Noninvasive measurement of cardiac output using partial CO2 rebreathing

Monitoring CO2 kinetics as a marker of cardiopulmonary efficiency

PURPOSE OF REVIEW

To describe current and near future developments and applications of CO2 kinetics in clinical respiratory and cardiovascular monitoring.

RECENT FINDINGS

In the last years, we have witnessed a renewed interest in CO2 kinetics in relation with a better understanding of volumetric capnography and its derived parameters. This together with technological advances and improved measurement systems have expanded the monitoring potential of CO2 kinetics including breath by breath continuous end-expiratory lung volume and continuous noninvasive cardiac output. Dead space has slowly been gaining relevance in clinical monitoring and prognostic evaluation. Easy to measure dead space surrogates such as the ventilatory ratio have demonstrated a strong prognostic value in patients with acute respiratory failure.

SUMMARY

The kinetics of carbon dioxide describe many relevant physiological processes. The clinical introduction of new ways of assessing respiratory and circulatory efficiency based on advanced analysis of CO2 kinetics are paving the road to a long-desired goal in clinical monitoring of critically ill patients: the integration of respiratory and circulatory monitoring during mechanical ventilation.

Clinical validation of a capnodynamic method for measuring end-expiratory lung volume in critically ill patients

RATIONALE

End-expiratory lung volume (EELV) is reduced in mechanically ventilated patients, especially in pathologic conditions. The resulting heterogeneous distribution of ventilation increases the risk for ventilation induced lung injury. Clinical measurement of EELV however, remains difficult.

OBJECTIVE

Validation of a novel continuous capnodynamic method based on expired carbon dioxide (CO2) kinetics for measuring EELV in mechanically ventilated critically-ill patients.

METHODS

Prospective study of mechanically ventilated patients scheduled for a diagnostic computed tomography exploration. Comparisons were made between absolute and corrected EELVCO2 values, the latter accounting for the amount of CO2 dissolved in lung tissue, with the reference EELV measured by computed tomography (EELVCT). Uncorrected and corrected EELVCO2 was compared with total CT volume (density compartments between - 1000 and 0 Hounsfield units (HU) and functional CT volume, including density compartments of - 1000 to - 200HU eliminating regions of increased shunt. We used comparative statistics including correlations and measurement of accuracy and precision by the Bland Altman method.

MEASUREMENTS AND MAIN RESULTS

Of the 46 patients included in the final analysis, 25 had a diagnosis of ARDS (24 of which COVID-19). Both EELVCT and EELVCO2 were significantly reduced (39 and 40% respectively) when compared with theoretical values of functional residual capacity (p < 0.0001). Uncorrected EELVCO2 tended to overestimate EELVCT with a correlation r2 0.58; Bias - 285 and limits of agreement (LoA) (+ 513 to - 1083; 95% CI) ml. Agreement improved for the corrected EELVCO2 to a Bias of - 23 and LoA of (+ 763 to - 716; 95% CI) ml. The best agreement of the method was obtained by comparison of corrected EELVCO2 with functional EELVCT with a r2 of 0.59; Bias - 2.75 (+ 755 to - 761; 95% CI) ml. We did not observe major differences in the performance of the method between ARDS (most of them COVID related) and non-ARDS patients.

CONCLUSION

In this first validation in critically ill patients, the capnodynamic method provided good estimates of both total and functional EELV. Bias improved after correcting EELVCO2 for extra-alveolar CO2 content when compared with CT estimated volume. If confirmed in further validations EELVCO2 may become an attractive monitoring option for continuously monitor EELV in critically ill mechanically ventilated patients.

TRIAL REGISTRATION

clinicaltrials.gov (NCT04045262).

Capnodynamics - noninvasive cardiac output and mixed venous oxygen saturation monitoring in children

Hemodynamic monitoring in children is challenging for many reasons. Technical limitations in combination with insufficient validation against reference methods, makes reliable monitoring systems difficult to establish. Since recent studies have highlighted perioperative cardiovascular stability as an important factor for patient outcome in pediatrics, the need for accurate hemodynamic monitoring methods in children is obvious. The development of mathematical processing of fast response mainstream capnography signals, has allowed for the development of capnodynamic hemodynamic monitoring. By inducing small changes in ventilation in intubated and mechanically ventilated patients, fluctuations in alveolar carbon dioxide are created. The subsequent changes in carbon dioxide elimination can be used to calculate the blood flow participating in gas exchange, i.e., effective pulmonary blood flow which equals the non-shunted pulmonary blood flow. Cardiac output can then be estimated and continuously monitored in a breath-by-breath fashion without the need for additional equipment, training, or calibration. In addition, the method allows for mixed venous oxygen saturation (SvO₂) monitoring, without pulmonary artery catheterization. The current review will discuss the capnodyamic method and its application and limitation as well as future potential development and functions in pediatric patients.

Capnodynamics-Measuring cardiac output via ventilation

Recent studies have identified stable hemodynamics as a contributing factor to improve outcome in pediatric anesthesia. So far, most of the hemodynamic monitoring methods applied in children have been complex to apply and often not satisfactory validated. Standard mainstream carbon dioxide analysis in combination with real-time mathematical analysis of the measured capnography data has enabled the development of dynamic capnography, a non-invasively cardiac output monitoring method that can be applied without user practice or need for calibrations. Capnodynamic cardiac output assessment has been extensively validated against gold standard reference methods, both in experimental and clinical settings. This review will describe the principle behind dynamic capnography measurement of cardiac output and mixed venous oxygen saturation. Additionally, the methods limitations and challenges when applied in children will be delineated.

Non-Invasive Continuous Measurement of Haemodynamic Parameters-Clinical Utility

The evaluation and monitoring of patients’ haemodynamic parameters are essential in everyday clinical practice. The application of continuous, non-invasive measurement methods is a relatively recent solution. CNAP, ClearSight and many other technologies have been introduced to the market. The use of these techniques for assessing patient eligibility before cardiac procedures, as well as for intraoperative monitoring is currently being widely investigated. Their numerous advantages, including the simplicity of application, time- and cost-effectiveness, and the limited risk of infection, could enforce their further development and potential utility. However, some limitations and contradictions should also be discussed. The aim of this paper is to briefly describe the new findings, give practical examples of the clinical utility of these methods, compare them with invasive techniques, and review the literature on this subject.

