[Effect of body posture on heart rate and cardiocirculatory parameters in stress--implications for frequency-adapted...

In order to study the influence of body position on cardiodynamics and the consequences for the regulation of rate-adaptive pacing, we examined variations of cardiocirculatory parameters with passive graded tilt from the erect to the supine position (90 degrees, 75 degrees, 60 degrees, 45 degrees, 30 degrees, 0 degrees) in 25 healthy subjects of both sexes under resting conditions and under conditions of exercise on a bicycle ergometer. Using impedance cardiography combined with phonocardiography, it was possible to record heart rate, cardiac output, contractility (Heather index), and left-ventricular ejection time (LVET). Under the condition of rest, some parameters showed evident tilt dependency: tilting from erect to supine induced a decrease of heart rate (85.1 to 65.7 bpm), an increase of stroke volume (84.4 to 111.4 ml), and a prolongation of LVET (255.8 to 330.3 ms). Cardiac output and the index of contractility were not changed by this manipulation, however. Tilting under the condition of exercise clearly had less influence on all cardiodynamic parameters than tilting under the condition of rest. The only significant increase of heart rate (89.4 to 109.2 bpm), cardiac output (10.4 to 13.1 l/min), and Heather index (24.2 to 29.8 ohm/s2) was found between 30 degrees or 15 degrees, respectively, and the supine position. Thus, in addition to indicators of physical activity, body position respective stroke volume should be taken into consideration. Consequently, rate-adaptive pacing may guarantee optimal physiological hemodynamics.