Cardiac patients often have sinus arrhythmia of non-respiratory origin [erratic sinus rhythm" (ESR)]. ESR was quantified using hourly Poincaré and power spectral HRV plots from normal-to-normal interbeat intervals (N-Ns) and hourly values of the short-term fractal scaling exponent and correlations of N-Ns in N=60 non-survivors and N=66 randomly-selected survivors in the Cardiac Arrhythmia Suppression Trial. Hours were coded (ABN) as normal (0), borderline (0.5) or ESR (1). T-tests compared ABN for N=2413 paired hours at baseline and on therapy. ABN was higher in nonsurvivors (0.38 ± 0.44 vs. 0.28 ± 0.40, baseline and 0.51 ± 0.45 vs. 0.34 ± 0.43, on therapy, p<0.001). Increased ABN with treatment was greater in non-survivors. Normal hours at baseline (RR=0.77, 095% CI=0.62−0.96, p=0.018) and on treatment (RR=0.47, 95%CI=0.39−0.58) were significantly associated with decreased mortality compared to ESR. Quantification of ESR may identify more vulnerable patients or help monitor the effects of pharmacological treatment. Keywordsheart rate; risk factor; mortality; post-MI; anti-arrhythmic The analysis of heart rate variability (HRV) provides information about cardiac autonomic function. However, we have found that many older subjects and patients with cardiovascular disease have an increased beat-to-beat variability that is irregular and does not reflect underlying respiratory sinus arrhythmia (1). We have called this erratic rhythm. The presence of erratic rhythm, which is frequently episodic, increases values for short-term HRV indices such as rMSSD [root mean square of successive differences of normal-to-normal (N-N) interbeat intervals in ms], pNN50 [the % of N-N intervals >50 ms different from the previous interval) and high frequency power (the amount of variance in N-N intervals at respiratory frequencies, i.e., 0.15−0.4 Hz) that quantify beat-to-beat variability. Thus, in the presence of erratic rhythm, increased short-term HRV does not, as it does for younger people, necessarily reflect greater parasympathetic control of heart rate. A high prevalence of erratic rhythm may also explain why HRV measures that are believed to reflect parasympathetic function have relatively little predictive value for outcomes. At the same time, some "non-linear" HRV measures like the short-term fractal scaling exponent, (DFA1), and (SD12), the ratio of the axes of an ellipse fitted to the Poincaré plot do capture the presence of erratic rhythm (2,3).