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Emily Lovern


  • A condition where the oxygen supply is inadequate either to the body as a whole (general hypoxia) or to a specific region (tissue hypoxia)

  • Two major ways to measure oxygenation, which are similar but distinct:

    • SpO2 or "pulse ox" - most common, measures the O2 saturation of Hgb at capillary level - <95% is abnormal, but do not need supplemental O2 unless <90-92% in most pts
    • PaO2 - the partial pressure of oxygenation measured on an ABG
      • <80 mmHg is abnormal
    • The relationship between SpO2 and PaO2 is the classic S-shaped curve.
      • SpO2 of <88% if PaO2 begins to fall off dangerously fast

Mechanisms of Hypoxia

Mechanism Pathophysiology
Decreased barometric pressure

Normal A-a gradient

Unlikely to be seen except at high altitudes


Normal A-a gradient

Hypoxia easily correctable with supplemental O2

V/Q Mismatch

Increased A-a gradient

Processes that lead to areas of lung where V/Q <1

Common examples include PNA, ARDS, pulmonary edema

Right-to-left Shunt

Increased A-a gradient

Can be anatomic (e.g. intracardiac, AVMs) or physiologic (water/pus/blood filling alveoli)

Classically does not easily correct with supplemental O2

Diffusion Limitation

Increased A-a gradient

Often related to diseases affecting the interstitium -- e.g., ILD

Differential diagnosis for hypoxia based on anatomical location

Anatomical Location Differential Diagnosis

COPD most common, Asthma in very severe cases

CF, bronchiectasis in patients with appropriate history



Pus: infection from bacterial, viral, fungal agents

Water: pulmonary edema

Protein/Cells/Other: ARDS, pneumonitis (e.g., aspiration, drug-induced)

Interstitium/Parenchyma Interstitial Lung Disease

Pulmonary Emboli

Suspect in patients with significant hypoxia and a clear CXR

Pleural Space and

Chest Wall

Pleural Effusions, Pneumothorax, Neuromuscular weakness, tense ascites

More likely to cause dyspnea, need to be severe to cause hypoxia


  • Confirm true hypoxia with good pleth

  • CXR

  • Labs: CBC, BMP, BNP, troponin, ABG/VBG

  • EKG

  • TTE: obtain with bubble if shunt on ddx

  • Lung US: B lines, lung sliding, effusions, consolidations (hepatization), diaphragm paralysis

  • Chest CT without contrast for evaluation of lung parenchyma

  • CTA chest if suspicion for PE (see PE section)

  • Outpatient PFTs if suspected obstructive or restrictive disease


  • Should be directed at underlying cause

  • If acutely decompensating, Duonebs, IV lasix, antibiotics depending on clinical picture

  • Supplemental O2 for goal SpO2:

    • 90-96% for most pts

    • Between 88-92% for patients with chronic hypoxia from COPD (i.e., on home O2)