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A Method for Increasing Confidence in Respiratory Gas Exchange Measurements in Mechanically Ventilated Patients
Author(s) -
Sue Darryl Y.,
Wolff Craig
Publication year - 1991
Publication title -
journal of parenteral and enteral nutrition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.935
H-Index - 98
eISSN - 1941-2444
pISSN - 0148-6071
DOI - 10.1177/0148607191015006625
Subject(s) - fraction of inspired oxygen , dilution , mixing (physics) , volumetric flow rate , chemistry , anesthesia , analytical chemistry (journal) , mechanical ventilation , medicine , mechanics , chromatography , thermodynamics , physics , quantum mechanics
Measurement of O 2 uptake (VO 2 ) and C0 2 output (VCO 2 ) 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 C0 2 and nitrogen is introduced into the inspired gas, simulating VO 2 by dilution and VCO 2 by addition of CO 2 . The ratio of VCO 2 :VO 2 (respiratory gas exchange ratio, R) in the “expired gas” should be [FGco 2 /(1‐ FGcO 2 )] x [(1 ‐ FIO 2 )/FIO 2 ], where FGCO 2 is the fraction of CO 2 in the diluting gas and FIO 2 is the fraction of O 2 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 CO 2 and O 2 concentrations in the simulated mixed expired gas closely matched the predicted R for FIO 2 = 0.21 — 0.50. On the other hand, when an automated gas exchange measurement device was tested, R was sometimes excessively high, especially for FIO 2 > 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)