We sought to determine if damage to the myocardial collagen network was the cause of infarct expansion.

Using polarized light microscopy, we examined sections from rat hearts obtained at 1, 2, 3, and 4 days after permanent coronary artery occlusion. Three features of the collagen network likely to be important in resisting infarct expansion were assessed: collagen quality, collagen quantity, and collagen organization. We observed a decrease in the number of normally birefringent collagen fibers in the infarct as early as 1 day after infarction. This decrease correlated significantly with time (r = -0.989, p less than 0.001). In addition, we found that the fewer normally birefringent collagen fibers present, the greater the degree of infarct expansion (assessed by measurement of total left ventricular cross-sectional area). At 4 days after infarction, we noted a loss of intermyocyte collagen struts and loss of interstitial space. These changes coincided with the onset of pronounced infarct expansion. The loss of collagen struts is consistent with the concept that expansion proceeds via slippage of myocytes previously tethered by the struts. The loss of interstitial space may represent the resolution of interstitial edema, which could further weaken the ventricular wall.

The correlation of infarct expansion with collagen damage and the loss of support provided by collagen struts suggest that collagen is important in maintaining structural integrity after acute myocardial infarction.