Imbalances in autonomic nervous system function have been posed as a possible mechanism that produces ventricular fibrillation and sudden cardiac arrest in patients with cardiovascular disease. Heart rate (HR) variability is described in survivors and nonsurvivors of sudden cardiac arrest within 48 hours after resuscitation using time and frequency domain analytic approaches. HR data were collected using 24-hour ambulatory electro-cardiograms in 16 survivors and 5 nonsurvivors of sudden cardiac arrest, and 5 control subjects. Survivors of sudden cardiac arrest were followed for 1 year, with recurrent cardiac events occurring in 4 patients who died within that year. Analysis of 24-hour electrocardiograms demonstrated that control subjects had the highest HR variability (standard deviation of all RR intervals = 155.2 +/- 54 ms), with nonsurvivors demonstrating the lowest HR variability (standard deviation of all RR intervals = 52.3 +/- 6.1 ms) and survivors of sudden cardiac arrest falling between the other 2 groups (standard deviation of all RR intervals = 78 +/- 25.5 ms, p less than or equal to 0.0000). Two other indexes of HR variability (mean number of beat to beat differences in RR intervals greater than 50 ms/hour and root-mean-square of successive differences in RR intervals) did not demonstrate the expected pattern in this sample, indicating that perhaps patterns of HR variability differ between groups of patients with cardiovascular disorders. Spectral analytic methods demonstrated that survivors of sudden cardiac arrest had reduced low- and high-frequency spectral power, whereas nonsurvivors demonstrated a loss of both low- and high-frequency spectral power.(ABSTRACT TRUNCATED AT 250 WORDS)