VALIDATION OF A VOLUMELESS BREATH-BY-BREATH METHOD FOR MEASUREMENT OFRESPIRATORY QUOTIENT

Measurement of respiratory quotient (RQ) is useful as an indicator of changes in CO2 equilibrium or metabolism. Because of technical difficu lties when volumetric or mixed expired methods are applied to clinical practice, we sought to validate a method of volumeless breath-by-brea th RQ measurement based on real-time analysis of inspired and alveolar concentrations of O2, CO2, and N2 alone. We derived our volumeless me thod from a modified three-compartment lung model and tested the valid ity of the model by comparing it with a standard volumetric open-circu it method. Data from 1,736 breaths from 25 healthy adult volunteers fo r a total of 78 epochs were collected. Inspired, end-tidal, and mixed expired gas compositions were analyzed by Raman spectroscopy, and RQ v alues from 0.6 to 2.4 were obtained. Linear regression of the volumele ss breath-by-breath method against the open-circuit method yielded a l ine with a slope of 1.002 (95% confidence interval 0.928 to 1.076) and an intercept of 0.066 (95% confidence interval -0.008 to 0.146) with an R2 of 0.91 (P < 0.001). We analyzed agreement by several methods an d used mathematical modeling to predict precision with variations in e poch length and breath volume, which were verified by Monte Carlo tech niques. We demonstrated that RQ car- be measured reliably and easily u sing this volumeless breath-by-breath technique.