Arterial Blood Gas Interpretation

Master systematic ABG interpretation including primary disorders, compensation, and mixed disturbances.

intermediate75 min estimated3 lessons

Normal Values and Systematic Approach

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%.

pH 7.35–7.45, PaCO2 35–45, HCO3 22–26, PaO2 80–100 mmHg
pH down + PaCO2 up = respiratory acidosis
pH up + PaCO2 down = respiratory alkalosis
pH down + HCO3 down = metabolic acidosis
pH up + HCO3 up = metabolic alkalosis

Clinical tip

Always state the pH first ('acidosis' or 'alkalosis'), then identify respiratory vs metabolic, then check compensation. Never skip a step.

Compensation Rules

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.

Acute resp acidosis: HCO3 +1 per 10 mmHg PaCO2 rise
Chronic resp acidosis: HCO3 +3.5 per 10 mmHg PaCO2 rise
Metabolic acidosis: expected PaCO2 = 1.5×HCO3 + 8 ± 2 (Winter's)
Compensation never normalizes pH — if pH is normal with abnormal values, consider mixed
Acute vs chronic: check clinical history — COPD patient likely chronically compensated

Clinical tip

If the measured compensation does not match the expected compensation, a mixed disorder is likely. Calculate expected and compare to actual.

Oxygenation Assessment

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.

A-a gradient: PAO2 – PaO2; normal 5–15 mmHg on room air
Elevated A-a gradient with hypoxemia = V/Q mismatch or shunt
P/F ratio: >400 normal, <300 mild ARDS, <200 moderate, <100 severe
Normal PaO2 decreases with age: expected PaO2 ≈ 104 – (0.27 × age)
OI >20–25 in neonates suggests ECMO consideration

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).

Clinical cases

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.

Know this for NBRC

Normal ABG values: memorize cold
Acute/chronic compensation rules: all four
Winter's formula for metabolic acidosis
P/F ratio ARDS criteria
MUDPILES for anion gap metabolic acidosis
Mixed disorder = pH 7.40 with abnormal PaCO2 AND HCO3

Key facts

Normal: pH 7.35–7.45, PaCO2 35–45, HCO3 22–26, PaO2 80–100, SaO2 95–100%
Acute resp acidosis: HCO3 +1 per ↑10 PaCO2; Chronic: HCO3 +3.5
Acute resp alkalosis: HCO3 -2 per ↓10 PaCO2; Chronic: HCO3 -5
Winter's formula (met acidosis): Expected PaCO2 = 1.5×HCO3 + 8 ± 2
Metabolic alkalosis compensation: Expected PaCO2 = 0.7×HCO3 + 21 ± 2
A-a gradient normal: 5–15 mmHg on room air
P/F ratio: >400 normal; <300 mild ARDS; <200 moderate; <100 severe
Respiratory acidosis causes: hypoventilation, COPD, opioids, neuromuscular disease
Anion gap = Na – (Cl + HCO3); normal 8–12 mEq/L
pH 7.40 ± 0.05 = well compensated; pH 7.40 with abnormal values = mixed disorder

NBRC pearls

NBRC always tests compensation — memorize acute vs chronic rules
If pH is normal but CO2 and HCO3 are both abnormal = mixed disorder
Winter's formula for metabolic acidosis compensation is frequently tested
P/F ratio ARDS cutoffs: mild <300, moderate <200, severe <100
Anion gap metabolic acidosis: MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates)

Memory tricks

ROME: Respiratory Opposite, Metabolic Equal (pH and primary value move opposite in respiratory, same in metabolic)
MUDPILES for anion gap acidosis causes
Compensation never overshoots: pH never crosses 7.40 due to compensation alone

Common mistakes

Skipping compensation check — always verify expected vs actual
Calling compensated ABG 'normal' — it's compensated, not normal
Using actual weight for IBW calculations in ventilator settings
Confusing acute vs chronic compensation (COPD patients are chronically compensated)
Forgetting to check oxygenation — don't just interpret acid-base
Missing a mixed disorder when pH is 7.40 but PaCO2 and HCO3 are both abnormal

Arterial Blood Gas Interpretation

Normal Values and Systematic Approach

Compensation Rules

Oxygenation Assessment

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Arterial Blood Gas Interpretation

Normal Values and Systematic Approach

Compensation Rules

Oxygenation Assessment

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Lessons

Normal Values and Systematic Approach

Compensation Rules

Oxygenation Assessment

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Lessons

Normal Values and Systematic Approach

Compensation Rules

Oxygenation Assessment

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics