To test thehypothesis that in the failing volume-overloaded ventricle, theextracellular matrix and fibrillar collagen in particular are major determinants of the architectural remodeling of the myocardium, this histopathological study of the dilated, postmortem canine left ventricle secondary to rapid ventricular pacing or aortocaval fistula was undertaken. Using the picrosirius-polarization technique to enhance collagen birefringence, we sought to examine the structural integrity of the collagen matrix and interstitium. In the dilated failing ventricle secondary to rapid pacing, we found 1) interstitial edema and a disruption or disappearance of collagen fibers that were apparent within 6 hours of pacing, persisted for weeks, and subsequently were associated with muscle fiber disorganization within the endomyocardium, 2) interstitial fibrosis that was present in the midwall and epimyocardium with chronic pacing, and 3) an early remodeling of intramyocardial coronary arteries that included medial swelling with smooth muscle degeneration followed by proliferative lesions involving fibroblasts and a subsequent perivascular and medial fibrosis. Many of these findings were still evident 48 hours after pacing had been discontinued. In contrast, the collagen matrix and interstitium seen with ventricular dilatation secondary to the circulatory overload that accompanies an aortocaval fistula were indistinguishable from that in sham-operated controls. Thus, we conclude that unlike the chamber enlargement and preserved ventricular function that accompany an aortocaval fistula, ventricular dilatation and failure caused by rapid pacing are based on an architectural remodeling of the myocardium. This structural dilatation involves the extracel-lular matrix and interstitium and appears to be related to altered permeability of intramyocardial coronary arteries. The mechanism or mechanisms involved in the pathogenesis of myocardial remodeling with rapid ventricular pacing require further investigation.(Circulation 1990; 82: 1387-1401)

It has been proposed12 that the failure of the dilated left ventricle is based on a thinning of its myocardium mediated by muscle fiber slippage. More recently, Weber3 suggested that the anatomical requisite for slippage in either eccentric or concentric hypertrophy resides in the disruption of collagen tethers that normally maintain muscle fiber align-ment. 4 A disruption and disappearance of collagen