Real-time measurement of cardiac vagal tone in conscious dogs.

Rapid changes in heart rate are caused by changes in parasympathetic tone. The NeuroScope is an electronic device designed to offer an objective real-time measure of instantaneous cardiac vagal tone by phase demodulation of a high-resolution time domain of R-R wave intervals. Data are displayed against an arbitrary but linear scale, the cardiac index of parasympathetic activity (CIPA). To validate this method, 10 conscious healthy dogs were each given six incremental doses of atropine (0.01 mg/kg) to a total dose of 0.06 mg/kg or equal volumes of saline. A dose-response curve was constructed. At the maximum dose of atropine, CIPA values fell to 1.3 ± 0.7% (SD) of baseline, whereas R-R intervals fell to 51.5 ± 11.5% of baseline, and standard deviation of the R-R wave interval fell to 10.6 ± 6.5% of baseline. These findings show that the NeuroScope can provide a specific real-time index of cardiac vagal tone in dogs without need for recourse to atropine.