Monitoring of O₂ Uptake and CO₂ Elimination During Anesthesia and Surgery

Abstract

Indirect calorimetry, the measurement of pulmonary oxygen uptake (${\dot{V}}_{{\mathrm{O}}_2}$) and carbon dioxide elimination (${\dot{V}}_{{\mathrm{CO}}_2}$), is used extensively in the critical care medicine unit and in exercise physiology testing. However, in anesthesia, metabolic monitoring is mostly confined to the inspired oxygen fraction (to preclude the delivery of hypoxic gas mixture) and to tidal ${P}_{{\mathrm{CO}}_2}$ monitoring (to ensure a patent airway and to estimate the alveolar ${P}_{{\mathrm{CO}}_2}$ during mechanical ventilation). The main reason, we believe, for the lack of understanding of metabolic monitoring during anesthesia is that the measurements of ${\dot{V}}_{{\mathrm{O}}_2}$ and ${\dot{V}}_{{\mathrm{CO}}_2}$ are challenging in the rebreathing anesthesia circle circuit and there are few accurate measurement devices. In this chapter, we describe the development and implementation of the bymixer, an in-line flow-averaging hydraulic gas mixer, and the fast response humidity and temperature airway sensor. Along with a fast response and accurate airway flow sensor, we have demonstrated accurate and precise bymixer-flow measurements of the ${\dot{V}}_{{\mathrm{O}}_2}$ and ${\dot{V}}_{{\mathrm{CO}}_2}$ during extensive validations with a metabolic lung simulator. We believe that there are at least three main areas of clinical interest for the measurements of airway ${\dot{V}}_{{\mathrm{O}}_2}$ and ${\dot{V}}_{{\mathrm{CO}}_2}$ during anesthesia and surgery: First, there are many acute perturbations during anesthesia and surgery that can be quickly and noninvasively detected and diagnosed by non-steady-state changes in airway ${\dot{V}}_{{\mathrm{O}}_2}$ and ${\dot{V}}_{{\mathrm{CO}}_2}$. Second, we hypothesize that indirect calorimetry can first detect onset of anaerobic lactic acidosis during anesthesia and surgery. Third, we predict that airway ${\dot{V}}_{{\mathrm{O}}_2}$ and ${\dot{V}}_{{\mathrm{CO}}_2}$ will be directly affected by the level of anesthesia depth. We plan to test the hypothesis that indirect calorimetry measurements of airway ${\dot{V}}_{{\mathrm{O}}_2}$ and ${\dot{V}}_{{\mathrm{CO}}_2}$ will help diagnose and drive treatment of these pathophysiology perturbations and improve patient outcome.