Effect of pulmonary arterial PCO2 on breathing pattern.

We studied breathing patterns and tidal volume (VT)-inspiratory time (TI) relationships at three steady-state levels of pulmonary arterial PCO2 (PpCO2) in 10 anesthetized dogs. To accomplish this we isolated and then separately pump perfused the pulmonary and systemic circulations, which allowed us to control blood gases in each circuit independently. To ventilate the lungs at a rate and depth determined by central drive, we used an electronically controlled positive-pressure ventilator driven by inspiratory phrenic neural activity. Expiratory time (TE) varied inversely with PpCO2 over the range of PpCO2 from approximately 20 to 80 Torr. VT and TI increased with rising PpCO2 over the range from approximately 20 to 45 Torr but did not change further as PpCO2 was raised above the middle level of approximately 45 Torr. Thus minute ventilation increased as a function of TE and VT as PpCO2 was increased over the lower range and increased solely as a function of TE as PpCO2 was increased over the upper range. The VT-TI relationship shifted leftward on the time axis as PpCO2 was lowered below the middle level but did not shift in the opposite direction as PpCO2 was raised above the middle level. In addition to its effect on breathing pattern, we found that pulmonary hypocapnia depressed inspiratory drive.