What occurs in the dependent lung when a patient is anesthetized and placed in the lateral decubitus position?

When a patient is anesthetized and placed in the lateral decubitus position, the dependent lung, which is the lung that is lower in relation to gravity, experiences increased perfusion due to the gravitational effects on blood flow. In this position, the blood vessels in the lower lung are more distended and receive more blood from the systemic circulation.

However, due to the mechanics of ventilation and potential compression of the alveoli in the dependent lung, this region may not be adequately ventilated. Factors such as the position of the diaphragm and the effect of anesthesia on respiratory drive can lead to poor aeration of the dependent lung. This results in a mismatch between perfusion and ventilation, where the dependent lung receives more blood flow but has reduced gas exchange capacity.

Thus, the combination of increased perfusion and compromised ventilation characterizes the physiological changes in the dependent lung when a patient is positioned laterally under anesthesia. These considerations are crucial in understanding ventilation-perfusion relationships during anesthesia and how to best manage patients in this position.