Impact of Propranolol on Hemodynamic Response and Blood Cyanide Levels during Nitroprusside Infusion

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Abstract
Ten patients receiving N2O-morphine anesthesia for spinal fusion operations were given one or more doses of propranolol, 0.01 mg/kg IV, after an hour of nitroprusside (SNP) hypotension at a mean arterial pressure of 70 torr. During the hour before propranolol was administered, SNP dose requirement increased from 1.8 μg/kg/min±0.4 to 3.5 μg/kg/min±0.4 (mean±SE, p<0.05) and heart rate increased from 91 beats/min±3 to 108 beats/min±6 (mean±SE, p<0.05). Blood cyanide levels increased from a control value of 0.9 μM/L±0.2 to 7.2 μM/L±0.2 (mean±SE, p<0.05) and base excess decreased from-0.5 mEq/L±0.6 to-2.6±0.4 (mean±SE, p<0.05). Nine patients responded to propranolol with a significant (p<0.05) reduction in heart rate (to 92±9 beats/min) and nitroprusside requirement (to 1.8±0.4 μg/kg/min, mean±SE). No further increase in cyanide levels was observed, and base excess rose to-0.3 mEq/L±0.5 (mean±SE, p<0.05). One patient did not respond to a single dose of propranolol with a decrease in heart rate, and continued to require the same SNP dose (5.5 μg/kg/min). in this individual, blood cyanide levels increased from 6.8 μM/L to 9.6 μM/L during the hour after receiving propranolol, while base excess fell further, from-4.4 to-4.8 mEq/L. Judicious administration of propranolol in a dose sufficient to lower heart rate during nitroprusside-induced hypotension reduces both the nitroprusside dose-requirement and also the likelihood of developing cyanide toxicity.

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