Analysis of error in the determination of respiratory gas exchange at varying FIO2

Abstract

On-line gas exchange monitors have been based on the measurement of both inspired and expired flows (method A) or on the measurement of expired flow only (method B). In both methods, the cancellation of systematic errors is appreciable if calibration errors in the inspiration variables are matched with calibration errors in the corresponding expiration variables. The determination of oxygen consumption (VO2) results in a considerable amplification of random measurement error, which becomes larger as the inspired oxygen fraction (FIO2) is made larger. For example, when in method A all gas compositions and flows are in error by +/- 0.01, the computed value of VO2 will be in error by +/- 0.16 when FIO2 = 0.21 and by +/- 0.40 if FIO2 = 0.50. When air is breathed, method B creates smaller errors than method A, but as FIO2 is increased the error in method B increases sharply and eventually overtakes that of method A. Even at high FIO2 values, the random error in VO2 may be reduced to an acceptable value by estimating it from a sequence of 100 measurement samples requiring a minimum of 5 min to acquire. There is no error amplification in the computation of carbon dioxide production.

Measured pulmonary oxygen consumption: difference between systemic oxygen uptake measured by the reverse Fick method and indirect calorimetry in cardiac surgery

“…Given that net gas uptake by the lungs in surgical patients is often only a small proportion of total flows, accuracy in its measurement is difficult. Use of an insoluble marker gas, as employed by the Haldane transformation, is an established means to reduce this source of error, but its limitations have been demonstrated by previous theoretical and clinical studies [12,13]. In line with the findings of these studies, we kept the inspired N 2 concentration between 30% and 70% in our patients to improve the reliability of our _ V O 2 Haldane.…”

Pulmonary Hemodynamics and Right Heart Catheterization

“…Many catheterization laboratories use an "assumed" value (typically 125 ml/min/m 2 for most adult patients, and 110 ml/min/m 2 for elderly patients) for oxygen consumption. The use of the "assumed" rather than the directly measured O 2 consumption can be a major source of error in the Fick method, but when performed correctly, the total error indetermination of the cardiac output is about 10 % [95][96][97].…”

An easy way to estimate energy expenditure from hemodynamic data in septic patients

“…Indirect calorimetry, although being currently considered to be the "gold standard", has technical limitations such as requiring trained personnel with available time, the need for an inspired oxygen fraction of less than 60% and equipment of high cost. 3,4 On the other hand, other existing methods involve predictive equations such as the classical equation of Harris & Benedict. 5 , plus various stress factors, whose purpose is to predict the energy cost of metabolic changes in this patient population.…”