Clinical use of volumetric capnography in mechanically ventilated patients

Capnography is a first line monitoring system in mechanically ventilated patients. Volumetric capnography supports noninvasive and breath-by-breath information at the bedside using mainstream CO₂ and flow sensors placed at the airways opening. This volume-based capnography provides information of important body functions related to the kinetics of carbon dioxide. Volumetric capnography goes one step forward standard respiratory mechanics and provides a new dimension for monitoring of mechanical ventilation. The article discusses the role of volumetric capnography for the clinical monitoring of mechanical ventilation.

Performance of a capnodynamic method estimating effective pulmonary blood flow during transient and sustained hypercapnia

The capnodynamic method is a minimally invasive method continuously calculating effective pulmonary blood flow (CO_EPBF), equivalent to cardiac output when intra pulmonary shunt flow is low. The capnodynamic equation joined with a ventilator pattern containing cyclic reoccurring expiratory holds, provides breath to breath hemodynamic monitoring in the anesthetized patient. Its performance however, might be affected by changes in the mixed venous content of carbon dioxide (C_vCO₂). The aim of the current study was to evaluate CO_EPBF during rapid measurable changes in mixed venous carbon dioxide partial pressure (P_vCO₂) following ischemia–reperfusion and during sustained hypercapnia in a porcine model. Sixteen pigs were submitted to either ischemia–reperfusion (n = 8) after the release of an aortic balloon inflated during 30 min or to prolonged hypercapnia (n = 8) induced by adding an instrumental dead space. Reference cardiac output (CO) was measured by an ultrasonic flow probe placed around the pulmonary artery trunk (CO_TS). Hemodynamic measurements were obtained at baseline, end of ischemia and during the first 5 min of reperfusion as well as during prolonged hypercapnia at high and low CO states. Ischemia–reperfusion resulted in large changes in P_vCO₂, hemodynamics and lactate. Bias (limits of agreement) was 0.7 (−0.4 to 1.8) L/min with a mean error of 28% at baseline. CO_EPBF was impaired during reperfusion but agreement was restored within 5 min. During prolonged hypercapnia, agreement remained good during changes in CO. The mean polar angle was −4.19° (−8.8° to 0.42°). Capnodynamic CO_EPBF is affected but recovers rapidly after transient large changes in P_vCO₂ and preserves good agreement and trending ability during states of prolonged hypercapnia at different levels of CO.

A modified breathing pattern improves the performance of a continuous capnodynamic method for estimation of effective pulmonary blood flow

In a previous study a new capnodynamic method for estimation of effective pulmonary blood flow (CO_EPBF) presented a good trending ability but a poor agreement with a reference cardiac output (CO) measurement at high levels of PEEP. In this study we aimed at evaluating the agreement and trending ability of a modified CO_EPBF algorithm that uses expiratory instead of inspiratory holds during CO and ventilatory manipulations. CO_EPBF was evaluated in a porcine model at different PEEP levels, tidal volumes and CO manipulations (N = 8). An ultrasonic flow probe placed around the pulmonary trunk was used for CO measurement. We tested the CO_EPBF algorithm using a modified breathing pattern that introduces cyclic end-expiratory time pauses. The subsequent changes in mean alveolar fraction of carbon dioxide were integrated into a capnodynamic equation and effective pulmonary blood flow, i.e. non-shunted CO, was calculated continuously breath by breath. The overall agreement between CO_EPBF and the reference method during all interventions was good with bias (limits of agreement) 0.05 (-1.1 to 1.2) L/min and percentage error of 36 %. The overall trending ability as assessed by the four-quadrant and the polar plot methodology was high with a concordance rate of 93 and 94 % respectively. The mean polar angle was 0.4 (95 % CI -3.7 to 4.5)°. A ventilatory pattern recurrently introducing end-expiratory pauses maintains a good agreement between CO_EPBF and the reference CO method while preserving its trending ability during CO and ventilatory alterations.

Capnodynamic assessment of effective lung volume during cardiac output manipulations in a porcine model

A capnodynamic calculation of effective pulmonary blood flow includes a lung volume factor (ELV) that has to be estimated to solve the mathematical equation. In previous studies ELV correlated to reference methods for functional residual capacity (FRC). The aim was to evaluate the stability of ELV during significant manipulations of cardiac output (CO) and assess the agreement for absolute values and trending capacity during PEEP changes at different lung conditions. Ten pigs were included. Alterations of alveolar carbon dioxide were induced by cyclic reoccurring inspiratory holds. The Sulphur hexafluoride technique for FRC measurements was used as reference. Cardiac output was altered by preload reduction and inotropic stimulation at PEEP 5 and 12 cmH₂O both in normal lung conditions and after repeated lung lavages. ELV at baseline PEEP 5 was [mean (SD)], 810 (163) mL and decreased to 400 (42) mL after lavage. ELV was not significantly affected by CO alterations within the same PEEP level. In relation to FRC the overall bias (limits of agreement) was -35 (-271 to 201) mL, and percentage error 36 %. A small difference between ELV and FRC was seen at PEEP 5 cmH₂O before lavage and at PEEP 12 cmH₂O after lavage. ELV trending capability between PEEP steps, showed a concordance rate of 100 %. ELV was closely related to FRC and remained stable during significant changes in CO. The trending capability was excellent both before and after surfactant depletion.

A novel continuous capnodynamic method for cardiac output assessment in a porcine model of lung lavage

BACKGROUND

We have evaluated a new method for continuous monitoring of effective pulmonary blood flow (COEPBF ), i.e. cardiac output (CO) minus intra-pulmonary shunt, during mechanical ventilation. The method has shown good trending ability during severe hemodynamic challenges in a porcine model with intact lungs. In this study, we further evaluate the COEPBF method in a model of lung lavage.

