Respiratory responses to external resistive loads during vagal blockade in awake dogs.

The present study examined the possibility that mechanoreceptors in the chest wall structures (rib cage and diaphragm) contribute to the increase in the neuromuscular drive to breathe (occlusion pressure) when the load on the respiratory muscles is increased in conscious animals and humans. Studies were carried out in 4 awake dogs in which external resistive loads (12 cmH2O/L/s) were applied during inspiration to increase the load on the respiratory muscles. Loads were applied via a tracheostomy during complete vagal blockade performed by cooling exteriorized cervical vagal loops. The ventilatory and occlusion pressure responses to the load were compared over the same range of chemical drive by applying loads during CO2 rebreathing. During vagal blockade, inspiratory resistive loads had no consistent effect on the duration of inspiration or expiration and decreased the ventilatory response to hypercapnia by decreasing average inspiratory flow rate. In all animals, however, inspiratory loading increased the occlusion pressure (P100) response to hypercapnia. The P100 at PCO2 = 55 mmHg increased during flow loading in all 4 animals, and the slope of the change in P100 produced for a given change in PCO2 (delta P100/delta PCO2) increased in 3 of the 4. Flow loading had no effect on end-expiratory lung volume at rest and did not influence the decrease in lung volume observed during hypercapnia. The present study indicated that the neuromuscular drive to breathe, as assessed from the occlusion pressure, is increased in conscious animals subjected to ventilatory loads during vagal blockade.(ABSTRACT TRUNCATED AT 250 WORDS)