Ultrasound examination of pleural effusion is quantitative and is generally classified as small, moderate and large. A small amount of pleural effusion presents as a long, echogenic zone between the base of the lung and the diaphragm, or a triangular-shaped echogenic zone in the posterior rib-diaphragmatic angle, which varies in shape and width with changes in breathing and body position. A moderate pleural effusion refers to the upper boundary of the fluid dark zone, which usually does not exceed the level of the sixth posterior rib. Pleural fluid extends upward beyond the angle of the rib diaphragm and compresses the lower lobe of the lung. The extent of the fluid dark zone increases and its depth widens, and the depth and extent of the fluid dark zone change with respiration and postural changes. Massive pleural effusion means that the upper boundary of the fluid dark zone exceeds the level of the sixth posterior rib, the lung is pressed partially or completely toward the mediastinum of the pulmonary hilum, the volume of the lung becomes smaller, the diaphragm is shifted downward, and the heart is also shifted toward the healthy side; at this time, changes in respiration and body position have little effect on the pleural effusion.