Oxygen Consumption and Carbon Dioxide Production: Physiological Basis and Practical Application in Intensive Care

Abstract

The metabolic production of carbon dioxide and consumption of oxygen reflect fundamental functions of the tissues. In the intensive care setting, carbon dioxide production (VCO₂) and oxygen consumption (VO₂) can be monitored from the respiratory gases [1-3] . When the body pool of CO₂ and O₂ are in a steady state, the VCO₂ and VO₂ measured from the respiratory gases represent the metabolic processes. VCO₂ and VO₂ are often considered merely as metabolic variables, and their monitoring by gas exchange measurements was originally introduced to intensive care as “metabolic monitoring”. The potential applications of gas exchange monitoring in intensive care are much broader. VCO₂-monitoring can be used to monitor alveolar ventilation [4], the efficiency of CO₂-removal (dead space to tidal volume ratio) and changes in ventilatory demand and their etiology [5] . VO₂-monitoring can provide information on the adequacy of circulation or oxygen transport, especially when combined with mixed venous oximetry [6], and it can be used to evaluate the causes of arterial hypoxemia. In addition, gas exchange monitoring is useful in estimating the nutritional requirements and response to nutrition support in patients requiring prolonged intensive care [7]. Of all the potential applications of gas exchange monitoring, ventilation and oxygen transport related problems are probably the most frequent circumstances, where gas exchange monitoring can provide clinically relevant information.