Changes in spatial ventricular gradient and QRS-T angle induced by strict 5-days immobilization, and effects of exercise countermeasure

Abstract

Purpose: The spatial QRS-T angle (SA) and ventricular gradient (SVG) are classical ECG parameters that provide information on cardiac conduction system function and on ventricular action potential duration heterogeneity. Wide SA and reduced SVG are related to pathological conditions. Our aims were: 1) to assess the effects of 5 days immobilization (-6 degrees bed-rest, BR) on SA and SVG; 2) to evaluate the effectiveness of 25 min/day standing exercise (unilateral and bilateral heel raise, bilateral squatting (60°, 90° and 120°), bilateral and cross-hopping) as possible countermeasure. Methods: 10 healthy men (mean age 30±6) were enrolled in a cross-over design (wash out period: 2 months; control, CTRL, and exercise, EXE, groups) at DLR (Cologne, Germany) as part of the European Space Agency BR studies. High fidelity (1000 Hz) Holter ECG (12-leads, Mortara Instrument) was acquired before (PRE), the last day of BR (HDT5), and four days after the BR conclusion (POST). The night period (23:00-06:30) was selected for the analysis. X,Y,Z leads were derived (inverse Dower matrix) and vectorcardiogram computed. Selective beat averaging was used to obtain averages of P-QRS-T complexes preceded by the same stable heart rate (10 msec bin amplitude, in the range 900-1200 msec). For each averaged waveform (i.e., one for each bin), Twave maximum amplitude (Tmax), VG and SVG were computed. Results: Compared to PRE, a reduction in SVG (11%±3% and 9%±1%) and corresponding widening in SA (27%±9% and 21%±5%) were found at HDT5 in both CTRL and EXE, respectively. The EXE countermeasure significantly reduced both SVG and SA changes compared to CTRL. Tmax presented the same changes, being reduced of 8%±3% in CTRL and only of 4%±2% in EXE. At POST, in both groups SA was restored to PRE values; conversely, while in EXE the SVG returned to PRE values, in CTRL it was found still reduced by 3%. Also, while Tmax was back to PRE values in EXE, it was increased by 5% in CTRL. Conclusions: Despite the short-term -6° BR immobilization, cardiac adaptation to this deconditioning condition affected ventricular repolarization during the night period, thus widening SA and reducing SVG as result of increased heterogeneity, thus increasing the risk of arrhythmia development. Selective beat averaging allowed quantifying these changes. Application of 25 min/day standing exercise reduced, but not canceled, these effects. This should be taken into account in patients with cardiovascular diseases, when immobilized in bed, to proper adjust the pharmacological therapy or set the rehabilitation training, in order to avoid further complications.