Master systematic ABG interpretation including primary disorders, compensation, and mixed disturbances.
75 min estimated
A systematic approach prevents errors. Always follow these steps:
1. pH: Normal 7.35–7.45. Below 7.35 = acidosis; above 7.45 = alkalosis. 2. PaCO2: Normal 35–45 mmHg. Elevated = respiratory acidosis; low = respiratory alkalosis. 3. HCO3: Normal 22–26 mEq/L. Elevated = metabolic alkalosis; low = metabolic acidosis. 4. Determine primary disorder: match the pH change with the PaCO2 or HCO3 that moves in the same direction. 5. Check for compensation: is the compensatory response appropriate? 6. Oxygenation: PaO2 normal 80–100 mmHg; SaO2 normal 95–100%.
Clinical Tip
Always state the pH first ('acidosis' or 'alkalosis'), then identify respiratory vs metabolic, then check compensation. Never skip a step.
Compensation is the body's attempt to normalize pH by adjusting the system opposite to the primary disorder. Compensation never overcorrects pH.
For respiratory acidosis: kidneys retain HCO3. - Acute: HCO3 rises 1 mEq/L per 10 mmHg rise in PaCO2 - Chronic: HCO3 rises 3.5 mEq/L per 10 mmHg rise in PaCO2
For respiratory alkalosis: kidneys excrete HCO3. - Acute: HCO3 falls 2 mEq/L per 10 mmHg fall in PaCO2 - Chronic: HCO3 falls 5 mEq/L per 10 mmHg fall in PaCO2
For metabolic acidosis: lungs blow off CO2. Expected PaCO2 = 1.5 × HCO3 + 8 ± 2 (Winter's formula). For metabolic alkalosis: lungs retain CO2. Expected PaCO2 = 0.7 × HCO3 + 21 ± 2.
Clinical Tip
If the measured compensation does not match the expected compensation, a mixed disorder is likely. Calculate expected and compare to actual.
The PaO2 tells us about oxygenation, but the A-a gradient provides more information about gas exchange efficiency.
A-a gradient = PAO2 – PaO2 PAO2 = FiO2 × (Patm – 47) – PaCO2/0.8
Normal A-a gradient: 5–15 mmHg on room air (increases with age: Age/4 + 4).
P/F ratio (PaO2/FiO2): quick oxygenation index. - Normal: >400 - Mild ARDS: 200–300 - Moderate ARDS: 100–200 - Severe ARDS: <100
Oxygen index (OI) = (FiO2 × Paw × 100) / PaO2 — used in neonates; OI >20–25 suggests need for ECMO.
Clinical Tip
A patient with normal A-a gradient and low PaO2 has hypoventilation as the cause of hypoxemia — check PaCO2 (it will be elevated).
COPD Patient in Acute Exacerbation
A 72-year-old with COPD presents with worsening dyspnea. ABG on 2 L/min O2: pH 7.28, PaCO2 72, HCO3 32, PaO2 56, SaO2 87%.
Interpret this ABG and identify whether this is acute, chronic, or acute-on-chronic.
Respiratory acidosis with metabolic compensation. Likely acute-on-chronic: HCO3 32 suggests chronic retention, but pH 7.28 indicates acute worsening.
Chronic respiratory acidosis: HCO3 rises 3.5 per 10 mmHg CO2. With PaCO2 72 (32 above normal), expected chronic HCO3 = 24 + (3.2 × 3.5) = ~35. Actual HCO3 32 is less than expected for full chronic compensation, and pH 7.28 (should be closer to normal if fully chronic) — this is acute-on-chronic respiratory acidosis with incomplete compensation.
Oxygen Therapy
Delivery systems, FiO2 calculations, indications, complications, and oxygen toxicity.
Mechanical Ventilation
Master ventilator modes, initial settings, troubleshooting, weaning, and complications.
Cardiopulmonary Diagnostics
CXR interpretation, ECG basics, cardiac markers, and diagnostic procedures.
Laboratory Values
CBC, BMP, CMP, coagulation studies, cultures, and critical lab values for respiratory care.