Situation: A 60-year-old man is admitted to the medical ward with a right pleural effusion. He has been taking antibiotics for pneumonia for 10 days but still has fever. The physician plans a bedside diagnostic and therapeutic thoracentesis. During the procedure, 1,200 mL of cloudy fluid is removed. One hour later he reports sudden sharp right chest pain and breathlessness. His respiratory rate is 30/min and his oxygen saturation is 88%. Breath sounds are absent over the upper right chest, which is hyperresonant to percussion. He has no cough and no frothy sputum. What is the MOST likely complication?

Pneumothorax is the most common complication of thoracentesis. Sudden pleuritic pain, breathlessness, falling saturation, and absent breath sounds with hyperresonance show air in the pleural space. Re-expansion pulmonary edema after large-volume drainage causes cough, crackles, and frothy sputum, and reaccumulated fluid would be dull to percussion.

Clinical context
A thoracentesis that removes a large volume of fluid can injure the visceral pleura or introduce air through the needle or catheter. When air enters the pleural space, the lung on that side collapses. The patient’s sudden sharp pleuritic chest pain, tachypnea, falling oxygen saturation, absent breath sounds, and hyperresonance over the right upper chest are classic signs of a pneumothorax. The absence of cough and frothy sputum makes re-expansion pulmonary edema unlikely, and hyperresonance rules out reaccumulated fluid, which would sound dull to percussion ^[1].

Why pneumothorax fits this presentation
During thoracentesis, the needle or catheter can lacerate the visceral pleura, or air can be pulled into the pleural space if the system is not sealed. The result is loss of negative intrapleural pressure. The lung recoils away from the chest wall, and the affected hemithorax becomes filled with air rather than lung tissue. On physical examination, air in the pleural space produces hyperresonance to percussion and absent or diminished breath sounds because sound is not transmitted through air as well as through aerated lung or fluid. The sudden onset of sharp pleuritic pain after the procedure strongly suggests a procedure-related complication rather than a slowly evolving process such as reaccumulation of effusion .

A post-thoracentesis pneumothorax typically presents within the first hour with pleuritic chest pain, dyspnea, hypoxemia, and hyperresonance with absent breath sounds on the affected side. The respiratory rate of 30/min and oxygen saturation of 88% indicate significant respiratory compromise. The volume removed, 1,200 mL, is large enough to raise concern for re-expansion pulmonary edema, but the clinical findings point away from that diagnosis ^[1].

Differential diagnosis

Complication	Key clinical features	Why it does not fit here
Pneumothorax	Sudden pleuritic pain, dyspnea, hypoxemia, hyperresonance, absent breath sounds	Matches all findings; most common thoracentesis complication
Re-expansion pulmonary edema	Cough, crackles, frothy sputum, diffuse alveolar infiltrates after rapid large-volume drainage	Patient has no cough and no frothy sputum; percussion is hyperresonant, not dull [1]
Reaccumulation of pleural fluid	Gradual dyspnea, dullness to percussion, decreased breath sounds over fluid level	Hyperresonance is the opposite finding; onset would be slower
Pulmonary embolism	Sudden dyspnea, pleuritic pain, but breath sounds usually remain present; no hyperresonance	Absent breath sounds and hyperresonance localize the problem to the pleural space, not the pulmonary vasculature

Watch out! Re-expansion pulmonary edema is a serious concern after draining a large effusion, but it produces pulmonary edema findings — cough, crackles, frothy sputum, and radiographic infiltrates — not the hyperresonance and absent breath sounds of a pneumothorax. The absence of cough and frothy sputum is a key negative finding that shifts the diagnosis toward pneumothorax ^[1].

Key point! Hyperresonance plus absent breath sounds after thoracentesis is a pneumothorax until proven otherwise. A chest radiograph or point-of-care lung ultrasound should be obtained immediately, and if tension physiology develops, urgent needle decompression or chest tube placement is required .

Pathophysiology of the percussion finding
Percussion transmits sound through the chest wall into the underlying structure. Normal aerated lung produces resonance. Fluid produces dullness because liquid conducts sound poorly. Air trapped in the pleural space produces hyperresonance because the large air-filled cavity vibrates more freely. This single physical finding effectively distinguishes pneumothorax from reaccumulated pleural effusion, which would be dull to percussion. In pneumothorax ex vacuo, a special form seen when the lung cannot fully expand after fluid removal, the trachea may deviate toward the affected side, but the air collection still produces hyperresonance .

Clinical priority after recognition
Once pneumothorax is suspected, the patient should be placed on supplemental oxygen to treat hypoxemia and to accelerate reabsorption of pleural air. A chest radiograph or lung ultrasound confirms the diagnosis. If the patient develops hypotension, distended neck veins, or tracheal deviation away from the affected side, tension pneumothorax is present and requires immediate decompression. Small, asymptomatic pneumothoraces may be observed, but this patient is symptomatic with hypoxemia and will likely require a chest tube or small-bore catheter drainage .