A Method for Increasing Confidence in Respiratory Gas Exchange Measurements in Mechanically Ventilated Patients

Abstract

Measurement of O₂ uptake (VO₂) and C0₂ output (VCO₂) with automated instruments can be used to optimize nutritional management of critically ill patients. However, these measurements may be made infrequently because of calibration problems or suspected inaccuracies, especially when the patient is given supplemental oxygen. A simple method is described for periodic testing of automated gas exchange measurements. While a test lung is mechanically ventilated an accurately known mixture of C0₂ and nitrogen is introduced into the inspired gas, simulating VO₂ by dilution and VCO₂ by addition of CO₂. The ratio of VCO₂:VO₂ (respiratory gas exchange ratio, R) in the "expired gas" should be [FGco₂/(1- FGcO₂)] x [(1 - FIO₂)/FIO₂], where FGCO₂ is the fraction of CO₂ in the diluting gas and FIO₂ is the fraction of O₂ in the inspired gas. R is independent of the flow rate of the diluting gas or the rate of ventilation of the test lung. Using a mixing chamber-mass spectrometer, we found that R calculated from measurement of CO₂ and O₂ concentrations in the simulated mixed expired gas closely matched the predicted R for FIO₂ = 0.21 — 0.50. On the other hand, when an automated gas exchange measurement device was tested, R was sometimes excessively high, especially for FIO₂ > 0.35. This method, using a single diluting gas and without precision flowmeters, may be useful for periodic testing of respiratory gas exchange instruments in the intensive care unit. If a discrepancy is found between measured and predicted R, measurements should not be relied upon until further calibration or repairs can be effected. (Journal of Parenteral and Enteral Nutrition 15 :625-629, 1991 )