Left atrial booster pump function produces variable effects on cardiac output. Generally, cardiac output decreases by only 15-20% when atrial fibrillation occurs, however, in some cases, hemodynamic collapse occurs through loss of left atrial contraction. We evaluated the relative significance of left atrial booster pump function in acute or chronic load and in myocardial ischemia using the left ventricular volume curve. Blood entering into the left ventricle during the left atrial contraction phase (FVLA) represents the left atrial volume work, and the ratio of FVLA to the left ventricular filling volume during one cardiac cycle (%FVLA) represents the relative significance of left atrial booster pump function in cardiac output. In dog experiments, we calculated the change in FVLA and %FVLA by measuring the the left ventricular internal minor axis diameter and using Pombo's method. We also measured the change of the left atrial segment length as a direct indicator of left atrial contraction. In the acute change in preload, FVLA changed with stroke volume, but %FVLA remained unchanged. The change in FVLA correlated with the direct indicator of the left atrial excursion; the extent of the left atrial segment length (LASL). During acute change of left ventricular afterload, both FVLA and %FVLA were unchanged. In regional myocardial ischemia, both FVLA and %FVLA were increased, suggesting an increase in the left atrial booster pump function. In clinical study, we calculated FVLA and %FVLA from the left ventricular diameter using M-mode echocardiography. In chronic volume overloading (aortic regurgitation), FVLA increased while %FVLA was maintained unchanged. The same FVLA-%FVLA relationship was observed in acute volume loading. In cases of left ventricular hypertrophy (LVH) and old myocardial infarction (MI), both FVLA and %FVLA were increased, suggesting the increased left atrial booster pump function. In these cases, the left ventricular rapid filling velocity decreased, suggesting that impairment of rapid filling caused the increase of left atrial preload and hence increased left atrial volume work. The results of this study show that in old MI and in LVH, both left atrial volume work and the relative significance of left atrial booster pump function increase. We concluded that prevention of atrial fibrillation may be very important in these diseases.