Comparative influence of proprioceptors and chemoreceptors in the control of respiratory muscles.

Previously we have demonstrated that continuous positive pressure breathing (PB) depresses diaphragm activity and excites expiratory activity in the abdominal muscle (AMR) via vagal proprioceptive impulses. During prolonged PB the AMR persists at a constant level whereas inhibition of the diaphragm wanes, possibly as a result of CO2 retention. This study measures CO2 retention during PB and compares the responses of the abdominal muscle and diaphragm to chemostimulation alone and to chemostimulation and PB in combination. Continuous recordings of minute ventilation, integrated EMGs of the diaphragm and abdominal muscle, and mass spectrometer analysis of airway gases were obtained during PB on air, 5.25 percent CO2 and 12.4 percent O2 in eight Dial-anaesthetized cats. Between 0 and 15 cm H2O the steady-state end-tidal CO2 rises about 0.6 mm Hg/km H2O, diaphragm activity decreases and AMR increases exponentially with each increment in PB. When 5.25 percent CO2 is inspired, diaphragm activity is augmented at every pressure suggesting algebraic summation of proprioceptive and chemoreceptive effects at the respiratory centre. In contrast, the AMR is not significantly altered by hypercapnia. The absence of all abdominal muscle expiratory activity after bilateral vagotomy suggests that the role of active expiration is to regulate thoracic-lung volume, not blood gases.