ABG Interpretation · Arterial Blood Gas Reference for Medical Students

🩸 Arterial Blood Gas (ABG) Interpretation A Stepwise Guide for Medical Students · Clinical Reference

1. Normal ABG Values & Key Concepts

Parameter	Normal Range (Arterial)	Clinical Significance
pH	7.35 – 7.45	Acidemia <7.35; Alkalemia >7.45
PaCO₂	35 – 45 mmHg	Respiratory component: >45 = respiratory acidosis; <35 = respiratory alkalosis
PaO₂	80 – 100 mmHg (age‑dependent)	Hypoxemia: mild 60‑79, moderate 40‑59, severe <40
HCO₃⁻	22 – 26 mEq/L	Metabolic component: <22 = metabolic acidosis; >26 = metabolic alkalosis
Base Excess	−2 to +2 mEq/L	Reflects metabolic disturbance independent of respiratory
SaO₂	95 – 100%	Oxygen saturation (calculated or measured)

🧠 Key Rule: pH is determined by the ratio of HCO₃⁻ to PaCO₂ (Henderson‑Hasselbalch). The body never fully "over‑compensates" — pH remains in the direction of the primary disorder unless a mixed disorder is present.

2. The 5‑Step Approach to ABG Interpretation

Step 1: Assess Oxygenation (PaO₂ & A‑a Gradient)
Is the patient hypoxemic? Calculate A‑a gradient if FiO₂ known.
PAO₂ = (FiO₂ × 713) − (PaCO₂ / 0.8) → A‑a = PAO₂ − PaO₂ (normal < age/4 + 4)
Step 2: Determine pH — Acidemia or Alkalemia?
pH <7.35 = acidemia; pH >7.45 = alkalemia.
Step 3: Identify the Primary Disorder — Respiratory or Metabolic?
If PaCO₂ and pH move in opposite directions → Respiratory (↑PaCO₂ + ↓pH = respiratory acidosis).
If HCO₃⁻ and pH move in same direction → Metabolic (↓HCO₃⁻ + ↓pH = metabolic acidosis).
Step 4: Assess Compensation
Is the compensation appropriate? (see formulas below). If not → mixed disorder.
Step 5: Calculate Anion Gap (AG) & Delta Gap (if metabolic acidosis)
AG = Na⁺ − (Cl⁻ + HCO₃⁻). Normal ≈ 8–12 (adjust for albumin).

3. Primary Acid‑Base Disorders & Expected Compensation

Disorder	Primary Change	Compensatory Response	Expected Compensation Formula
Respiratory Acidosis	↑ PaCO₂	↑ HCO₃⁻ (renal, slow)	Acute: ↑ HCO₃⁻ 1 mEq per 10 mmHg CO₂ above 40 Chronic: ↑ HCO₃⁻ 4 mEq per 10 mmHg CO₂ above 40
Respiratory Alkalosis	↓ PaCO₂	↓ HCO₃⁻ (renal)	Acute: ↓ HCO₃⁻ 2 mEq per 10 mmHg CO₂ below 40 Chronic: ↓ HCO₃⁻ 5 mEq per 10 mmHg CO₂ below 40
Metabolic Acidosis	↓ HCO₃⁻	↓ PaCO₂ (hyperventilation)	Winter's Formula: Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
Metabolic Alkalosis	↑ HCO₃⁻	↑ PaCO₂ (hypoventilation)	Expected PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24)

⚠️ If measured compensation differs significantly from expected → Mixed Disorder. For example, in metabolic acidosis, if PaCO₂ is higher than Winter's formula predicts, there is a concurrent respiratory acidosis.

4. Anion Gap (AG) & Delta Gap in Metabolic Acidosis

Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻) · Normal: 8–12 (correct for albumin: AG corrected = AG + 2.5 × [4.0 − albumin])

🔺 High AG Metabolic Acidosis

MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
Also: Alcoholic ketoacidosis, starvation ketosis.

🔹 Normal AG (Hyperchloremic) Metabolic Acidosis

USED CARP: Ureteral diversion, Saline infusion, Endocrine (Addison's), Diarrhea, Carbonic anhydrase inhibitors, RTA, Pancreatic fistula.

