Cross-spectral analysis of cardiovascular parameters whilst supine may identify subjects with poor orthostatic tolerance.

An easy and low-cost method for identification of subjects prone to orthostatic vasovagal syncope would be of clinical benefit. An orthostatic test with 60 degrees head-up tilt and progressive lower-body negative pressure was performed on 79 patients with histories of unexplained syncope and 26 control subjects. The test was stopped at the onset of presyncope and time to presyncope was taken as a measure of orthostatic tolerance. Spectral and cross-spectral analysis was performed on the supine time series of the R-R interval (ECG) and systolic pressure (Finapres) recorded before the beginning of the test. According to reference values, 38 patients and 11 controls were classified as having poor orthostatic tolerance (PPT and CPT respectively), whereas 41 patients and 15 controls displayed normal orthostatic tolerance (PNT and CNT respectively). The central frequency of the low-frequency (LF approximately equal to 0.1 Hz.) oscillations in PNT and CNT was significantly higher than that in PPT and CPT. In addition, it was significantly linearly correlated with the time of presyncope. Using our test of orthostatic tolerance as a comparison, the LF central frequency allows the classification of subjects with poor or normal tolerance with 80% sensitivity and 82% specificity. These results suggest that the LF central frequency in the supine position may provide a useful index in the diagnosis of orthostatic intolerance.