Hypoxemia
(Redirigido desde «Hypoxic»)
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)
- Occurs when blood enters the systemic arteries with out traversing ventilated lung
- 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
- Increased minute ventilation
- Pulmonary arterial vasoconstriction
- Increased sympathetic tone
Differential Diagnosis
Acute dyspnea
Emergent
- Pulmonary
- Airway obstruction
- Anaphylaxis
- Angioedema
- Aspiration
- Acute respiratory distress syndrome (ARDS)
- Asthma
- Cor pulmonale
- Epiglottitis
- Inhalation exposure
- Noncardiogenic pulmonary edema
- Pneumonia
- Pneumocystis Pneumonia (PCP)
- Pulmonary embolism
- Pulmonary hypertension
- Tension pneumothorax
- Idiopathic pulmonary fibrosis acute exacerbation
- Cystic fibrosis exacerbation
- Cardiac
- Other Associated with Normal/↑ Respiratory Effort
- Other Associated with ↓ Respiratory Effort
Non-Emergent
- ALS
- Ascites
- Uncorrected ASD
- Congenital heart disease
- COPD exacerbation
- Deconditioning
- Fever
- Hyperventilation
- Interstitial lung disease
- Neoplasm
- Obesity
- Obstructive sleep apnea
- Panic attack
- Pleural effusion
- Polymyositis
- Porphyria
- Pregnancy
- Rib fracture
- Spontaneous pneumothorax
- Thyroid Disease
- URI
- Vocal cord dysfunction
Evaluation
Management
Disposition
Calculators
A-a O₂ Gradient
| Parameter | Value |
|---|---|
| Age (years) | |
| FiO₂ (%) | |
| PaCO₂ (mmHg) | |
| PaO₂ (mmHg) | |
| A-a Gradient | mmHg |
| Expected A-a | mmHg (age-adjusted normal) |
|
|