The Accuracy and Precision of an Open-Circuit System To Measure Oxygen Consumption and Carbon Dioxide Production in Neonates

摘要

We measured the oxygen consumption, carbon dioxide production, and respiratory quotient during the combustion of a known mass of anhydrous ethanol and methanol to assess the accuracy of an open-circuit flowthrough system. Continuous measurements were made of the mass of alcohol burned, the velocity of gas flow through the apparatus, and simultaneous measurements of the fractional concentration of oxygen, carbon dioxide and nitrogen of the inlet and outlet gas using paramagnetic oxygen analyzer, infrared carbon dioxide meter, and mass spectrometer. Standard respiratory and stoichiometric equations were used to calculate the oxygen consumption, carbon dioxide production and RQ for the mass of absolute alcohol combustion per unit time. In a series of 12 consecutive laboratory experiments (on 7 days), the measured values of gas exchange (similar to the rate of respiratory gas exchange by an infant of 1-4 kg) were in agreement within 5% of the true values for ethanol and methanol combustion, confirming the validity of the open-circuit method. The paramagnetic oxygen analyzer and the mass spectrometer gave similar oxygen consumption results and differed very little when the rate of absolute alcohol combustion was used to quantify the accuracy of the complete measurement system. A positive measurement error was observed for the carbon dioxide production results from both the IR meter and mass spectrometer, with the result that the respiratory quotient measurements were 3.4-4.7% higher than the true value. The mass spectrometer gave more precise oxygen consumption results, whereas smaller variance of carbon dioxide production measurements was observed using the infrared CO2 meter. The sources of error and methods to reduce the overall system error were considered. By using a rigorous set of calculations we showed that the rate of combustion of a known mass of absolute alcohol was a suitable laboratory method for validation of respiratory gas exchange measurements made in the neonate.