Gas Exchange

Gas exchange and indirect calorimetry appliguide

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Introduction

Recent advancements in medical technology have made possible accurate measurement of respiratory gas exchange in a wide variety of clinical conditions. The modern indirect calorimetry devices are portable and easy to use, which facilitates routine clinical measurements. The clinical applications range from assessment of energy requirements and response to nutrition in malnutrition and obesity to comprehensive analysis of ventilation and oxygen transport in patients with complex cardiorespiratory problems. Though the measurements can be made easily, accuracy and reproducibility of results requires understanding of the basic principles of the measurement and related physiology. Furthermore, indirect calorimetry is sensitive to measurement errors; the need for routine procedures of quality control is therefore emphasized.

Despite accurate measurement, several clinical and physiological factors influence the results of gas exchange measurements and should be considered in interpretation. In this respect, the relationship between ventilation and gas exchange is of crucial importance. Any acute change in alveolar ventilation will be immediately reflected in CO2 -production, which will not measure the metabolic production of CO2, until a new steady-state has been achieved. Similar, but shorter transient will be seen also in O2 - consumption. Analogously, acute changes in tissue perfusion may influence both tissue oxygen uptake and removal of CO2 from the tissues. These issues will be discussed in more detail in "Physiological aspects of gas exchange measurements".

Last updated: 1 March 2001Created
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