RESPIRATORY RESPONSE TO POSITIVE INSPIRATORY PRESSURE IN THE CAT - EFFECTS OF CO2 AND VAGAL INTEGRITY

Effects of inspiratory pressure support (IFS) on respiration and activity of inspiratory muscles were tested in eight anesthetized cats by recording the diaphragmatic electromyogram (EMGdi) and respiratory variables at four levels of positive inspiratory airway pressure (5, 10, 15, and 20 cmH(2)O); onset of IFS was triggered by the inspiratory effort of the animal. When IFS was applied with room air (IPSAir) the respiratory frequency (f) was reduced compared with spontaneous breathing and the tidal volume (V-T) was significantly increased, which resulted in a fall of arterial PCO2 (Pa-co2) at IFS airway pressures (Paw) above 5 cmH(2)O. Despite this increase in VT, the amplitude of the integrated EMGdi (A(di)) was reduced during IFS at all levels of Paw. When arterial hypocapnia is corrected by addition of CO2 to the inspirate, the values of VT at any given Paw were virtually identical with those during IPSAir, but IFS-mediated changes in f and A(di) were smaller than those during IPSAir. IPS was also performed after bilateral vagotomy. Vagotomy itself caused VT and Adi to increase, and f to decrease, during spontaneous breathing. In comparison with the corresponding treatment before vagotomy, IPSAir led to a less severe reduction in A(di). As a result, V-T was more enhanced and Pa-co2, was more reduced after vagotomy than before, both during spontaneous breathing and during IPSAir at all levels of Paw. When, however, isocapnia was restored with IPS with CO2 in the vagotomized animal, diaphragmatic activity and became very similar to their values during spontaneous breathing, whereas V-T remained elevated as a result of the high positive airway pressure. Our data suggest that in anesthetized cats IPS leads to a diminution of diaphragm activity and that this reduction can be entirely attributed to 1) the hypocapnia, resulting from increased VT, and 2) the stimulation of pulmonary vagal afferent fibers at positive airway pressure.