Capnotracking of cardiac output or effective pulmonary blood flow during mechanical ventilation

Abstract

A capnotracking method for continuous determination of cardiac output or EPBF of a mechanically ventilated subject includes the steps of measuring expiratory CO2 of the subject and determining a first value of cardiac output or EPBF of the subject at a first point in time; controlling the mechanical ventilation of the subject to keep a level of venous CO2 of the subject substantially constant between the first point in time and a second point in time; determining from the expiratory CO2 measurements a change in alveolar CO2 of the subject between the first and second points in time; and determining a second and updated value of cardiac output or EPBF of the subject based on the first value and the change in alveolar CO2.

Claims

1. A method for controlling a breathing apparatus and determining a cardiac output or an Effective Pulmonary Blood Flow (EPBF) of a mechanically ventilated subject, comprising: determining, at a first point in time, a first value of cardiac output or EPBF of the subject by a control unit of the breathing apparatus; measuring, at the first point in time, a first expiratory CO.sub.2 content of the subject, wherein the first expiratory CO.sub.2 content is measured by at least one CO.sub.2 sensor; controlling a mechanical ventilation of the subject by the control unit adjusting one or more settings of the breathing apparatus so as to keep a level of venous CO.sub.2 of the subject substantially constant between the first point in time and a second point in time; measuring, at the second point in time, a second expiratory CO.sub.2 content of the subject, wherein the second expiratory CO.sub.2 content is measured by the at least one CO.sub.2 sensor; determining a change in alveolar CO.sub.2 content of the subject between the first and second points in time by the control unit based on the first expiratory CO.sub.2 content and the second expiratory CO.sub.2 content measured by the at least one CO.sub.2 sensor, and determining a second value for the cardiac output or the EPBF of the subject by the control unit based on the first value and the change in alveolar CO.sub.2 content between the first and second points in time.

2. The method of claim 1, wherein a measure of CO.sub.2 elimination of the subject is used by the control unit as control parameter to keep the level of venous CO.sub.2 substantially constant between the first and second points in time.

3. The method of claim 2, wherein the level of venous CO.sub.2 is kept substantially constant between the first and second points in time by the control unit controlling the mechanical ventilation to keep the CO.sub.2 elimination of the subject substantially constant between the first and second points in time, or to keep the CO.sub.2 elimination of the subject substantially proportional to a measured oxygen consumption of the subject measured by an oxygen sensor.

4. The method of claim 1, wherein the first value of cardiac output or EPBF is a first value of EPBF and the second value of cardiac output or EPBF is a second value of EPBF, which second value is calculated based on the following relationship: ${\mathrm{EPBF}}_n={\mathrm{EPBF}}_0.Math.\frac{\mathrm{VCO}{2}_n^{\mathrm{target}}}{\mathrm{VCO}{2}_0+{\mathrm{EPBF}}_0.Math.S_{\mathrm{CO}2}.Math.\left(\mathrm{PACO}{2}_0-\mathrm{PACO}{2}_n\right)}$ where EPBF.sub.n is the second value of EPBF, EPBF.sub.0 is the first value of EPBF, VCO2.sub.0 and PACO2.sub.0 are the CO2 elimination and partial pressure of alveolar CO2, respectively, of the ventilated subject at the first point in time, PACO2.sub.n, is the partial pressure of alveolar CO2 of the ventilated subject at the second point in time, S.sub.CO2 is the coefficient of CO2 solubility in blood, and VCO2.sub.n.sup.target is a target value for the CO.sub.2 elimination at the second point in time.

5. The method of claim 4, wherein VCO2.sub.n.sup.target is set to VCO2.sub.0, assuming constant metabolic production of CO2 of the ventilated subject, or calculated based on a measured change in oxygen consumption of the ventilated subject between the first and second points in time.

6. A non-transitory, computer readable data storage medium with an executable program stored thereon, wherein the program instructs a processing unit of a breathing apparatus to perform operations for controlling the breathing apparatus and determining a cardiac output or an Effective Pulmonary Blood Flow (EPBF) of a mechanically ventilated subject, when executed by the processing unit, the operations comprising steps to: determine, at a first point in time, a first value of cardiac output or EPBF of the subject; receive a measure of a first expiratory CO.sub.2 content of the subject, measured at the first point in time; control a mechanical ventilation of the subject by adjusting one or more settings of the breathing apparatus so as to keep a level of venous CO.sub.2 of the subject substantially constant between the first point in time and a second point in time; receive a measure of a second expiratory CO.sub.2 content of the subject, measured at the second point in time; determine a change in alveolar CO.sub.2 content of the subject between the first and second points in time based on the first expiratory CO.sub.2 content and the second expiratory CO.sub.2 content, and determine a second value for the cardiac output or the EPBF of the subject based on the first value and the change in alveolar CO.sub.2 content between the first and second points in time.

