Beat‐to‐Beat Heart Rate and QT Variability in Patients with Congestive Cardiac Failure: Blunted Response to Orthostatic Challenge

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Abstract
Background: Congestive cardiac failure is associated with increased sympathetic activity and impaired baroreflex function. We sought to test the hypothesis that these patients also have blunted response of beat‐to‐beat QT interval variability during orthostatic challenge. Methods: We compared beat‐to‐beat heart rate and QT interval data in 17 patients with congestive cardiac failure and 17 age‐matched normal controls in supine normal breathing, supine controlled breathing, and standing controlled breathing conditions. The ECG data were acquired in lead II configuration at a sampling rate of 1000 Hz. Results: Supine controlled breathing was associated with an increase in spectral HF power (0.15–0.5 Hz) of HR and QT interval time series compared to spontaneous breathing condition only in controls. While there were significant changes in HR, HR LF power, HR LF/HF ratios, and QT variability measures in standing posture in controls, there were no such changes in patients. Conclusions: This impairment of postural changes of HR variability is most likely due to an impaired baroreceptor function in patients with congestive heart failure. The etiology of this is likely due to an increased cardiac sympathetic and a decreased vagal function. However, the relationship of postural changes in beat‐to‐beat QT interval variability and baroreflex need further investigation.