METHODS

COEPBF was compared to a reference method for CO during hemodynamic and PEEP alterations, 5 and 12 cmH2 O, before and after repeated lung lavages in 10 anaesthetised pigs. Bland-Altman, four-quadrant and polar plot methodologies were used to determine agreement and trending ability.

RESULTS

After lung lavage at PEEP 5 cmH2 O, the ratio of arterial oxygen partial pressure related to inspired fraction of oxygen significantly decreased. The mean difference (limits of agreement) between methods changed from 0.2 (-1.1 to 1.5) to -0.9 (-3.6 to 1.9) l/min and percentage error increased from 34% to 70%. Trending ability remained good according to the four-quadrant plot (concordance rate 94%), whereas mean angular bias increased from 4° to -16° when using the polar plot methodology.

CONCLUSION

Both agreement and precision of COEPBF were impaired in relation to CO when the shunt fraction was increased after lavage at PEEP 5 cmH2 O. However, trending ability remained good as assessed by the four-quadrant plot, whereas the mean polar angle, calculated by the polar plot, was wide.

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.

Lung volume assessments in normal and surfactant depleted lungs: agreement between bedside techniques and CT imaging

Background

Bedside assessment of lung volume in clinical practice is crucial to adapt ventilation strategy. We compared bedside measures of lung volume by helium multiple-breath washout technique (EELV_MBW,He) and effective lung volume based on capnodynamics (ELV) to those assessed from spiral chest CT scans (EELV_CT) under different PEEP levels in control and surfactant-depleted lungs.

Methods

Lung volume was assessed in anaesthetized mechanically ventilated rabbits successively by measuring i) ELV by analyzing CO₂ elimination traces during the application of periods of 5 consecutive alterations in inspiratory/expiratory ratio (1:2 to 1.5:1), ii) measuring EELV_MBW,He by using helium as a tracer gas, and iii) EELV_CT from CT scan images by computing the normalized lung density. All measurements were performed at PEEP of 0, 3 and 9 cmH₂O in random order under control condition and following surfactant depletion by whole lung lavage.

Results

Variables obtained with all techniques followed sensitively the lung volume changes with PEEP. Excellent correlation and close agreement was observed between EELV_MBW,He and EELV_CT (r = 0.93, p < 0.0001). ELV overestimated EELV_MBW,He and EELV_CT in normal lungs, whereas this difference was not evidenced following surfactant depletion. These findings resulted in somewhat diminished but still significant correlations between ELV and EELV_CT (r = 0.58, p < 0.001) or EELV_MBW,He (0.76, p < 0.001) and moderate agreements.

Conclusions

Lung volume assessed with bedside techniques allow the monitoring of the changes in the lung aeration with PEEP both in normal lungs and in a model of acute lung injury. Under stable pulmonary haemodynamic condition, ELV allows continuous lung volume monitoring, whereas EELV_MBW,He offers a more accurate estimation, but intermittently.

Hybrid measurement to achieve satisfactory precision in perioperative cardiac output monitoring

Advanced haemodynamic monitoring employing minimally invasive cardiac output measurement may lead to significant improvements in patient outcomes in major surgery. However, the precision (scatter) of measurement of available generic technologies has been shown to be unsatisfactory with percentage error of agreement with bolus thermodilution (% error) of 40% to 50%. Simultaneous measurement and averaging by two or more technologies may reduce random measurement scatter and improve precision. This concept, called the hybrid method, was tested by comparing accuracy and precision of measurement relative to bolus thermodilution using combinations of three component methods. Thirty patients scheduled for either elective cardiac surgery or liver transplantation were studied. Agreement with simultaneous bolus thermodilution of hybrid combinations of continuous thermodilution (QtCCO) or Vigeleo™/FloTrac™ pulse contour measurement (QtFT) with pulmonary Capnotracking (QtCO2) was assessed pre- and post-cardiopulmonary bypass or pre- and post-reperfusion of the donor liver and compared with that of the component methods alone. Hybridisation of QtCO2 (% error 42.2) and QtCCO (% error 51.3) achieved significantly better precision (% error 31.3) than the component methods (P=0.0004) and (P=0.0195). Due to poor inherent precision of QtFT (% error 82.8), hybrid combination of QtFT with QtCO2 did not result in better precision than QtCO2 alone. Hybrid measurement can approach a 30% error, which is recommended as the upper limit for acceptability. This is a practical option where at least one component method, such as Capnotracking, is automated and does not increase the cost or complexity of the measurement process.

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.

Comparison of Static End-expiratory and Effective Lung Volumes for Gas Exchange in Healthy and Surfactant-depleted Lungs

Background:

Effective lung volume (ELV) for gas exchange is a new measure that could be used as a real-time guide during controlled mechanical ventilation. The authors established the relationships of ELV to static end-expiratory lung volume (EELV) with varying levels of positive end-expiratory pressure (PEEP) in healthy and surfactant-depleted rabbit lungs.

Methods:

Nine rabbits were anesthetized and ventilated with a modified volume-controlled mode where periods of five consecutive alterations in inspiratory/expiratory ratio (1:2–1.5:1) were imposed to measure ELV from the corresponding carbon dioxide elimination traces. EELV and the lung clearance index were concomitantly determined by helium wash-out technique. Airway and tissue mechanics were assessed by using low-frequency forced oscillations. Measurements were collected at PEEP 0, 3, 6, and 9 cm H2O levels under control condition and after surfactant depletion by whole-lung lavage.

Results:

ELV was greater than EELV at all PEEP levels before lavage, whereas there was no evidence for a difference in the lung volume indices after surfactant depletion at PEEP 6 or 9 cm H2O. Increasing PEEP level caused significant parallel increases in both ELV and EELV levels, decreases in ventilation heterogeneity, and improvement in airway and tissue mechanics under control condition and after surfactant depletion. ELV and EELV exhibited strong and statistically significant correlations before (r = 0.84) and after lavage (r = 0.87).

Conclusions:

The parallel changes in ELV and EELV with PEEP in healthy and surfactant-depleted lungs support the clinical value of ELV measurement as a bedside tool to estimate dynamic changes in EELV in children and infants.

