Objective: To describe the relationship between vagally mediated bradycardia and heart rate variability indices in young and older healthy individuals.
Design: Cross-sectional, physiologic study.
Setting: Outpatient cardiovascular research laboratory.
Participants: A total of 34 young (mean age 24 years) and 27 older (mean age 63 years) healthy adults.
Methods: Eight bolus injections of atropine sulfate were given intravenously to participants while in a supine position (cumulative doses from 0.4 to 7.2 microg/kg). Each dose was followed by a 3-minute data collection period in which subjects controlled their breathing frequency at a rate of 15 breaths per minute.
Main outcome measurements: Chronotropic responses were assessed from average RR interval and blood pressure was assessed by automated brachial cuff. Heart rate variability (HRV) indices were calculated to represent both time domain measures (RR interval standard deviation and root mean squared of successive differences) and frequency domain measures (respiratory sinus arrhythmia and total power).
Results: RR interval responses exhibited the expected curvilinear pattern to atropine administration with all subjects exhibiting a bradycardia with at least one dose and RR interval returning to baseline or decreasing in most subjects as atropine dosing progressed. RR interval was closely related to vagotonic atropine dose with an r(2) greater than 0.70 in 89% of subjects. Heart rate variability indices were not consistently correlated with the bradycardic effect of vagotonic atropine and ranged from highly positive to highly negative with almost one-fifth of correlations less than 0.5.
Conclusions: The relationship between HRV and vagal tone is likely complex and has a large interindividual variation.