Learning objectives

• Describe the etiology and causes of hypercapnia
• Diagnose hypercapnia
• Manage hypercapnia

Background

• 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

Etiology

• 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

Underlying pathologies

Pathologies that lead to hypercapnia include:

• Acute respiratory distress syndrome
• Asthma exacerbation
• Central or obstructive sleep apnea
• Chronic obstructive pulmonary disorder
• Drug overdose
• Exogenous CO2 inhalation
• Head or cervical cord injury
• Myasthenia gravis
• Myxedema
• Obesity-hypoventilation syndrome (Pickwickian syndrome)
• Polyneuropathy
• Poliomyelitis
• Primary muscle disorders
• Porphyria
• Primary alveolar hypoventilation
• Pulmonary edema
• Tetanus

Signs & symptoms

• Flushed skin
• Lethargy
• Inability to focus
• Mild headache
• Disorientation
• Dizziness
• Dyspnea
• Nausea
• Vomiting
• Fatigue
• Delirium
• Paranoia
• Depression
• Abnormal muscle twitches
• Palpitations
• Hyperventilation
• Hypoventilation
• Seizures
• Anxiety
• Syncope

Diagnosis

• Signs on physical examination may include:
  • Fever
  • Tachycardia
  • Tachypnea
  • Dyspnea
  • Altered mental status
  • Wheezing on auscultation
  • Rales on auscultation
  • Rhonchi on auscultation
  • Decreased breath sounds
  • hyper-resonant chest on percussion
  • Increased anterior-posterior diameter of chest
  • Cardiac murmur 
  • Signs of hypoxia
  • Hepatosplenomegaly
  • neurological deficit
  • Confusion
  • Somnolence
  • Muscular weakness
  • Peripheral edema
  • Asterixis
  • Papilledema
  • superficial vein dilation
• Diagnostic testing:
  • Complete blood count to determine anemia presence
  • Complete metabolic panel (sodium, potassium, chloride, HCO3)
  • Thyroid stimulating hormone to determine underlying hypo– or hyperthyroidism
  • Arterial or venous blood gas (pH status, serum CO2, serum HCO3, anion gap)
  • Spirometry (forced expiratory volume over 1 second, forced vital capacity)
  • Chest X-ray
  • Chest CT
  • Echocardiography if cardiopulmonary abnormality is suspected
  • ECG to evaluate central nervous system malfunctions
  • Electromyography to evaluate neuromuscular disorders
  • Polysomnography for suspected central or obstructive sleep apnea

Management

• 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

Suggested reading

• Rawat D, Modi P, Sharma S. Hypercapnea. [Updated 2022 Jul 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK500012/
• Tiruvoipati R, Gupta S, Pilcher D, Bailey M. Management of hypercapnia in critically ill mechanically ventilated patients-A narrative review of literature. J Intensive Care Soc. 2020;21(4):327-333.