Changes in sympathetic and parasympathetic cardiac activation during mental load: an assessment by spectral analysis of heart rate variability.

Spectral analyses of heart rate (HR) and blood pressure (BP) fluctuations yield three typical peaks at a low (0.02-0.06 Hz), a mid (0.07-0.14 Hz) and a high (around the respiratory frequency) frequency area. These so-called bands attract the interest of researchers because they seem to offer the facility of non-invasively studying autonomic cardiovascular control mechanisms. The high frequency component is solely under vagal control, the influence of sympathetic/vagal efferents on the low and mid frequency band is unclear. We therefore investigated in a single case study (23 year old male) the effects of propanol (0.06 mg/kg, 30 min. interruption, 0.12 mg/kg i.v.), dobutamine (1.14 micrograms/kg/min for 30 minutes, then 2.21 micrograms/kg/min i.v., then 4.42 micrograms/kg/min), atropine (0.01 mg/kg within 5 minutes, 30 minutes later 0.02 mg/kg within 5 minutes), and carbachol (0.125 mg, 30 min. interruption, 0.25 mg s.c.) upon HR, HR-variability spectra, BP and respiratory parameters at rest and during 5 minutes of a mental task. Under all four drug conditions BP is elevated at rest and mental stress, the latter always giving higher results than the former. Atropine shortens interbeat intervals (IBI) by almost 50 percent (from 939 msec to 514 msec), the high dose of dobutamine reduces IBI from 725 to 580 msec, propranolol increases interval length by 10 percent. Drug effects on spectral bands give clear results with atropine: It reduces spectral energy in all three frequency bands at rest and during mental stress. The other drugs show no clear-cut effects on HR-variability spectra. Even though results of a single case study should be interpreted with great caution we believe that the following conclusion can be made: At rest and during short-lasting mental stress all frequency bands in HR-variability spectra are to a large extent under parasympathetic control.