Am. J. Respir. Crit. Care Med., Volume 156, Number 4, October 1997, 1205-1209

Ventilation-Perfusion Mismatch in Patients with Pleural Effusion
Effects of Thoracentesis

ALVAR G. N. AGUSTÍ, JAUME CARDÚS, JOSEP ROCA, JOSEP M. GRAU, ANTONI XAUBET, and ROBERT RODRIGUEZ-ROISIN

Serveis de Pneumologia i Al.lèrgia Respiratòria and Medicina Interna General, Departament de Medicina, Hospital Clínic, Universitat de Barcelona, Barcelona, Spain

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-perfusion ( A/) 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, Pa_O2 was 82.3 ± 10.2 mm Hg and AaPO₂ was 28.7 ± 10.0 mm Hg. Patients had broadened unimodal A/ distributions with small amounts of blood flow perfusing lung units with low A/ 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 A/ 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, AaPO₂, and shunt remained unchanged; only the amount of blood flow perfusing low A/ 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. Agustí AGN, Cardús J, Roca J, Grau JM, Xaubet A, Rodriguez-Roisin R. Ventilation-perfusion mismatch in patients with pleural effusion: effects of thoracentesis.

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