Clinical meaning
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.
Exam relevance
Risk factors: - Thoracic surgery (post-lobectomy, pneumonectomy) - Trauma (penetrating or blunt chest trauma) - Spontaneous pneumothorax (tall, thin males 20-40 years) - Iatrogenic pneumothorax (post-central line, thoracentesis, lung biopsy) - Malignant pleural effusion - Empyema (infected pleural space) - Tuberculosis - COPD with bullous disease