Non-invasive pulmonary blood flow measurement by means of CO2 analysis of expiratory gases

Inference of alveolar capillary network connectivity from blood flow dynamics

The intricate lung structure is crucial for gas exchange within the alveolar region. Despite extensive research, questions remain about the connection between capillaries and the vascular tree. We propose a computational approach combining three-dimensional (3-D) morphological modeling with computational fluid dynamics simulations to explore alveolar capillary network connectivity based on blood flow dynamics. We developed three-dimensional sheet-flow models to accurately represent alveolar capillary morphology and conducted simulations to predict flow velocities and pressure distributions. Our approach leverages functional features to identify plausible system architectures. Given capillary flow velocities and arteriole-to-venule pressure drops, we deduced arteriole connectivity details. Preliminary analyses for nonhuman species indicate a single alveolus connects to at least two 20-µm arterioles or one 30-µm arteriole. Hence, our approach narrows down potential connectivity scenarios, but a unique solution may not always be expected. Integrating our blood flow model results into our previously published gas exchange application, Alvin, we linked these scenarios to gas exchange efficiency. We found that increased blood flow velocity correlates with higher gas exchange efficiency. Our study provides insights into pulmonary microvasculature structure by evaluating blood flow dynamics, offering a new strategy to explore the morphology-physiology relationship that is applicable to other tissues and organs. Future availability of experimental data will be crucial in validating and refining our computational models and hypotheses.NEW & NOTEWORTHY The alveolus is pivotal for gas exchange. Its complex, dynamic nature makes structural experimental studies challenging. Computational modeling offers an alternative. We developed a data-based three-dimensional (3-D) model of the alveolar capillary network and performed blood flow simulations within it. Choosing a novel perspective, we inferred structure from function. We systematically varied the properties of vessels connected to our capillary network and analyzed simulation results for blood flow and gas exchange to obtain plausible vessel configurations.

Variations in respiratory excretion of carbon dioxide can be used to calculate pulmonary blood flow

Background

A non-invasive means of measuring pulmonary blood flow (PBF) would have numerous benefits in medicine. Traditionally, respiratory-based methods require breathing maneuvers, partial rebreathing, or foreign gas mixing because exhaled CO₂ volume on a per-breath basis does not accurately represent alveolar exchange of CO₂. We hypothesized that if the dilutional effect of the functional residual capacity was accounted for, the relationship between the calculated volume of CO₂ removed per breath and the alveolar partial pressure of CO₂ would be reversely linear.

Methods

A computer model was developed that uses variable tidal breathing to calculate CO₂ removal per breath at the level of the alveoli. We iterated estimates for functional residual capacity to create the best linear fit of alveolar CO₂ pressure and CO₂ elimination for 10 minutes of breathing and incorporated the volume of CO₂ elimination into the Fick equation to calculate PBF.

Results

The relationship between alveolar pressure of CO₂ and CO₂ elimination produced an R² = 0.83. The optimal functional residual capacity differed from the “actual” capacity by 0.25 L (8.3%). The repeatability coefficient leveled at 0.09 at 10 breaths and the difference between the PBF calculated by the model and the preset blood flow was 0.62 ± 0.53 L/minute.

Conclusions

With variations in tidal breathing, a linear relationship exists between alveolar CO₂ pressure and CO₂ elimination. Existing technology may be used to calculate CO₂ elimination during quiet breathing and might therefore be used to accurately calculate PBF in humans with healthy lungs.

Pulmonary carbon dioxide elimination for cardiac output monitoring in peri-operative and critical care patients: history and current status

Minimally invasive measurement of cardiac output as a central component of advanced haemodynamic monitoring has been increasingly recognised as a potential means of improving perioperative outcomes in patients undergoing major surgery. Methods based upon pulmonary carbon dioxide elimination are among the oldest techniques in this field, with comparable accuracy and precision to other techniques. Modern adaptations of these techniques suitable for use in the perioperative and critical are environment are based on the differential Fick approach, and include the partial carbon dioxide rebreathing method. The accuracy and precision of this approach to cardiac output measurement has been shown to be similar to other minimally invasive techniques. This paper reviews the underlying principles and evolution of the method, and future directions including recent adaptations designed to deliver continuous breath-by-breath monitoring of cardiac output.

Non-Invasive Cardiac Output Measurement using a Fast Mixing Box to Measure Carbon Dioxide Elimination

This study investigated the accuracy of a new technique for measuring cardiac output using the derivative Fick principle based on the ratio of change in the partial pressures of end-tidal and mixed expired carbon dioxide produced by short periods of partial rebreathing.

A prospective clinical study involving 24 patients following cardiopulmonary bypass for coronary artery bypass grafting or valvular surgery was undertaken in the intensive care unit of a university-affiliated hospital.