7. A breathing apparatus configured to provide mechanical ventilation to a subject and to determine a cardiac output or an Effective Pulmonary Blood Flow (EPBF) of the mechanically ventilated subject, comprising a control unit configured to: determine, at a first point in time, a first value of cardiac output or EPBF of the subject; receive a measure of a first expiratory CO.sub.2 content of the subject, measured by at least one CO.sub.2 sensor at the first point in time; control the mechanical ventilation of the subject to keep a level of venous CO.sub.2, of the subject substantially constant between the first point in time and a second point in time; receive a measure of a second expiratory CO.sub.2 content of the subject, measured by the at least one CO.sub.2 sensor at the second point in time; determine a change in alveolar CO.sub.2 of the subject between the first and second points in time based on the first expiratory CO.sub.2 content and the second expiratory CO.sub.2 content measured by the at least one CO.sub.2 sensor, and determine a second value for the cardiac output or the EPBF of the subject based on the first value and the change in alveolar CO.sub.2 content between the first and second points in time.

8. The breathing apparatus of claim 7, wherein the control unit is configured to use a measure of CO.sub.2 elimination as a control parameter to keep the level of venous CO.sub.2 substantially constant between the first and second points in time.

9. The breathing apparatus of claim 8, wherein the control unit is configured to keep the level of venous CO.sub.2 substantially constant between the first and second points in time by controlling the mechanical ventilation of the subject to keep the CO.sub.2 elimination of the subject substantially constant between the first and second points in time, or to keep the CO.sub.2 elimination of the subject substantially proportional to a measured oxygen consumption of the subject, measured by an oxygen sensor of the breathing apparatus.

10. The breathing apparatus of claim 7, wherein the first value of cardiac output or EPBF is a first value of EPBF and the second value of cardiac output or EPBF is a second value of EPBF, the control unit being configured to calculate the second value based on the following relationship: ${\mathrm{EPBF}}_n={\mathrm{EPBF}}_0.Math.\frac{\mathrm{VCO}{2}_n^{\mathrm{target}}}{\mathrm{VCO}{2}_0+{\mathrm{EPBF}}_0.Math.S_{\mathrm{CO}2}.Math.\left(\mathrm{PACO}{2}_0-\mathrm{PACO}{2}_n\right)}$ where EPBF.sub.n is the second value of EPBF, EPBF.sub.0 is the first value of EPBF, VCO2.sub.0 and PACO2.sub.0 are the CO.sub.2 elimination and partial pressure of alveolar CO.sub.2, respectively, of the ventilated subject at the first point in time, PACO2.sub.n is the partial pressure of alveolar CO.sub.2 of the ventilated subject at the second point in time, S.sub.CO2 is the coefficient of CO.sub.2 solubility in blood, and VCO2.sub.n.sup.target is a target value for the CO.sub.2 elimination at the second point in time.

11. The breathing apparatus of claim 10, wherein the control unit is configured to set VCO2.sub.n.sup.target to VCO2.sub.0, assuming constant metabolic production of CO.sub.2 of the ventilated subject, or to calculate VCO2.sub.n.sup.target based on a measured change in oxygen consumption of the ventilated subject between the first and second points in time.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of this disclosure will become more fully understood from the detailed description provided hereinafter and the accompanying drawings which are given by way of illustration only. In the different drawings, same reference numerals correspond to the same element.

(2) FIG. 1 illustrates a breathing apparatus according to an exemplary embodiment of the present disclosure, and

(3) FIG. 2 is a flow chart illustrating a method for determination of cardiac output or EPBF of a mechanically ventilated subject, according to an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

(4) FIG. 1 illustrates a breathing apparatus 1 configured for determination of cardiac output or EPBF of a mechanically ventilated subject 3, hereinafter sometimes referred to as the patient, in accordance with a non-limiting, illustrating embodiment of the present disclosure. The breathing apparatus 1 may be a ventilator, an anaesthesia machine or any other breathing apparatus adapted for mechanical ventilation of a subject in need of respiratory support.

