Unloading of baroreceptors by carotid occlusion does not increase heart rate in fetal sheep

Unloading of baroreceptors by carotid occlusion does not increase heart rat e in fetal sheep; Objectives: To test the hypothesis that in fetal sheep re duction of carotid sinus pressure by carotid occlusion increases heart rate . Study design: Fetal sheep (gestational age 121-132 days) were chronically instrumented with bilateral carotid occluders, catheters and electrodes (E CG) to measure systemic arterial and carotid sinus (CSP) blood pressures, a nd fetal heart rate. Results: Bilateral carotid occlusion (BCO) increased m ean arterial blood pressure from 46 +/- 7 mm Hg to 53 +/- 8 mm Hg (mean +/- S.D.) while CSP decreased from 44 +/- 7 mm Hg to 17 +/- 7 mm Hg. Fetal hea rt rate fell during occlusion significantly from 186 +/- 34 bpm to 159 +/- 26 bpm (n = 20 animals). Infusion of phenylephrine (8.5-20 mu g min(-1) kg( -1)) or methoxamine (60-200 mu g min(-1) kg(-1)) increased mean blood press ure from 44 +/- 6 to 61 +/- 9 mm Hg, and fetal heart rate decreased from 18 6 +/- 30 to 132 +/- 31 bpm (n = 12). BCO increased systemic arterial pressu re further to 70 +/- 11 mm Hg whereas carotid sinus pressure was reduced to 31 +/- 13 mm Hg. However, average heart rate did not increase significantl y (136 +/- 28 bpm). Conclusion: We conclude that in contrast to adult anima ls, in fetal sheep carotid occlusion with subsequent unloading of barorecep tors does not increase heart rate even when the baroreflex had been activat ed by arterial hypertension. It seems likely that stimulation of carotid ch emoreceptors prevents the expected baroreceptor mediated heart-rate respons e. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.