Baroreflex and non-baroreflex modulation of vagal cardiac control after myocardial infarction

Vagal control of sinus node exerted by arterial baroreceptors is markedly impaired 48 hours after acute myocardial infarction (AMI), but it recovers 10 days later. However, it is unknown whether this recovery is peculiar to baroreceptor vagal control or reflects normalization of the overall vagal modulation of heart rate. In 21 untreated patients (aged 51 ± 3 years, mean ± SEM) studied 10 ± 1 and 21 ± 1 days after an AMI and in 13 healthy controls (aged 47 ± 2 years), we examined the increases in RR interval (electrocardiogram) induced by carotid baroreceptor stimulation via a neck chamber and by immersion of the face in iced water for 15 seconds (diving reflex). Both 10 and 21 days after AMI, baseline blood pressure and RR interval values were superimposable to those obtained in controls. Ten days after AMI, the bradycardic responses to carotid baroreceptor stimulation were similar to those seen in controls (maximal RR interval lengthenings: 248 ± 34 vs 270 ± 31 ms, respectively, p = NS) and remained virtually unchanged later. In contrast, the bradycardic response to diving was reduced in patients after AMI compared with controls (maximal RR interval lenghtenings: 203 ± 43 vs 325 ± 52 ms, respectively, p <0.05) and did not improve later. Thus, in AMI recovery of the early impairment of baroreceptor–heart rate control does not reflect normalization of vagal cardiac control, which remains lower than normal values at a time when the baroreflex is restored.