Fractal nature of short-term systolic BP and HR variability during lower body negative pressure

We have shown previously that the heart rate variability (HRV) signal is fractal in nature with a high degree of complexity, as given by the calculated fractal dimension (DF). We have also reported that loss of complexity, as indicated by a reduction in DF of HRV, is associated with orthostatic hypotension and impending syncope. To extend this investigation of cardiovascular responses, we have investigated the signal characteristics of short-term systolic blood pressure variability (BPV) coincident with measurements of HRV during orthostatic stress. Eight healthy men completed a test protocol of 20 min supine rest followed sequentially by 10 min at each of -5, -15, -25, -40, and -50 mmHg lower body negative pressure (LBNP) and 10 min supine recovery. We found that resting BPV and HRV were fractal with approximately 70% of both variables in the fractal component of the variability signal. The slope of the 1/f beta relationship was 1.16 +/- 0.12 for HRV and 2.31 +/- 0.17 for BPV. With increasing levels of orthostatic stress, the 1/f beta slope of HRV increased significantly to 1.68 +/- 0.08 at -50 mmHg LBNP, whereas the 1/f beta slope was unchanged for BPV. Indicators of parasympathetic and sympathetic nervous system activity derived from heart rate variability suggested reduced and increased values, respectively, as the LBNP increased. These data indicate important differences in heart rate and blood pressure control under orthostatic stress.