EFFECTS OF HIGH-FREQUENCY VENTILATION AND PEEP ON CAROTID BARORECEPTOR REFLEXES

We tested the hypothesis that altering the pattern and/or magnitude of discharge of pulmonary stretch receptors (PSRs) would alter barorecep tor reflexes in anesthetized aortic-denervated cats. Carotid barorecep tor control of mean arterial pressure (MAP), heart rate (HR), and hind limb perfusion pressure (PPhl) was examined by changing carotid sinus pressure (CSP) from 50 to 225 mmHg. The pattern of PSR discharge was c hanged by switching conventional mechanical ventilation (CMV) to high- frequency ventilation (HFV). Magnitude of PSR discharge was altered by changing positive end-expiratory pressure (PEEP). Altering the discha rge pattern of PSR had no effect on CSP-MAP or CSP-PPhl relationships; small changes in HR were observed. Increasing PSR activity by increas ing PEEP during CMV (from 3 to 9 cmH2O) depressed CSP-MAP relationship , set point, and threshold pressure. However, the depression in CSP-MA P relationship and set point during PEEP was unrelated to PSR activati on, because these changes were not abolished after bilateral vagotomy. CSP-PPhl relationship was significantly elevated during PEEP before a nd after vagotomy, suggesting activation of a nonvagally mediated vaso constrictory mechanism instead of PSR-mediated depressor reflex. CSP-H R relationship during PEEP showed a slight elevation, which was abolis hed after vagotomy, We conclude that despite minor increases in HR, al tering the pattern or magnitude of PSR activity with HFV and PEEP has no significant effect on carotid baroreceptor regulation of systemic c irculation. Hemodynamic changes observed during PEEP were likely due t o its mechanical effect on cardiac output and activation of other card iopulmonary receptors rather than to the increase in PSR activity.