₂ so that arterial blood will acquire adequate properties with respect to these gases. For oxygen, focus is often on saturation of haemoglobin that can easily be measured in the periphery, S_P0₂. For C0₂, the arterial partial pressure of C0₂, PaC0₂, is the parameter of primary interest. PaC0₂ cannot be accurately estimated without analysis of arterial blood samples. Then, apart from blood sampling and costs, a further problem is that the change in PaC0₂ occurs slowly after a change of alveolar ventilation caused by ventilator resetting. This is due to large C0₂ stores in body fluids. It may take 30 minutes or more before a new steady state is reached after a change of alveolar ventilation. An alternative to PaC0₂ is to measure end tidal partial pressure of C0₂, E_TC0₂. E_TC0₂ is easily obtained from the transducer 14. However, in lung disease E_TC0₂ often differs greatly from PaC0₂. Furthermore, a change of E_TC0₂ occurs after a similar delay as PaC0₂ and is influenced by several physiological effects which change with time and with ventilator setting. There is no available method to directly measure or accurately estimate PaC0₂ at bedside in conjunction with ventilator resetting. However, a change in PaC0₂ in conjunction with ventilator resetting, APaC0₂, can according to the invention be estimated within short after resetting as explained below.

V_M|_NC0₂ averaged over so long time that body stores of C0₂ may be regarded as constant represents the rate of C0₂ production that is proportional to aerobic metabolic rate. A change of V_M|_NC0₂ observed after ventilator resetting over such a short time that metabolic rate and body stores of C0₂ can be regarded as constant reflects a change of efficient alveolar ventilation, which equals total ventilation minus physiological dead space ventilation. Therefore, the determination of the change in V_MINC0₂ occurring immediately after resetting, AV_M|_NC0₂, is of particular interest. In stable conditions, such as in controlled ventilation of a sedated patient, AV_M|_NC0₂ can often be roughly estimated from observation of the recorded signal Figure 3, Panel C. Very commonly the signal representing V_M|_NC0₂ is affected by noise from many sources. Then, an accurate estimation of AV_MINC0₂ cannot be made from visual analysis of the V_M|_NC0₂ signal.

Figure 4 with associated text illustrates how the influence of noise on measured AV_MINC0₂ according to a preferred embodiment of the invention is reduced to a level that under most circumstances is adequately low. V_M|_NC0₂baseline is calculated from statistical analysis of values during a period preceding ventilator resetting. This analysis not only minimizes breath by breath variation but also allows determination of slow drift. Values after ventilator resetting are corrected for the slow drift characterized before resetting. V_MINC0₂reset is according to preferred embodiments of the invention obtained by statistical analysis of data after resetting. This analysis serves to characterize the slow asymptotic return towards steady state corresponding to the metabolic production of C0₂. According to a preferred embodiment of the invention an exponential return is anticipated, but similar results may be obtained using alternative mathematical expressions.

Ventilator resetting does not always occur at a specific moment, e.g. when more than one parameter is changed as in the example behind Figure 3. Furthermore, a feature of some ventilators is that ventilator setting is not immediately executed at the moment of resetting but implemented over some breaths. Accordingly, the first breaths following the initiation of resetting must pass before data defining the course of V_M|_NC0₂ after resetting is characterized. 5 breaths or 15 s after the last resetting is according to a preferred embodiment of the invention the default for delay of analysis, which by the operator can be prolonged but preferably not to more than about 30 s after last resetting. If longer, the accuracy may decline because of changing C0₂ stores in body fluids and because data which could lessen the influence of noise are lost. When the course of V_M|_NC0₂ after resetting has been characterized, the equation is extrapolated backwards to the time of resetting to obtain V_MiNC0₂reset representing the time of resetting and to calculate AV_MINC0₂ as the difference between V_M|_NC0₂reset and V_M|_NC0₂baseline. AV_M)NC0₂ denotes the change of C0₂ elimination at the time of ventilator resetting.