(5) The breathing apparatus 1 is connected to the patient 3 via an inspiratory line 7 for supplying breathing gas to the patient 3, and an expiratory line 9 for conveying expiration gas away from the patient 3. The inspiratory line 7 and the expiratory line 9 are connected to a common line 11, via a so called Y-piece 12, which common line is connected to the patient 3 via a patient connector 13, such as a facemask or an endotracheal tube.

(6) The breathing apparatus 1 further comprises a control unit 14, such as a control computer, for controlling the ventilation of the patient 3 based on preset parameters and/or measurements obtained by various sensors of the breathing apparatus. The control unit 14 controls the ventilation of the patient 3 by controlling a pneumatic unit (i.e. a gas regulator) 15 of the breathing apparatus 1, which pneumatic unit 15 is connected, on one hand, to one or more gas sources 17, 19 and, on the other hand, to the inspiratory line 7 for regulating a flow and/or pressure of breathing gas delivered to the patient 3. To this end, the pneumatic unit 15 may comprise various gas mixing and regulating means well known in the art of ventilation, such as gas mixing chambers, controllable gas mixing valves and one or more controllable inspiration valves.

(7) The control unit 14 comprises a processing unit 21 and a non-volatile memory device 23 storing a computer program for determining the cardiac output or EPBF of the patient 3 according to the principles described herein. Unless stated otherwise, actions and method steps described hereinafter are performed by, or caused by, the control unit 14 of the breathing apparatus 1 upon execution by the processing unit 21 of different code segments of the computer program stored in the memory 23.

(8) The breathing apparatus 1 further comprises at least one flow sensor 27 for measuring at least an expiratory flow of expiration gas exhaled by the patient 3, and at least one CO2 sensor 29 for measuring the CO2 content of at least the expiration gas exhaled by the patient. The control unit 14 is configured to determine the cardiac output or EPBF of the patient 3 based on the CO2 measurements obtained by the CO2 sensor, as will be described in more detail below. Also the measurements of expiratory flow obtained by the flow sensor 27 may be used by the control unit 14 in the cardiac output or EPBF determination. Preferably, the flow and CO2 sensors 27, 29 are configured to measure also inspiratory flow and CO2 content.

(9) In the illustrated embodiment, the flow sensor 27 and the CO2 sensor 29 form parts of a capnograph 31 configured for volumetric capnography measurements. The capnograph 31 is arranged in the proximity of the airway opening of the patient 3, namely, in the common line 11 of the breathing circuit in which it is exposed to all gas exhaled and inhaled by the patient 3. The capnograph 31 is connected to the breathing apparatus 1 via a wired or wireless connection 33, and configured to communicate the result of the flow and CO2 measurements to the breathing apparatus for further processing by the processing unit 21. The breathing apparatus 1 may be configured to generate a volumetric capnogram 35 from the flow and CO2 measurements received from the capnograph 31, and, additionally, to display the volumetric capnogram 35 on a display 37 of the breathing apparatus.

(10) Additionally, the breathing apparatus 1 may comprise an oxygen sensor 39 for measuring inspiratory O2, i.e., the O2 content of the breathing gas delivered to the patient 3 during inspiration, as well as expiratory O2, i.e., the O2 content of the expiration gas exhaled by the patient during expiration. The oxygen sensor 39 is connected to the breathing apparatus 1 via a wired or wireless connection 41, and configured to communicate the result of the O2 measurements to the breathing apparatus for further processing by the processing unit 21. In embodiments taking variations in the metabolic production of CO2 into account, the processing unit 21 may be configured to use the O2 measurements obtained by the O2 sensor 39 to determine the O2 consumption of the patient 3, and to use the O2 consumption in the determination of the cardiac output or EPBF of the patient 3, as will be described in more detail below.

(11) In a preferred embodiment, the control unit 14 is configured to determine a first value, or baseline value, of cardiac output or EPBF of the patient 3 from the flow and CO2 measurements obtained by the flow and CO2 sensors 27, 29 using a non-invasive respiratory based method, preferably a Fick method.

(12) Fick based determination of cardiac output or EPBF typically requires the level of expired CO2 to change with at least 0,2% and preferably around 0,5% or more during the analysed sequence of breaths. To this end, the control unit 14 is configured to introduce a change in the effective ventilation of the patient 3 by changing one or more breathing apparatus settings controlling the ventilation of the patient 3, and to determine the cardiac output or EPBF of the patient based on the flow and CO2 measurements obtained during an analysed sequence of breaths during which the change in effective ventilation occurs.

