Prolonged Bradycardia After Phenylephrine Administration

Background: Phenylephrine is the first line vasopressor for prophylaxis and treatment in cases of hypotension due to spinal anesthesia in obstetrics. Phenylephrine binds to alpha-1 receptors leading to venous and arterial vasoconstriction. Because of its selectivity, reflex bradycardia can result from an increased baroreceptor firing rate. The duration of bradycardia has not been well characterized. Here we present a case of bradycardia persisting for 87.8 hours after the discontinuation of phenylephrine. Case Information: A 29-year-old female was admitted to the hospital for premature rupture of membranes at 27 weeks gestation of twins. After undergoing C-section, she became hypotensive and hypokalemic and was started on septic shock protocol. The patient was transferred to the ICU and was given antibiotics, IV fluids, and potassium repletion. She was started on a phenylephrine drip and was weaned off within 24 hours. The patient's HR was noted to be 49. TSH was within normal limits, and medication list was negative for beta blockers. She was asymptomatic and was monitored without intervention for 87 hours until HR consistently reached 60. Conclusion: Though reflex bradycardia is a well documented effect of phenylephrine administration, the duration of this effect is less clear. This case describes bradycardia persisting for 87.8 hours, which is the longest duration of this effect we have found in literature review. Because phenylephrine is used both prophylactically and for spinal anesthesia induced hypotension in obstetrics, further studies are needed to specifically characterize the duration and mechanism of this prolonged effect.