| Cause of hypoxemia* | Definition | Etiologies | A-a gradient | PaCO2 | Response to oxygen | Treatment |
| V/Q mismatch | - Imbalance of lung perfusion relative to ventilation:
- Low V/Q – Regions underventilated relative to perfusion (shunt is extreme form of low V/Q mismatch; refer to below)
- High V/Q – Regions that are overventilated relative to perfusion (dead space is extreme form of high V/Q mismatch)
| - Embolic disease (venous pulmonary embolism, fat embolism, amniotic fluid embolism, malignant cells)
- Pulmonary vascular disease (destroys capillary beds and limits perfusion to alveoli)
- Chronic obstructive lung disease/emphysema (destroys the pulmonary capillary-alveolar interface)
- Interstitial diseases (eg, pulmonary edema, pulmonary inflammation)
- Low flow cardiac output states
| High | Typically normal
(unless associated with hypoventilation or significant increase in dead space) | Yes
(unless due to shunt) | - Supplemental oxygen
- Treat underlying cause
|
| Right-to-left shunt | - Extreme form of low V/Q mismatch:
- Anatomic (ie, mechanical shunt)
- Physiologic (ie, typically capillary shunt)
| - Anatomic:
- Cardiac shunt (eg, ASD, PFO, VSD)
- Intravascular shunt (pulmonary or other AVM, hepatopulmonary syndrome)
- Physiologic:
- Capillary shunt (eg, atelectasis, edema, pneumonia)
- Alveolar filling disorders (eg, fluid, hemorrhage, pus, cells, protein, lipid)
- Complete airway obstruction (including angioedema)
| High | Normal | No | - Oxygen; however, it is frequently minimally responsive or unresponsive to oxygen
- Treat underlying cause
|
| Hypoventilation | - Reduced minute ventilation "can't breathe, won't breathe, can't breathe enough"¶
| - Central neurologic disease (eg, stroke, sedation, hypothyroidism)
- Respiratory neuromuscular and chest wall diseases (eg, cervical-spine or phrenic nerve injury, Guillain-Barré syndrome, bilateral diaphragmatic weakness)
- Airway/alveolar disease (acute exacerbation of COPD, COPD, asthma, ILD)
| Normal | Elevated | Yes | - Supplemental oxygen (oxygen will improve oxygenation but will not resolve hypercapnia)
- Increase minute ventilation (eg, noninvasive ventilation or invasive ventilation)
- Treat underlying cause
|
| Diffusion defect | - Impairment of gas exchange between alveoli and pulmonary capillaries
- Often overlaps with shunt or V/Q mismatch pathophysiology
| - ILD
- Lung resection
- Emphysema
- Pulmonary vascular disease
| High
(occasionally normal) | Normal | Yes | - Supplemental oxygen
- Treat underlying cause
|
| Reduced PiO2 | | - Sudden change in elevation or new altitude
- Reduction in pressure during air travel in aircraft
| Normal | Normal or low
(if hyperventilation present) | Yes | - Descent if altitude is the cause
- Supplemental oxygen
- Consider acetazolamide
|
| Increased oxygen extraction | - Reduced mixed venous oxygen content of blood returning to the right side of the heart
| - Decreased cardiac output
- Anemia
- Hypermetabolism (eg, fever, sepsis, hyperthyroidism, burns, trauma)
| High | Normal | Yes | - Supplemental oxygen
- Treat underlying cause
|
