Plasma endogenous opioid levels in acute myocardial infarction patients, with and without pain.

Plasma levels of beta-endorphin, met-enkephalin and dynorphin were assessed in acute myocardial infarction (AMI) patients, with and without pain (group I: no pain, N = 12; group II: severe pain, N = 16). Plasma opioid peptide concentration was measured on admission to hospital (between 1 and 3 h after the myocardial infarction onset), at 7, 12, 24 h and at 2, 3 and 4 days. A transient increase in plasma beta-endorphin levels was found in AMI patients with severe pain, the levels normalizing within 12-18 h when pain had ceased. No changes in beta-endorphin concentration were observed in AMI patients without pain. Compared with healthy subjects, low levels of met-enkephalin were found in both groups of AMI patients throughout the study. Low levels of dynorphin were observed in patients with no pain while in the other patients initial low levels of dynorphin normalized when pain ceased. Blood pressure, heart rate and central venous pressure values were normal and did not correlate with plasma opioid levels. The results suggest that endogenous opioids do not affect pain in the early phase of myocardial infarction. The rise in beta-endorphin concentration observed in patients with severe pain seems to be induced by pain stress.