Accuracy of intermittent metabolic gas exchange recordings extrapolated for diurnal variation

In the treatment of critically ill patients, it may be important to know the values of total diurnal O2 consumption and CO2 production. Often, diurnal values are obtained by extrapolation from the easily obtained short interval values. However, both stochastic and systematic errors can be introduced. This study analyzes the systematic influence of a possible diurnal variation of gas exchange and quantifies the extrapolation accuracy of 16 commonly used recording protocols (one to four times a day for 5, 15, 30, and 60 min). Continuous gas exchange measurements were performed for 24 h in 50 ventilated patients and compared to extrapolated results. Only a small diurnal variation was found, and extrapolation accuracy depended on the duration and, especially, the number of recording periods. In clinical practice, 24-h values can be estimated with sufficient accuracy by extrapolation from two 15-min measurements per day.