ETCO₂ in Tension Pneumothorax During Mechanical Ventilation
In a mechanically ventilated patient who develops tension pneumothorax, ETCO₂ will initially increase due to impaired CO₂ elimination from reduced pulmonary blood flow and ventilation-perfusion mismatch, followed by a potential decrease if cardiac output becomes severely compromised. 1, 2, 3
Pathophysiology of ETCO₂ Changes
The development of tension pneumothorax creates a complex physiologic derangement that affects ETCO₂ in a biphasic pattern:
Initial Phase: ETCO₂ Increases
- Decreased lung compliance from pneumothorax causes immediate reduction in effective ventilation, leading to CO₂ retention and elevated ETCO₂ 1, 2
- Increased airway pressures develop simultaneously with rising ETCO₂, creating a characteristic triad: elevated peak inspiratory pressure, decreased compliance, and increased end-tidal CO₂ 2, 3
- Enhanced CO₂ absorption occurs when CO₂ under pressure in the pleural space increases systemic CO₂ load, particularly during laparoscopic procedures where CO₂ pneumothorax develops 2
- Research in newborn piglets demonstrated that tension pneumothorax at +20 mbar caused marked hypercapnia (PaCO₂ 61.2 ± 5.9 mmHg) with corresponding ETCO₂ elevation 1
Late Phase: ETCO₂ May Decrease
- Severe cardiac output compromise from mediastinal shift and decreased venous return reduces pulmonary blood flow, which becomes the primary determinant of ETCO₂ during mechanical ventilation 4
- Critically low cardiac output (analogous to cardiac arrest physiology) will cause ETCO₂ to fall below 10 mmHg, indicating inadequate tissue perfusion 5, 4
- The American Heart Association notes that during low-flow states, ETCO₂ directly reflects cardiac output, as CO₂ delivery to the lungs depends on pulmonary blood flow 5, 4
Diagnostic Recognition
Monitor the simultaneous triad for immediate diagnosis: 2, 3
- Sudden increase in ETCO₂ (initial finding)
- Decrease in dynamic lung-thorax compliance
- Abnormal increase in peak inspiratory airway pressure
This combination allows early detection before hemodynamic collapse occurs 2, 3.
Critical Pitfall to Avoid
Do not assume ETCO₂ accurately reflects PaCO₂ during acute pneumothorax. While ETCO₂ normally correlates well with PaCO₂ in stable mechanically ventilated patients (r = 0.893 in SIMV mode) 6, research demonstrates that during decreasing lung compliance from tension pneumothorax, "end-tidal PCO₂ does not reflect the true ventilation," making arterial blood gas analysis mandatory for accurate assessment 1.
Management Algorithm
Step 1: Immediate Recognition
- Identify the diagnostic triad: rising ETCO₂, increased airway pressures, decreased compliance 2, 3
- Obtain arterial blood gas to confirm true PaCO₂, as ETCO₂ becomes unreliable 1
Step 2: Definitive Treatment
- Perform needle decompression immediately if tension pneumothorax is clinically suspected 5
- Consider chest tube placement for definitive management in most cases 2
Step 3: Temporizing Measures (if drainage delayed)
- Apply PEEP to partially correct respiratory derangements and improve compliance 2
- Increase respiratory rate to compensate for hypoventilation if hypercapnia develops 1