Noninvasive determination of cardiac output using single breath CO2 analysis

Volumetric capnography: reliability and reproducibility in spontaneously breathing patients

Volumetric capnography provides a breath-by-breath analysis of ventilation-perfusion imbalances and deadspace volumes. The technique has been best described in intubated and ventilated patients, but promising clinical applications also concern spontaneously breathing patients. The objective of the study was to verify the reliability and reproducibility of a new capnographic program in various types of clinical conditions. In a first step, 56 patients, either healthy or with acute respiratory disorders, were connected to a sidestream gas sampler and flow sensor through a mouthpiece. An acquisition software synchronized expired CO2 and flow data to create volumetric capnographic curves. Mixed expired CO2 partial pressure, corresponding to the exhaled CO2 of the effective tidal volume, was simultaneously collected in a neoprene bag for comparison. In a second step, changes in airway deadspace before and after the adjunction of known spacer volumes were compared in six healthy volunteers. The mean difference between both methods in measuring mixed expired CO2 partial pressure was -0.9 mmHg (SE 0.2 mmHg, P<0.001). The limits of agreement extended from -4.4 to 2.5 mmHg. The interobserver correlation coefficient for reproducibility was 0.98. Airway deadspace volume, after correction for extra volumes, was not statistically different than the basic value (P=0.89). In conclusion, volumetric capnography can be compared with references when used in spontaneously breathing patients. Future developments and clinical applications should clarify its role as a non-invasive method for deadspace and ventilation-perfusion imbalances analysis.

Monitoring cardiac function in intensive care

Systolic cardiac function results from the interaction of four interdependent factors: heart rate, preload, contractility, and afterload. Heart rate can be quantified easily at the bedside, while preload estimation has traditionally relied on invasive pressure measurements, both central venous and pulmonary artery wedge. These have significant clinical limitations; however, adult literature has highlighted the superiority of several novel preload measures. Measurement of contractility and afterload is difficult; thus in clinical practice the bedside assessment of cardiac function is represented by cardiac output. A variety of techniques are now available for cardiac output measurement in the paediatric patient. This review summarises cardiac function and cardiac output measurement in terms of methodology, interpretation, and their contribution to the concepts of oxygen delivery and consumption in the critically ill child.

Role of lung function testing in the management of mechanically ventilated infants

The mechanical characteristics of the ventilated lung can only be interpreted when the volume of the lung, the elastic properties, and the degree of airway obstruction have been accurately quantified by pulmonary function testing. More gentle ventilation strategies (permissive hypercapnia) are used, and the efficacy of mechanical ventilation can be verified in the intensive care unit. Pulmonary function testing brings new insights, awareness, and applications, but its limitations need to be taken into account when interpreting the acquired data.

Measurement of cardiac output and tissue perfusion

Recent technologic innovations have allowed a greater scope for cardiac output measurement in critically ill children. There is a move toward both less invasive and continuous methods, several of which also offer novel measures of preload. Many of the new methods are still undergoing preliminary evaluation in the pediatric population and will be summarized in this article.

Noninvasive monitoring in the pediatric intensive care unit

The best ICU monitors are physicians and nurses, who integrate all of the physiologic parameters of patients with the known pathophysiology of the disease process. Over-reliance on raw electronic data, with their inherent errors, jeopardizes the safe and efficient care of patients. Data must be interpreted in the context of the history, repetitive physical examinations, response to therapy, and a background of experience. New modalities and the application of artificial intelligence may facilitate the interpretation of data, but the role of the bedside medical practitioner remains as the heart of pediatric critical care.

PaCO(2)/ETCO(2) gradient: early indicator of thrombolysis efficacy in a massive pulmonary embolism

End tidal CO(2) measurement may be helpful in detecting the efficacy of thrombolysis after a massive pulmonary embolism. We report the case of a 76-year-old man with a massive pulmonary embolism, who required early intubation and mechanical ventilation. Thrombolysis with rtpA (total dosage: 60 mg) was initiated. During this procedure, clinical data, arterial blood gases and end-tidal CO(2) with a capnograph were recorded. Before thrombolysis the P(a-ET)CO(2) gradient was raised to 25 mmHg. During thrombolysis, the clinical data improved and the P(a-ET) gradient fell to 14 mmHg. We postulate that the P(a-ET)CO(2) gradient seems to be a reasonable indicator of efficacy of thrombolysis in this setting. However, the gradient did not return to normal values (4-5 mmHg). The possible reasons for this may be that during mechanical ventilation there was a large ventilation-perfusion ratio and the cardiac output may have still reduced. With these limitations, we conclude that the P(a-ET)CO(2) gradient should be evaluated as an indicator of pulmonary reperfusion in massive pulmonary embolism.