(13) As in most Fick based methods for cardiac output determination, the analysed sequence of breaths may comprise any number of breaths but typically comprises 4 to 20 breaths, and preferably 4 to 12 breaths. The analysed sequence of breaths comprises at least one phase of increased ventilation and at least one phase of decreased ventilation, wherein each phase of increased and decreased ventilation comprises at least one breath, typically at least two breaths, and preferably two to six breaths. The transition from the phase of increased ventilation to the phase of decreased ventilation, and vice versa, is effectuated by the change in effective ventilation of the patient 3. The change in effective ventilation may be caused by the control unit 14 in any manner known in the art, e.g., by changing the duration and/or the tidal volume of the breaths delivered to the patient by the breathing apparatus.

(14) Preferably, in order to determine cardiac output or EPBF continuously using a Fick based technique, the breathing apparatus 1 is configured to ventilate the patient 3 using a cyclic ventilation pattern comprising alternating phases of decreased and increased ventilation, wherein each phase of decreased ventilation is immediately followed by a phase on increased ventilation, and vice versa. Preferably but not necessarily, the number of breaths in each cycle of the cyclic ventilation pattern corresponds to the number of breaths in the analysed sequence of breaths.

(15) Thus, the breathing apparatus 1 is preferably configured to ventilate the patient 3 using a cyclic ventilation pattern comprising alternating phases of increased and decreased ventilation, and to determine the cardiac output or EPBF of the patient 3 from expiratory flow and CO2 measurements obtained during an analysed sequence of breaths, e.g., a sequence of ten breaths. For example, the control unit 14 may be configured to determine the cardiac output or EPBF of the patient 3 from the analysed sequence of breaths using any of the techniques described in WO 2006/119546, U.S. Pat. No. 7,135,001, WO2013/141766, EP2799008, PCT/SE2015/051357, or the co-pending application PCT/SE2016/050402. By replacing the measurements obtained during the oldest breath in the analysed sequence of breaths with measurements obtained during the most recent breath delivered by the breathing apparatus, an updated value of cardiac output or EPBF can be determined continuously, i.e., on a breath-by-breath basis.

(16) If, due to the clinical situation at hand, the cyclic ventilation pattern can no longer be used or in the event the Fick based technique fails to determine or reliably determine the cardiac output or EPBF of the ventilated patient 3, the control unit 14 of the breathing apparatus 1 is configured to switch from the above described Fick based determination of cardiac output or EPBF to a capnotracking technique for determination of cardiac output or EPBF in accordance with the principles of the present disclosure. Hereinafter, the phase during which cardiac output or EPBF is determined using the Fick based technique will be referred to as the Fick phase, whereas the following phase during which cardiac output or EPBF is determined using the proposed capnotracking technique will be referred to as the capnotracking phase.

(17) In the capnotracking phase, the control unit 14 may be configured to use a value of cardiac output or EPBF determined during the preceding Fick phase as a baseline value of cardiac output or EPBF, which value serves as a starting point for the capnotracking. For example, the baseline value may be the most recent value of cardiac output or EPBF determined during the preceding Fick phase. In one embodiment, the Fick based determination of cardiac output or EPBF is made using any of the techniques disclosed in WO2013/141766 and PCT/SE2015/051357, which is advantageous in that these methods allow the effective lung volume (ELV) and the venous CO2 content (CvCO2) of the patient 3 to be determined at the same time as the cardiac output or EPBF. As will become apparent from the description following hereinafter, CvCO2 will remain constant during the entire capnotracking phase and so does not need to be calculated again.

(18) When switching from the Fick phase to the capnotracking phase, the control unit 14 starts to control the breathing apparatus 1 to keep the level of venous CO2 of the patient 3 substantially constant and equal to the level of venous CO2 at the time of determination of the baseline value of cardiac output or EPBF. This is typically achieved by the control unit 14 by controlling the breathing apparatus 1 based on the CO2 elimination (VCO2) of the patient 3, as determined from the flow and CO2 measurements obtained by the flow and CO2 sensors 27 and 29.

(19) If the metabolic production of CO2 of the patient is or can be assumed to be constant, the venous CO2 content of the patient 3 will remain constant as long as VCO2 remains constant, which allows the control unit 14 to keep the venous CO2 content of the patient 3 at a substantially constant level by controlling the breathing apparatus 1 to keep measured VCO2 substantially constant. In this scenario, the breathing apparatus 1 can be controlled based on expiratory flow and CO2 measurements alone.