Haemodynamic measurements were performed after admission to the intensive care unit. Cardiac output was measured simultaneously by bolus pulmonary artery thermodilution and by a noninvasive carbon dioxide partial rebreathing technique.

Cardiac output measurement using the new technique demonstrated a significant but consistent underestimate, with a bias of -0.60 ± 0.87 l/min.

This new adaptation of the partial rebreathing technique is reliable in measuring cardiac output in postoperative patients. Reasons for the consistent discrepancy between thermodilution and partial rebreathing techniques are discussed.

Non-invasive automated measurement of cardiac output during stable cardiac surgery using a fully integrated differential CO(2) Fick method

OBJECTIVES

To re-evaluate the accuracy and precision of a non-invasive method for measurement of cardiac output based on the differential CO(2) Fick approach using an automated change in respiratory rate delivered by a ventilator under control by a prototype measurement system.

METHODS

Twenty-four patients during coronary artery bypass surgery, pre- and postcardiopulmonary bypass were recruited. After routine cannulation including pulmonary artery catheter, relaxant general anesthesia was induced. After hemodynamic and ventilatory stability were achieved, simultaneous paired measurements were made by the differential Fick method and by bolus thermodilution. Measurements were generated by inducing a change in respiratory rate by the ventilator under computer control. In Group 1, this involved an increase in respiratory rate from 8 to 12 breaths/min. In Group 2, this involved a decrease from 12 to 6 breaths/min.

RESULTS

Nineteen measurements were made in each Group, 12 pre-CPB and 7 post-CPB. In Group 1 mean bias was -0.06 l/min, with a precision of agreement of 0.87 l/min, r = 0.91. In Group 2 (excluding one outlier) mean bias was -0.07 l/min, with a precision of 1.12 l/min, r = 0.71.

CONCLUSIONS

Acceptable agreement with thermo- dilution during surgery was found, particularly where the ventilatory change involved an increase in respiratory rate from a lower baseline. This approach has potential to be readily integrated into modern anesthesia delivery platforms, allowing routine non-invasive cardiac output measurement.

Measurement of cardiac output before and after cardiopulmonary bypass: Comparison among aortic transit-time ultrasound, thermodilution, and noninvasive partial CO2 rebreathing

OBJECTIVES

A noninvasive continuous cardiac output system (NICO) has been developed recently. NICO uses a ratio of the change in the end-tidal carbon dioxide partial pressure and carbon dioxide elimination in response to a brief period of partial rebreathing to measure CO. The aim of this study was to compare the agreement among NICO, bolus (TDCO), and continuous thermodilution (CCO), with transit-time flowmetry of the ascending aorta using an ultrasonic flow probe (UFP) before and after cardiopulmonary bypass (CPB).

DESIGN

Prospective, observational human study.

SETTING

Veterans Affairs Medical Center Hospital.

PARTICIPANTS

Sixty-eight patients.

METHODS

Matched sets of CO measurements between NICO, TDCO, CCO, and UFP were collected in 68 patients undergoing elective CABG at specific time periods before and after separation from CPB. After anesthetic induction, all patients had an NICO sensor attached between the endotracheal tube and the breathing circuit, a PAC floated into the pulmonary artery for TDCO and CCO monitoring, and a UFP positioned on the ascending aorta and used for the reference CO. Bland-Altman analysis was used to compare the agreement among the different methods.

MEASUREMENTS AND MAIN RESULTS

Bland-Altman analysis of CO measurements before CPB yielded a bias, precision, and percent error of 0.04 L/min +/- 1.07 L/min (44.8%) for NICO, 0.18 L/min +/- 1.01 L/min (41.7%) for TDCO, and 0.29 L/min +/- 1.40 L/min (57.5%) for CCO compared with simultaneous UFP CO measurements, respectively. After separation from CPB (average 29 mins), bias, precision, and percent error were -0.46 L/min +/- 1.06 L/min (37.3%) for NICO, 0.35 L/min +/- 1.39 L/min (46.1%) for TDCO, and 0.36 L/min +/- 1.96 L/min (64.7%) for CCO compared with UFP CO measurements, respectively.

CONCLUSIONS

Before initiation of CPB, the accuracy for all 3 techniques was similar. After separation from CPB, the tendency was for NICO to underestimate CO and for TDCO and CCO to overestimate it. NICO offers an alternative to invasive CO measurement.

Comparison of continuous cardiac output measurements in patients after cardiac surgery

OBJECTIVE

To investigate in a direct comparison accuracy and precision of continuous cardiac output measurements assessed by continuous pulmonary artery thermodilution technique (TDCCO), continuous pulse contour analysis (PCCO), and noninvasive partial CO(2)-rebreathing technique (NICO) in patients after coronary artery bypass grafting (CABG) during the postoperative period.

DESIGN

Prospective, controlled clinical study.

