Arterial pressure maintenance after haemorrhage in the pregnant rabbit.

The changes in arterial blood pressure and cardiac output following rapid removal of 10% of the blood volume have been compared in non-pregnant and pregnant anaesthetized rabbits close to term (28 or 29 days gestation). Immediately after the end of haemorrhage the fall in arterial pressure was greater in the pregnant group (27.7 vs. 17.8 mmHg), but as recovery proceeded the difference diminished and by the tenth minute the reduction from pre-haemorrhage levels was similar in pregnant and non-pregnant rabbits. Cardiac output fell by about 30% in both groups and increased only very slightly during the ensuing 10 min. 1 h after section of the carotid sinus and aortic nerves the haemorrhage was repeated. The fall in arterial pressure in the non-pregnant rabbits was now more than doubled (47.2 vs. 17.8 mmHg). By contrast it was little changed in the pregnant rabbits (32.0 vs. 27.7 mmHg). It is concluded that pregnancy diminishes the role of the baroreceptor reflexes in the response of the rabbit to haemorrhage. Other mechanisms, such as an increased compliance of the resistance vessels and/or a more effective chemically mediated vasoconstriction, probably become of more importance to the maintenance of arterial pressure.