Hypoxia¶
Henry Brems
Background¶
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Two major ways to measure oxygenation, which are similar but distinct:
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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
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PaO2 - the partial pressure of oxygenation measured on an ABG
- \<80 mmHg is abnormal
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The relationship between SpO2 and PaO2 is the classic S-shaped curve.
- SpO2 of \<88% 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¶
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Confirm true hypoxia with good pleth
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CXR
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Labs: CBC, BMP, BNP, troponin, ABG/VBG
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EKG
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TTE: obtain with bubble if shunt on ddx
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Lung US: B lines, lung sliding, effusions, consolidations (hepatization), diaphragm paralysis
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Chest CT without contrast for evaluation of lung parenchyma
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CTA chest if suspicion for PE (see PE section)
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Outpatient PFTs if suspected obstructive or restrictive disease
Management¶
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Should be directed at underlying cause
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If acutely decompensating, Duonebs, IV lasix, antibiotics depending on clinical picture
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Supplemental O2 for goal SpO2:
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>90-92% for most patients (There is generally no need to make SpO2 100%)
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Between 88-92% for patients with chronic hypoxia from COPD (i.e., on home O2)
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