Significance of long term components of heart rate variability for the further prognosis after acute myocardial infarction

STUDY OBJECTIVE: The study examined heart rate variability to find out whether shorter ECG records can predict long term mortality following acute myocardial infarction as efficiently as 24 h recordings. DESIGN: Heart rate variability was assessed in 24 h electrocardiograms recorded during the first 2 weeks following acute myocardial infarction and in separate 1 h portions of the complete recording. The spectral analysis of complete 24 h records was performed and different short and long term components of heart rate variability were used to distinguish between patients with and without later complications. SUBJECTS: 20 patients who initially survived acute myocardial infarction but later experienced serious events (death or symptomatic sustained ventricular tachycardia) during a 6 month follow up (group I) were compared with 20 patients (group II) who remained free of complications for more than 6 months after discharge and who were matched with group I for age, gender, infarct site, ejection fraction, and beta blocker treatment. MEASUREMENTS AND MAIN RESULTS: The distinction based on components limited to changes of heart rate within periods less than or equal to 1 h was as significant (p less than 0.001, paired t test) as when using the components limited to changes of periods less than or equal to 10 h. However, heart rate variability of separate 1 h portions of the complete 24 h records differed between the groups significantly only for certain 1 h intervals of the day (the p values varied from 0.2 to 0.0005). CONCLUSIONS: Whilst the maximum value of short term heart rate variability is sufficient for stratification of the high risk post-myocardial infarction patients, an arbitrarily selected short term ECG recording is unlikely to register the maximum heart rate variability. It is concluded that the heart rate variability assessed from arbitrary 1 h electrocardiographic records is not as prognostically important as the variability estimated from 24 h recordings.