Transmural grandients of experimental myocardial ischemia: Limited correlation of ultrastructure with epicardial S-T segment elevation

Abstract

A standardized preparation was developed to investigate the precise relationship between epicardial S-T segment elevation and myocardial ultrastructure. The distal left anterior descending coronary artery was occluded for 20 minutes and epicardial S-T segments were recorded at five-minute intervals. Immediate perfusion-fixation with glutaraldehyde preserved premortem anatomic relationships and allowed precise sampling of the myocardium immediately underlying sites of S-T segment recording. Ischemic and non-ischemic zones were defined both anatomically and by S-T segment mapping. Transmural samples at 0.2 cm. intervals were compared at sites in ischemic and non-ischemic zones. In non-ischemic zones, S-T segment elevations ranged from 0 to 2 mm., and myocardial ultrastructure was normal. In ischemic zones, S-T segment elevations ranged from 0 to 14 mm., and samples showed a gradient of ischemic injury with greatest change in subendocardial blocks. The extent of ultrastructural change at any point was not consistently proportional to the height of S-T segment elevation. S-T segment elevations greater than 2 mm. were always associated with a marked transmural gradient of change, but S-T segment elevations less than 2 mm. in ischemic zones could also be associated with severe subendocardial and mid-myocardial change. In a second group of dogs the coronary artery snare was released after 20 minutes of occlusion and recovery was allowed for 60 minutes before killing and subjecting to perfusion-fixation. These hearts exhibited no abnormality in myocardial ultrastructure when sampled in the same fashion as the first group. In this model a reproducible gradient of transmural, myocardial ultrastructural change was demonstrated under conditions of reversible injury. Prominent S-T segment elevation in ischemic zones always indicated the presence of underlying ultrastructural change, but marked changes were also present when S-T segment elevations were minimal, indicating that the S-T segment maps tended to underestimate the extent of early ischemic change. We were unable to establish a quantitative relationship between the extent of S-T segment elevation and the extent of transmural ultrastructural change.