SETTING

University hospital.

PARTICIPANTS

Twenty-two patients undergoing elective CABG surgery.

INTERVENTIONS

Hemodynamic measurements were performed after admission to the ICU and in sequence every 2 hours during the subsequent 6-hour period. Simultaneously, cardiac output (CO) was measured using a TDCCO, PCCO, and NICO. After the continuous cardiac output measurements were read, bolus thermodilution-derived cardiac output was obtained from thermodilution curves detected in the pulmonary artery (TDBCO(pa)). Four intermittent consecutive boli consisting of 10 mL of ice-cold saline were randomly injected over the ventilatory cycle.

MEASUREMENTS AND MAIN RESULTS

The comparison between the continuous cardiac output measurement methods TDCCO versus PCCO showed a bias of -0.12 L/min, between TDCCO versus NICO -0.17 L/min, and between PCCO versus NICO -0.44 L/min. The comparison to the reference technique between TDBCO(pa) versus TDCCO revealed a bias of -0.28 L/min, between TDBCO(pa) versus PCCO -0.40 L/min, and between TDBCO(pa) versus NICO -0.64 L/min.

CONCLUSIONS

The results of this clinical investigation show agreement between TDCCO and PCCO to satisfy clinical requirements in a setting of postoperative patients after cardiac surgery. In contrast, the NICO monitor is of very limited use in these patients.

Partial CO2 rebreathing indirect Fick technique for non-invasive measurement of cardiac output

OBJECTIVE

Evaluation in animals of a non-invasive and continuous cardiac output monitoring system based on partial carbon-dioxide (CO2) rebreathing indirect Fick technique.

METHODS

We have developed a non-invasive cardiac output (NICO) monitoring system, based on the partial rebreathing method. The partial rebreathing technique employs a differential form of the Fick equation for calculating cardiac output (QT) using non-invasive measurements. Changes in CO2 elimination (deltaVCO2) and partial pressure of end-tidal CO2 (deltaPETCO2) in response to a brief period of partial rebreathing are used to measure pulmonary capillary blood flow (Q(PCBF)). A non-invasive estimate of anatomic and intrapulmonary shunt fraction (Q(S)/Q(T)), based on oxygen saturation from pulse oximetry (SpO2) and inspired oxygen concentration (FIO2), is added to compute total cardiac output [Q(T) = Q(PCBF)/(1 - Q(S)/Q(T))]. The performance of the NICO was compared with iced 5% dextrose bolus thermodilution cardiac output (TDco) measurements in 6 dogs. Cardiac output was varied using dobutamine, and halothane, and by clamping of the inferior vena cava. Two hundred and forty-six (n = 246) paired measurements of TDco and NICO over a range of cardiac outputs (TDco range = 0.60-8.87 l/min) were compared using Bland-Altman analysis and weighted correlation coefficient.

RESULTS

The Bland-Altman technique yielded a NICO precision of +/- 0.70 l/min (13.8%) with a mean bias of -0.07 l/min (-1.4%) compared to TDco. The weighted correlation coefficient between TDco and NICO values was: r = 0.93 (n = 246).

CONCLUSION

The partial CO2 rebreathing technique for measurement of cardiac output is non-invasive, automated, and based on the well accepted Fick principle. The limits of agreement between NICO and TDco is within the recommended value for NICO to be a clinically acceptable method for cardiac output measurement. The results of this canine study show that NICO performed as well, and in some cases better, than other currently available non-invasive cardiac output techniques over a wide range of cardiac outputs.

Performance of the partial CO2 rebreathing technique under different hemodynamic and ventilation/perfusion matching conditions

OBJECTIVE

The partial CO2 rebreathing technique has been demonstrated to accurately measure the effective pulmonary capillary blood flow (PCBF) in different clinical situations. Usually, PCBF is calculated from changes in CO2 elimination (VCO2) and end-tidal partial pressure of CO2 (PetCO2 ), which can be obtained noninvasively. In this study, we investigated the performance of the partial CO2 rebreathing technique under different conditions of ventilation/perfusion matching and hemodynamic states. In addition, we investigated whether the determination of arterial blood gases combined with mathematical modeling of gas exchange can improve the performance of this method.

DESIGN

Prospective, controlled animal laboratory study.

SETTING

Experimental research facility of a university hospital.

SUBJECTS

Sixteen female sheep weighing 45-55 kg.

INTERVENTIONS

Cardiac output and ventilation/perfusion matching were manipulated during three phases: phase I, variation in cardiac output to achieve normal, hyperdynamic and hypodynamic states; phase II, increase of alveolar deadspace and variation in cardiac output; phase III, lung injury and increased alveolar deadspace. Partial CO2 rebreathing maneuvers were performed to obtain variations in VCO2 and PetCO2 between a nonrebreathing (NR) and a rebreathing (R) period.