The accuracy of AV_M)NC0₂ is reduced by noise of the V_MINC0₂ signal. The level of noise is according to a preferred embodiment of the invention statistically analyzed. Thereby the influence of noise on the determination of AV_M|_NC0₂ can be estimated by applying ordinary statistical methods. The fraction AV_MiNC0₂reset/V_MiNC0₂baseline shows to what extent alveolar ventilation changed in conjunction with ventilator resetting and can be expressed in percent, AV_M|_NC0₂%. In the example given in Figure 4, data sets for V_M)NC0₂ 2 minutes before and 15 s to 2 minutes after resetting were analyzed. Two minutes may be a suitable default value for those periods. In the presence of heavy noise in the tracing of V_MINC0₂ longer periods may be automatically or manually instituted in order to increase the accuracy of AV_MI C0₂ determination. At controlled ventilation respiratory rate is usually constant. Then, values of eliminated volumes of C0₂ per breath may be used instead of values for V_MINC0₂ for calculation of the change of C0₂ elimination in conjunction with ventilator resetting.

PaC0₂ is proportional to metabolic C0₂ production and indirectly proportional to efficient alveolar ventilation. As a consequence, the change of PaC0₂ that will follow resetting after equilibration of body fluid stores of C0₂ is according to a preferred embodiment of the invention estimated from AV_MINC0₂%. In the example in Figure 4, AV_MINC0₂% was +7% with an estimated range between +6 and +8%. The values for range were estimated using ordinary statistical methods as the 95% confidence interval. This interval serves as default according to a preferred embodiment of the invention. Accordingly, the expected relative change of PaC0₂ after equilibration was -7%, range -8 to -6%. An alternative to range is to report standard error of the estimated change of PaC0₂ as illustrated in Figure 5. If information of the PaC0₂ value before resetting is available in the system, the estimated PaCC½ value after resetting will according to a preferred embodiment of the invention be presented to the operator. If the computer 17 or 20 also has access to present pH value and the acid base status for example expressed as Base excess, the computer can also calculate the expected change in pH by using standard well known equations. The following serves as an example:

Present PaC0₂ 8.3 kPa. Estimated PaC0₂ after resetting 7.7 kPa (range 7.6-7.8).

Present pH 7.28. Estimated pH after resetting 7.32 It is worth noting that few studies have been published about relationships between observed change of V_M|_NC0₂ and change of Pa0₂ in various patient populations. Secondary feedback reactions in the body may under some circumstances lead to complementary mechanisms which to marginal extent may offset the calculation of PaC0₂ after ventilator resetting. It is therefore expected that refined algorithms used to predict a change of PaC0₂ and pH on the basis of AV_M|_NC0₂ will be developed for different patient populations.

In Figure 3 and 4 each data point represent a single breath. This is at controlled ventilation a preferred embodiment of the invention. At very irregular breathing V_MINC0₂ and other parameters like tidal volume may vary much between breaths. An alternative embodiment of the invention is to measure parameters over longer periods of time rather than per breath. Determination of volumes of e.g. C0₂ on the basis of gas flow rate and fraction of C0₂ are affected by conditions at which flow rate and C0₂ are measured. Conditions may vary before and after ventilator resetting, particularly with respect to pressure. According to a preferred embodiment of the invention, corrections are made to standardized conditions, for example BTPS (body temperature, atmospheric pressure and saturated with water vapour) or STPD (standard temperature and pressure, saturated). Which standard is chosen does not matter with respect to the present invention. Corrections to a standard is performed according to well known physics utilising information from the airway pressure transducer 15,19. The signal from the C0₂ transducer may be slightly affected by the oxygen concentration in respired air. When oxygen concentration is changed in association with ventilator resetting, the accuracy of AV_MINC0₂ determination is according to a preferred embodiment of the invention increased by correction of the C0₂ signal for variation in oxygen concentration. A signal for oxygen concentration in respired air is available in current advanced ventilators.

The analysis of other physiological parameters than V_M|_NC0₂ before and after resetting is made by statistical analysis of data sets for each signal sampled before and after resetting. For most parameters, a steady state is to be expected already within a few breaths after resetting. Accordingly, the level of a particular physiological parameter after resetting is determined on the basis of data during a period that according to default setting starts 5 breaths or 15 s after the last resetting and ends 2 minutes later, as for the analysis of V_M|_NC0₂. During that period the mean value, range or standard error of the parameter in question is reported. For parameters which attain a steady state after a few breaths, analysis of trends before and after resetting is in contrast to AV_M|_NC0₂ not necessary. As ventilator resetting does not occur at an exactly defined moment a change due to ventilator resetting is referred to as a "change in conjunction with ventilator resetting".