Effectiveness of end-tidal carbon dioxide tension for monitoring thrombolytic therapy in acute pulmonary embolism

OBJECTIVE

In acute massive pulmonary embolism with hemodynamic instability, monitoring of pulmonary artery pressure can be used to assess the efficacy of thrombolytic therapy. As a noninvasive alternative to pulmonary artery catheterization, we investigated the efficacy of continuous monitoring of end-tidal CO2 tension.

DESIGN

In 12 patients with massive pulmonary embolism who required mechanical ventilation, mean pulmonary arterial pressure (MPAP) and end-tidal carbon dioxide tension (ETCO2) were registered continuously during thrombolytic therapy. PaCO2, cardiac index as estimated by thermodilution catheter and respiratory ratio of arterial oxygen tension and inhaled oxygen concentration (PaO2/FIO2) were determined every 60 mins.

MEASUREMENTS AND MAIN RESULTS

Before thrombolysis, MPAP (34.5+/-9.8 mm Hg) and the difference between PaCO2 and ETCO2 (10.1+/-4.7 mm Hg) were markedly increased compared with normal values. Continuously monitored MPAP was related to ETCO2 for both all patients (r2 = .42; p < .001) and individually (mean r2 = .92; range, .79-.98; p < .001). In ten survivors, the mean cardiac index and PaO2/FIO2 increased during therapy from 1.7+/-0.4 to 2.8+/-0.6 L/min x m2 and 125+/-27 to 285+/-50 mm Hg (p < .01, respectively). In these patients, the difference between PaCO2 and ETCO2 decreased from 9.8+/-4.5 to 2.8+/-0.9 mm Hg (p < .001). Recurrent embolism was detected in two patients by sudden reduction of ETCO2.

CONCLUSIONS

Analysis of ETCO2 allows monitoring of the efficacy of thrombolysis and may reflect recurrent embolism. Thus, on the basis of this small study, analysis of ETCO2 appears to be useful for noninvasive monitoring in mechanically ventilated patients with massive pulmonary embolism.

Continuous cardiac output by femoral arterial thermodilution calibrated pulse contour analysis: comparison with pulmonary arterial thermodilution

OBJECTIVE

To compare two thermodilution methods for the determination of cardiac output (CO)-thermodilution in the pulmonary artery (COpa) and thermodilution in the femoral artery (COa)-with each other and with CO determined by continuous pulse contour analysis (COpc) in terms of reproducibility, bias, and correlation among the different methods. Good agreement between the methods would indicate the potential of pulse contour analysis to monitor CO continuously and at reduced invasiveness.

DESIGN

Prospective criterion standard study.

SETTING

Cardiac surgical intensive care unit in a university hospital.

PATIENTS

Twenty-four postoperative cardiac surgery patients.

INTERVENTIONS

Without interfering with standard hospital cardiac recovery procedures, changes in CO as a result of the postsurgical course, administration of vasoactive substances, and/or fluid administration were recorded. CO was first recorded after a 1-hr stabilization period in the intensive care unit and hourly thereafter for 6 hrs, and by subsequent determinations at 9, 12, and 24 hrs.

MEASUREMENTS AND MAIN RESULTS

There were 216 simultaneous determinations of COpa, COa, and COpc. COpc was initially calibrated using COa, and no further recalibration of COpc was performed. COpa ranged from 3.0 to 11.8 L/min, and systemic vascular resistance ranged from 252 to 2434 dyne x sec/cm5. The mean difference (bias) +/-2 SD of differences (limits of agreement) was -0.29+/-1.31 L/min for COpa vs. COa, 0.07+/-1.4 L/min for COpc vs. COpa, and -0.22+/-1.58 L/min for COpc vs. COa. In all but four patients COpc correlated with COa after the initial calibration. Correlation and precision of COpc vs. COa was stable for 24 hrs.

CONCLUSIONS

Femoral artery pulse contour CO correlates well with both COpa and COa even during substantial variations in vascular tone and hemodynamics. Additionally, CO determined by arterial thermodilution correlates well with COpa. Thus, COa can be used to calibrate COpc.

Single-breath CO2 analysis as a predictor of lung volume in a healthy animal model during controlled ventilation

OBJECTIVE

To examine the utility of single-breath CO2 analysis as a measure of lung volume.

DESIGN

A prospective, animal cohort study comparing 21 parameters derived from single-breath CO2 analysis with lung volume measurements determined by nitrogen washout in animals during controlled ventilation.

SETTING

An animal laboratory in a university-affiliated medical center.

SUBJECTS

Seven healthy lambs.

INTERVENTIONS

The single-breath CO2 analysis station consists of a mainstream capnometer, a variable orifice pneumotachometer, a signal processor and computer software with capability for both on- and off-line data analysis. Twenty-one derived components of the CO2 expirogram were evaluated as predictors of lung volume. Lung volume was manipulated by 3 cm H2O incremental increases in positive end-expiratory pressure from 0 to 21 cm H2O, and ranged between 147 and 942 mL.

