QT rate dependence in norm and myocardial infarction

To assess QT/RR relationship in healthy subjects and in patients with acute myocardial infarction (AMI) using long term 12-lead ECG monitoring.We studied separately diurnal and nocturnal QT interval rate-dependence using a linear model "QT=beta+alphaxRR" in 42 patients with AMI on 2-nd week of the disease and in 14 healthy volunteers. All necessary parameters were automatically calculated from 24-hour 12 lead ECG recordings using special programs. Myocardial perfusion and infarct size were assessed using 99mTc-MIBI gated SPECT.Long-term ECG monitoring permitted: 1) to detect the ECG lead with most pronounced QT/RR relationship delineated by alpha(max); 2) to determine value of "deltaalpha=alpha(max)-alpha(min)", that characterized spatial heterogeneity of QT/RR relationship. In norm diurnal values of alpha(max) were significantly higher than nocturnal ones (0.207+/-0.037 vs. 0.151+/-0.035, respectively; p<0.01). The alpha(max) values correlated with the size of MI (r=0.42-0.43, p<0.01). Therefore in patients with anterior AMI who demonstrated significantly larger defects of 99mTc-MIBI uptake than patients with inferior AMI, diurnal mean value of amax remained high despite therapy with b-blockers (0.225+/-0.037), and nocturnal alpha(max) value significantly exceeded respective amax value in healthy subjects (0.216+/-0.097 vs 0.151+/-0.035, respectively; p<0.01). Patients with AMI demonstrated an increased mean Da value when compared to normal subjects (in anterior AMI 0.098+/-0.057 vs. 0.039+/-0.014, respectively; p<0.001--diurnal period; 0.104+/-0,042 vs. 0.039+/-0.022, respectively; p<0.001--nocturnal period). In anterior AMI Da values correlated with infarct size (r=0.62, p=0.001).A new approach to the assessment of QT rate dependence with the use of 12-lead Holter monitoring widens possibilities of the method, firstly, because of selection of most informative ECG lead that shows maximal QT/RR relationship dynamic, and, secondly due, to discovery of the new field of ventricular repolarization research by assessment of space dispersion of QT/RR relationship. In patients with AMI when compared with healthy subjects we observed a decrease of QT/RR circadian modulation, increase of the slope and space dispersion of QT/RR rate dependence.