The lateral decubitus position causes both mechanical interference with chest wall motion and a decrease in lung expansion. Specifically, the dependent chest wall is constricted by the operating room table, the lateral movements are limited by the bolsters or the 'beanbag' used to hold the patient in position, whereas the upper expansion is reduced by the weight of the mediastinum. The flexion at the hips causes a theoretical improvement of the diaphragmatic motion, especially in morbidly obese patients. However, the caudad displacement of the abdominal organs is counteracted by the negative effects of general anesthesia and muscle relaxation. The final result is a gravity-induced mismatch of ventilation and perfusion while in the lateral position, with redistribution of blood flow toward the dependent lung, which is compressed by the mediastinal contents, resulting in an increase in atelectasis and hypoventilation. In the awake, spontaneously breathing subject, the lower (dependent) diaphragm is still able to contract more efficiently, maintaining ventilation in the dependent lung and matching the increased perfusion. In contrast, when the patient is anesthetized, with or without paralysis, most of the ventilation is preferentially switched to the more compliant upper lung, whereas perfusion increases in the lower lung. The result is an increased degree of ventilation/perfusion mismatch.