Noninvasive cardiac output determined with a new method based on gas exchange measurements and carbon dioxide rebreathing: A study in animals/pigs

A system has been designed to determine cardiac output noninvasively. The system's main component is a closed breathing circuit and it measures oxygen uptake (Vo₂), carbon dioxide elimination (Vco₂), and end-tidal CO₂ partial pressure (Pet). As an integral part of the system, periods of CO₂ rebreathing can be automatically implemented. The CO₂ partial pressure of oxygenated mixed venous blood (Pv) is obtained from the measured exponential rise of thePet value during such a CO₂ rebreathing maneuver. A new method is described for estimating the pulmonary blood flow, alveolar ventilation, cardiac output (CO), and mixed venous oxygen saturation (Svo₂) fromPv,Pet,Vo₂,Vco₂, tidal volume, and arterial oxygen saturation. The method was evaluated in 6 anesthetized and mechanically ventilated pigs. A wide range of cardiac output, shunt fractions, and dead space to tidal volume ratios were induced by combinations of bronchoalveolar lavage, hypervolemia, hypovolemia, and variable levels of positive end-expiratory pressure (PEEP). The bias between the CO obtained with the noninvasive technique (CO L/min) and the thermodilution CO (Qt L/min) was 0.13 L/min (SD=0.78 L/min) and the correlation was N=64; R=0.92; CO=0.95*Qt+0.38. The bias obtained for double determinations with the noninvasive CO technique was 0.3 L/min (SD=0.5 L/min). The bias between the noninvasive estimates ofSvo₂ and the directly measured values was 1.1% (SD=9.3%). For double determination with the noninvasive technique the bias was -0.9% (SD=4.7%). It is concluded that in mechanically ventilated pigs the proposed method produces good estimates of CO andSvo₂ also in the presence of significant ventilation/perfusion mismatch.