Clinical evaluation of a noninvasive method to measure cardiac output

Introduction

In the presence of severe complications associated with invasive measurement of cardiac output a noninvasive technique is desirable. This study was to evaluate a rebreathing method to determine cardiac output in mechanically ventilated patients.

Methods

The noninvasive method is based on a lung model consisting of a ventilated and a non-ventilated compartment, the first leading to effective pulmonary blood flow (PBF), the latter to shunt perfusion (QVA/Qt). In this model QVA/Qt can be calculated as QVA/Qt = (CcO₂ — CaO₂) x PBF/VO₂. Using a gas mixture of soluble nitrous oxide and insoluble sulfur hexafluoride in oxygen, PBF was calculated from the uptake of N₂O (PBF,rb) and oxygen uptake (VO₂) from the disappearence of oxygen (VO₂,rb) (AMIS 2001 Intensive Care Monitoring System, INNOVISION, Odense Denmark). CcO₂ was calculated from the alveolar gas equation and CaO₂ measured by gas analysis. Thus, QVA/Qt and cardiac output (CO) could be determined by means of the rebreathing data (QVA/Qt,rb) (CO,rb = PBF,rb / [1 — QVA/Qt]). Simultaneously, CO, VO₂ and QVA/Qt were determined by thermodilution (CO,thd), indirect calorimetry (VO₂,ic) (Deltatrac II, Datex-Engström, Finland) and by calculation from arterial and mixed venous blood gas analyses (QVA/Qt,bga), respectively. After approval by the local ethics committee of the medical faculty the investigations were performed in 40 patients on mechanical ventilation.

Results

CO,rb varied from 3 to 12 l/min, QVA/Qt,rb from 0.06 to 0.33 and VO₂,rb from 200 to 600 ml/min. Mean values of CO,thd and CO,rb, QVA/Qt,bga and QVA/Qt,rb and VO₂,ic and VO₂,rb were taken as reference values. Bias and precision were calculated as mean difference and standard deviation between the rebreathing and reference values in relation to the reference values. The mean difference between Co,rb and reference value was –3.6% (bias), with a standard deviation of ± 10.9% (precision). The mean difference between QVA/Qt,rb and reference value was 17.5%, with a standard deviation of 14.0% respectively; VO₂ was determined with a bias of 3.4% and a precision of ± 15.4%. The mean difference between duplicate measurments related to their mean value accounted for 3.5%.

Conclusion

The present results suggest, that cardiac output, venous admixture and oxygen consumption may be reliably estimated by the proposed rebreathing technique.

Supported by INNOVISION, Denmark, providing the gasanalyser and the computer software for the calculation of pulmonary blood flow and oxygen consumption (AMIS 2001 system).