Abstract
The pressure generated 100 ms after the onset of an occluded inspiratory effort (P0.1) is advocated and used as a measure of respiratory centre drive. We have re-examined P0.1, measured simultaneously in the mouth (Pmo0.1) and the oesophagus (Poes0.1), during carbon dioxide rebreathing, in eight patients with severe chronic obstructive pulmonary disease, to see whether either indicates central respiratory drive. Pmo0.1 was identical to Poes0.1 in 4 out of 61, greater than Poes0.1 in 18 out of 61, and less than Poes0.1 in 39 out of 61 measurements (overall Poes0.1-Pmo0.1, median +0.075, range -0.175 to +1.01 kPa). Within a rebreathing run in an individual patient, there was considerable variability in the relationship Pmo0.1/Poes0.1 (0.89 +/- 0.24), coefficient of variation (CoV%) 14.4 +/- 3.7%), in the end-expiratory oesophageal pressure (0.7 +/- 0.54 kPa, CoV% 105 +/- 106%), and in the time delay between the onset of a fall in oesophageal pressure (Poes) from the end-expiratory level to the beginning of inspiration, defined as starting when mouth pressure (Pmo) fell below atmospheric pressure (129 +/- 25 ms, CoV% 22.5 +/- 5.3%). We conclude that the problem of determining the true onset of inspiratory muscle activity from pressure data, and the likelihood that breaths are taken from different lung volumes, make it unlikely that Poes0.1 accurately represents central respiratory drive during rebreathing in chronic obstructive pulmonary disease. Furthermore, Pmo0.1 differed from Poes0.1 during rebreathing, and their relationship was not constant, so that Pmo0.1 is even less likely to be a useful reflection of central nervous system output or respiratory centre drive in such patients.