MEASUREMENTS AND MAIN RESULTS

PCBF was measured by the rebreathing method as PCBF = -DeltaVCO2/f(Pc'CO2 (R), Pc'CO2(NR), Hb), where f is the CO2 dissociation curve in blood, Pc'CO2 is the end-capillary partial pressure of CO2, Delta is the variation between NR and R periods, and Hb is hemoglobin concentration. Pc'CO2 was estimated from PetCO2 according to two algorithms. In the so-called "noninvasive algorithm," Pc'CO2 = PetCO2, with PetCO2(NR) and PetCO2(R) being determined as the mean PetCO2 value of the last 60 secs preceding rebreathing and within 15-30 secs of rebreathing, respectively. In the "semi-invasive algorithm," Pc'CO2(NR) was estimated as the PaCO2, and Pc'CO2(R) was estimated as follows: First, a monoexponential function was fitted to PetCO2 values during rebreathing and the asymptote represented PetCO2(R). Second, the Pc'CO2(R) to PetCO2(R) difference was calculated by means of a bicompartmental, tidal model of gas exchange, which showed that such differences decrease with the degree of rebreathing. PCBF values obtained with both algorithms were compared with thermodilution cardiac output minus intrapulmonary shunt flow. Bias and precision calculations with the noninvasive algorithm in phases I, II, and III were, respectively, -1.0 +/- 1.9, -2.1 +/- 2.6, and -2.4 +/- 1.2 L/min. The semi-invasive algorithm had an overall better performance in the phases investigated: -1.2 +/- 1.9, -0.6 +/- 2.0, and -0.2 +/- 3.0 L/min, respectively. The noninvasive algorithm showed a slight tendency to overestimate lower reference PCBF values and, importantly, to underestimate higher PCBF values in all three phases (r = -.66, p<.0001; r = -.75, p<.001; r = -.60, p<.0001, respectively). A similar figure was observed with the semi-invasive algorithm in phase I (r = -.47, p<.01) but not in phases II and III (r = -.1, p=.54; r =.62, p<.001, respectively).

CONCLUSIONS

Although PCBF is systematically underestimated during hyperdynamic cardiac output states and high alveolar deadspaces, the performance of the partial CO2 rebreathing technique can be improved by means of arterial blood gas sampling and an algorithm that takes in account the effects of nonequilibration of PetCO2 during rebreathing and the variation of Pc'CO2 to PetCO2 differences from the nonrebreathing to the rebreathing period. Such an algorithm may prove useful under moderately increased alveolar deadspace and normal to hypodynamic cardiac output states.

Non-invasive cardiac output determination: comparison of a new partial-rebreathing technique with thermodilution

This study compares a derivative Fick technique using carbon dioxide (CO2) with the thermodilution pulmonary artery catheter (PAC), for determination of cardiac output (CO). Subjects were sedated, mechanically ventilated adults following elective cardiac surgery Microprocessor controlled deadspace activation and side-stream capnography in a ventilator circuit enabled calculation of CO (CO(CO2)) every four minutes. Thermodilution CO (CO(TD)) was performed as clinically indicated and at 20-minute intervals. Simultaneous CO(TD)/CO(CO2) pairs were recorded from time of admission to ICU for a minimum period of two hours for each patient. There were 358 CO(TD)/CO(CO2) pairs recorded from 41 patients. Cardiac output measurements ranged from 2. 7 to 10.6 l/min. The bias (Bland-Altman) was 0.050 l/min (95% CI -0.024 to 0.125 l/min). The 95% limits of agreement were -1.354 to 1.455 l/min. This simple, non-invasive partial-rebreathing technique is a valid alternative to thermodilution for cardiac output determination in sedated, mechanically ventilated patients. There are significant implications for improved safety, reduced complexity and reduced cost in anaesthesia and intensive care.

Partial CO2 rebreathing cardiac output--operating principles of the NICO system

The partial rebreathing method of cardiac output estimation is reviewed with a particular focus on its application for continuous monitoring, rebreathing and implementations and from both a historical and technical perspective. The assumptions of the method are discussed as well as the various implementations. The NICO monitor and rebreathing valve are described from a functional view. The clinical data including (a) comparisons between bolus thermodilution and continuous thermodilution in patients in the OR setting, (b) comparisons to continuous thermodilution with both the Baxter and Abbott continuous cardiac output devices and (c) comparison between different means of shunt correction are presented. Compared to conventional cardiac output methods, the partial CO2 rebreathing technique is non-invasive, can easily be automated and can provide real-time and continuous cardiac output monitoring. Taking advantage of modern sophisticated sensor and signal processing technology and integrating multiple monitored physiological variables the NICO monitor is the first commercially available cardiac output system making use of the partial rebreathing of CO2.