Evaluation of a CO2 partial rebreathing functional residual capacity measurement method for use during mechanical ventilation

OBJECTIVE

There is a need for an automated bedside functional residual capacity (FRC) measurement method that does not require a step change in inspired oxygen fraction. Such a method can be used for patients who require a high inspired oxygen fraction to maintain arterial oxygenation and for patients ventilated using a circle breathing system commonly found in operating rooms, which is not capable of step changes in oxygen. We developed a CO(2) rebreathing method for FRC measurement that is based on the change in partial pressure of end-tidal carbon dioxide and volume of CO(2) eliminated at the end of a partial rebreathing period. This study was designed to assess the accuracy and precision of the proposed FRC measurement system compared to body plethysmography and nitrogen washout FRC.

METHODS

Accuracy and precision of measurements were assessed by comparing the CO(2) rebreathing FRC values to the gold standard, body plethysmography FRC, in twenty spontaneously breathing volunteers. The CO(2) rebreathing FRC measurements were then compared to nitrogen washout FRC in twenty intensive care patients whose lungs were mechanically ventilated. For each subject, an average value of CO(2) rebreathing FRC was compared to the average gold standard method. Measurements were accepted for statistical analysis if they had been recorded from periods of stable tidal ventilation, defined as a coefficient of variation of tidal volume of <0.13.

RESULTS

Compared to body plethysmography, the accuracy (average error) for the CO(2) rebreathing method during stable ventilation (n = 8) was 0.03 L and precision (1 standard deviation of the error) was 0.29 L (0.8 ± 7.6% of body plethysmography). During stable mechanical ventilation (n = 9), the accuracy was -0.02 L and precision was 0.26 L (-1.1 ± 12.6% of nitrogen washout).

CONCLUSIONS

The CO(2) rebreathing method for FRC measurement provides acceptable accuracy and precision during stable ventilation compared to the gold standards of body plethysmography and nitrogen washout. The results based on periods of stable ventilation best approximate the performance of the system in the likely areas of application during controlled mechanical ventilation. Further study of the CO(2) rebreathing method is needed to evaluate accuracy in a larger group of controlled mechanical ventilation patients, including patients with respiratory insufficiency and significant lung injury.

Cardiac Output and Propofol Concentrations in Prone Surgical Patients

The aim of this study was to compare cardiac output and plasma propofol concentrations in the supine and prone positions in healthy adult patients presenting for lumbar spine surgery. Patients received propofol and remifentanil via effect-site steered target-controlled infusions. Cardiac output and plasma propofol concentration were compared during 20 minutes in the supine position and 20 minutes after positioning on a Wilson frame. Cardiac output did not change significantly over 20 minutes in either position (P=0.37) and was similar at 20 minutes in the supine (6.1 [1.6] l/minute) and prone positions (6.1 [1.9] l/minute) (P=0.87). Propofol concentrations were similar in the supine and prone positions at 20 minutes (2.55 [0.89] and 2.53 [0.90] μg/ml; P=0.93). We conclude that prone positioning on the Wilson frame does not affect cardiac output or plasma propofol concentration.

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.

Nichtinvasives erweitertes hämodynamisches Monitoring

BACKGROUND

Cardiac output and the cardiac index (CI) are not routinely monitored during major abdominal surgery for economic as well as medical reasons. This practice, however, might be changed by the application of newer non-invasive technologies like the partial CO(2) rebreathing method based on the inverse Fick's principle. In this prospective randomized study we investigated the impact of a non-invasive monitoring of CI on the incidence of hemodynamic instability and interventions by the attending anesthesiologist during major abdominal surgery.

PATIENTS AND METHODS

Additionally to routine hemodynamic monitoring we measured CI using the partial CO(2) rebreathing method in 28 patients (9 female, 19 male) undergoing major abdominal surgery. In group I the anesthesiologists were aware of the results of the extended hemodynamic monitoring and in group II the attending anesthesiologist was blinded to the information obtained by these measurements of CI.

RESULTS

Groups did not differ with regard to the baseline hemodynamic parameters. We obtained 923 measurements in both groups and 95 situations of hemodynamic instability (CI<2.5 l/minxm(2)) were detected in group I compared to 147 situations in group II (p<0.05). There were significantly more hemodynamic interventions in group I than in group II (p<0.0001). The cardiac index remained higher in group I in comparison to group II (p<0.0001). Measurement of CI was the only method to detect situations of hemodynamic instability in our setting.

CONCLUSION

The incidence of hemodynamic instability was significantly reduced during major abdominal surgery when anesthesiologists were aware of the measurement results of extended hemodynamic monitoring.

Noninvasive technologies for tissue perfusion

As outlined in Table 1, the nonthermodilution techniques available to measure cardiac output are noninvasive and clinically applicable to a variable degree. The truly noninvasive monitors are bioimpedance and CO2 re-breathing. The latter, however, requires the patient to be intubated, and the former continues to be evaluated with regard to correlation with the thermodilution standard. Esophageal Doppler devices are relatively noninvasive in that they do not require vascular cannulation, but they do require an immobile patient and some user expertise. Pulse contour analysis requires an arterial catheter, and two of the three available monitors require external calibration, while the third has not been validated adequately. The reader can see that all four approaches continue to be refined, with new analysis algorithms and monitors continuing to appear on the market. In the absence of true tissue oxygenation monitors, it seems likely that some or all of these alternatives to thermodilution will play a greater role in the care of patients where measurement of cardiac output is desired.

The linearity-improvement sigma-delta modulator for single-ended biomedical sensors

A sigma-delta modulator which can improve the linearity of input original signal is proposed in this paper. The inverting and double sampling techniques are used in proposed modulator to convert the single-ended signal to fully differential signal. Therefore, the signal's linearity could be modified by the suppression of the even mode harmonic in fully differential circuit. With the proposed technique, the sensing biomedical signals could be more accurately acquired. The proposed modulator is designed in standard 0.18 microm 1P6M CMOS technology. The simulation results show that the linear errors of the CMOS thermal sensor and the PH electrochemical sensor are improved from 42.5% to 20.17%, 8.06% to 3.11%.

