Article Sections

Introduction

The pleural cavity is a potential space between the visceral and parietal pleural layers that normally contains a trace amount of fluid (~20 mL) serving as a lubricant during respiration ( Figure 1).  A pleural effusion is an abnormal accumulation of fluid in this space caused by a variety of hydrostatic, secretory, or inflammatory processes that disrupt the balance of fluid formation and resorption.

Pathogenesis and classification

Mechanisms

Pleural fluid formation is primarily attributed to 4 major mechanisms ( Figure 2).

  • Hydrostatic pressure gradient (pleural capillary − pleural space):  Increased pleural capillary pressure (eg, from left heart failure) drives extravasation into the pleural space.  Fluid builds up until it tamponades further formation (ie, higher left-sided filling pressures lead to larger effusions).
  • Oncotic pressure gradient (plasma proteins − pleural fluid proteins):  Decreased plasma oncotic pressure (eg, nephrotic syndrome) weakens the colloid drag that pulls fluid back into vessels.  However, plasma hypoproteinemia alone cannot induce an effusion.  Instead, it plays a minor role in maintaining effusions due to primary disease states associated with hypoalbuminemia (eg, renin-angiotensin-system–induced salt-water retention with nephrotic syndrome and cirrhosis, systemic inflammation and vascular permeability with malignancy and malnutrition).

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