Diferencia entre revisiones de «Hypoxemia»

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*Defined as PaO2 <60 mm Hg
*Defined as PaO2 <60 mm Hg
**Amount of oxygen available is a function of the arterial oxygen content and blood flow
**Amount of oxygen available is a function of the arterial oxygen content and blood flow
*Occurs in states of low CO, low Hb, or low SaO2 (arterial oxygen saturation)
*Occurs in states of low CO, low hemoglobin, or low SaO2 (arterial oxygen saturation)
*Relative Hypoxemia
*Relative Hypoxemia
**PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
**PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
Línea 17: Línea 17:
**Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
**Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
*V/Q mismatch
*V/Q mismatch
**PE, PNA, asthma, COPD
**PE, pneumonia, asthma, COPD
**Improves with supplemental O2
**Improves with supplemental O2
**A-a gradient increased
**A-a gradient increased
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==Diagnosis==
==Evaluation==


==Management==
==Management==
==Disposition==
== Calculators ==
{{Aa_Gradient_Calculator}}


==See Also==
==See Also==
*[[Shortness of breath]]
*[[Shortness of breath]]
*[[Hypercapnia]]
*[[Hypercapnia]]
*[[Post-hypoxic myoclonus]]


==References==
==References==

Revisión actual - 16:32 21 mar 2026

Background

  • Defined as PaO2 <60 mm Hg
    • Amount of oxygen available is a function of the arterial oxygen content and blood flow
  • Occurs in states of low CO, low hemoglobin, or low SaO2 (arterial oxygen saturation)
  • Relative Hypoxemia
    • PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
    • Can calculate amount of relative hypoxemia by A-a gradient
      • P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy patients)

Causes

  • Hypoventilation
    • Always associated with increased PaCO2
    • Normal A-a gradient
  • Right-to-left shunt
    • Occurs when blood enters the systemic arteries with out traversing ventilated lung
      • Occurs with pulmonary consolidation, atelectasis
    • Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
  • V/Q mismatch
    • PE, pneumonia, asthma, COPD
    • Improves with supplemental O2
    • A-a gradient increased
  • Diffusion impairment
    • Improves with supplemental O2
    • A-a gradient increased
  • Low inspired O2
    • Improves with supplemental O2
    • A-a gradient normal

Compensation

  1. Increased minute ventilation
  2. Pulmonary arterial vasoconstriction
  3. Increased sympathetic tone

Differential Diagnosis

Acute dyspnea

Emergent

Non-Emergent

Evaluation

Management

Disposition

Calculators

A-a O₂ Gradient

Alveolar-arterial (A-a) O₂ Gradient
Parameter Value
Age (years)
FiO₂ (%)
PaCO₂ (mmHg)
PaO₂ (mmHg)
A-a Gradient mmHg
Expected A-a mmHg (age-adjusted normal)
Interpretation
  • Normal A-a gradient ≈ (Age/4) + 4 on room air
  • Elevated A-a gradient suggests: V/Q mismatch, shunt, or diffusion impairment
  • Normal A-a gradient + hypoxia suggests: hypoventilation or low FiO₂
References
  • Formula: A-a = [FiO₂ × (Patm – PH2O)] – (PaCO₂/0.8) – PaO₂
  • Kanber GJ, et al. The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis. 1968;97(3):376-381. PMID 5637791.

See Also

References