Evaluation of a modified acetylene rebreathing method for the determination of cardiac output

Hemodynamic changes and baroreflex gain in the supine hypotensive syndrome

OBJECTIVE

The purpose of this study was to determine whether supine hypotensive syndrome is caused by a decreased baroreflex gain that results in inadequate compensatory responses to restore cardiac output.

STUDY DESIGN

Ten third-trimester gravid women with symptoms that were suggestive of supine hypotensive syndrome and 10 control subjects without symptoms were studied. Patients were studied initially for 30 minutes in the lateral recumbent position with continuous heart rate, blood pressure, external cardiotocography, and cardiac output determinations every 5 minutes. After 30 minutes, the patient was repositioned supine for up to an additional 30 minutes, and all measurements were repeated similarly. This lateral-supine sequence was repeated twice or until symptoms precluded further testing.

RESULTS

The two groups were demographically similar. With maternal position change to supine, cardiac output (-2.7 L/min vs -1.0 L/min, P =.04) and mean arterial blood pressure (-10.7 mm Hg vs -2.5 mm Hg, P =.03) were respectively lower, and heart rate (+26.9 vs +14.4 beats/min, P =.04) was higher in the subjects with supine hypotensive syndrome than in the control subjects. Baroreflex gain, however, did not differ between the subjects with supine hypotensive syndrome and control subjects (P =.26).

CONCLUSION

We were unable to demonstrate a decrease in baroreflex gain as a possible cause of supine hypotensive syndrome. Inadequate pelvic collateral circulation that leads to a greater decrease in cardiac output may be causative in the syndrome.

Cardiac output during exercise by the open circuit acetylene washin method: comparison with direct Fick

An open-circuit (OpCirc) acetylene uptake cardiac output (QT) method was modified for use during exercise. Two computational techniques were used. OpCirc1 was based on the integrated uptake vs. end-tidal change in acetylene, and OpCirc2 was based on an iterative finite difference modeling method. Six subjects [28-44 yr, peak oxygen consumption (VO(2)) = 120% predicted] performed cycle ergometry exercise to compare QT using OpCirc and direct Fick methods. An incremental protocol was repeated twice, separated by a 10-min rest, and subsequently subjects exercised at 85-90% of their peak work rate. Coefficient of variation of the OpCirc methods and Fick were highest at rest (OpCirc1, 7%, OpCirc2, 12%, Fick, 10%) but were lower at moderate to high exercise intensities (OpCirc1, 3%, OpCirc2, 3%, Fick, 5%). OpCirc1 and OpCirc2 QT correlated highly with Fick QT (R(2) = 0.90 and 0.89, respectively). There were minimal differences between OpCirc1 and OpCirc2 compared with Fick up to moderate-intensity exercise (<70% peak VO(2)); however, both techniques tended to underestimate Fick at >70% peak VO(2). These differences became significant for OpCirc1 only. Part of the differences between Fick and OpCirc methods at the higher exercise intensities are likely related to inhomogeneities in ventilation and perfusion matching (R(2) = 0.36 for Fick - OpCirc1 vs. alveolar-to-arterial oxygen tension difference). In conclusion, both OpCirc methods provided reproducible, reliable measurements of QT during mild to moderate exercise. However, only OpCirc2 appeared to approximate Fick QT at the higher work intensities.

Thermodilution versus inert gas rebreathing for estimation of effective pulmonary blood flow

OBJECTIVE

To compare measurements of the effective pulmonary blood flow (Qep, i.e., nonshunted fraction of cardiac output, Qt) by the inert gas rebreathing (RB) method and the thermodilution (TD) technique in critically ill patients.

DESIGN

Prospective, comparative study of a noninvasive method and an established invasive technique.

SETTING

An 11-bed general intensive care unit in a university hospital.

PATIENTS

A total of 14 critically ill patients, all mechanically ventilated and monitored with systemic and pulmonary artery catheters.

MEASUREMENTS AND MAIN RESULTS

Qep was determined in duplicate by RB using a mass spectrometer for gas analysis. For each determination, Qt was measured in triplicate by the cold water bolus TD technique and averaged. Simultaneously mixed venous and arterial blood samples were analyzed to calculate the intrapulmonary shunt fraction and thereby convert estimates of Qt to Qep. Mean difference between paired estimates (RB - TD) was 0.01 L/min, so for differences was 1.19 L/min, and 95% confidence interval for the bias was -0.45 to 0.47 L/min. Coefficients of variation for repeated Qep estimates were 8% (RB) and 12% (TD), respectively. Coefficients of variation for RB estimates of functional residual capacity and lung tissue volume were 6% and 17%, respectively.

CONCLUSIONS

The RB method is a promising method for simultaneous noninvasive estimation of Qep and functional residual capacity in mechanically ventilated patients. However, further investigations are needed to evaluate potential problems of the method before it can be recommended for clinical purposes.

Noninvasive estimation of cardiac output with nonprescribed breathing

A noninvasive method to estimate cardiac output Q without special patient cooperation was developed by modifying a previous acetylene-helium (C2H2-He) rebreathing technique (ART). Estimation of Q using ART is based on a single-compartment model that is valid only under prescribed breathing; e.g., fast, deep breathing, and emptying of the rebreathing bag on each breath. To make the ART less dependent on subject cooperation, a more sophisticated mathematical model and estimation method are needed. For this purpose, we modeled the C2H2 and He concentration dynamics at the mouth over successive breaths using a multi-compartment model. This model takes into account the effects of breathing pattern, compartmental volumes, and gas solubility. From computer simulations and sensitivity analysis, we found that Q could be estimated from the available data with adequate precision. Our model and estimation method were tested on a group of six normal adult subjects, at rest and during submaximal exercise (75 watts). Estimates of Q from our new method (6.5 +/- 0.4 L/min at rest, 12.5 +/- 0.4 L/min at 75 watts) were in agreement with those obtained using a previous ART (7.0 +/- 0.3 L/min at rest, 12.6 +/- 0.5 L/min at 75 watts). We conclude that this approach promises to provide reliable estimates of Q in patients (e.g., children and elderly), at rest and during exercise, without the need of prescribed breathing patterns or changes in rebreathing bag volume.