Temporal analysis of cardiac autonomic regulation during orthostatic challenge by short-term symbolic dynamics

The gradual shift of cardiac autonomic regulation toward sympathetic predominance and vagal withdrawal during graded head-up tilt test in young controls has been demonstrated by parameters from symbolic dynamics obtained from short-term heart rate variability (HRV) analysis. In this study, the influence of gender and vasovagal syncope (VVS) on the autonomic response to an orthostatic challenge was investigated by HRV analysis using short-term symbolic dynamics (STSD). This study included 24 healthy young subjects (12 males; 12 age-matched females) and 16 female patients diagnosed with VVS. All subjects were enrolled in a head-up tilt (HUT) test, breathing normally, including 5 minutes of supine position (baseline) and 20-40 minutes of 70° orthostatic phase. The STSD parameters were obtained following their behavior at different times during the HUT test, i.e., at baseline, early and middle orthostatic phases. Gender differences including increased sympathetic activity in men were already present in baseline and in the middle tilt phase. In baseline there were no differences between female controls and female patients, but parameters from STSD showed highly significantly (p=0.0007) reduced heart rate variability due to increased sympathetic activity in female patients in the middle tilt phase. Furthermore a new nonlinear index for the estimation of sympatho-vagal balance was introduced.