Pharmacological modulation of parasympathetic cardiac control: Dose-dependent effect of atropine on the power spectrum of heart-rate fluctuations

The dose-dependent effect of atropine on autonomic cardiac control was studied by spectral analysis of heart-rate (HR) fluctuations. A crossover placebo controlled study was performed in eight young volunteers who received increasing boluses of i.v. atropine or placebo (from 0.1 to 2.3 mg). Computing the HR power spectrum and integrating over specific frequency bands, the authors focused on the respiratory frequency band (usually between 0.2-0.4 Hz), which is purely of vagal mediation. At small atropine doses (<0.4 mg), the respiratory peak increased, relative to baseline, with maximal response at 0.2 mg (from 1.0 to 1.9 ± 0.9). Larger doses of atropine (≥0.5 mg) markedly reduced the power of the respiratory peak in a dose-dependent way. Corresponding changes were observed in mean HR but in the opposite direction. It is concluded that atropine has a bimodal dose-dependent effect on parasympathetic cardiac control. This phenomenon may be attributed to the combined peripheral and central parasympathetic effect. HR spectral analysis can be used as a noninvasive tool for in-vivo quantitative evaluation of parasympathetic effects of antimuscarinic drugs.