The effects of left ventricular stretch versus cavity pressure on intramyocardial pressure.

OBJECTIVE Since muscles, vessels and interstitial spaces are in close physical proximity in the heart wall, interstitial (i.e., intramyocardial) pressure (IMP) should be affected by the stresses of the vessels and/or the muscular tissue surrounding the interstitial spaces. Thus, we tested the hypothesis that increasing the stresses (or stiffness) of the surrounding tissues by muscle contraction or stretching--produced externally by stretching the LV cavity or internally by increasing coronary perfusion pressure--has a greater effect than LV cavity pressure per se on IMP. METHODS In isolated rabbit hearts we measured IMP with small (< 10 microns diam) glass micropipettes while stretching the vessels (by changing coronary perfusion pressure) and the wall (by inflating a balloon in the left ventricle) during the passive state as well as during barium contracture. RESULTS With LV cavity pressure equal to 0 (balloon open to air) or equal to 30 mmHg, a 20 mmHg increase in perfusion pressure increased IMP by 3.6 and 5 mmHg, respectively, in the passive state and by 7.6 and 7.9 mmHg, respectively, in the contracted state. This 30 mmHg increase in LV pressure produced a significant but small (3-5 mmHg) increase in IMP in the passive state but no effect in the contracture state. CONCLUSIONS These results can be explained by a unifying concept in which stretching of the tissues surrounding the intestinal spaces--produced externally by increasing ventricular cavity size or internally by pressurizing vessels--but not LV cavity pressure per se is the major determinant of IMP.