Hypercapnia

Learning objectives

Hypercapnia is defined as an elevation in the partial pressure of carbon dioxide (PaCO2) above 45 mmHg
Due to the role of CO2 in Ph buffering, hypercapnia can lead to acid-base imbalances

Hypercapnia is secondary to disease rather than a single etiology
Hypercapnia is caused by either increased metabolic CO2 production or respiratory failure
Metabolic processes that increase CO2 production:
- Fever
- Thyrotoxicosis
- Increased catabolism in sepsis or steroid use
- Overfeeding
- Metabolic acidosis
- Exercise
Respiratory failure: Failure to eliminate CO2 from the pulmonary system, hypoventilation secondary to decreased respiratory rate or decreased tidal volume
- Causes:
  - Decreased central nervous system respiratory drive
  - Anatomical defects
  - Decreased neuromuscular response
  - Increased dead space within the lung
Hypercapnia can be acute or chronic, depending on the etiology
- Acute hypercapnia: PaCO2 >45 mmHg, HCO3 within normal range (~30 mmHg), resulting decrease in pH <7.35
- Chronic hypercapnia: Renal compensation, PaCO2 >45 mmHg, HCO3 elevated proportionally, less severe pH imbalance

Pathologies that lead to hypercapnia include:

Treat the underlying pathology
Increase ventilation:
- Bi-level positive airway pressure
- Ventilation assist
- Continuous positive airway pressure ventilation
- Intubation with mechanical ventilation in critically ill patients
Maintain oxygen saturation at 90% or higher
Other options for critically ill ventilated patients:
- Increase minute ventilation
- Increase end-inspiratory pause prolongation
- Buffers: sodium bicarbonate, tromethamine
- Prone position ventilation
- Airway pressure release ventilation
- High-frequency oscillation ventilation
- Extracorporeal membrane oxygenation
- Low-flow extracorporeal CO2 removal devices