Purpose: To determine the relative potency in healthy individuals of the vagally mediated reflexes used clinically to inhibit sinoatrial and atrioventricular node function.
Subjects and methods: Twenty healthy volunteers with no history of heart disease performed face immersion in cold water and the Valsalva maneuver twice, to maximum endurance and to the subjective point of first discomfort, and face immersion in warm water and the Müller maneuver to maximum endurance only. Right and left carotid massage and left, right, and bilateral eyeball compression were each performed for 15 seconds. Change in heart rate was taken as baseline minus the rate over the slowest three consecutive QRS cycles elicited by each maneuver. Fisher's least-significant-difference multiple comparison procedure was used to analyze heart rate responses. Significance was defined as p less than or equal to 0.05.
Results: Maximum pulse decrements from baseline and 95% confidence intervals in beats/minute were as follows: cold-water face immersion to maximum endurance 15.5 (12.3 to 18.5), cold-water face immersion to first discomfort 10.1 (6.7 to 13.1), Valsalva maneuver to maximum endurance 9.2 (6.3 to 12.4), Valsalva maneuver to first discomfort 8.3 (5.0 to 11.3), right carotid massage 7.3 (4.3 to 10.3), left carotid massage 5.2 (2.3 to 8.4), right eyeball compression 6.0 (3.1 to 9.2), left eyeball compression 6.6 (3.6 to 9.5), bilateral eyeball compression 6.0 (3.1 to 9.2), warm-water face immersion 7.0 (3.2 to 9.8), and Müller maneuver 1.6 (-1.3 to 4.9). Bradycardia was significantly greater for cold-water immersion of the face performed to maximum endurance than for all other maneuvers.
Conclusion: In healthy subjects, the diving reflex is the most potent of the vagally mediated reflexes utilized in clinical practice. Immersion of the face in cold water may prove effective at the bedside when other maneuvers fail to augment vagal tone adequately.