Noninvasive measurements of cardiac output in sheep: an improved thermometry method.

In 25 sheep and 5 goats, which were anesthetized, intubated and mechanically ventilated a sudden decrease in the inspired gas humidity was used to cool the lungs. The dynamics of the temperature of expired gas and its relationship to ventilation rate and cardiac output measured by thermodilution were investigated. In six animals minute ventilation was changed at a stable cardiac output and in 14 animals cardiac output was changed by infusion of saline or by bleeding at a constant ventilation. The difference between the blood temperature and the expired gas temperature at a steady state is proportional to minute ventilation and is inversely proportional to the cardiac output. The inverse time constant of the decay of temperature of the expired gas is proportional to the cardiac output and does not depend on ventilation. The lungs function as a natural humidifier of the respiratory gases with an inner heat source from the pulmonary circulation and an outer heat sink to the expired gas. A simple lumped heat capacity model of non-steady state heat exchange in the lungs was developed, which may be used as a basis for the non-invasive method for determining cardiac output. The coefficient of the lung thermal conductivity (KT/(rho WCpW) = 0.156 +/- 0.056) was determined and applied to measure cardiac output in a separate group, designed as a prospective study. When calculations of cardiac output were done based on the lung mass, estimated from the body weight (12 g/kg), bias and precision compared with thermodilution were -0.27 l/min and 0.38 l/min, respectively in 15 animals. Measurements of blood flow by the air thermometry correlated very well with thermodilution cardiac output (r = 0.92). Thermometry of the expired gas is a promising approach to measure the cardiac output non-invasively.