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Neurocardiological basis for intraindividual ECG variability
1Cardiology Department, Leiden University Medical Center, The Neitherlands. c.a.swenne@lumc.nl
Insights
Different autonomic nervous system inputs can produce the same heart rate but alter electrocardiogram (ECG) properties. This study explores ECG variability under various physiological stressors in healthy men.
Area of Science:
- Cardiovascular Physiology
- Autonomic Nervous System Regulation
- Electrocardiography
Background:
- Cardiac autonomic control involves complex interplay between sympathetic and parasympathetic outflows.
- Varying combinations of these outflows can maintain heart rate while altering ventricular electrophysiology.
- This electrophysiological variation may contribute to intra-individual electrocardiogram (ECG) variability.
Purpose of the Study:
- To investigate intra-individual ECG variability under different physiological stress conditions.
- To determine if distinct autonomic nervous system activation patterns, despite similar heart rates, result in measurable ECG differences.
- To explore the clinical relevance of autonomic-induced ECG variability.
Main Methods:
- Healthy male subjects underwent two distinct stress tests: isometric handgrip (ergoreflex activation) and progressive leg lowering (gravitational stress).
- Cardiac preload, afterload, sympathetic, and parasympathetic outflow were modulated by the stressors.
- ECG parameters including QRS azimuth, T vector, and QT interval were measured and compared between conditions.
Main Results:
- Heart rate was matched between handgrip and leg lowering conditions (within 1%).
- Significant differences were observed in multiple ECG parameters, including QRS duration, T vector components, and QT interval (435 ± 21 ms vs. 418 ± 15 ms).
- These findings demonstrate that different autonomic inputs, even with matched heart rates, induce distinct ventricular electrophysiological profiles.
Conclusions:
- Autonomic nervous system-driven changes in cardiac preload and afterload can cause significant intra-individual ECG variability.
- The study highlights a potential source of variability in ECG interpretation that may be clinically relevant.
- Further research into these autonomic influences on ECG is warranted, particularly in more extreme conditions or sensitive populations.
Abstract:
Various combinations of 1) cardiac preload, 2) sympathetic outflow to the heart, and 3) parasympathetic outflow to the heart may yield the same heart rate, while the resulting ventricular electrophysiological properties differ. This may be an important source of intra-individual electrocardiogram (ECG) variability. Isometric stress, by invoking the ergoreflex, increases sympathetic outflow. By an increased peripheral resistance this increases cardiac afterload, activating the arterial baroreflex. This reflex, in turn, reduces the sympatho-excitation and increases parasympathetic outflow. Progressive gravitational stress (sitting-standing) causes a progressively decreasing cardiac preload. Decreasing cardiac preload activates the cardiopulmonary baroreflex. This causes an increase of sympathetic outflow, while parasympathetic outlow changes in opposite direction. An example of intra-individual ECG variability in the sitting position is given for healthy men. Two stressors were applied: handgrip (ergoreflex) with the legs in horizontal position, and progressive leg lowering (gravitational load). Thirteen subjects had a heart rate match better than 1%. QRS azimuth, QRS duration, maximal T vector, T azimuth, T elevation, ST duration, QRS-T angle and QT interval differed significantly between handgrip and leg lowering (QT interval 435 +/- 21 vs. 418 +/- 15 ms, P <.01). More extreme experimental conditions and/or more sensitive substrates than healthy young men are likely to entail larger ECG differences. It may be clinically relevant to further explore this source of intra-individual ECG variability.