Hypoxemia

Step 1: Understand the A-a (PAO2 – PaO2) gradient

  • PAO2 = PiO2 – (PaCO2/R) = (FiO2)(PB-PH2O) – (PaCO2/R)
    • PAO2 = Partial pressure of oxygen in the alveoli
    • PiO2 = Partial pressure of oxygen in inhaled air
    • FiO2 = Fraction of oxygen concentration in inhaled air
    • PB = Barometric pressure (760 mmHg at sea level)
    • PH2O = Water vapor pressure (47 mmHg)
    • PaCO2 = Partial pressure of carbon dioxide in arterial blood
    • R = Respiratory quotient (0.8)
  • Thus, PAO2 = (FiO2)(760-47) – (PaCO2/0.8)
    • Use the patient’s PaCO2 (from an ABG) and their FiO2 to calculate their PAO2
  • A-a gradient = (PAO2 calculated as above) minus (PaO2 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

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