Hypoxia

Hypoxia – Henry Brems

Background

  • Two major ways to measure oxygenation, which are similar but distinct: 
    • SpO2 or "pulse ox" - most common measurement in the hospital or clinic, which measures the O2 saturation of Hgb at capillary level
      • <95% is abnormal, but may not need supplemental O2 until <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.
  • Below an SpO2 of ~88%, PaO2 begins to fall off dangerously fast thus necessitating quick intervention for pts with SpO2 below this level


Mechanisms of Hypoxia:

Mechanism

Pathophysiology

Decreased barometric pressure

- Normal A-a gradient

- This is 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, i.e. where there is relatively less ventilation than perfusion.

- 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 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

- Any process that fills the alveoli with:

      • 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 Diseases (see section)

Vascular

- Pulmonary Emboli

- particularly suspect these in patients with significant hypoxia and a clear CXR

Pleural Space and

Chest Wall

- Pleural Effusions, Pneumothorax, Neuromuscular weakness, tense ascites

- These are more likely to cause significant dyspnea and usually need to be quite severe to cause hypoxia

 

Evaluation

  • Confirm true hypoxia with good pleth. Can press pulse-ox on pt’s finger for better transmission
  • CXR is almost always the first step
  • Other common initial workup: CBC, BMP, BNP, troponin, EKG, ABG/VBG
  • TTE -- particularly useful if suspected new pulmonary edema or pulmonary HTN
    • Obtain with 'bubble study' if any concern for shunt
    • Obtain “limited” if patient had recent “complete” TTE
  • Chest CT
    • Order CTA ('CTA PE' in Epic) if concerned for PE
      • V/Q scan if unable to receive IV contrast
    • CT Chest without contrast otherwise generally preferred for evaluation of lung parenchyma
  • PFTs are useful if there is suspected obstructive (e.g. COPD) or restrictive disease (e.g. ILD), or if the cause remains otherwise unknown after initial workup
    • These are almost always done as an outpatient

 

Management

  • Should be directed at underlying cause. See other sections for information relevant to particular diagnoses
  • If acutely decompensating, Duonebs, IV lasix, Antibiotics depending on clinical picture
  • Supplemental O2 for goal SpO2:
    • >90-92% for most patients (There is generally no need to make SpO2 100%)
    • Between 88-92% for patients with chronic hypoxia from COPD (i.e., on home O2)