Sedentary Long-Duration Head-Down Bed Rest and ECG Repolarization Heterogeneity

Abstract

We studied the effects of 90 d of 6 degrees head-down bed rest (HDBR) on cardiac autonomic and advanced electrocardiographic (ECC) function, especially on repolarization heterogeneity as assessed by beat-to-beat QT interval variability (QTV), T-wave complexity, and 3-dimensional ECG. Based on prior observations of lengthening of the QTc interval during long-duration spaceflight, we hypothesized that abnormalities in ECG repolarization would also occur during long-duration HDBR. During controlled breathing, 5-min supine high-fidelity 12-lead ECGs were obtained from 20 healthy subjects (14 men and 6 women) together with measurements of plasma volume and electrolytes at 5 points in time: within 10 d before; 28-30, 60, and 90 d into; and 3-5 d after HDBR. By repeated measures ANOVA, 90 d of sedentary HDBR significantly increased the QTV index (from -1.87 +/- 0.33 to - 1.53 +/- 0.39 units), the index of unexplained QTV (from 0.61 +/- 0.48 to 1.21 +/- 0.40 units), the T-wave complexity intradipolar ratio (from 0.344 +/- 0.260 to 2.04 +/- 4.01%), and the spatial QRS-T angle (from 49.1 +/- 23.8 to 58.7 +/- 31.0 degrees), and significantly decreased the spatial ventricular gradient (from 91.3 +/- 26.5 to 59.1 +/- 23.0 mV x ms(-1)). These changes resolved in part by 3-5 d after resumption of ambulation, but unlike concomitant changes in the QTc interval itself and in heart rate variability, they did not significantly relate to changes in electrolytes or plasma volume. Sedentary, long-duration HDBR reversibly increases ECG repolarization heterogeneity and by inference ventricular arrhythmic risk.