Noninvasive assessment of cardiac output using thoracic electrical bioimpedance in hemodynamically stable and unstable patients after cardiac surgery: a comparison with pulmonary artery thermodilution

OBJECTIVE

To compare noninvasive cardiac output (CO)measurement obtained with a new thoracic electrical bioimpedance (TEB) device, using a proprietary modification of the impedance equation, with invasive measurement obtained via pulmonary artery thermodilution.

DESIGN

Prospective, observational study.

SETTING

Surgical intensive care unit (ICU) of a university-affiliated community hospital.

PATIENTS AND PARTICIPANTS

Seventy-four adult patients undergoing elective cardiac surgery with routine pulmonary artery catheter placement.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Simultaneous paired CO and cardiac index (CI) measurements by TEB and thermodilution were obtained in mechanically ventilated patients upon admission to the ICU. For analysis of CI data the patients were subdivided into a hemodynamically stable group and a hemodynamically unstable group. The groups were analyzed using linear regression and tests of bias and precision. We found a significant correlation between thermodilution and TEB (r = 0.83; n < 0.001), accompanied by a bias of -0.01 l/min/m(2) and a precision of +/-0.57 l/min/m(2) for all CI data pairs. Correlation, bias, and precision were not influenced by stratification of the data. The correlation coefficient, bias, and precision for CI were 0.86 (n< 0.001), 0.03 l/min/m(2), and +/-0.47 l/min/m(2) in hemodynamically stable patients and 0.79 (n< 0.001), 0.06 l/min/m(2), and +/-0.68 l/min/m(2) in hemodynamically unstable patients.

CONCLUSIONS

Our results demonstrate a close correlation and clinically acceptable agreement and precision between CO measurements obtained with impedance cardiography using a new algorithm to calculate CO from variations in TEB, and those obtained with the clinical standard of care, pulmonary artery thermodilution, in hemodynamically stable and unstable patients after cardiac surgery.

The effect of bi-level positive airway pressure mechanical ventilation on gas exchange during general anaesthesia

BACKGROUND

Atelectasis may occur and ventilation-perfusion mismatch may increase during general anaesthesia with neuromuscular paralysis and mechanical ventilation, though preservation of some intermittent muscle contraction might mitigate this process. There is still no ideal manoeuvre to minimize such mismatch or atelectasis. Bi-level positive airway pressure (BiPAP) ventilation adjusts to extra breaths and improves gas exchange during recovery of diaphragm function after neuromuscular paralysis. We hypothesize that BiPAP ventilation may limit the development of pulmonary shunt and may improve ventilation-perfusion mismatch when compared with standard IPPV, with or without PEEP when neuromuscular paralysis has been used during surgery.

METHODS

Twenty ventilated patients either on BiPAP or IPPV with or without PEEP were studied randomly using the multiple inert gas elimination technique (MIGET) at 60 and 120 min after rocuronium at induction and after 60 min. Non-invasive cardiac output (NICO) monitoring and plasma concentrations of rocuronium were measured. We compared the data of MIGET, gas exchange, haemodynamic variables and pulmonary mechanics measurements between the different ventilatory modes.

RESULTS

Intrapulmonary shunt (blood flow to V(A)/Q < 0.005) did not increase at 60 min of anaesthesia in any of the different ventilation modes compared with the shunt value before anaesthesia. Log standard deviation of perfusion increased in IPPV, with and without PEEP groups, compared with the baseline (P< 0.05) but did not increase in the BiPAP group. BiPAP ventilation generated a higher level of Pa(O2)than IPPV with or without PEEP (P<0.05).

CONCLUSION

BiPAP ventilation was beneficial in decreasing ventilation-perfusion mismatch and improving oxygenation when compared with conventional IPPV (with or without PEEP).

Messung des Herzzeitvolumens

Diagnosis and therapy of hemodynamic instability are of the utmost importance in the treatment of critically ill patients during surgery and in intensive care. For both diagnosis and therapy, adequate and preferably continuous hemodynamic monitoring is essential. Besides the assessment of cardiac preload and blood pressure, cardiac output represents an important clinical marker of cardiac performance and global perfusion. Since its clinical introduction by Swan and Ganz in 1970, the standard technique for measuring cardiac output has been the pulmonary arterial thermodilution technique using a pulmonary artery catheter. The ongoing discussion on the risk-benefit ratio of such a pulmonary artery catheter has led to the introduction of several less invasive methods for determining cardiac output. The aim of this review is to provide background information on these alternative methods and to discuss the individual advantages and disadvantages of each method in the context of their clinical applicability.

Physiologically precise simulation of multiple lung gas exchange during anaesthesia by simultaneous gas infusion and extraction

A lung gas exchange simulator was tested which produces simultaneous uptake and/or elimination of multiple gases by an artificial test lung with physiologically realistic gas expired and exhaust gas flows, using a combination of infusion of diluting/enriching gases into the lung with lung gas extraction. A deterministic algorithm is incorporated which calculates required gas infusion and extraction flow rates for any set of possible target gas exchange values with any given set of fresh gas flows and concentrations. Six different scenarios were simulated, comprising a range of gas exchange values for each gas species which lie within a physiologically realistic range for anaesthetized patients. For each of these experiments the system was tested for 15 consecutive measurements over 25 min by measurement of gas exchange in the system using the Haldane transformation.

RESULTS

the mean bias and standard error of the mean bias (SE, in parentheses) relative to the target value was: +0.001 (0.002) l min(-1) for O(2) uptake, -0.002 (0.005) l min(-1) for CO(2) production, -0.001 (0.002) l min(-1) for uptake of nitrous oxide and +0.3 (0.1) ml min(-1) for uptake of a volatile anaesthetic agent (isoflurane). The confidence limits of the mean bias were within 5% of the target value for all gases and scenarios with the exception of those where a low uptake of anaesthetic gas was specified. The confidence limits of the mean bias for the lower uptakes of isoflurane were within 10% of the target value for these scenarios and within 15% for the low uptake of N(2)O. Good accuracy and precision of this approach to lung gas exchange simulation were demonstrated, resulting in a versatile simulator.

