Resetting of Baroreflex Sensitivity after Induced Hypotension

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Abstract
The effect of sodium nitroprusside(SNP)-induced hypotension on the sensitivity of the baroreflex was studied in 11 patients anesthetized with morphine (M) or halothane (H). PaCO2 [arterial partial pressure of CO2] was controlled at 28-32 mm Hg with a respirator. Baroreflex sensitivity was assessed with a depressor test using a small dose of SNP (4-6 .mu.g/kg, i.v.) to decrease the systolic pressure (SP) rapidly by .apprx. 30 mm Hg. The slope of the regression line (in ms/mm Hg) relating SP and the succeeding pulse interval (PI, R-R interval) was used as an index for the sensitivity of baroreflex control of heart rate. In control measurements, SP-PI slopes were 6.4 ms/mm Hg for the M group and 3.2 ms/mm Hg for the H group, indicating that baroreflex sensitivity is greater during M than during H anesthesia. This difference in baroreflex sensitivity might explain the differences in dose requirements for SNP in patients anesthetized with either M or H. Following a control test, SNP was infused at a rate adjusted to maintain the mean arterial pressure between 55-60 mm Hg. The duration of hypotension was in accordance with surgical needs. SNP infusion was then discontinued and SP allowed to recover spontaneously. When SP recovered to its control level, PI was significantly prolonged by 18% in patients anesthetized with M and by 13% in those anesthetized with H over the respective control values. Immediately after the recovery of SP following the discontinuation of SNP infusion, another baroreflex sensitivity test was made; the SP-PI slopes increased markedly by 105% in patients anesthetized with M and by 179% in those with H over the respective control values, indicating a resetting of the baroreflex. These changes may have significant implications in hemodynamic adjustments ffollowing induced hypotension in patients during general anesthesia.

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