Prolonged PR Interval Is a Risk Factor for Bradycardia During Spinal Anesthesia

Abstract

Comment: The pathogenesis of bradycardia during spinal anesthesia is probably multifactorial. Contributing factors are thought to include decreased cardiac sympathetic tone, reduced venous return, reflex decrease in heart rate resulting from a reduction of pacemaker stretch, and reflex decrease in heart rate mediated via ventricular mechanoreceptors. Regardless of etiology, intraoperative bradycardia is of concern because asystole may follow. Thus, the ability to identify patients at risk for bradycardia during spinal anesthesia may improve patient care. Indeed, the potential seriousness of bradycardia during spinal anesthesia has been vividly documented in the ASA Closed Claims Studies.1 Risk factors previously identified for the development of bradycardia include baseline heart rate <60 bpm, ASA physical status 1 vs. 3 or 4, use of P-blocking drugs, sensory block height >T5, and age <50 years. A previous surveillance study in patients undergoing epidural anesthesia with controlled hypotension for elective hip replacement also demonstrated several ECG predictors for intraoperative bradycardia. These were heart rate <60 bpm, evidence of inferior wall myocardial infarction, and evidence of left ventricular hypertrophy.5 The duration of the PR interval depends on sympathetic and parasympathetic balance, age, and heart rate. Typically the PR interval increases throughout childhood until stabilizing in adults at a duration between 0.12 and 0.2 sec. (The authors appropriately warn that, because there were only 14 patients under 40 yr old in their study population, the findings may not be applicable to younger patients). A prolonged PR interval is said to occur in approximately 1% of healthy men and usually does not lead to the development of complete heart block; it is not typically associated with ischemic heart disease. Although the exact relationship between preoperative prolongation of the PR interval and intraoperative occurrence of bradycardia during spinal anesthesia is speculative, the authors suggest that a prolonged PR interval may be a marker for patients with high parasympadietic tone, and this parasympathetic predominance combined with spinal anesthe-