End tidal C0₂, E_TC0₂, has its own particular behaviour after ventilator resetting. As a first approximation it will fall at a rate and to a degree similar to that of PaC0₂, following an exponential course. However, E_TC0₂ is affected by more physiological factors than PaC0₂. When for example the respiratory rate is increased and tidal volume decreased, the difference between E_TC0₂ and PaC0₂ will under most circumstances increase. E_TC0₂ is also affected by the cardiac output and intrapulmonary shunt fraction, which are often affected after ventilator resetting. As variation of E_TC0₂ is complexly affected by both slow and fast phenomena it is in general not useful to predict its upcoming steady state value after ventilator resetting but rather to trace its variation with time as in Figure 3. In spite of the complexity of E_TC0₂ changes it is of value to display this parameter. E_TC0₂ may suddenly fall at an important suppression of pulmonary perfusion that may happen after unsuitable ventilator resetting. At such an event, also V_MINC0₂ falls suddenly. A sudden fall of both E_TC0₂ and V_M|_NC0₂ warns against suppressed circulation.

Examples given in Figure 3 and Figure 4 refer to controlled ventilation in a sedated patient. Under such circumstances noise in the signals is in general low. The signal for Respiratory rate and Tidal volume are virtually free from noise. The invention can be applied for other modes of ventilation, for example different types of supported ventilation. Then, the variability of parameters leading to noise in the observations is often much more important. The observation periods over which calculations are performed should then be adapted to the noise level. When the noise becomes so high that values of AV_M|_NC0₂% cannot be statistically determined, the prediction of change in PaC0₂ is according to a preferred embodiment of the invention suppressed.

Apart from a graphical presentation such as that in Figure 3, data before and after ventilator resetting are presented in a numerical format. Figure 5 shows an example based upon the data in Figure 3. According to a preferred embodiment of the invention, the parameters displayed graphically and numerically can be selected in a set up procedure of systems like those in Figure 1 and 2. This option includes parameters not shown in Figure 3. Examples are mean airway pressure and total PEEP. Total PEEP is the pressure in alveoli at the end of expiration that can be measured during a post-expiratory pause or estimated according to principles described by Jonson et al. (Bull. Physiopath. Resp. 1975, v 11 , pp 729-743). According to a preferred embodiment of the invention, signals from haemodynamic monitoring systems and meters for S_P0₂ are transmitted to the computer 17, 20. Such parameters are monitored and analyzed in analogy with parameters from inherent transducers.

Both graphically and numerically presented information are according to a preferred embodiment of the invention stored and may be retrieved for documentation of treatment and for research.

Claims

A monitoring apparatus for mechanical ventilation comprising transducers for measurement of at least airway flow rate (5, 13, 18) and C0₂ (14) and a computer (17, 20) that monitors physiological parameters derived from the transducers such as C0₂ elimination per unit time calculated by integration of the product between flow rate and C0₂ concentration in expired air, characterized in that the relative change of arterial partial pressure of C0₂, which after ventilator resetting will occur during establishment of steady state, is estimated on the basis of the relationship between measured value of C0₂ elimination and the change of C0₂ elimination measured at the time of ventilator resetting.

A monitoring apparatus according to claim 1 characterized in that the relative change of arterial partial pressure of C0₂, which after ventilator resetting will occur during establishment of steady state, is estimated on the basis of the quotient between the change of C0₂ elimination measured at the time of ventilator resetting and a measured value of C0₂ elimination.

A monitoring apparatus according to claim 1 or 2 characterized in that the value of arterial PC0₂, which ventilator resetting will lead to after establishment of steady state, is estimated on the basis of the relationship between the change of C0₂ elimination measured at the time of ventilator resetting and measured value of C0₂ elimination compiled with a value of arterial PC0₂.

A monitoring apparatus according to claim 3, characterized in that arterial PC0₂ combined with other acid/base data measured before resetting are compiled with estimated value of arterial PC0₂ following the resetting for calculation of arterial pH after establishment of steady state.

