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Impaired T-wave amplitude adaptation to heart-rate induced by cardiac deconditioning after 5-days of head-down bed-rest

Abstract:

The study of QT/RR relationship is important for the clinical evaluation of possible risk of acquired or congenital ventricular tachyarrhythmia, pbkp_redisposing to life-threatening arrhythmias. We hypothesised that microgravity exposure could induce changes in the repolarization mechanisms, and thus in the QT/RR relationship and repolarization heterogeneity, with potential effects on increasing the risk of arrhythmia susceptibility. Accordingly, our aim was to test if even a short 5-days strict 6c head-down bed-rest (HDBR) manoeuvre could induce alterations on the QT/RR relationship and repolarization heterogeneity. 12 healthy men (mean age 35±3) were enrolled at MEDES (Toulouse, France) 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 HDBR (HDT5), and four days after its conclusion (POST). The night period (23:00-06:30) was selected for analysis. X,Y,Z leads were derived using inverse Dower matrix and vectorcardiogram computed. Selective beat averaging was used to obtain averages of P- QRS-T complexes preceded by the same RR (10 msec bin amplitude, in the range 900-1200 msec). For each averaged waveform (i.e., one for each bin), T-wave maximum amplitude (Tmax), T-wavc area (Tarea), RT-apex and -end were measured. Also, the ventricular gradient (VG) magnitude and the spatial QRS-T angle, reflecting the difference in directions of propagation of depolarization and repolarization, were computed. Non-parametric Friedman test was applied to evaluate the effect of HDBR on the computed parameters among timepoints (PRE, HDT5 and POST); to study the relationship with RR, median values for each bin among all subjects were linearly correlated, and Pearson coefficient computed. Changes both in QT temporal, amplitude, VG and QRS-T angle parameters were detected showing that, despite the short duration HDBR, alterations in the cardiac electrophysiology were present. In particular, the decrease in VG together with the increase in QRS-T angle found at HDT5 underlines augmented repolarization heterogeneity, potentially associated with risk of life-threatening arrhythmias. Also, a marked loss in strength of the linear regression with RR, and a reduced slope, was found at HDT5 in Tmax and Tarea parameters, evidencing and impaired T-wave amplitude adaptation to heart-rate compared with PRE, that could represent a new dynamic marker of increased risk for life-threatening arrhythmias. Additional studies conducted on longer periods (2Id and 90d) could confirm what observed in this short-term HDBR. ©(2012) by the International Astronautieal Federation.