Multi-site-frequencyelectromechanocardiography for the prediction of ejection fraction and stroke volume in heart failure

BACKGROUND

Methods for stroke volume (SV) and ejection fraction (EF) measurements require the presence of qualified physicians and are not suited for continuous monitoring.

AIM

To develop an automated non-invasive method for the measurement and continuous monitoring of SV and EF.

METHODS

We have designed a method for the measurement of EF and SV using multiple-site-impedance (z0) measurements, applying multiple frequencies of 5, 40 and 200 kHz whereby various segments of the human body, including volume changes within these segments, could be defined electrically. The obtained variables were used to train neuronal nets and related by multiple regression analyses to cardiac output (CO) as measured by a partial rebreathing Fick method (CO(r-fick)) or EF as measured by echocardiography (EF(echo)), respectively. A total of 129 subjects (48 with normal heart function and 81 with CHF, NYHA I-IV) were investigated.

RESULTS

The multiply derived values of z0 and of change of impedance (dz/dt) were shown, by multiple regression analysis, to be significantly related to CO(r-fick) and to EF(echo), (total r=0.77, n=35, p<0.001, and r=0.81, n=47, p<0.001, respectively.). By training a neuronal net with the electrical data of 67 (out of 94) subjects, EF(echo) could be predicted in the remaining 27 subjects which were unknown to the neuronal net with a combined r=0.71 (p<0.001,n=27). In contrast, conventional impedance cardiography (ICG) was unable to predict either CO(r-fick) or EF(echo).

CONCLUSION

The new method, which we call multi-site-frequency electromechanocardiography (msf-ELMC) appears promising for the automated electrical measurement of the mechanical heart action in patients with normal and reduced cardiac function.

Effects of reduced rebreathing time, in spontaneously breathing patients, on respiratory effort and accuracy in cardiac output measurement when using a partial carbon dioxide rebreathing technique: a prospective observational study

Introduction

New technology using partial carbon dioxide rebreathing has been developed to measure cardiac output. Because rebreathing increases respiratory effort, we investigated whether a newly developed system with 35 s rebreathing causes a lesser increase in respiratory effort under partial ventilatory support than does the conventional system with 50 s rebreathing. We also investigated whether the shorter rebreathing period affects the accuracy of cardiac output measurement.

Method

Once a total of 13 consecutive post-cardiac-surgery patients had recovered spontaneous breathing under pressure support ventilation, we applied a partial carbon dioxide rebreathing technique with rebreathing of 35 s and 50 s in a random order. We measured minute ventilation, and arterial and mixed venous carbon dioxide tension at the end of the normal breathing period and at the end of the rebreathing periods. We then measured cardiac output using the partial carbon dioxide rebreathing technique with the two rebreathing periods and using thermodilution.

Results

With both rebreathing systems, minute ventilation increased during rebreathing, as did arterial and mixed venous carbon dioxide tensions. The increases in minute ventilation and arterial carbon dioxide tension were less with 35 s rebreathing than with 50 s rebreathing. The cardiac output measures with both systems correlated acceptably with values obtained with thermodilution.

Conclusion

When patients breathe spontaneously the partial carbon dioxide rebreathing technique increases minute ventilation and arterial carbon dioxide tension, but the effect is less with a shorter rebreathing period. The 35 s rebreathing period yielded cardiac output measurements similar in accuracy to those with 50 s rebreathing.

An evaluation of a noninvasive cardiac output measurement using partial carbon dioxide rebreathing in children

Cardiac output (CO) is an important hemodynamic measure that helps to guide the therapy of critically ill patients. Invasive CO assessment in infants and children is often avoided because of the inherent risks. A noninvasive CO monitor that uses partial rebreathing has been recently developed to determine CO via the Fick principle for carbon dioxide. There have been no clinical studies confirming its accuracy in pediatric patients. This is a prospective observational study of 37 children <12 yr of age who underwent cardiac catheterization. Under general anesthesia via an endotracheal tube without a leak, we made multiple CO measurements using thermodilution and compared them with noninvasively determined CO measurements. Paired measurements were analyzed for bias, precision, and correlation via Bland-Altman plot and linear regression. Noninvasive measurements showed a linear correlation with thermodilution CO assessment with an r value of 0.83 (P < 0.03). Bland-Altman analysis yielded a bias of -0.27 L/min and a precision +/-1.49 L/min. Cardiac index measurements demonstrated a decreased r value of 0.67 (P = 0.15) and a bias of -0.18 L . min(-1) . m(-2) and precision of +/-2.13 L . min(-1) . m(-2). Differences between partial rebreathing measurements and thermodilution measurements were largest in children with a body surface area of </=0.6 m(2) ventilated with tidal volumes <300 mL. Based on these findings, noninvasive CO measurement using partial rebreathing may be clinically acceptable in children with >0.6 m(2) body surface area and >300 mL tidal volume.

Comparison between cardiac output values measured by thermodilution and partial carbon dioxide rebreathing in patients with acute lung injury

CONTEXT

Thermodilution, which is considered to be a standard technique for measuring the cardiac output in critically ill patients, is not free from relevant risks. There is a need to find alternative, noninvasive, automatic, simple and accurate methods for monitoring cardiac output at the bedside.

OBJECTIVE

To compare cardiac output measurements by thermodilution and partial carbon dioxide rebreathing in patients with acute lung injury at two levels of severity (lung injury score, LIS: below 2.5, group A; and above 2.5, group B).

TYPE OF STUDY

Comparative, prospective and controlled study.

SETTING

Intensive Care Units of two university hospitals.

METHODS

Cardiac output was measured by thermodilution and partial carbon dioxide rebreathing. Twenty patients with acute lung failure (PaO2/FiO2 < 300) who were under mechanical ventilation and from whom 294 measurements were taken: 164 measurements in group A (n = 11) and 130 in group B (n = 9), ranging from 14 to 15 determinations per patient.

