Noninvasive Estimates of Cardiac Performance during and Early after Single and Repeated Passive Upright Tilt in Normal Men: Volume Dependency of Systolic Time Intervals and Maximum Velocity of Transthoracic Impedance Changes

The combination of electrocardiography, impedance cardiography and phonocardiography allows to measure systolic time intervals and maximum velocity of transthoracic impedance changes (dZ/dt(max)) as estimates of (changes in) cardiac performance. Such assessment was performed in 35 normal male subjects undergoing single gradual passive upright tilting and 11 subjects undergoing repeated upright tilting. During tilting a cardio-acceleratory and vasopressor response is evident in compensation for the largely reduced stroke volume as a consequence of the reduced preload. During upright tilting, heart rate-corrected preejection pressure (PEP) was prolonged, heart rate-corrected left ventricular ejection time (LVET) was shortened and PEP/LVET was increased, and dZ/dt(max) and related parameters were reduced during upright tilting. These changes most likely reflect the preload dependency of these parameters even to such an extent that they could not reflect net inotropic changes related to the increased adrenergic activity during tilting. During the first 10 min after upright tilt, dZ/dt(max) and estimated stroke volume were increased versus supine baseline prior to tilt, whereas the return of the estimated thoracic blood volume back to baseline was delayed. This was associated with a relative bradycardia and slight reduction in diastolic blood pressure. These changes reached their maximum on average 3–4 min after the supine position was resumed. This enhanced cardiac pump performance early after tilt, suggested by dZ/dt(max) and related parameters, was not associated with corresponding changes in systolic time intervals. Although both systolic time intervals and dZ/dt(max) may be volumedependent, dZ/dt(max) and related parameters may offer additional information on cardiac performance not derivable from systolic time intervals.