[Transient behaviour of the heart rate in man depending on respiratory cycle during fast tilt]

The initial heart rate response to rapid passive changing of posture was analysed in 43 normal subjects. The tilting (mean tilt time: 1.7 s) was applied during expiration or inspiration. As compared to inspiration, the 70 degrees head-up tilt during expiration resulted in significantly higher values of heart rate increase and longer duration of the "initial complex", i.e. the immediate increase and the following decrease of heart rate to a minimum after tilting. The heart rate response to tilting from erect to supine position was not symmetrical to that during head-up tilt: at first the heart rate increased, about 6 s after the change of body position heart rate decreased suddenly, mostly below the recumbent control value, which was attained again after about 20-30 s. The distinct initial heart rate response to the head-up tilt probably can be explained in part by the hydrostatically caused drop of the mural pressure at the level of carotid artery and the different sensitivity of arterial baroreflex during the respiratory cycle. Other mechanisms, e.g. the participation of chronotropic autoregulation are discussed.