RESULTS

There was a poor positive correlation between the methods studied for the patients from groups A (r = 0.52, p < 0.001) and B (r = 0.47, p < 0.001). The application of the Bland-Altman test made it possible to expose the lack of agreement between the methods (group A: -0.9 +/- 2.71 l/min; 95% CI = -1.14 to -0.48; and group B: -1.75 +/- 2.05 l/min; 95% CI = -2.11 to -1.4). The comparison of the results (Student t and Mann-Whitney tests) within each group and between the groups showed significant difference (p = 0.000, p < 0.05).

DISCUSSION

Errors in estimating CaCO2 (arterial CO2 content) from ETCO2 (end-tidal CO2) and situations of hyperdynamic circulation associated with dead space and/or increased shunt possibly explain our results.

CONCLUSION

Under the conditions of this study, the results obtained allow us to conclude that, in patients with acute lung injury, the cardiac output determined by partial rebreathing of CO2 differs from the measurements obtained by thermodilution. This difference becomes greater, the more critical the lung injury is.

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.

Continuous measurement of gas uptake and elimination in anesthetized patients using an extractable marker gas

Measurement of pulmonary gas uptake and elimination is often performed, using nitrogen as marker gas to measure gas flow, by applying the Haldane transformation. Because of the inability to measure nitrogen with conventional equipment, measurement is difficult during inhalational anesthesia. A new method is described, which is compatible with any inspired gas mixture, in which fresh gas and exhaust gas flows are measured using carbon dioxide as an extractable marker gas. A system was tested in eight patients undergoing colonic surgery for automated measurement of uptake of oxygen, nitrous oxide, isoflurane, and elimination of carbon dioxide with this method. Its accuracy and precision were compared with simultaneous measurements made with the Haldane transformation and corrected for predicted nitrogen excretion by the lungs. Good agreement was obtained for measurement of uptake or elimination of all gases studied. Mean bias was −0.003 l/min for both oxygen and nitrous oxide uptake, −0.0002 l/min for isoflurane uptake, and 0.003 l/min for carbon dioxide elimination. Limits of agreement lay within 30% of the mean uptake rate for nitrous oxide, within 15% for oxygen, within 10% for isoflurane, and within 5% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas uptake and elimination in an anesthetic breathing system with any inspired gas mixture.

A comparative evaluation of thermodilution and partial CO2 rebreathing techniques for cardiac output assessment in critically ill patients during assisted ventilation

OBJECTIVE

To evaluate the reliability and clinical value of partial noninvasive CO2 (NICO2) rebreathing technique for measuring cardiac output compared with standard thermodilution in a group of intensive care nonpostoperative patients.

DESIGN AND SETTING

Clinical investigation in a university hospital ICU.

PATIENTS

Twelve mechanically ventilated patients with high (n=6) and low (n=6) pulmonary shunt fractions.

MEASUREMENTS AND RESULTS

Thirty-six paired measurements of cardiac output were carried out with NICO2 and thermodilution in patients ventilated in pressure-support mode and sedated with a sufentanil continuous infusion to obtain a Ramsay score value of 2. The mean cardiac output was: thermodilution 7.27+/-2.42 l/min; NICO2 6.10+/-1.66 l/min; r2 was 0.62 and bias -1.2 l/min+/-1.5. Mean values of cardiac output were similar in the low shunt group (Qs/Qt < 20), with r2=0.90 and a bias of 0.01 l/min+/-0.4; conversely, in the high pulmonary shunt group (Qs/Q > 35%) the mean was 9.32+/-1.23 l/min with thermodilution and a mean NICO2CO value was 6.97+/-1.53 l/min, with r2 of 0.38 and a bias of -2.3 l+/-1.2 min.

CONCLUSIONS

The partial CO2 rebreathing technique is reliable in measuring cardiac output in nonpostoperative critically ill patients affected by diseases causing low levels of pulmonary shunt, but underestimates it in patients with shunt higher than 35%.

A new method for measurement of gas exchange during anaesthesia using an extractable marker gas

A new method for the measurement of pulmonary gas exchange during inhalational anaesthesia is described which measures fresh gas and exhaust gas flows using carbon dioxide as an extractable marker gas. The theoretical precision of the method was compared by Monte Carlo modelling with other approaches which use marker gas dilution. A system was constructed for automated measurement of uptake of oxygen, nitrous oxide, volatile anaesthetic agent and elimination of carbon dioxide by an anaesthetized patient. The accuracy and precision of the method was tested in vitro on a lung gas exchange simulator, by comparison with simultaneous measurements made using nitrogen as marker gas and the Haldane transformation. Good agreement was obtained for measurement of simulated uptake or elimination of all gases studied over a physiologically realistic range of values. Mean bias for oxygen and nitrous oxide uptake was 0.003 l min(-1), for isoflurane 0.0001 l min(-1) and for carbon dioxide 0.001 l min(-1). Limits of agreement lay within 10% of the mean uptake rate for nitrous oxide, within 5% for oxygen and isoflurane and within 1% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas exchange in an anaesthetic breathing system with any inspired gas mixture.

Cardiac output technologies with special reference to the horse

Critical illness, anesthesia, primary cardiovascular disease, and exercise may result in marked hemodynamic alterations. Measuring cardiac output (CO) is central to defining these alterations for both clinician and researcher. In the past 10 years, several new methods of measuring CO have been developed for the human medical market. Some of these methods are now validated in the horse and are in clinical use. The Fick method has been used in equine research for more than a century. It depends on simultaneous measurement of mixed venous (pulmonary arterial) and peripheral arterial oxygen content and oxygen uptake by the lungs. The technique is technically demanding, which restricts its clinical use. Indicator dilution techniques, with indocyanine green, cold (thermodilution), or lithium as the marker, have also been widely used in the horse. The indocyanine technique is cumbersome, and thermodilution requires right heart catheterization, which is not a benign procedure, making both of these methods less than ideal for clinical use. Lithium dilution requires catheterization of a peripheral artery and a jugular vein. It has recently been validated in anesthetized adult horses and neonatal foals. Doppler echocardiography is a noninvasive ultrasound-based technique. More accurate measurements are obtained with transesophageal than with transthoracic measurements; however, both methods require considerable technical expertise. Bioimpedance and pulse contour analysis are 2 new methods that have yet to be validated in the horse. With the currently available technology, lithium dilution appears to be the method of measuring CO best suited to the equine clinic.

