Power Spectral Changes of Spontaneous Beat-to-Beat Variability of the RT Interval in Patients with Coronary Artery Disease

Objective: This study sought to evaluate the spontaneous beat-to-beat variability of the RT interval (RTV) as a marker of autonomic balance at the ventricle in patients with coronary artery disease (CAD), as heart rate variability (HRV) primarily reflects sympathovagal interactions at the sinus node. Methods: ECG and peripheral blood pressure signals were analyzed in supine and passive head-up tilt in 19 patients with CAD. RT intervals were measured between the maximum of the QRS complex and the maximum of the T wave. Power spectral analysis of RR intervals, RT intervals, systolic and diastolic pressure were performed using discrete Fourier transformation. We calculated the area under the spectral density curve in the low (LF = 0.05–0.18 Hz) and high frequency range (HF = 0.18–0.50 Hz). Results: Tilting results in a decrease of HF in HRV to 45%± 10% (P < 0.05) and an increase of HF in RTV to 266%± 83% (P < 0.01). Total RTV was 2.4% of total HRV. In systolic blood pressure variability HF increased to 396%± 124% (P < 0.05), in diastolic blood pressure variability to 231%± 69% (P < 0.05). Conclusion: If the transfer-function of sympathetic activity to the ventricle and to the sinus node are similar, the increase of HF in RTV cannot be explained by the increase in sympathetic activity induced by tilting. It is most likely a mechanical phenomenon induced by respiration synchronous changes of the position of the heart in the thorax and transferred variabilities of the central venous pressure that can also be identified in systolic and diastolic pressure variabilities.