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Time-frequency analysis of cardiac rhythm to reveal patterns of adaptation to orthostatic challenge

Beat-to-beat changes in cardiac rhythm or heart rate variability (HRV), evoked by changes in the autonomic nervous system activity (ANSA), are complex and often remain unexposed by the traditional HRV analysis. In particular, this analysis does not reveal individual ANSA adjustments to changes in the body position during head-up tilt. Although tilt-related changes in the traditional HRV indices have been reported, the differences between successful and unsuccessful ANSA adaptation remain uncertain. To investigate HRV patterns in tilt-symptomatic and tilt-asymptomatic subjects, we used a time-frequency representation of the HRV signals. This representation, recently developed by the authors, is based on an orthogonal linear decomposition.