The fall in blood pressure which all observers agree is commonly found accompanying spinal anesthesia has been ascribed in the main to two causes: (1) dilatation of the arterioles in the anesthetized area due to a block of vasoconstrictor fibers in the anesthetized roots1; (2) decreased cardiac output.2
The evidence in the literature for diminished vasoconstriction is as follows:
Burch and Harrisonle showed that spinal anesthesia increases the rate of flow of perfusion fluid through the femoral and brachial arteries of dogs. They showed also that compensatory vasoconstriction following hemorrhage by bleeding as shown by a marked diminution in the perfusion rate does not occur in the animals which receive spinal anesthesia, since in these animals the perfusion rate is unchanged or increased by hemorrhage. They concluded that
... in subjects under spinal anesthesia, the power of active vasoconstriction is, however, more or less completely absent because of
KOSTER H. BLOOD PRESSURE CHANGES DURING SPINAL ANESTHESIA IN NONOPERATIVE CASES. Arch Surg. 1942;45(4):596–605. doi:10.1001/archsurg.1942.01220040092010
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