REFLEX VASCULAR-RESPONSES IN THE ANESTHETIZED DOG TO LARGE RAPID CHANGES IN CAROTID-SINUS PRESSURE

This study examined reflex vascular responses to large rapid increases and decreases in carotid sinus pressure to determine whether delayed or inappropriate vascular responses might be obtained that, if they oc curred in people, could lead to hypotension during exposure to rapidly alternating gravitational forces. In chloralose-anesthetized open-che st dogs, a perfusion circuit controlled carotid sinus and thoracic aor tic pressures and blood flows to both the vascularly isolated abdomina l circulation and a hindlimb (perfusion pressure changes denoted resis tance). When carotid pressure was increased and decreased over the ran ge of 60-180 mmHg, the resulting reflex vasodilatation occurred signif icantly more rapidly than the vasoconstriction (P < 0.001). In the abd ominal vascular bed, time constants for vasodilatation and vasoconstri ction were 4.2 +/- 0.5 and 7.5 +/- 1.0 s, respectively. Decreases in c arotid pressure in pulses of 10-s duration or less failed to elicit ma ximal vasoconstriction, whereas increases in carotid pressure lasting as little as 5 s did elicit maximal vasodilatation. ''Square-wave'' al ternations in carotid pressure with periods of 10 s or less (5 s high, 5 s low) resulted in attenuation of the vasoconstriction, and at a 4- s period, both vascular beds remained almost maximally vasodilated thr oughout. The failure of vascular resistance to follow carotid pressure changes was not due to a failure of the response of sympathetic effer ent activity, since the time constants for the reduction and increase in discharge were much shorter at 0.56 +/- 0.13 and 0.43 +/- 0.10 s, r espectively. These results indicate that rapid changes in carotid pres sure could result in inappropriate vasodilatation and hypotension and might, in some circumstances, such as in pilots flying high-performanc e aircraft, predispose to syncope.