Ventilation-perfusion mismatch in patients with pleural effusion - Effects of thoracentesis

Pleural effusion (PE) often causes abnormal pulmonary gas exchange, Thoracentesis is commonly used to relieve dyspnea in patients with PE, but its effect upon arterial oxygenation is varied and poorly understood. This investigation sought to: (1) characterize the distribution of ventilation-per fusion ((V) over dotA/(Q) over dot) ratios in patients with PE and (2) assess the effects of PE drainage by thoracentesis upon pulmonary gas exchange. We studied nine patients (two females) with a mean age of 39 +/- 20 (SD) yr. All of them had PE of recent clinical onset (< 2 wk of symptoms), without other apparent medical conditions. Before thoracentesis, PaO2 was 82.3 +/- 10.2 mm Hg and AaPO(2) was 28.7 +/- 10.0 mm Hg. Patients had broadened unimodal (V) over dotA/(Q) over dot distributions with small amounts of blood flow perfusing lung units with low (V) over dotA/(Q) over dot ratios (< 0.1) (1.4 +/- 2.2%) and mild intrapulmonary shunt (6.9 +/- 6.7%). Pa-O2 was significantly related to the amount of shunt (rho = -0.82; p < 0.01) but not to the percentage of blood flow perfusing low (V) over dotA/(Q) over dot units. While thoracentesis drained 693 +/- 424 ml of fluid and caused a significant fall in mean pleural pressure (by -10.7 +/- 7.1 mm Hg; p < 0.01), Pa-O2, AaP(O2), and shunt remained unchanged; only the amount of blood flow perfusing low (V) over dotA/(Q) over dot ratios increased slightly (2.4 +/- 2.6%; p < 0.05). This study shows that: (1) intrapulmonary shunt is the main mechanism underlying arterial hypoxemia in patients with PE and (2) effective thoracentesis has minor short-term effects upon pulmonary gas exchange, These findings are in accord with delayed (> 30 min) pulmonary volume re-expansion after thoracentesis with or without the coexistence of mild ex vacuo pulmonary edema.