HEART-RATE AND BLOOD-PRESSURE VARIABILITIES DURING GRADED HEAD-UP TILT

We investigated the responses of the frequency components of heart rat e (HR) and blood pressure (BP) variabilities to progressive changes in autonomic activity induced by the graded head-up tilt technique in 12 normal subjects (age 19-27 yr) under the condition of frequency-contr olled respiration (0.25 Hz). During low-level tilt (0-30 degrees), the R-R interval was unchanged and the amplitude of the high-frequency (H F; 0.25 Hz) component of HR variability showed only a slight insignifi cant decrease. The amplitude of the low-frequency (LF; 0.04-0.15 Hz) c omponent of HR variability increased progressively as the angle increa sed (P < 0.05). During high-level tilt (30-90 degrees), the RR interva l and the HF amplitude of HR variability decreased progressively with tilt angle (P < 0.001 for both). The LF amplitude of HR variability pe aked at a tilt angle of 30 degrees. The LF-to-HF ratio of HR variabili ty and the LF amplitude of systolic and diastolic BP variabilities inc reased progressively as the tilt angle increased from 0 to 60 degrees (P < 0.001), although systolic and diastolic BPs were unchanged. These results suggest that mixed autonomic responses to orthostatic stress, which are thought to be mediated by both cardiopulmonary and arterial baroreflex mechanisms, can be distinguished by changes in the frequen cy components of HR and BP variabilities.