Relationship between baseline sympatho-vagal balance and the autonomic response to cardiac rehabilitation after a first uncomplicated myocardial infarction

Background: After a first uncomplicated myocardial infarction, cardiac rehabilitation increases the parasympathetic tone, in a direction linked to a reduced risk of sudden cardiac death. This change in sympatho-vagal balance may be related to other clinical variables. The aim of this study was to define the factors implicated in determining the autonomic response to cardiovascular rehabilitation after myocardial infarction. Methods: In 55 patients (39-80 years) we evaluated the modulation of the autonomic profile induced by 8 weeks of rehabilitation: we analyzed the changes in pNN50 derived from time-domain analysis (ΔpNN50) and in the low frequency/high frequency (LF/HF) ratio derived from autoregressive power spectral analysis (ΔLF/HF). A control group of 15 patients not undergoing rehabilitation was also studied. Variables considered at 4 weeks postinfarction and related to ΔpNN50 and ΔLF/HF ratio were: age, site of myocardial infarction, previous thrombolysis, ejection fraction, stress test duration, baseline LF/HF ratio. Results: Patients not undergoing rehabilitation did not change their autonomic profile. On the contrary, rehabilitation induced a higher vagal tone (pNN50 from 6.5 ± 1.5 to 16.2 ± 3.1; LF/HF ratio from 8.3 ± 5.2 to 5.1 ± 2.9, p <0.05). Eleven patients (20%) had baseline LF/HF ratio exceeding the mean value by 1.5 SD (19.4 ± 1.4): in this subgroup, pNN50 was very low. In these patients, rehabilitation increased pNN50 and decreased LF/HF ratio. Indeed, both ΔpNN50 and ΔLF/HF ratio were significantly related to their baseline values (p <0.001), even considering thrombolysis, site of myocardial infarction, age, and β-blocker therapy. Conclusions: After a first uncomplicated myocardial infarction, sympatho-vagal balance may be very disturbed in some patients, despite a preserved ventricular function, good exercise capability and β-blockers. These patients should be encouraged to undergo rehabilitation, since the significant improvement in the parasympathetic tone may protect them against subsequent arrhythmic events.