Mechanisms of Retarded Apical Filling in Acute Ischemic Left Ventricular Failure

Abstract

Background —We examined the hypothesis that retardation of apical filling as measured by color M-mode Doppler echocardiography in the diseased left ventricle (LV) reflects a decrease in the intraventricular mitral-to-apical pressure gradient. Methods and Results —In 9 open-chest anesthetized dogs, micromanometers were placed near the mitral tip and in the apical region. From the color M-mode Doppler images, the time delay (TD) between peak velocity at the mitral tip and the apical region was determined as an index of LV flow propagation. Acute ischemic LV failure was induced by coronary microembolization. Induction of ischemia caused a marked increase in LV end-diastolic pressure and a decrease in LV ejection fraction. The time constant of LV isovolumic apical pressure decay (τ) increased from 31±8 to 49±16 ms ( P <0.001). The peak early diastolic mitral-to-apical pressure gradient (ΔPLV mitral-apex ) decreased from 1.9±0.9 to 0.7±0.5 mm Hg ( P <0.01), and TD increased from 5±3 to 57±26 ms ( P <0.001). The slowing of flow propagation was limited to the apical portion of the LV cavity. The TD correlated with ΔPLV mitral-apex ( r =−0.94, P <0.01) and with τ ( r =0.92, P <0.01). Before ischemia, the mitral-to-apical flow propagation velocity far exceeded the velocity of the individual blood cells, whereas during ischemia, flow propagation velocity approximated the blood velocity. Conclusions —Retardation of apical filling in acute ischemic failure was attributed to a decrease in the mitral-to-apical driving pressure, reflecting slowing of LV relaxation. The slowing of flow propagation appeared to represent a shift in apical filling from a pattern of column motion to a pattern dominated by convection.