Key Concepts
Introduction
The pleural space normally contains 5-15 mL of serous fluid that maintains a negative intrapleural pressure (-4 to -8 cmH2O during quiet breathing), keeping the lungs expanded against the chest wall. Disruption of this negative pressure through air entry (pneumothorax), fluid accumulation (pleural effusion), or blood (hemothorax) causes lung collapse. Chest tubes restore negative intrapleural pressure by draining air and fluid. The water seal chamber creates a one-way valve: air can exit the pleural space (bubbling in the water seal) but cannot re-enter. Suction (typically -20 cmH2O) actively facilitates drainage and lung re-expansion. Tidaling (fluid oscillation in the water seal) reflects normal respiratory pressure changes: rises with inspiration (more negative pleural pressure) in spontaneous breathing, and falls with inspiration in mechanically ventilated patients. Absence of tidaling may indicate lung re-expansion, tube occlusion, or tube dislodgement. On the exam, writers often pair stable-sounding options with unstable data—notice the mismatch before you commit. If the stem names a license or role, reread that line; scope errors are classic trap answers even when the clinical topic is familiar. Run a 60-second scan: breathing...