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Reading an ABG at the Bedside - A Pulmonologist's Approach

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Normal ABG Values (Memorize These)

Goldman-Cecil Medicine, block 12: Normal PaO₂ by age sitting = 104.2 - (0.27 × age in years)

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THE 7-STEP BEDSIDE METHOD

STEP 1 - Is the Patient Acidemic or Alkalemic?

• pH < 7.35 = Acidemia
• pH > 7.45 = Alkalemia
• pH 7.35-7.45 = Normal (but do NOT stop here - a mixed disorder can hide a normal pH)

Clinical pearl: A pH of exactly 7.40 in a critically ill patient should immediately raise suspicion for a mixed disorder, not reassurance.

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STEP 2 - Identify the Primary Disorder

Current Surgical Therapy 14e, p.1608: In respiratory processes, PaCO₂ changes in the opposite direction of pH. In metabolic processes, PaCO₂ typically changes in the same direction.

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STEP 3 - Check Compensation (Is It Appropriate?)

Compensation Formulas (You MUST Know These)

Harrison's Principles of Internal Medicine 22E (2025), p.413: Chronic respiratory alkalosis when prolonged is an exception - it may return pH fully to normal value, unlike all other compensatory responses.

• If measured PaCO₂ = 29 → pure metabolic acidosis with appropriate compensation
• If measured PaCO₂ = 40 → additional respiratory acidosis (the lungs aren't compensating)
• If measured PaCO₂ = 22 → additional respiratory alkalosis (over-compensation = mixed disorder)

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STEP 4 - Calculate the Anion Gap (AG)

Harrison's 22E, p.414: The "normal" AG has declined with improved electrolyte methodology and ranges from 6-12 mmol/L, with an average of approximately 10 mmol/L.

High Anion Gap Acidosis - Causes (MUDPILES / GOLDMARK)

Normal Anion Gap (Hyperchloremic) Acidosis - Causes

• Diarrhea (most common)
• Renal tubular acidosis (RTA)
• Carbonic anhydrase inhibitors (acetazolamide)
• Post-hypocapnia
• Dilutional (rapid saline infusion)
• Ureteral diversion (ileal conduit)

Symptom to Diagnosis, p.69: Processes that produce acid (eg. ketoacidosis) also produce associated unmeasured anions which accumulate, resulting in an elevated AG. Processes that lose HCO₃⁻ (eg. diarrhea) do not generate unmeasured anions - the AG remains normal.

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STEP 5 - Delta-Delta (Δ/Δ) Ratio: Is There a Hidden Second Disorder?

Barash Clinical Anesthesia 9e, p.1159: A Δ/Δ between 1 and 2 indicates pure anion-gap acidosis; <1.0 indicates concurrent non-anion-gap metabolic acidosis; >2.0 indicates either concurrent metabolic alkalosis or compensated chronic respiratory acidosis.

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STEP 6 - Assess Oxygenation

PaO₂ - Basic Interpretation

• Normal: 80-100 mmHg (age-dependent)
• Age-adjusted: PaO₂ sitting = 104.2 - (0.27 × age)
• Mild hypoxemia: PaO₂ 60-79 mmHg
• Moderate hypoxemia: PaO₂ 40-59 mmHg
• Severe hypoxemia: PaO₂ < 40 mmHg

The A-a Gradient (Alveolar-Arterial O₂ Difference)

Goldman-Cecil Medicine, block 12: The A-a gradient can be approximated by the simple equation P(A-a)O₂ = (age/4) + 4.

P/F Ratio (PaO₂/FiO₂) - The ICU Workhorse

Clinical pearl: A P/F ratio < 300 on 5 cmH₂O PEEP is one of the Berlin ARDS criteria.

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STEP 7 - Synthesize and Identify Mixed Disorders

1. Is compensation appropriate? (If not - mixed disorder)
2. Is the Δ/Δ outside 1-2? (Hidden second metabolic process)
3. Does the clinical picture fit?

Common Mixed Disorder Patterns at Bedside

Harrison's 22E, p.413: Metabolic acidosis due to endogenous acid production lowers HCO₃⁻ and decreases pH. This stimulates medullary chemoreceptors to increase ventilation and return pH toward but not typically to normal value.