(20) If, in this scenario, the cardiac output or EPBF of the patient 3 should change during capnotracking, the alveolar CO2 of the patient will also change while VCO2 is kept constant by adjusting the alveolar ventilation of the patient 3 through active control of the breathing apparatus 1, e.g., by adjusting the respiratory rate or the tidal volume of breaths delivered by the breathing apparatus. This allows the control unit 14 to determine a new and updated value of cardiac output or EPBF from the baseline value of cardiac output or EPBF and the change in alveolar CO2 content, as determined from the CO2 measurements obtained by the CO2 sensor 29. The rationale behind this will be better understood in view of the following relationships.

(21) First it should be noted that, given a constant level of CO2 in the lungs of the patient 3, the respiratory based CO2 elimination, VCO2, balancing out the delivery of CO2 from the blood to the lungs, depends on the difference between venous CO2 content, CvCO2, and pulmonary capillary CO2 content, CcCO2, according to:
VCO2=EPBF.Math.(CvCO2-CcCO2)Eq. 3 where CvCO2CcCO2 (=CvcCO2) is the difference between venous and pulmonary capillary CO2 content.

(22) If VCO2 is kept constant while the EPBF and the alveolar CO2 content of the ventilated patient vary, equation 3 can be used to describe a relation between two different states between which the venous CO2 content, CvCO2, is also assumed to be kept constant and equal to a value CvCO2.sub.0, according to:
VCO2.sub.0=EPBF(t).Math.(CvCO2.sub.0-CcCO2(t))=EPBF.sub.0.Math.(CvCO2.sub.0-CcCO2.sub.0)Eq. 4

(23) In equation 4, subscript 0 indicates values of the respective quantities at the point in time for determination of baseline value of EPBF, EPBF.sub.0, i.e., the point in time for the Fick based determination serving as starting point for capnotracking. EPBF.sub.0 is typically the most recent EPBF value determined during the Fick phase, or the most recent sufficiently reliable EPBF value determined during the Fick phase.

(24) The new and updated value of EPBF is the quantity denoted EPBF(t) in equation 4. By rearranging equation 4, EPBF(t) can be expressed in terms of quantities with subscript 0 and the varying pulmonary capillary CO2 content, CcCO2 (t), which can be determined breath by breath from the expiratory CO2 measurements, in accordance with:

(25) $\begin{array}{cc}\mathrm{EPBF}\left(t\right)={\mathrm{EPBF}}_0.Math.\frac{\mathrm{CvCO}{2}_0-\mathrm{CcCO}{2}_0}{\mathrm{CvCO}{2}_0-\mathrm{CcCO}2\left(t\right)}& \mathrm{Eq}.5\end{array}$

(26) By introducing CvcCO2.sub.0=CvCO2CcCO2.sub.0, equation 5 can be expressed as:

(27) $\begin{array}{cc}\mathrm{EPBF}\left(t\right)={\mathrm{EPBF}}_0.Math.\frac{\mathrm{CvcCO}{2}_0}{\mathrm{CvcCO}{2}_0+\mathrm{CcCO}{2}_0-\mathrm{CcCO}2\left(t\right)}& \mathrm{Eq}.6\end{array}$

(28) An advantage of this expression is that the pulmonary capillary CO2 difference, CcCO2.sub.0CcCO2(t), can be expressed in terms of partial pressure of pulmonary capillary CO2 (PcCO2) and a coefficient of CO2 solubility in blood (S.sub.CO2):

(29) $\begin{array}{cc}\mathrm{EPBF}\left(t\right)={\mathrm{EPBF}}_0.Math.\frac{\mathrm{CvcCO}{2}_0}{\mathrm{CvcCO}{2}_0+S_{\mathrm{CO}2}.Math.\left(\mathrm{PcCO}{2}_0-\mathrm{PcCO}2\left(t\right)\right)}& \mathrm{Eq}.7\end{array}$

(30) If it is assumed that alveolar CO2 is in equilibrium with pulmonary capillary CO2, a new and updated value of EPBF for breath n, EPBF.sub.n, can be determined on a breath-by-breath basis from measured variations in alveolar CO2 (PACO2) together with quantities that are obtainable at the point in time for determination of EPBF.sub.0:

(31) $\begin{array}{cc}{\mathrm{EPBF}}_n={\mathrm{EPBF}}_0.Math.\frac{\mathrm{CvcCO}{2}_0}{\mathrm{CvcCO}{2}_0+S_{\mathrm{CO}2}.Math.\left(\mathrm{PACO}{2}_0-\mathrm{PACO}{2}_n\right)}& \mathrm{Eq}.8\end{array}$ where PACO2.sub.0 is the partial pressure of alveolar CO2 at the time for determination of EPBF.sub.0 and PACO2.sub.n is the partial pressure of alveolar CO2 at breath n, directly derivable from the measurements of expiratory CO2 obtained by the CO2 sensor 29.

