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

Effects of inspiratory pressure support (IFS) on respiration and activ ity of inspiratory muscles were tested in eight anesthetized cats by r ecording the diaphragmatic electromyogram (EMGdi) and respiratory vari ables at four levels of positive inspiratory airway pressure (5, 10, 1 5, and 20 cmH(2)O); onset of IFS was triggered by the inspiratory effo rt of the animal. When IFS was applied with room air (IPSAir) the resp iratory 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) abo ve 5 cmH(2)O. Despite this increase in VT, the amplitude of the integr ated EMGdi (A(di)) was reduced during IFS at all levels of Paw. When a rterial hypocapnia is corrected by addition of CO2 to the inspirate, t he values of VT at any given Paw were virtually identical with those d uring IPSAir, but IFS-mediated changes in f and A(di) were smaller tha n those during IPSAir. IPS was also performed after bilateral vagotomy . Vagotomy itself caused VT and Adi to increase, and f to decrease, du ring spontaneous breathing. In comparison with the corresponding treat ment 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 IPS Air at all levels of Paw. When, however, isocapnia was restored with I PS with CO2 in the vagotomized animal, diaphragmatic activity and beca me 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 attribu ted to 1) the hypocapnia, resulting from increased VT, and 2) the stim ulation of pulmonary vagal afferent fibers at positive airway pressure .