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

Vagal control of sinus node exerted by arterial baroreceptors is markedly i mpaired 48 hours after acute myocardial infarction (AMI), but it recovers 1 0 days later. However, it is unknown whether this recovery is peculiar to b aroreceptor vagal control or reflects normalization of the overall vagal mo dulation of heart rate, In 21 untreated patients (aged 51 +/- 3 years, mean a SEM) studied 10 +/- 1 and 21 +/- 1 days after an AMI and in 13 healthy c ontrols (aged 47 +/- 2 years), we examined the increases in RR interval (el ectrocardiogram) induced by carotid baroreceptor stimulation via a neck cha mber and by immersion of the face in iced water for 15 seconds (diving refl ex). Both 10 and 21 days after AMI, baseline blood pressure and RR interval values were superimposable to those obtained in controls. Ten days after A MI, the bradycardic responses to carotid baroreceptor stimulation were simi lar to those seen in controls (maximal RR interval lengthenings: 248 +/- 34 vs 270 +/- 31 ms, respectively, p = NS) and remained virtually unchanged l ater. In contrast, the bradycardic response to diving was reduced in patien ts 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 c ontrol does not reflect normalization of vagal cardiac control, which remai ns lower than normal values at a time when the baroreflex is restored. (C) 1999 by Excerpta Medica, Inc.