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PUTTING IT ALL TOGETHER - The 90-Second Bedside Script

"pH 7.28 - acidemia. PaCO₂ 20, HCO₃⁻ 9 - both low, so metabolic acidosis is the primary. Expected PaCO₂ by Winter's = (1.5 × 9) + 8 = 21.5 ± 2 - compensation is appropriate. AG = 138 - (100 + 9) = 29, corrected for albumin 2.5: 29 + 2.5 × (4.5 - 2.5) = 34 - high AG. Δ/Δ = (34-12)/(24-9) = 22/15 = 1.47 - pure high-AG acidosis. PaO₂ 72, FiO₂ 0.5, P/F = 144 - moderate ARDS. Diagnosis: High-AG metabolic acidosis (likely lactic/sepsis) with moderate ARDS."

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QUICK REFERENCE CARD

STEP 1: pH < 7.35 = Acidosis | pH > 7.45 = Alkalosis
STEP 2: PaCO₂ ↑ with pH ↓ = Resp Acidosis
        HCO₃⁻ ↓ with pH ↓ = Met Acidosis
        PaCO₂ ↓ with pH ↑ = Resp Alkalosis
        HCO₃⁻ ↑ with pH ↑ = Met Alkalosis
STEP 3: CHECK COMPENSATION
        Met Acidosis: Expected PaCO₂ = 1.5×HCO₃ + 8 ± 2
        Met Alkalosis: PaCO₂ = 40 + 0.7×(HCO₃-24) ± 5
        Acute Resp Acidosis: HCO₃ rises 1 per 10 mmHg CO₂
        Chronic Resp Acidosis: HCO₃ rises 3.5 per 10 mmHg CO₂
STEP 4: AG = Na - (Cl + HCO₃); Normal 8-12
        Correct for albumin: +2.5 per g/dL below 4.5
STEP 5: Δ/Δ = (AG-12)/(24-HCO₃)
        <1 = non-AG acidosis hiding; >2 = met alkalosis hiding
STEP 6: A-a gradient = PAO₂ - PaO₂; Normal = age/4 + 4
        P/F ratio: >400 normal; <200 ARDS; <100 severe ARDS
STEP 7: SYNTHESIZE - name the disorder(s)

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SPECIAL CLINICAL SCENARIOS

1. COPD Exacerbation

• Chronic respiratory acidosis: pH 7.35, PaCO₂ 60, HCO₃⁻ 34
• Renal compensation is chronic (HCO₃⁻ rises ~3.5 per 10 mmHg of PaCO₂)
• An acute-on-chronic: pH will drop more sharply than the HCO₃⁻ would suggest
• Key: compare to patient's baseline ABG if available

2. Sepsis

• Classic: metabolic acidosis (lactic) + respiratory alkalosis (tachypnea from fever/pain)
• pH can be near-normal, masking both disorders
• Clue: both PaCO₂ and HCO₃⁻ are low; a normal pH in this context = mixed disorder

3. DKA

• High-AG metabolic acidosis, Kussmaul breathing compensates
• Check Δ/Δ: if > 2, suspect vomiting-induced alkalosis
• Glucose corrects Na⁺: for every 100 mg/dL glucose above 100, add 1.6 mEq/L to Na⁺

4. Salicylate Toxicity

• Classic: metabolic acidosis + respiratory alkalosis
• Salicylates directly stimulate medullary respiratory center → primary respiratory alkalosis
• Also generate organic acids → primary metabolic acidosis
• Both disorders coexist - do not be fooled by a near-normal pH

5. Osmolal Gap (When to Add It)

Harrison's 22E, p.414: Most importantly, in the face of an elevated osmolar gap and anion gap, treatment should not be delayed while awaiting return of ethylene glycol or methanol levels from the laboratory.

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URINE ANION GAP - When You Need It

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BASE EXCESS - The Copenhagen Approach

• Positive BE (> +2) = metabolic alkalosis
• Negative BE (base deficit, < -2) = metabolic acidosis

Miller's Anesthesia 10e, p.6495: The BE is defined as the amount of strong acid or base required to return pH to 7.4, assuming PaCO₂ is constant at 40 mmHg and temperature is 37°C.

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SOURCES

• Goldman-Cecil Medicine International Edition, Vol. 1 - Systemic Arterial Blood Gas Analysis
• Harrison's Principles of Internal Medicine, 22E (2025) - Acid-Base Disturbances (Ch. 58)
• Barash Clinical Anesthesia, 9e - Diagnosis of Acid-Base Disorders and ABG Interpretation
• Current Surgical Therapy, 14e - Steps to ABG Interpretation
• Symptom to Diagnosis: An Evidence Based Guide, 4e - Stepwise Approach to Acid-Base Disorders
• Miller's Anesthesia, 10e - Base Excess (Copenhagen) Approach
• Murray & Nadel's Textbook of Respiratory Medicine