FUNCTIONAL BEHAVIOR OF THE LOW PRESSURE SYSTEM UNDER HIGH SPINAL ANESTHESIA

In order to investigate the functional behavior of low pressure system in hypotension (68±2.1% of the control value of systolic pressure of the aorta) due to high spinal anesthesia, mean circulatory filling pressure (Pms), venous to arterial capacitance ratio (CV/CA), effective compliance (EC) of the low pressure system and, in addition, dynamic performance of right atrium were measured on thoracotomized dogs. Pms and EC were decreased, whereas CV/CA was increased, by spinal anesthesia and restored by administration of plasma expander with an elevation of systolic pressure of the aorta. Among the dynamic performances of the right atrium, the passive filling phase was noted to be an important factor in the venous return mechanism. The control value of filling time ratio was 52±7.6% (average±S.E.) on thoracotomized dogs under nitrous oxide and pancuronium anesthesia. The time ratio was decreased by spinal anesthesia and restored or even increased with an administration of plasma expander. In order to keep the maintenance of the systolic blood pressure well above the control level in the condition of hypotension due to high spinal anesthesia, the vasopressors, such as nor-epinephrine and phenylephrine were needed to be administered after a certain amount of plasma expander had been given (20 ml/Kg). Under such a condition, nor-epinephrine (0.8 μg/Kg/min.) maintained the time ratio of right atrial filling more favorably than phenylephrine (0.03 mg/Kg/min.). Besides, nor-epinephrine immediately decreased an extreme elevation of right atrial wall tension which had been caused by volume effect of plasma expander. With these results, we considered that the important factors for maintaining the functional behavior of the low pressure system in hypotension due to spinal anesthesia would be a volume (its filling pressure) and the compliance.