COMPARISON OF HEART-RATE AND ARTERIAL-PRESSURE SPECTRA DURING HEAD-UP TILT AND A MATCHED LEVEL OF LBNP

Lower Body Negative Pressure (LBNP) can be used to stimulate cardiovascular regulation by inducing blood shifts similar to those produced during head up tilt (HUT). It is unclear, however, whether similar blood shifts produced by these two stresses evoke similar cardiovascular regulatory responses. Hence, we compared the autonomic components of cardiovascular responses to 50 degrees HUT and a matched level of LBNP. A level of LBNP that produced changes in calf circumference similar to those produced during the first 3 min of 50 degrees HUT was considered to be a matched level. Autonomic components of cardiovascular responses were determined by spectral analysis of heart rate and blood pressure. Results from nine subjects showed that in terms of changes in calf circumference at the end of 3 min, 50 degrees HUT and 48 mm Hg LBNP were similar (2.13% and 1.94%). During 20-min exposures to HUT and LBNP, the increase in heart rate during LBNP was greater (+7 bpm) than HUT, while blood pressure increases were similar. For heart rate and blood pressure spectra, power in the respiratory frequency region (0.25 Hz) decreased and power in the low frequency region (0.03 Hz) increased similarly during HUT and LBNP. These results indicated that 50 degrees HUT and a matched level of LBNP evoked similar autonomic responses in cardiovascular regulation, with the autonomic balance shifted toward increased sympathetic and decreased parasympathetic influence.