Delta Gap (ΔAG / ΔHCO₃⁻) — Unmasking Mixed Disorders

Delta Gap = (Measured AG − 12) / (24 − Measured HCO₃⁻)
Ratio <0.8: Concurrent normal AG metabolic acidosis (e.g., diarrhea + lactic acidosis).
Ratio 0.8–1.2: Pure high AG metabolic acidosis.
Ratio >1.2: Concurrent metabolic alkalosis (e.g., vomiting + DKA) OR pre‑existing high HCO₃⁻.

💡 Example: Patient with DKA: pH 7.25, HCO₃⁻ 12, AG 24. Delta gap = (24−12)/(24−12) = 1.0 → pure AG acidosis. If HCO₃⁻ were 18 with same AG, delta gap = 12/6 = 2.0 → concurrent metabolic alkalosis (likely from vomiting).

5. Mixed Acid‑Base Disorders: Recognition

Scenario	ABG Clues	Common Causes
Metabolic Acidosis + Respiratory Alkalosis	PaCO₂ lower than Winter's prediction	Salicylate toxicity, sepsis, liver failure
Metabolic Acidosis + Respiratory Acidosis	PaCO₂ higher than Winter's prediction	Cardiac arrest, severe pneumonia with shock
Metabolic Alkalosis + Respiratory Acidosis	PaCO₂ higher than expected for HCO₃⁻	COPD patient on diuretics or vomiting
Metabolic Alkalosis + Respiratory Alkalosis	PaCO₂ lower than expected for HCO₃⁻	Pregnancy with vomiting, liver disease on diuretics
Triple Disorder	High AG + normal AG acidosis + metabolic alkalosis	DKA + vomiting + renal tubular acidosis

6. Hypoxemia & A‑a Gradient

Alveolar Gas Equation (Sea Level, FiO₂ = 0.21):

    PAO₂ = (FiO₂ × 713) − (PaCO₂ / 0.8)    (at sea level, room air ≈ 150 − PaCO₂/0.8)
  

A‑a Gradient = PAO₂ − PaO₂ · Normal < age/4 + 4 (e.g., age 40 → normal <14 mmHg).

A‑a Gradient	Mechanism	Examples
Normal	Hypoventilation, low FiO₂	Opiate overdose, high altitude, neuromuscular weakness
Increased	V/Q mismatch, shunt, diffusion impairment	Pneumonia, PE, ARDS, ILD, pulmonary edema

🫁 Shunt vs. V/Q Mismatch: Shunt (e.g., lobar pneumonia, ARDS) does not improve significantly with 100% O₂. V/Q mismatch (COPD, asthma) usually responds to supplemental O₂.

7. Clinical Case Application

Case 1: COPD Exacerbation

ABG: pH 7.29, PaCO₂ 68, PaO₂ 52, HCO₃⁻ 32 (on room air).

Step 1: Hypoxemic (PaO₂ 52), A‑a gradient likely elevated.
Step 2: pH 7.29 → acidemia.
Step 3: ↑ PaCO₂ + ↓ pH → Respiratory acidosis.
Step 4: HCO₃⁻ 32. Acute compensation expected HCO₃⁻ = 24 + 1×(68−40)/10 ≈ 27. Chronic expected = 24 + 4×(28/10) ≈ 35. HCO₃⁻ 32 indicates partially compensated / acute‑on‑chronic respiratory acidosis.
Interpretation: Acute‑on‑chronic respiratory acidosis with hypoxemia. Likely COPD exacerbation.

Case 2: DKA

ABG: pH 7.21, PaCO₂ 25, PaO₂ 98, HCO₃⁻ 10. Na 136, Cl 100.

Step 1: PaO₂ 98 → normal oxygenation.
Step 2: pH 7.21 → acidemia.
Step 3: ↓ HCO₃⁻ + ↓ pH → Metabolic acidosis.
Step 4: Winter's formula: Expected PaCO₂ = (1.5×10) + 8 ± 2 = 21–25. PaCO₂ 25 → appropriate respiratory compensation (pure metabolic acidosis).
Step 5: AG = 136 − (100+10) = 26 → High AG metabolic acidosis. Delta gap = (26−12)/(24−10) = 14/14 = 1.0 → pure high AG acidosis.
Interpretation: High AG metabolic acidosis with appropriate respiratory compensation. Consistent with DKA.

