Cardiac arrhythmias in acute myocardial infarction. I. Complete heart block and its natural history

The clinical significance of atrial pacing-induced Mobitz I atrioventricular block in patients with coronary artery disease

Two hundred consecutive patients with a history of chest pain undergoing cardiac catheterization, coronary angiography and atrial pacing have been analyzed to assess the clinical significance of Mobitz I atrioventricular (A-V) block developing with the stress of atrial pacing. Of 160 patients with coronary artery disease, 26 (16%) developed Mobitz I A-V block at rates below 140 beats/min. Eighteen of these 26 patients (69%) had electrocardiographic evidence of old inferior wall myocardial infarction, compared to only 34 of the remaining 134 patients (29%) (P = less than 0.01). During the mean follow-up of 29.5 months (range 18-50 months) none of the 26 patients has developed spontaneous second or third degree A-V block. Twenty-three of the 26 patients had an exercise test within one week of the pacing study. No A-V block was noticed during or immediately following exercise, although the mean heart rate attained during exercise was higher than the mean pacing rate at which the A-V block occurred (136 +/- 5 vs 122 +/- 3, P = less than 0.01). Eighteen of these 23 achieved heart rates equal to or higher than the pacing rate at which A-V block developed. Nineteen (83%) shortened their P-R interval during exercise and 4 (17%) did not change the P-R length. Although atrial pacing-induced Mobitz I A-V block may indicate a latent A-V nodal conduction abnormality in some cases, its demonstration does not necessarily predict the occurrence of spontaneous advanced A-V block. Exercise should not be restricted in these patients on the basis of such a finding during a pacing study.

Site of myocardial infarction. A determinant of the cardiovascular changes induced in the cat by coronary occlusion

The influence of site of acute myocardial infarction on heart rate, blood pressure, cardiac output, total peripheral resistance (TPR), cardiac rhythm, and mortality was determined in 58 anesthetized cats by occlusion of either the left anterior descending (LAD), left circumflex or right coronary artery. LAD occlusion resulted in immediate decrease in cardiac output, heart rate, and blood pressure, an increase in TPR, and cardiac rhythm changes including premature ventricular beats, ventricular tachycardia, and occasionally ventricular fibrillation. The decrease in cardiac output and increase in TPR persisted in the cats surviving a ventricular arrhythmia. In contrast, right coronary occlusion resulted in a considerably smaller decrease in cardiac output. TPR did not increase, atrioventricular condition disturbances were common, and sinus bradycardia and hypotension persisted in the cats recovering from an arrhythmia. Left circumflex ligation resulted in cardiovascular changes intermediate between those produced by occlusion of the LAD or the right coronary artery. Mortality was similar in each of the three groups. We studied the coronary artery anatomy in 12 cats. In 10, the blood supply to the sinus node was from the right coronary artery and in 2, from the left circumflex coronary artery. The atrioventricular node artery arose from the right in 9 cats, and from the left circumflex in 3. The right coronary artery was dominant in 9 cats and the left in 3. In conclusion, the site of experimental coronary occlusion in cats is a major determinant of the hemodynamic and cardiac rhythm changes occurring after acute myocardial infarction. The cardiovascular responses evoked by ligation are related in part to the anatomical distribution of the occluded artery.

Heart block complicating acute inferior wall myocardial infarction

Heart block was noted in 60 (35 complete and 25 second-degree) of 410 patients with acute inferior wall myocardial infarction. This group with heart block was compared to a control group of 30 patients with acute inferior wall infarction without heart block. The incidences of prior myocardial infarction and hypertension, in addition to the highest level of serum creatine phosphokinase and a maximum degree of ST-segment elevation in the inferior leads, were all greater in patients with heart block, as compared to the controls. The incidences of various complications, including dizziness and syncope, transient hypotension, cardiogenic shock, and congestive heart failure, were also higher in the group with heart block, while sinus nodal distrubances and atrial arrhythmias occurred with equal frequency. The mortality in those with heart block was 28 percent compared to 13 percent for the control. It is concluded that patients with heart block complicating acute inferior myocardial infarction have a greater amount of myocardial necrosis, a higher incidence of complications, and a higher mortality. Insertion of a temporary pacemaker should be considered when specific indications are present and not routinely.

Significance of high degree atrioventricular block in acute posterior myocardial infarction. The importance of clinical setting and mechanism of block

This report evaluates the morbidity and mortality, during hospitalization and follow-up, of a subgroup of patients with posterior or diaphragmatic myocardial infarction (PDMI) who developed high degree A-V block via a type I mechanism and in the absence of power failure (pulmonary edema or cardiogenic shock). This subgroup was not at any higher risk of hospital morbidity, hospital mortality, or 1-year mortality than three other groups: (a) patients with PDMI but neither high degree A-V block nor initial power failure; (b) patients with other infarct sites who developed high degree A-V block in the absence of power failure; and (c) patients with other infarct sites but neither high degree A-V block nor initial power failure. The significance of subgrouping patients with high degree A-V block by the quantity of clinical heart failure is exemplified by a review of the literature and the present study.