A monitoring apparatus according to any of claim 1-4, characterized in that the change in C0₂ elimination per unit time occurring at ventilator resetting is calculated by statistical analysis of separate data volumes of C0₂ elimination observed before and after the resetting.

A monitoring apparatus according to claim 4, characterized in that the change in C0₂ elimination per unit time occurring at ventilator resetting is based upon an equation describing a slow trend of changing C0₂ elimination per unit time statistically characterised from data measured before ventilator resetting.

A monitoring apparatus according to claim 4, characterized in that the change in C0₂ elimination per unit time occurring at ventilator resetting is based upon an equation describing a course towards a value that represents steady state, which course is statistically characterised from data measured after ventilator resetting.

A monitoring apparatus according to claim 1 , characterized in a transducer for measurement of airway pressure (15, 19) and in that changes of parameters related to lung mechanics such as postinspiratory plateau pressure and end-expiratory pressure are calculated by statistical analysis of separate data volumes of each parameter before and after the resetting.

A monitoring apparatus according to any of claim 1 to 7, characterized in complementary monitoring equipment transmitting signals to the computer (17, 20), which signals in conjunction with ventilator resetting are monitored on the basis of statistical analysis of separate data volumes of each parameter before and after the resetting, in analogy with parameters from transducers incorporated in the apparatus.

10. A method for monitoring of mechanical ventilation based on measurement of at least airway flow rate (5, 13, 18) and C0₂ (14) and a computer (17, 20) that monitors physiological parameters derived from the transducers such as C0₂ elimination per unit time calculated by integration of the product between flow rate and C0₂ concentration in expired air, characterized in that the relative change of arterial partial pressure of C0₂, which after ventilator resetting will occur during establishment of steady state, is estimated on the basis of the relationship between measured value of C0₂ elimination and the change of C0₂ elimination measured at the time of ventilator resetting.

11. A method for monitoring of mechanical ventilation according to claim 10 characterized in that the relative change of arterial partial pressure of C0₂, which after ventilator resetting will occur during establishment of steady state, is estimated on the basis of the quotient between the change of C0₂ elimination measured at the time of ventilator resetting and a measured value of C0₂ elimination.

12. A method for monitoring of mechanical ventilation according to claim 10 or 11 characterized in that the value of arterial PC0₂, which ventilator resetting will lead to after establishment of steady state, is estimated on the basis of the relationship between the change of C0₂ elimination measured at the time of ventilator resetting and measured value of C0₂ elimination compiled with a value of arterial PC0₂.

13. A method for monitoring of mechanical ventilation according to claim 12, characterized in that arterial PC0₂ combined with other acid/base data measured before resetting are compiled with estimated value of arterial PC0₂ following the resetting for calculation of arterial pH after establishment of steady state.

14. A method for monitoring of mechanical ventilation according to any of claim 10-13, characterized in that the change in CC½ elimination per unit time occurring at ventilator resetting is calculated by statistical analysis of separate data volumes of C0₂ elimination observed before and after the resetting.

15. A method for monitoring of mechanical ventilation according to claim 14, characterized in that the change in C0₂ elimination per unit time occurring at ventilator resetting is based upon an equation describing a slow trend of changing C0₂ elimination per unit time statistically characterised from data measured before ventilator resetting.

16. A method for monitoring of mechanical ventilation according to claim 14, characterized in that the change in C0 elimination per unit time occurring at ventilator resetting is based upon an equation describing a course towards a value that represents steady state, which course is statistically characterised from data measured after ventilator resetting.

17. A method for monitoring of mechanical ventilation according to claim 10, characterized in a transducer for measurement of airway pressure (15, 19) and in that changes of parameters related to lung mechanics such as postinspiratory plateau pressure and end-expiratory pressure are calculated by statistical analysis of separate data volumes of each parameter before and after the resetting.

18. A method for monitoring of mechanical ventilation according to any of claim 10 to 17, characterized in complementary monitoring equipment transmitting signals to the computer (17, 20), which signals in conjunction with ventilator resetting are monitored on the basis of statistical analysis of separate data volumes of each parameter before and after the resetting, in analogy with parameters from transducers incorporated in the apparatus.