Time-dependent variations of transcutaneous PCO2 level in normal subjects.

Transcutaneous PCO2 (PtcCO2), which is linearly related to arterial PCO2, was continuously recorded in healthy, adult, normal volunteers for 8-h periods. Recording this variable with the apparatus employed permits measurement of changes in the level of ventilation while subjects are freely ambulant and unencumbered by invasive and flow-resistive respiratory apparatus. The time series obtained exhibited marked periodicities. The frequencies and amplitudes varied between subjects. Peak-to-peak variation was 10-20% of mean values. There was no apparent association between fluctuations in PCO2 and activity other than formal exercise. Visual inspection of the time series and preliminary statistical analysis of digitally converted data suggested that the time-dependent changes of PtcCO2 were normally distributed. However, more rigorous statistical examination revealed that in no case was PtcCO2 actually normally distributed.