For breathing/oxygenation in a unilateral pleural effusion, place the patient with the unaffected (good) lung down and the affected effusion side up.

Example: right pleural effusion -> left lateral position (left side down).

This directs more blood flow to the better-ventilated lung. A semi-Fowler position may also help relieve dyspnea.

For a thoracentesis, the usual position is sitting upright and leaning slightly forward.

For a unilateral pleural effusion, place the patient with the affected side down (lateral decubitus) if the goal is to improve oxygenation.

Example: right pleural effusion -> right side down.

This can reduce blood flow to the poorly ventilated, fluid-compressed lung and improve ventilation-perfusion matching. For dyspnea, a semi-Fowler or upright position is often also more comfortable.