Efficacy of Carotid Collateral Perfusion in Anaesthetized Sheep Fetuses

Abstract The purpose of this study was to learn to what extent carotid collateral circulation is efficient in maintaining cephalic blood flow in the sheep fetus. Under halothane anaesthesia six fetal sheep at 124–135 days of gestation were instrumented with inflatable occluders around both common carotid arteries, an inductive flow probe around one external carotid artery, and arterial catheters to measure carotid sinus and aortic pressure. In acute experiments, the occluders were inflated on one side, or the other, or both sides simultaneously, while carotid blood flow, driving pressures and fetal heart rate were determined. Ipsi- and bilateral occlusion reduced carotid blood flow from about 42 ml/min to 10–0 ml/min and decreased sinus pressure from 39 mmHg to 29.1±2.9 (mean±s.d.) and 16.7±3.7 mmHg, respectively. Occlusion of the contralateral carotid artery increased ipsilateral carotid blood flow from 45±10 ml/min to 64±14 ml/min within 0.2 sec. Heart rate and aortic and sinus pressures did not change appreciably. Analysis of an analogue resistance network demonstrated that the observed carotid flow increases (less than twice normal) do not require changes of vascular resistances. It is concluded that the fetal sheep, as in the adult of many species, possesses an efficient carotid collateral system.