Case 3: Salicylate Toxicity

ABG: pH 7.44, PaCO₂ 20, PaO₂ 105, HCO₃⁻ 14. Na 140, Cl 105.

Step 1: PaO₂ normal.
Step 2: pH 7.44 → normal/slightly alkalemic.
Step 3: ↓ PaCO₂ (alkalosis) and ↓ HCO₃⁻ (acidosis) → Mixed disorder.
Step 4: AG = 140 − (105+14) = 21 (high). Winter's for HCO₃⁻ 14 = PaCO₂ 27–31. Actual PaCO₂ 20 (lower) → concurrent primary respiratory alkalosis.
Interpretation: High AG metabolic acidosis + primary respiratory alkalosis. Classic for salicylate poisoning (stimulates respiratory center directly).

Case 4: Vomiting + Diuretic Use

ABG: pH 7.52, PaCO₂ 48, PaO₂ 88, HCO₃⁻ 38. Na 138, Cl 88.

Step 1: Mild hypoxemia? PaO₂ 88 (age 60 → normal A‑a ~19).
Step 2: pH 7.52 → alkalemia.
Step 3: ↑ HCO₃⁻ + ↑ pH → Metabolic alkalosis.
Step 4: Expected PaCO₂ = 40 + 0.7×(38−24) ≈ 50. Actual PaCO₂ 48 → appropriate respiratory compensation (hypoventilation).
Step 5: AG = 138 − (88+38) = 12 (normal).
Interpretation: Metabolic alkalosis with appropriate respiratory compensation. Likely from vomiting or diuretics (check urine Cl⁻ to differentiate: <20 = vomiting; >20 = diuretics).

8. Quick Reference: ABG Patterns at a Glance

pH	PaCO₂	HCO₃⁻	Likely Primary Disorder
↓ (<7.35)	↑ (>45)	N or ↑	Respiratory acidosis
↓	↓ (<35)	↓ (<22)	Metabolic acidosis
↑ (>7.45)	↓	N or ↓	Respiratory alkalosis
↑	↑	↑ (>26)	Metabolic alkalosis
Normal (7.35‑7.45)	Abnormal	Abnormal	Fully compensated or mixed disorder

Compensation Mnemonics

💰 "One‑Five‑Two‑Four" for Respiratory:
Acute Resp Acidosis: HCO₃⁻ ↑ 1 per 10 CO₂
Chronic Resp Acidosis: HCO₃⁻ ↑ 4 per 10 CO₂
Acute Resp Alkalosis: HCO₃⁻ ↓ 2 per 10 CO₂
Chronic Resp Alkalosis: HCO₃⁻ ↓ 5 per 10 CO₂

❄️ Winter's Formula:
Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
(If measured PaCO₂ differs → mixed disorder)

9. Clinical Pearls & Common Pitfalls

Venous Blood Gas (VBG): pH ~0.03 lower, PvCO₂ ~5‑6 mmHg higher than arterial. Can be used for acid‑base assessment but not oxygenation.
Hypoalbuminemia: Lowers AG; correct formula: AG_corrected = AG + 2.5 × (4.0 − albumin).
Hyperventilation in Metabolic Acidosis: PaCO₂ rarely drops below ~10 mmHg regardless of HCO₃⁻.
Post‑hypercapnic Metabolic Alkalosis: After rapid correction of chronic respiratory acidosis (e.g., mechanical ventilation), HCO₃⁻ remains elevated → metabolic alkalosis.
Pseudohypoxemia: Leukocyte larceny (extreme leukocytosis consumes O₂ in sample tube) → send on ice and run STAT.

🩸 ABG Interpretation Reference · High‑yield for medical students, USMLE, and clinical rotations.
Stepwise approach, compensation formulas, anion gap, delta gap, mixed disorders, and case applications.