Respiratory effects of brief baroreceptor stimuli in the anesthetized dog

To quantify the immediate isocapnic respiratory response to baroreceptor stimulation, pressure in the isolated externally perfused carotid sinuses (CS) of 24 vagotomized alpha-chloralose-anesthetized dogs was increased selectively during either inspiration or expiration as a step (from time of onset to end of respiratory phase) or a pulse (500 ms). The rise time (150 ms), base-line pressure (80 mmHg), and stimulus magnitude (40 mmHg) were similar for the two stimuli. The time of stimulus onset (delay), expressed as a percent of control time of inspiration (TI) or expiration (TE), was varied. TI, TE, and tidal volume (VT) were expressed as percent changes from control. Stimuli delivered early in inspiration lengthened TI [23.5 +/- 6.4% (SE) for step and 11.7 +/- 6.3% for pulse stimuli at 5% delay] more effectively than late stimuli. VT was essentially unaltered. In contrast, step stimuli delivered during expiration caused a lengthening of TE (32.7 +/- 6.3% at 5% delay) that did not depend on the delay (up to 75%). Very late (85%) pulse stimuli lengthened TE (15.2 +/- 5.7%) more effectively than early stimuli. For both stimuli, the expiratory VT was unaltered. When the responses are compared before and after separation of the blood supply of the carotid bodies from the CS region and when they are compared before and after inhibition of reflex systemic hypotension by ganglionic blockade, the observed responses were shown to be due solely to CS baroreceptor stimulation and not to alterations in carotid body blood flow or reflex changes in systemic cardiovascular variables.