Continuous measurement of cardiac output by inert gas throughflow: comparison with thermodilution

OBJECTIVE

The throughflow method is a new technique for continuous and minimally invasive measurement of cardiac output by the Fick principle, which uses ventilation of the 2 lungs with unequal inspired gas concentrations by means of a double-lumen endobronchial tube. It exploits steady-state gas exchange and thus permits rapid repetition of measurement.

DESIGN

Comparison of paired measurements by the throughflow method using N(2)O exchange with bolus thermodilution.

SETTING

Departments of anesthesiology in 2 university teaching hospitals.

PARTICIPANTS

Nine patients undergoing cardiac surgery in the precardiopulmonary bypass period.

INTERVENTIONS

Patients intubated with a double-lumen endobronchial tube were ventilated with 45% nitrous oxide (N(2)O) to the left lung (zero to the right lung). Arterial blood gas samples were taken to measure alveolar deadspace to allow correction for the alveolar-arterial N(2)O difference and to correct for the presence of unmeasured shunt perfusion.

MEASUREMENTS AND MAIN RESULTS

Throughflow measurements correlated with thermodilution (r = 0.719, p < 0.05) with a mean bias of -0.208 L/min (-5.2%). The standard error of the bias was 0.060 L/min, with 95% confidence limits for the bias of -0.088 L/min and -0.328 L/min. The limits of agreement between the 2 methods were +0.960 L/min and -1.376 L/min.

CONCLUSIONS

The throughflow method showed good agreement with thermodilution. It permits continuous cardiac output measurement without the need for sampling of mixed venous blood, using techniques of lung isolation, which are readily available in clinical anesthetic practice.

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.

Development and in vitro validation of a device for measuring non-shunt cardiac output by nitrous oxide throughflow

A system has been developed for measuring non-shunt cardiac output by the throughflow technique, using nitrous oxide in patients undergoing general anaesthesia. The throughflow measurement technique is a non-invasive method based on inert gas throughflow theory. In vitro validation of the measurement system was performed using a lung gas exchange simulator. The accuracy and precision of the throughflow measurement system was assessed by comparing measured and target values for five simulated values of non-shunt cardiac output, from 2.88 to 9.86 l min(-1). This showed an overall mean bias of -0.031 min(-1) (range -0.00 to -0.101 min(-1)), with a mean coefficient of variation of the difference of 1.39% (1.20-1.93%). These results indicate that the measurement system is suitable for monitoring the non-shunt cardiac output in patients undergoing general anaesthesia using nitrous oxide throughflow.

Noninvasive cardiac output assessment during heart surgery

OBJECTIVE

To evaluate the reliability of a new noninvasive method for the assessment of cardiac output with the partial carbon dioxide rebreathing technique.

METHODS

This technique was applied to patients undergoing heart surgery. Values of cardiac index obtained with this equipment were compared with the artero-venous CO2 gradient, a reliable index of cardiovascular status. Positive and negative predictive values of the test were assessed.

RESULTS

A total of 21 simultaneous measurement of the cardiac index and of the artero-venous CO2 gradient were obtained. The positive predictive value of the test was 67% while the negative predictive value was 100%, indicating that a normal value of cardiac index recorded with the rebreathing technique predicts with a good reliability a normal cardiovascular state.

CONCLUSIONS

Working through a series of mathematical algorithms, accuracy in the computation of cardiac output can be decreased with this equipment; however, this limitation seems to be outweighed by the simplicity and noninvasive nature of the methods.

Noninvasive cardiac output by partial CO2 rebreathing after severe chest trauma

BACKGROUND

In multiple trauma patients, early continuous cardiac output (CCO) monitoring is frequently desired but is difficult to routinely employ in most emergency departments because it requires invasive procedures. Recently, a noninvasive cardiac output (NICO) technique based on the Fick principle and partial CO2 rebreathing has shown promise under a variety of conditions. Since this method has not been tested after lung damage, we evaluated its utility in a clinically relevant model.

METHODS

Anesthetized, ventilated swine (n = 11, 35-45 kg) received a unilateral blunt trauma via a captive bolt gun followed by a 25% hemorrhage. After 60 min of shock, crystalloid resuscitation was given as needed to maintain heart rate < 100 beats/min and mean arterial pressure > 70 mm Hg. Standard CCO by thermodilution (Baxter Vigilance, Irvine, CA) was compared with NICO (Novametrix Medical Systems Inc., Wallingford, CT) for 8 hr.

RESULTS

The severity of the injury is reflected by seven deaths (average survival time = 4.25 hr). Trauma increased dead space ventilation (19%), airway resistance (30%), and lactate (3.2 mmol/L), and decreased dynamic compliance (48%) and Pao2/Fio2 (54%). In these extreme conditions, the time course and magnitude of change of CCO and NICO were superimposed. Bland-Altman analysis reveal a bias and precision of 0.01 +/- 0.69 liters/min. The linear relationship between individual CCO and NICO values was significant (p < 0.0001) and was described by the equation NICO = (0.74 +/- 0.1)CCO + (0.65 +/- 0.16 liters/min) but the correlation coefficient (r2 = 0.541) was relatively low. The cause for the low correlation could not be attributed to increased pulmonary shunt, venous desaturation, anemia, hypercapnia, increased dead space ventilation, or hyperlactacidemia.

CONCLUSION

NICO correlated with thermodilution CCO, but underestimated this standard by 26% in extreme laboratory conditions of trauma-induced cardiopulmonary dysfunction; 95% of the NICO values fall within 1.38 liters/min of CCO; and with further improvements, NICO may be useful in multiple trauma patients requiring emergency intubation during initial assessment and workup.

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.