NUMERIC PROCESSING OF LORENZ PLOTS OF R-R INTERVALS FROM LONG-TERM ECGS - COMPARISON WITH TIME-DOMAIN MEASURES OF HEART-RATE-VARIABILITY FOR RISK STRATIFICATION AFTER MYOCARDIAL-INFARCTION

The so-called ''Lorenz plots'' are scatterplots that show the R-R inte rval as a function of the preceding R-R intervals. Repeatedly, it has been proposed chat these plots might be used for visualizing thp varia bility, of the heart rate and that the assessment of heart rare variab ility (HRV) from these plots might be superior to conventional measure s of HRV. However, a precise numeric evaluation of the images of Loren z plots has never been suggested. To classify the images of Lorenz plo ts, a computer package that measures their density was developed. For each rectangular area of the plot, the relative number of R1/R2 sample s in that area is established and a function is created that assigns t he maximum relative number of samples (ie, the maximum density) to eac h size of an area of the plot. Plots that are very compact result in a sharply failing density function, while plots that are more diffuse l ead to a flat density function. The distinction between such types of density function may be expressed as a logarithmic integral of the den sity function to express the ''compactness'' of the plot numerically. As the computational demands of this approach are intensive, an approx imate method that restricts the measurement of the density to the area around the peak of the plot was also developed. The results of this a pproximate method correlate strongly with the full results (r = .98), and approximate measurement of one plot requires less than 1 minute of computer time. The approximate method has been applied to a set of 24 -hour Holter records obtained from 637 survivors of acute myocardial i nfarction. For each record, the SDNN and SDANN values were also calcul ated as conventional measures of HRV. Both the density of the Lorenz p lots and the conventional measures of HRV were used to investigate the differences among 48 patients who suffered an arrhythmic event (sudde n death or sustained symptomatic ventricular tachycardia) during a 2-y ear follow-up period and the remaining 589 patients without arrhythmic postinfarction complications. At a sensitvity of 30%, the Lorenz plot density distinguished the patients with events with a positive predic tive accuracy of 58%, while the SDNN and SDANN led to a positive predi ctive accuracy of only 23 and 18%, respectively. Thus, a detailed anal ysis of Lorenz plots is feasible and more clinically useful than the c onventional measures of HRV.