MEASUREMENTS AND MAIN RESULTS

Fifty-five measurements of lung volume were available for comparison with derived variables from the CO2 expirogam. Stepwise linear regression identified four variables that were most predictive of lung volume: a) dynamic lung compliance; b) the slope of phase 3; c) the slope of phase 2 divided by the mixed expired CO2 tension; and d) airway deadspace. The multivariate equation was highly statistically significant and explained 94% of the variance (adjusted r2 =.94, p < .0001). The bias and precision of the calculated lung volume was .00 and 51, respectively. The mean percent difference for the lung volume estimate derived from the single-breath CO2 analysis station was 0.79%.

CONCLUSIONS

Our data indicate that analysis of the CO2 expirogram can yield accurate information about lung volume. Specifically, four variables derived from a plot of expired CO2 concentration vs. expired volume predict changes in lung volume in healthy lambs with an adjusted coefficient of determination of .94. Prospective application of this technology in the setting of lung injury and rapidly changing physiology is essential in determining the clinical usefulness of the technique.

Can quantitative capnometry differentiate between cardiac and obstructive causes of respiratory distress?

STUDY OBJECTIVE

To determine whether quantitative measurement of end-tidal carbon dioxide (ETCO2) can differentiate between cardiac and obstructive causes of respiratory distress.

DESIGN

Prospective observational study.

SETTING

Emergency department (ED) of a tertiary care hospital.

PATIENTS

Adult patients who presented to the ED with moderate-to-severe dyspnea. Patients were excluded if they were unable to cooperate with the performance of peak expiratory flow rate (PEFR) or ETCO2 tests, were younger than 18 years of age, or had received prehospital intervention for their respiratory distress.

INTERVENTIONS

Physicians obtained an ETCO2 level and PEFR prior to ED pharmacologic intervention. A hand-held capnometer with digital read-out was used to obtain the ETCO2 level. The patient's age, sex, initial vital signs, breath sounds and medication history, the presence or absence of diaphoresis and/or orthopnea, the duration of symptoms, the chest radiograph interpretation, and final diagnosis were also recorded.

MEASUREMENTS AND RESULTS

Forty-two patients were eligible for inclusion in the analysis. The mean ETCO2 level was 31.1+/-9.4 mm Hg; the mean PEFR was 161.3+/-53.1 L/min. The ETCO2 levels for pulmonary edema/congestive heart failure (CHF) patients differed significantly from those of asthma/COPD patients (27.1+/-7.8 mm Hg vs 33.4+/-9.6 mm Hg; p=0.0375). However, no single ETCO2 level was found to be a reliable predictor of diagnosis.

CONCLUSION

ETCO2 levels for pulmonary edema/CHF patients differ significantly from those of asthma/COPD patients. However, no single ETCO2 level reliably differentiates between the two disease processes.

Noninvasive determination of cardiac output in a model of acute lung injury

OBJECTIVE

To examine the utility of single breath CO2 analysis as a noninvasive measure of cardiac output in a model of acute lung injury.

SETTING

An animal laboratory in a university-affiliated medical center.

DESIGN

A prospective, animal cohort study comparing 21 parameters derived from single breath CO2 analysis with cardiac output determined by an ultrasonic flow probe.

SUBJECTS

Six adult sheep with saline lavage-induced acute lung injury.

INTERVENTIONS

Animals were treated with repetitive saline lavage to achieve a uniform degree of acute lung injury (PaO2 of < 100 torr [< 13.32 kPa] on an FIO2 of 1.0). Cardiac output was manipulated by successive injections of an hydraulic constrictor placed around the inferior vena cava and measured using an ultrasonic flow probe. Twenty-one derived components of the CO2 expirogram were evaluated as predictors of cardiac output.

MEASUREMENTS AND MAIN RESULTS

Thirty-eight measurements of cardiac output were available for comparison with derived variables from the CO2 expirogam. Stepwise linear regression identified four variables for the equation predicting cardiac output: a) PaO2/FIO2 ratio; b) the angle between the slope lines for phases II and III divided by the tidal volume; c) mixed expired CO2 tension; and d) physiologic deadspace to tidal volume ratio. The multivariate equation was highly statistically significant and explained 80% of the variance (adjusted R2 = .80, p < .0001). The blas and precision of the calculated cardiac output were .00 and .38, respectively. The mean percent difference for the cardiac output estimates derived from the single breath CO2 analysis station was -0.01%.

CONCLUSIONS

Our results indicate that changes in cardiac output can be determined using components of the CO2 expirogram with a high degree of reliability in animals with induced acute lung injury. Specifically, the use of four parameters derived from a plot of expired CO2 concentration vs. expired volume predict changes in cardiac output in adult sheep with induced lung injury with an adjusted coefficient of determination of .80. Prospective application of this technology in the clinical setting with the rapidly changing physiology that is characteristic of the acutely ill patient will be essential in determining the clinical usefulness of single breath CO2 analysis as a noninvasive measure of cardiac output.