(32) If the CO2 level in the lungs of the patient 3 is in equilibrium at the time for determination of EPBF.sub.0, and if a baseline level of CO2 elimination (VCO2.sub.0) is determined at the same time from the expiratory flow and CO2 measurements, equation 1 can be used to calculate dCvcCO2.sub.0 as:

(33) $\begin{array}{cc}\mathrm{CvcCO}{2}_0=\frac{\mathrm{VCO}{2}_0}{{\mathrm{EPBF}}_0}& \mathrm{Eq}.9\end{array}$

(34) Combining equations 8 and 9 yields the following relation which may be advantageously used by the control unit 14 of the breathing apparatus 1 to calculate a new and updated value of EPBF for any given breath n within the capnotracking phase following determination of a baseline value for EPBF, EPBF.sub.0:

(35) $\begin{array}{cc}{\mathrm{EPBF}}_n={\mathrm{EPBF}}_0.Math.\frac{\mathrm{VCO}{2}_0}{\mathrm{VCO}{2}_0+{\mathrm{EPBF}}_0.Math.S_{\mathrm{CO}2}.Math.\left(\mathrm{PACO}{2}_0-\mathrm{PACO}{2}_n\right)}& \mathrm{Eq}.10\end{array}$

(36) Thus, according to one embodiment of the present disclosure, the control unit 14 may be configured to determine a first baseline value of EPBF, EPBF.sub.0, at a first point in time using any known technique for EPBF determination, such as any of the above discussed Fick based techniques; control the mechanical ventilation of the patient 3 to keep the level of venous CO2 in the patient substantially constant between the first and a second point time, e.g., by adjusting the alveolar ventilation of the patient to keep VCO2 substantially constant; determining a change in alveolar CO2 (PACO2.sub.0PACO2.sub.n) of the patient between the first and second points in time from the expiratory CO2 measurements, and determining a new and updated value of EPBF, EPBF.sub.n, based on the baseline value of EPBF, EPBF.sub.0, and the change in alveolar CO2, e.g., by using equation 10.

(37) If using equation 10, the control unit 14 would also need to use an estimate of the CO2 solubility in blood in the determination of the new and updated EPBF value, EPBF.sub.n. How to estimate S.sub.CO2 is well-known in the art, and the control unit 14 may be configured to use any known constant estimate of S.sub.CO2, or to estimate S.sub.CO2 from available data using any known technique for S.sub.CO2 estimation. Constant S.sub.CO2 estimates and techniques for estimating S.sub.CO2 have been discussed, e.g., in Gedeon et al., A new method for noninvasive bedside determination of pulmonary blood flow, Med Biol Eng Comput 1980; 18:411-418, Capek et al., Noninvasive Measurement of Cardiac Output Using Partial CO2 Rebreathing, IEEE Transactions on Biomedical Engineering, Vol. 35, No. 9, September 1988, and Cecchini et al., Non-invasive Estimation of Cardiac Output in Mechanically Ventilated Patients: A prolonged Expiration Method, Annals of Biomedical Engineering, August 2012, Volume 40, Issue 8, pp 1777-1789. Thus, it should be appreciated that the control unit 14 may be configured to estimate S.sub.CO2 from available data, e.g., using any of the techniques discussed in the above mentioned publications, or to use a constant estimate of S.sub.CO2, e.g., an estimate that is manually input to the control unit 14 via a user interface of the breathing apparatus.

(38) The control unit 14 may be configured to use any suitable control algorithm for keeping VCO2 of the patient 3 substantially constant (in case of constant metabolic production of CO2) during the capnotracking phase. In a basic implementation, the control unit 14 may be configured to control the mechanical ventilation of the patient 3 in proportion to VCO2, according to:

