Regional myocardial function at the papillary muscle insertion site

The importance of the mitral apparatus to the global left ventricular (LV) function has been suggested in several clinical studies. One recent study reported that chordal transsection induced an unloading of myocardium at the papillary muscle insertion site. We hypothesized that the regional response for afterloading at this site with intact mitral apparatus was different from that at the free wall. We investigated the end-systolic pressure-regional segment length relations (ESPLR) in two anterior LV sites, free wall (FREE) and the papillary muscle insertion site (PAP), during an increasing afterload by aortic occlusion in 7 anesthetized open-chest dogs. To measure the regional segment length at FREE and PAP, two sets of the pair of sonomicrometer crystals were implanted in the same midwall depth at the same circumferential hoop by using an echocardiographic guide. ESPLR both at FREE and PAP were always highly linear in a physiological range (r > or = 0.9). The slope of this relation at FREE (274 +/- 164 mmHg/mm) was significantly steeper than that at PAP (157 +/- 118 mmHg/mm) for each dog (p < 0.05). These data indicate that the regional response for afterloading at PAP loaded by chordal tension is different from that at FREE in the same heart.