STUDIES ON AUTONOMIC BLOCKADE. II. OBSERVATIONS ON THE NATURE OF BLOOD PRESSURE FALL WITH HIGH SELECTIVE SPINAL ANESTHESIA IN PREGNANT WOMEN 1

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Abstract
The fall in blood pressure with high selective spinal anesthesia was studied on 17 normal pregnant subjects and 4 patients with pre-existing essential hypertension associated with pregnancy. The fall in blood pressure was found not to be related to loss of skeletal muscle tone or to anoxia. A statistically significant decrease in the cardiac output was encountered at the max. fall in blood pressure. Spinal hypotension was found to be related to venous stagnation in the lower extremities. It could be prevented by "exclusion" of the legs from the systemic circulation by cuffs or by 90 degree elevation. The fact that only normal pregnant patients in the prepartum and patients with pre-existing essential hypertension responded with a marked fall in blood pressure to autonomic blockade with high selective spinal anesthesia indicates the presence of an increased neurogenic tone. The present data show that the blood pressure fall in pregnant females is related to venous stagnation which is not related to loss of skeletal muscle tone or to anoxia. 2 hypotheses are postulated to explain the mechanism by which venous stagnation may lead to spinal hypotension: (1) The blood pressure of normal pregnant women at term is maintained by increased neurogenic tone. Such neurogenic tone possibly serves to compensate for the increased tendency to venous pooling in the lower extremities. When this tone is blocked by high selective spinal anesthesia, both arteriolar and venous systems collapse. (2) Spinal hypotension could be caused by blockade of the compensatory homeostatic vasoconstrictor reflexes in different areas of the body. The bradycardia following high selective spinal anesthesia could be explained on the basis of venous stagnation in the lower extremities with impairment of venous return to the heart Its disappearance with atropine and oxygen makes it more likely to be related to vagal stimulation and anoxia.