Skip to content

Hypoxia

Emily Lovern

Background

  • 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
Hypoventilation

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
Airways

COPD most common, Asthma in very severe cases

CF, bronchiectasis in patients with appropriate history
Alveoli

Blood

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
Vascular

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

Evaluation

  • 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

Management

  • 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)