(39) $\begin{array}{cc}V{A}_{n+1}=V{A}_n.Math.\frac{\mathrm{VCO}{2}_0}{\mathrm{VCO}{2}_n}& \mathrm{Eq}.11\end{array}$ where VCO2 is the CO2 elimination of the patient 3 at the point in time for determination of EPBF.sub.0, VA.sub.n and VCO2 is the alveolar ventilation and the CO2 elimination, respectively, of the patient 3 for a subsequent breath n, and VA.sub.n+1 is the alveolar ventilation to be provided to the patient by the breathing apparatus 1 in a breath n+1 following the subsequent breath n. This means that the mechanical ventilation may be controlled on a breath-by-breath basis during the capnotracking phase such that the alveolar ventilation of the patient 3 for any breath n+1 is based on, and proportional to, the relationship between the baseline VCO2 value, VCO2.sub.0, and the VCO2 value, VCO2.sub.n, determined for the preceding breath n. As mentioned above, the alveolar ventilation of the patient 3 is preferably adjusted by the control unit 14 by adjusting the respiratory rate (RR) or the tidal volume (VTi) of breaths delivered by the breathing apparatus 1. Most preferably, the respiratory rate is adjusted to achieve the desired alveolar ventilation.

(40) The alveolar ventilation may, as well known in the art, be determined from the tidal volume and the airway deadspace of the patient 3. As also well known in the art, the airway deadspace can be derived using volumetric capnography, and may thus be determined using the capnograph 31 of the breathing apparatus 1.

(41) The above calculations are based on the assumption that the metabolic production of CO2 of the ventilated patient 3 remains substantially constant during the capnotracking phase. If, however, the metabolic CO2 production varies, the venous CO2 content of the patient 3 may vary in an unknown manner even if controlling the breathing apparatus 1 to keep VCO2 substantially constant and equal to VCO.sub.0.

(42) Therefore, the proposed capnotracking technique may involve the steps of measuring also the O2 content of respiration gases, and taking the O2 content into account to keep venous CO2 substantially constant during the capnotracking phase.

(43) To this end, the control unit 14 may be configured to determine the metabolic consumption of O2 of the ventilated patient 3 from measured flow and O2 content, and to keep the venous CO2 content of the patient 3 substantially constant during the capnotracking phase by controlling the mechanical ventilation of the patient 3 in a manner causing VCO2 to vary in proportion to the metabolic O2 consumption.

(44) The metabolic production of CO2 is proportional to the metabolic consumption of O2 according to:
VCO2.sub.met=RQ.Math.VO2.sub.metEq. 12 where VCO2 .sub.met is the metabolic production of CO2 of the ventilated subject, VO2 met is the oxygen consumption of the ventilated subject, and RQ is the so called respiratory quotient having a typical value in the range of 0.7-1.0, depending on the composition of the nutrition.

(45) In order to keep the venous CO2 content of the ventilated patient 3 substantially constant during the capnotracking phase, taking variations in the metabolic production of CO2 into account, the control unit 14 may be configured to control the mechanical ventilation of the patient 3 to keep measured VCO2 substantially equal to a variable target value that is calculated based on measured variations in O2 consumption during the capnotracking phase, e.g., according to:

(46) $\begin{array}{cc}\mathrm{VCO}{2}^{\mathrm{target}}\left(t\right)=\mathrm{VCO}{2}_0.Math.\frac{\mathrm{VO}2\left(t\right)}{\mathrm{VO}{2}_0}& \mathrm{Eq}.13\end{array}$ where VCO2.sup.target(t) is the target value for CO2 elimination at time t,VO2(t) is the measured O2 consumption at time t, and VCO2.sub.0 and VO2.sub.0 are the baseline values of VCO2 and VO2 at the time for determination of EPBF.sub.0.

(47) The new and updated value of EPBF may then be determined for any given breath n during the capnotracking phase by using the following relation, which corresponds to equation 10 with the exception that the term VCO2.sub.0 in the numerator has been replaced with the VO2 dependent target value of CO2 elimination for breath n, VCO2.sub.n.sup.target:

(48) 0$\begin{array}{cc}{\mathrm{EPBF}}_n={\mathrm{EPBF}}_0.Math.\frac{\mathrm{VCO}{2}_n^{\mathrm{target}}}{\mathrm{VCO}{2}_0+{\mathrm{EPBF}}_0.Math.S_{\mathrm{CO}2}.Math.\left(\mathrm{PACO}{2}_0-\mathrm{PACO}{2}_n\right)}& \mathrm{Eq}.14\end{array}$

(49) In other words, a new and updated value of EPBF, EPBF.sub.n, may be derived from a baseline value of EPBF, EPBF.sub.0, and a measured change in alveolar CO2 (PACO2.sub.0PACO2.sub.n) using equation 14 which, assuming constant metabolic CO2 production, may be simplified in accordance with equation 10.

