Contribution to Heart Rate Variability by Mechanoelectric Feedback

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Abstract
Background Heart rate variability is an important prognostic indicator for sudden death. An increased risk of sudden death and arrhythmia is associated with reduced heart rate variability in heart failure. In heart failure, there is also dilatation of the atria, which raises the prospect that there could be some physiological basis to possibly link heart rate variability with atrial dilatation. We therefore investigated whether sustained atrial stretch could modulate heart rate variability directly. Methods and Results Pigs were anesthetized and their hearts exposed. A specially built device stretched the sinoatrial node before and after vagal section and then after administration of propranolol. Stretch of the sinoatrial node decreases heart rate variability in the following ways: The standard deviation of the beat-to-beat interval decreases (4.2 to 2.6 ms; P=.004), and the high-frequency components are reduced (control, 6.5±2.2 ms2; during stretch, 1.4±0.3 ms2; P=.003). After section of both vagi, the high-frequency components are reduced by stretch of the sinoatrial node (2.8±0.9 ms2 for control versus 1.2±0.3 ms2 during stretch; P=.05). Similarly, after both vagal section and β-blockade, stretch of the sinoatrial node reduces the high-frequency components (10.6±3.5 ms2 for control versus 3.0±1.5 ms2 during stretch; P=.01). Conclusions We conclude that stretch of the sinoatrial node reduces high-frequency heart rate variability. This may account in part for the reduced heart rate variability seen in clinical conditions in which the right atrium is dilated, such as congestive cardiac failure.

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