Work of Breathing During Mechanical Ventilation

Abstract

The quantification of breathing effort can be approached in three different but related ways. The first is to measure the O₂ consumption difference resulting from the application or removal of a breathing stress, attributing the difference in total oxygen consumption (Δ VO₂) to the activity of the respiratory pump [1]. In theory, this method has the advantage of quantifying effort at the basic level of cellular metabolism. Because the breathing effort involves muscular activity not strictly associated with useful chest movement, the A V0₂ technique helps account for the grossly inefficient nature of the ventilatory musculature that often characterizes critical illness. Unfortunately, it also includes a component related to the energy expended by nonrespiratory tissues as well as a component related to the expiratory work of breathing. However theoretically attractive, this methodology has several major drawbacks for clinical practice. First, the energy cost of breathing normally constitutes such a small fraction of the total body O₂ requirement that precise measurement may not be easy [2]. This inherent problem of signal detection is multiplied in the clinical setting by the need to use supplemental oxygen. High inspired fractions of O₂ provided at an elevated minute ventilation may be required to support the patient suffering an oxygenation crisis. As a result, large volumes of O₂ are inspired and expired with every breath. By comparison, even total body O₂ consumption is quite small. Therefore, the O₂ sensor applied to the inspired and expired gas must be extremely accurate to prevent large percentage errors from occurring [3]. Equipment that bases the O₂ consumption estimate on volumetric measurements may circumvent some of these problems, but has not yet been thoroughly evaluated.