Efficacy of carotid collateral perfusion in anaesthetized sheep fetuses

The purpose of this study was to learn to what extent carotid collateral ci rculation is efficient in maintaining cephalic blood flow in the sheep fetu s. Under halothane anaesthesia six fetal sheep at 124-135 days of gestation were instrumented with inflatable occluders around both common carotid art eries, an inductive flow probe around one external carotid artery, and arte rial catheters to measure carotid sinus and aortic pressure. In acute exper iments, the occluders were inflated on one side, or the other, or both side s simultaneously, while carotid blood flow, driving pressures and fetal hea rt rate were determined. Ipsi- and bilateral occlusion reduced carotid bloo d flow from about 42 ml/min to 10-0 ml/min and decreased sinus pressure fro m 39 mmHg to 29.1 +/- 2.9 (mean +/- s.d.) and 16.7 +/- 3.7 mmHg, respective ly. Occlusion of the contralateral carotid artery increased ipsilateral car otid blood flow From 45 +/- 10 ml/min to 64 +/- 14 ml/min within 0.2 sec. H eart rate and aortic and sinus pressures did not change appreciably. Analys is 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, poss esses an efficient carotid collateral system. (C) 2000 Harcourt Publishers Ltd.