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Validation of a volumeless breath-by-breath method for measurement of respiratory quotient. J Appl Physiol (1985) 1993 Oct;75(4):1903-10



Pubmed ID




Scopus ID

2-s2.0-0027493854   8 Citations


Measurement of respiratory quotient (RQ) is useful as an indicator of changes in CO2 equilibrium or metabolism. Because of technical difficulties when volumetric or mixed expired methods are applied to clinical practice, we sought to validate a method of volumeless breath-by-breath RQ measurement based on real-time analysis of inspired and alveolar concentrations of O2, CO2, and N2 alone. We derived our volumeless method from a modified three-compartment lung model and tested the validity of the model by comparing it with a standard volumetric open-circuit method. Data from 1,736 breaths from 25 healthy adult volunteers for a total of 78 epochs were collected. Inspired, end-tidal, and mixed expired gas compositions were analyzed by Raman spectroscopy, and RQ values from 0.6 to 2.4 were obtained. Linear regression of the volumeless breath-by-breath method against the open-circuit method yielded a line 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 and used mathematical modeling to predict precision with variations in epoch length and breath volume, which were verified by Monte Carlo techniques. We demonstrated that RQ can be measured reliably and easily using this volumeless breath-by-breath technique.

Author List

Hoffman GM, Torres A, Forster HV


Hubert V. Forster PhD Professor in the Physiology department at Medical College of Wisconsin
George M. Hoffman MD Chief, Professor in the Anesthesiology department at Medical College of Wisconsin

MESH terms used to index this publication - Major topics in bold

Blood Gas Analysis
Evaluation Studies as Topic
Monte Carlo Method
Pulmonary Gas Exchange
Regression Analysis
Respiratory Function Tests
jenkins-FCD Prod-466 5b81815b8b3d1f46bfec16512ed5f574613f59c5