Carbon monoxide and lethal arrhythmias in conscious dogs with a healed myocardial infarction

Environmental studies suggested that exposure to carbon monoxide (CO) increases cardiovascular mortality among patients with coronary artery disease. We investigated whether, in dogs with a healed anterior myocardial infarction at low and high risk for ventricular fibrillation, acute exposure to CO has adverse effects during acute myocardial ischemia combined with exercise. One month after myocardial infarction, 17 dogs had ventricular fibrillation and 16 survived during the combined exercise and ischemia test. These tests were then repeated in all dogs with different concentrations of carboxyhemoglobin (COHb) (from 5% to 15%). With 15% COHb, heart rate (HR) at rest and during exercise was higher (p less than 0.05) than in the control tests. Surprisingly, the reflex HR response to acute ischemia was also altered; namely, the HR reduction characteristic of the low-risk animals was anticipated and accentuated (-31 +/- 25 versus 2 +/- 30 beats/min, p less than 0.05). Conversely, the HR increase characteristic of the high-risk group was reduced by CO (44 +/- 52 versus 72 +/- 43 beats/min, p less than 0.05). With 15% COHb, malignant arrhythmias occurred in two of the low-risk dogs and in none of the high-risk dogs. In the latter, CO was tested with a combination of exercise work load and myocardial ischemia duration not associated with ventricular fibrillation (VF) in the control condition. This study demonstrated that brief exposure to CO (1) profoundly alters the reflex HR response to exercise and to acute myocardial ischemia and (2) does not enhance the occurrence of malignant arrhythmias in conscious dogs with a healed myocardial infarction.