Hypoxemia

Step 1: Understand the A-a (P_AO₂ – P_aO₂) gradient

• P_AO₂ = P_iO₂ – (P_aCO₂/R) = (F_iO₂)(P_B-P_H2O) – (P_aCO₂/R)
  • P_AO₂ = Partial pressure of oxygen in the alveoli
  • P_iO₂ = Partial pressure of oxygen in inhaled air
  • F_iO₂ = Fraction of oxygen concentration in inhaled air
  • P_B = Barometric pressure (760 mmHg at sea level)
  • P_H2O = Water vapor pressure (47 mmHg)
  • P_aCO₂ = Partial pressure of carbon dioxide in arterial blood
  • R = Respiratory quotient (0.8)

• Thus, P_AO₂ = (F_iO₂)(760-47) – (P_aCO₂/0.8)
  • Use the patient’s P_aCO₂ (from an ABG) and their F_iO₂ to calculate their P_AO₂

• A-a gradient = (P_AO₂ calculated as above) minus (P_aO₂ from patient’s ABG)
  • Normal A-a gradient = (Age + 10) / 4 

• A-a gradient tells you about gas exchange, and helps narrow the differential diagnosis, as below

Mechanisms of Hypoxemia

Reduced FiO2

• High altitude

Hypoventilation

• CNS depression
  • Medications (opioids, benzodiazepines)
  • Illicit drugs
  • CNS lesions (ischemic or mass)

• PNS depression
  • C-spine injury
  • Phrenic nerve palsy

• Muscular
  • Myasthenia Gravis
  • Muscular Dystrophy
  • Polymyositis
  • Tetanus
  • Hypothyroidism

• Skeletal/Structural
  • Scoliosis
  • Flail chest
  • Obesity hypoventilation syndrome (“Pickwickian” syndrome)

V/Q mismatch

• Increased Dead Space
  • Pulmonary hypertension
  • Pulmonary embolism

• Intraalveolar Process – can be seen on chest radiography
  • Alveolar filling
    • Blood (trauma, autoimmune)
    • Pus (infection)
    • Water (heart failure, renal failure, ARDS)
    • Cells (bronchoalveolar cell carcinoma, lymphoma)
  • Alveolar collapsing
    • Atelectasis

• Shunt (extreme V/Q mismatch – blood passes from the right to the left side of the heart without being oxygenated)
  • Anatomic shunt: Blood bypasses alveoli
    • Intracardiac shunt (PFO, ASD, VSD)
    • Pulmonary arteriovenous malformation
    • Hepatopulmonary syndrome
  • Physiologic shunt: Severe intraalveolar process in which no gas exchange is participating at the alveolar-pulmonary capillary interface

Diffusion limitation

• Impairment of diffusion of oxygen from alveolus to pulmonary capillary
  • Interstitial Lung Disease