Pressure-length loop area: Its components analyzed during graded myocardial ischemia

The changes in total pressure-length loop area were compared with changes in effective shortening area, systolic lengthening area and postsystolic shortening area (defined with respect to enddiastolic and end-systolic lengths) of the pressure-length loop during myocardial ischemia in seven anesthetized dogs instrumented for measurement of left ventricular pressure and regional segmental wall motion (sonomicrometry) in the minor axis of the apical region of the left ventricle.Ischemia was induced by gradual tightening of a micrometercontrolled snare around the left anterior descending coronary artery, which supplied the apical myocardium. Data were obtained at normal flow, after critical constriction (loss of pulsatile coronary flow), mild ischemia (ischemia 1: onset of regional dysfunction, i.e., postsystolic shortening and mild hypokinesia) and moderate ischemia (ischemia 2: marked hypokinesia). At each stage, acute afterloading was performed by partially occluding the descending thoracic aorta.The pressure-length loops were analyzed in terms of four areas: total loop area, effective shortening area, postsystolic shortening area and systolic lengthening area. Total loop area decreased only when marked hypokinesia was present (176 ± 18.3 mm Hg x mm at ischemia 2 versus 245.1 ± 26.9 mm Hg × mm at ischemia 1, p < 0.05). However, effective shortening area (98.2 ± 0.8% of total loop area at baseline; 93.8 ± 2.4% at critical constriction; 76.3 ± 7.2% at ischemia 1; 51.9 ± 12.2% at ischemia 2) and postsystolic shortening area (1.8 ± 0.8% of total loop area at baseline; 5.2 ± 1.9% at critical constriction; 14.3 ± 3/4% at ischemia 1; 23.8 ± 5.1% at ischemia 2) changed significantly with each progressive stage of ischemia. Afterloading significantly decreased total loop area at all stages compared with normal loading, whereas effective shortening and postsystolic shortening areas changed only during ischemia (ischemia 1 and ischemia 2).Thus, the individual component areas of the pressure-length loop, in particular the effective shortening area, are more sensitive to ischemic changes than is the total loop area