Heart period variability provides useful prognosticinformation on autonomic cardiac control, and a strong association has been demonstrated after myocardial infarction (MI) between cardiac mortality, sudden death, and reduced total power, ultralow-frequency (ULF) power, and very-lowfrequency (VLF) power. Converting enzyme inhibitors are widely used in MI patients, but their influence on heart period variability emains to be defined.Methods and ResultsTime- and frequency-domain measures of heart period variability were calculated from 24-hour Holter monitoring in 40 patients with a first uncomplicated MI. After baseline examination between 48 and 72 hours after symptom onset, patients were randomly assigned to placebo or captopril administration, and on the third day, 24- hour Holter monitoring was repeated. No changes in time and frequency domain were detectable after placebo. After captopril, the SD of all normal RR (NN) intervals (SDNN) increased from 90±29 to 105±30 milliseconds (P< .01); the SD of the average NN intervals for all 5-minute segments (SDANN index) and the mean of the SDsof all NN intervals for all 5-minute segments (SDNN index) also increased from 74±24 to 90±26 milliseconds (P< .01) and from 45±17 to 49±15 milliseconds (P< .05), respectively. The root mean square successive difference (r-MSSD) and the percent of differences between adjacent NN intervals >50 milliseconds (pNN50) remained unchanged. In regard to frequency-domain measures, after captopril, total power (ln unit) increased from 8.28±0.42 to 8.47±0.30 (P< .01); considering the frequency bands, a significant increase was observed in ULF (P< .01), VLF (P< .05), and low-frequency (LF) power (P< .05), whereas high-frequency (HF) power remained unchanged.ConclusionsThis study supports the hypothesis that the renin-angiotensin system modulates the amplitude of ULF and VLF power. Furthermore, it demonstrates that in MI patients, converting enzyme inhibition favorably modifies measures of heart period variability strongly associated with a poor prognosis.