Mechanism of Propofol-Induced Bradycardia
Propofol causes bradycardia primarily because it lacks vagolytic activity and may exert a central vagotonic effect, making it prone to causing bradycardia particularly when combined with other medications that stimulate the vagus nerve. 1, 2
Primary Mechanisms
Propofol lacks central vagolytic activity that other induction agents like thiopentone possess, and may actually exert a central vagotonic effect that enhances parasympathetic tone 2
The bradycardia results from centrally mediated sympatholytic and/or vagotonic actions rather than direct cardiac effects, as demonstrated by studies showing atropine can prevent the heart rate decrease but not the blood pressure drop 3
Direct vasodilating actions contribute to hypotension, while the bradycardia is mediated through central autonomic mechanisms 3
What Propofol Does NOT Do
Propofol-induced bradycardia does not involve the Bezold-Jarisch reflex, as demonstrated in animal studies where propofol failed to produce the characteristic vagally-mediated responses seen with 5-HT and veratridine 4
The bradycardia is not due to simple vagomimetic actions, since atropine pretreatment does not attenuate propofol-induced bradycardia when propofol is given alone 5
Propofol has vagolytic effects on the airway (producing bronchodilation) but paradoxically does not have vagolytic effects on the cardiovascular system 5
Clinical Context and Risk Factors
The combination of propofol with suxamethonium can produce severe sinus bradycardia in unpremedicated patients, as propofol's lack of vagolytic activity allows the muscarinic effects of suxamethonium to be exaggerated 2
Pediatric patients are particularly susceptible, especially when fentanyl is administered concomitantly with propofol 1
The FDA label explicitly states that propofol has no vagolytic activity and that reports of bradycardia, asystole, and rarely cardiac arrest have been associated with its use 1
Hemodynamic Profile
Propofol produces decreased heart rate, decreased preload, decreased afterload, and decreased cardiac output 6
These cardiovascular effects are more pronounced than with benzodiazepines, which maintain heart rate and cardiac output more effectively 6
Prevention Strategies
Prophylactic anticholinergic agents (atropine or glycopyrrolate) should be considered before propofol administration, especially in patients with known bradycardia risk factors 7, 1
Atropine premedication (0.6 mg intramuscularly) can prevent severe bradycardia when propofol is followed by suxamethonium 2
Avoid rapid bolus administration in hemodynamically unstable patients; instead use small incremental doses or slow infusion 7