Resetting of Baroreflex Sensitivity after Induced Hypotension

The effect of sodium nitroprusside (SNP)-induced hypotension on the sensitivity of the baroreflex was studied in 11 informed patients anesthetized with morphine (M) or halothane (H). PaCO2 was controlled at 28 to 32 mmHg with a respirator. Baroreflex sensitivity was assessed with a depressor test using a small dose of SNP (4–6 μg/kg, iv) to decrease the systolic pressure (SP) rapidly by about 30 mmHg. The slope of the regression line (in ms/mmHg) 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/mmHg for the morphine group and 3.2 ms/mmHg for the halothane group, indicating that baroreflex sensitivity is greater during morphine than during halothane anesthesia. This difference in baroreflex sensitivity might explain the differences in dose requirements for SNP in patients anesthetized with either morphine or halothane. Following a control test, SNP was infused at a rate adjusted to maintain the mean arterial pressure between 55 and 60 mmHg. The duration of hypotension was in accordance with surgical needs. SNP infusion was then discontinued and SP was allowed to recover spontaneously. When SP recovered to its control level, PI was significantly prolonged by 18 per cent in patients anesthetized with morphine and by 13 per cent in those anesthetized with halothane 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 per cent in patients anesthetized with morphine and by 179 per cent in those with halothane over the respective control values, indicating a resetting of the baroreflex. These changes may have significant implications in hemodynamic adjustments following induced hypotension in patients during general anesthesia.