The heart rate spectrum in simulated flight: reproducibility and effects of atropine.

The heart rate spectrum (HRS) was computed on eight pilots during simulated flight. Although respiratory-coupled heart rate variability (i.e., that produced by vagal inputs to the heart) varied three-fold across volunteers, each individual volunteer's data were very stable on three test days (median coefficient of variation = 5.4%). Atropine rapidly reduced heart rate variability at the respiratory frequency to the noise level (i.e., 74% decrease from predrug levels). Although there was no significant difference in the reduction induced by a 2 mg or 4 mg dose, there was a tendency for variability at this component of the spectrum to return to normal faster following 2 mg injection than after 4 mg injection. By the morning after injection at either dose, heart rate variability at the respiration component of the HRS was back to normal--indicating complete washout of the drug. This study revealed no technical limitation to using the HRS in real life situations outside of the laboratory and suggests that the test can be used to test the efficacy of other treatments or manipulations that reduce vagal tone to the heart.