(50) FIG. 3 is a flow chart illustrating a method for determination of cardiac output or EPBF of a subject being mechanically ventilated by means of a breathing apparatus, according to an embodiment of the present disclosure.

(51) In a first step, S1, expiratory CO2 of the ventilated subject is measured. In the event VCO2 is used as control parameter for controlling the mechanical ventilation of the subject (see step S3), or in the event expiratory flow is used together with expiratory CO2 in determination of a change in alveolar CO2 (see step S4), also expiratory flow may be measured. As mentioned above, expiratory flow and CO2 may be measured using a capnograph, or the like, such as the capnograph 31 schematically illustrated in FIG. 1, devised to measure flow and CO2 content of expiration gases exhaled by the subject.

(52) In a second step, S2, a first value or baseline value of cardiac output or EPBF of the subject is determined at a first point in time. As mentioned above, this baseline value may be determined using any known technique for cardiac output or EPBF determination, but is preferably determined at least partly from the expiratory flow and CO2 measurements using a non-invasive Fick technique.

(53) In a third step, S3, a capnotracking phase is initiated by starting to control the breathing apparatus to keep the venous CO2 content of the subject at a substantially constant level. As discussed above, this is typically achieved by controlling the breathing apparatus based on measured VCO2 by adjusting the respiratory rate and/or the tidal volume of breaths delivered by the breathing apparatus to keep the measured VCO2 constant, or proportional to measured oxygen consumption by the subject.

(54) In a fourth step, S4, a change in alveolar CO2 content of the subject between the first and second points in time is determined from the expiratory CO2 measurements obtained in step S1. The change may, for example, be determined as a change in partial pressure of CO2 of expired alveolar gas, e.g., measured by the capnograph 31. Expiratory flow measurements may also be used in addition to the expiratory CO2 measurements in the determination of the change in alveolar CO2 content.

(55) In a fifth step, S5, a new and updated value of cardiac output or EPBF is determined based on the first or baseline value of cardiac output or EPBF and the change in alveolar CO2 content. The new and updated value of cardiac output or EPBF may, for example, be calculated based on the relationship expressed by equation 14, possibly simplified in accordance with equation 10.

(56) In a subsequent step (not shown), the new and updated value of cardiac output or EPBF determined in step S5 may be compared with one or more threshold values, defining a recommended and pre-set range for cardiac output or EPBF, whereupon an alarm signal may be generated in response to the comparison should the determined cardiac output or EPBF value fall outside the recommended range.

(57) The method is typically a computer-implemented method, meaning that it is performed through execution of a computer program. As mentioned above, the various method steps are typically performed by, or caused by, the control unit 14 of the breathing apparatus 1 upon execution by the processing unit 21 of different code segments of the computer program, which may be stored in the hardware memory device 23.

(58) Although the proposed capnotracking technique has been described above in conjunction with a Fick based technique for determination of the baseline value of cardiac output or EPBF, it should be emphasized that any known technique for cardiac output or EPBF determination can be used to generate the baseline value. In alternative embodiments, the baseline value may, for example, originate from invasive techniques for cardiac output monitoring, such as invasive pulse contour analysis. Furthermore, the baseline value may be determined automatically by the control unit 14 of the breathing apparatus 1 based on available data, or it may be determined by the control unit 14 based on user input that is input to the control unit 14 by a user or operator via a user interface of the breathing apparatus. Thus, in yet alternative embodiments, the control unit 14 may be configured to receive user input indicating a value of cardiac output or EPBF, and to use this value as a baseline value for cardiac output or EPBF during a subsequent phase of capnotracking of cardiac output or EPBF according to the principles described herein.

(59) Furthermore, although the above calculations have been performed for capnotracking of EPBF, it should be noted that the cardiac output of the ventilated subject can be continuously determined using the same principles. In accordance with equation 2, cardiac output is directly proportional to EPBF in case of constant shunt. The above equations for calculation of EPBF may hence be readily adapted for calculation of cardiac output, taking the pulmonary shunt fraction, fs, into account. The shunt fraction may be estimated by the control unit 14 in any manner known in the art, or an estimate of the shunt fraction may be provided to the control unit 14 by an external device to which the breathing apparatus is connectable, or by a user or operator via a user interface of the breathing apparatus. The shunt fraction may either be assumed to remain constant during the period of capnotracking, or the equations for calculation of a new and updated value of cardiac output may be adapted to take variations in shunt fraction during the capnotracking phase into account in order to further increase the accuracy in cardiac output determination.