The contribution of chemoreflex drives to resting breathing in man

The contribution of automatic drives to breathing at rest, relative to beha vioural drives such as 'wakefulness', has been a subject of debate. We meas ured the combined central and peripheral chemoreflex contribution to restin g ventilation using a modified rebreathing method that included a prior hyp erventilation and addition of oxygen to maintain isoxia at a P-ET,P-O2 (end -tidal partial pressure of oxygen) of 100 mmHg. During rebreathing, ventila tion was unrelated to P-ET,P-CO2 (end-tidal partial pressure of carbon diox ide) in the hypocapnic range, but after a threshold P-ET,P-CO2 was exceeded , ventilation increased linearly with P-ET,P-CO2. We considered the sub-thr eshold ventilation to be an estimate of the behavioural drives to breathe ( mean +/- S.E.M. = 3.1 +/- 0.5 1 min(-1)), and compared it to ventilation at rest (mean +/- S.E.M. = 9.1 +/- 0.7 1 min(-1)). The difference was signifi cant (Student's paired t test, P < 0.001). We also considered the threshold P-CO2 observed during rebreathing to be an estimate of the chemoreflex thr eshold at rest (mean +/- S.E.M. = 42.0 +/- 0.5 mmHg), However, P-ET,P-CO2 d uring rebreathing estimates mixed venous or tissue P-CO2, whereas the resti ng P-ET,P-CO2 during resting breathing estimates P-a,P-CO2 (arterial partia l pressure of carbon dioxide). The chemoreflex threshold measured during re breathing was therefore reduced by the difference in P-ET,P-CO2 at rest and at the start of rebreathing (the plateau estimates the mixed venous P-CO2 at rest) in order to make comparisons. The corrected chemoreflex thresholds (mean +/- S.E.M. = 26.0 +/- 0.9 mmHg) were significantly less (paired Stud ent's t test, P < 0.001) than the resting P-ET,P-CO2 values (mean +/- S.E.M . = 34.3 +/- 0.5 mmHg). We conclude that both the behavioural and chemorefl ex drives contribute to resting ventilation.