ABNORMALITIES IN BEAT-TO-BEAT DYNAMICS OF HEART-RATE BEFORE THE SPONTANEOUS ONSET OF LIFE-THREATENING VENTRICULAR TACHYARRHYTHMIAS IN PATIENTS WITH PRIOR MYOCARDIAL-INFARCTION

Background Beat-to-beat analysis of RR intervals can reveal patterns o f heart-rate dynamics, which are not easily detected by summary measur es of heart-rate variability, This study was designed to test the hypo thesis that alterations in RR-interval dynamics occur before the spont aneous onset of ventricular tachyarrhythmias (VT). Methods and Results Ambulatory ECG recordings from 15 patients with prior myocardial infa rction (MI) who had spontaneous episodes of sustained VT during the re cording and VT inducible by programmed electrical stimulation (VT grou p) were analyzed by plotting each RR interval of a sinus brat as a fun ction of the previous one (Poincare plot). Poincare plots were also ge nerated for 30 post-MI patients who had no history of spontaneous VT e vents and no inducible VT (MI control subjects) and for 30 age-matched subjects without heart disease (normal control subjects). The MI cont rol subjects and VT group were matched with respect to age and severit y of underlying heart disease. All the healthy subjects and MI control subjects showed fan-shaped Poincare plots characterized by an increas ed next-interval difference for long RR intervals relative to short on es. All the VT patients had abnormal plots: 9 with a complex pattern, 3 ball-shaped, and 3 torpedo-shaped, Quantitative analysis of the Poin care plots showed the SD of the long-term RR-interval variability (SD2 ) to be smaller in all VT patients (52 +/- 14 ms; range, 31 to 75 ms) than in MI control subjects (110 +/- 24 ms; range, 78 to 179 ms, P < . 001) or the normal control subjects (123 +/- 38 ms, P < .001), but the SD of the instantaneous beat-to-beat variability (SD1) did not differ between the groups, The complex plots were caused by periods of alter nating sinus intervals, resulting in an increased SD1/SD2 ratio in the VT group, This ratio increased during the 1-hour period preceding the onset of 27 spontaneous VT episodes (0.43 +/- 0.20) compared with the 24-hour average ratio (0.33 +/- 0.19) (P < .01). Conclusions Reduced long-term RR-interval variability: associated with episodes of beat-to -beat sinus alternans, is a highly specific sign of a propensity for s pontaneous onset of VT, suggesting that abnormal beat-to-beat heart-ra te dynamics may reflect a transient electrical instability favoring th e onset of VT in patients conditioned by structurally abnormal hearts.