The most appropriate management is to perform needle aspiration of the pneumothorax. This patient has a primary spontaneous pneumothorax, because it occurred in the absence of trauma with no clinically apparent lung disease. He is clinically stable but has breathlessness associated with a large pneumothorax (>2 cm from the lung margin to the chest wall at the level of the hilum). High-flow supplemental oxygen is usually given to most patients with pneumothorax to facilitate absorption of the pleural air. Emergent needle thoracostomy followed by thoracostomy tube placement is indicated in patients with a large, hemodynamically significant (tension) pneumothorax. However, in patients such as this one with a large pneumothorax that is symptomatic but stable, it is reasonable to perform needle aspiration of the pneumothorax. If the lung expands after aspiration, the patient may be appropriately observed for several hours with reimaging and discharge if stable. Placement of a one-way (Heimlich) valve is another option with outpatient follow-up.

Insertion of a small-bore thoracostomy tube (<14 Fr [4.7 mm]) is indicated as initial treatment for larger (>2 cm) secondary pneumothoraces in patients who are symptomatic, but it would not be a recommended initial treatment for this patient with a primary pneumothorax.

Noninvasive positive pressure ventilation would not be appropriate, because it does not have an established role in the management of pneumothorax.

Observation alone is not appropriate because this patient is symptomatic, with increased breathlessness, and has a large pneumothorax.