Vagal tone decreases following intravenous diazepam.

To investigate the relationship between anxiety and parasympathetic nervous system activity, cardiac vagal tone was assessed in six healthy volunteers after the intravenous administration of the anxiolytic diazepam. Vagal tone was determined by quantifying the amplitude of respiratory sinus arrhythmia. We observed a significant dose-dependent attenuation in vagal tone, accompanied by an increase in heart rate. There was a nonstatistically significant decrease in plasma norepinephrine concentrations and subjective anxiety ratings at higher doses of diazepam. We discuss a possible role of the parasympathetic nervous system in the anxiolytic effects of the benzodiazepines.