Beat-to-beat variability of QRS amplitude and heart rate in ischemic heart disease : diagnostic potentials

Abstract

The diagnostic potential of beat-to-beat QRS amplitude variability in patients with coronary artery disease (CAD) was evaluated with variance ECG and compared to exercise stress test and myocardial scintigraphy (SPECT). Two high prevalence populations of 199 and 166 patients each were investigated. The discriminative accuracy of the different methods was assessed employing receiver operating curves constructed by successive consideration of several variance ECG generated CAD index cut point values and various threshold criteria based on ST segment depression with or without exertional chest pain.

The CAD index provided a significant discrimination which was matched by exerse stress test only when chest pain variable was added to ST segment depression as a discriminating criterion. Overall pathological findings at SPECT was better identified by the CAD index than by symptom limited exercise stress test. While the index was strongly diagnostic for persistent perfusion defects or combined defects, exer se stress test was diagnostic for transient defects only. Relative to pathology at coronary angiography, the CAD index had a diagnostic capa ty at least as good as that of SPECT and better than that of exer se stress test. Beat-to-beat QRS amplitude variability during dobutamine provoked myocardial ischemia was investigated in 15 patients with angiographically verified CAD. Compared to healthy controls, dobutamine infusion induced a significant increase in variability in the patients, as a sign of electrical instability of the myocardium.

Variance ECG was evaluated six months after myocardial infarction (AMI)(n=73) and after coronary by pass grafting (CABG) (n=56). Patients with AMIand initially low index values increased their index. In patients subjected to CABG, the CAD index increased significantly during the six months which was prominent and reversible in character in patients with initial low index.

The changes in CAD index following ischemic cardiac events may provide new insights into the dynamics of ischemic heart disease. Beat-to-beat variability of heart rate (750 beats) was recorded in healthy individuals and in patients with recent AMI, in the basal state and afterautonomic blockade. The variability was analysed in time- and frequency domain (linear methods) together with quantitative measurements of Lyapunov exponent, correlation and fractal dimension and qualitative assessment by the evolution in phase space (nonlinear methods). Heart rate variability was found to exhibit characteristics of low dimensional chaos. Patients with AMI showed significantly less complex behaviour than healthy controls but the difference was eliminated by atropine, suggestive of vagal toneas a major cause. Heart rate variability and its modulation can be described equally by linear and nonlinear methods. Nonlinear methods are deterministic thus possessing a potential to predict and control electrical instability.