Higuchi´s fractal complexity of RR and QT interval series during transient myocardial ischemia

Myocardial ischemia may lead to significant changes in autonomic control of heart rate (HR) affecting its variability and alter beat-to-beat ventricular repolarization periods. We hypothesized that transient myocardial ischemia affects the complex dynamics of the HR and QT. The aim of this study was to assess the RR and QT interval time series complexity using Higuchi´s fractal dimension (HFD) during prolonged balloon occlusion of one of the major coronary arteries. Eighty-five patients who underwent elective percutaneous coronary intervention procedures were selected. Patients were classified into 2 groups according to the presence of prior healed myocardial infarction (INF) (n = 29) or not (No_NF) (n = 56). RR, QT and QTc (Bazzet´s formula) time series were obtained from continuous ECG. Three 3-minute stages were chosen: (1) preinflation as baseline (pre), (2) from the start of occlusion (pci), and (3) immediately post deflation (post..ci). HFD was calculated on each 3-minute stage time series to quantifY the changing complexity and self-similarity of RR, QT and QTc time series (HFD RR, HFD QT and HFD QTc, respectively). Cohen´s d statistics were included to measure the effect size of the procedure. HFD values decreased significantly from baseline to pci and post_pci in the three time-series. HFD RR, 1.76 ± 0.13 vs. 1.69 ± 0.15 (p<;0.001, Cohen´s d = -0.64); HFD QT, 1.90 ± 0.11 vs 1.82 ± 0.13 (p<;0.001, d = -0.67); HFD QTc, 1.88 ± 0.09 vs. 1.83 ± 0.11 (p<;0.01, d= -0.46). The HFD reduction was greater from baseline to post _pci (p<;0.001, d = -0.99 (RR), d = -1.02 (QT)). HFD of QT intervals decreases during the procedure predominantly in the No jNF group. The time series studied, related to autonomic control of HR and to the variability of ventricular repolarization, exhibit a reduced complexity provoked by transient myocardial ischemia.