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I now have all the information needed to give a thorough, example-rich explanation of the full ABG interpretation chart from the images. Here it is:
ABG Interpretation - Complete Guide with Live Examples
This chart from Srikamatchi Medical Centre lays out a 5-step systematic approach to reading an Arterial Blood Gas (ABG). Let's go through each step with real clinical examples.
Normal ABG Values (the baseline you compare against)
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mmHg |
| PaO2 | 80 - 100 mmHg |
| HCO3- | 20 - 28 mEq/L |
STEP 1: Identify the Primary Process
Follow the decision tree using pH first, then check PaCO2 or HCO3:
pH
├── Low (<7.35) → ACIDOSIS
│ ├── High PaCO2 → Respiratory Acidosis
│ └── Low HCO3 → Metabolic Acidosis
├── Normal (7.35-7.45) → Normal OR Mixed disorder
└── High (>7.45) → ALKALOSIS
├── Low PaCO2 → Respiratory Alkalosis
└── High HCO3 → Metabolic Alkalosis
Live Examples:
Example A - Respiratory Acidosis:
pH = 7.22, PaCO2 = 68, HCO3 = 26
- pH is LOW → Acidosis
- PaCO2 is HIGH → Respiratory Acidosis
- Clinical cause: COPD exacerbation, patient hypoventilating and retaining CO2
Example B - Metabolic Acidosis:
pH = 7.20, PaCO2 = 22, HCO3 = 10
- pH is LOW → Acidosis
- HCO3 is LOW → Metabolic Acidosis
- Clinical cause: Diabetic Ketoacidosis (DKA)
Example C - Respiratory Alkalosis:
pH = 7.52, PaCO2 = 28, HCO3 = 23
- pH is HIGH → Alkalosis
- PaCO2 is LOW → Respiratory Alkalosis
- Clinical cause: Anxiety hyperventilation, early sepsis, pulmonary embolism
Example D - Metabolic Alkalosis:
pH = 7.50, PaCO2 = 44, HCO3 = 34
- pH is HIGH → Alkalosis
- HCO3 is HIGH → Metabolic Alkalosis
- Clinical cause: Severe vomiting (losing HCl), over-use of antacids
STEP 2: Is Compensation Present?
The body always tries to bring the pH back toward normal. The key rule:
In compensation: PaCO2 and HCO3 move in the SAME direction. If they move in OPPOSITE directions → a mixed disorder is present.
| Primary Disorder | Compensatory Response |
|---|---|
| Metabolic Acidosis | Lungs blow off CO2 (PaCO2 falls) |
| Metabolic Alkalosis | Lungs retain CO2 (PaCO2 rises) |
| Respiratory Acidosis | Kidneys retain HCO3 (HCO3 rises) |
| Respiratory Alkalosis | Kidneys excrete HCO3 (HCO3 falls) |
Live Example:
pH = 7.32, PaCO2 = 28, HCO3 = 14
- Low pH = Acidosis; Low HCO3 = Metabolic Acidosis (primary)
- PaCO2 is also low = lungs are compensating by blowing off CO2
- Both moving DOWN (same direction) = compensated metabolic acidosis - this is a simple disorder
pH = 7.30, PaCO2 = 55, HCO3 = 10
- Low pH = Acidosis; PaCO2 high = Respiratory Acidosis; HCO3 low = Metabolic Acidosis
- CO2 and HCO3 moving in opposite directions = Mixed disorder (both respiratory + metabolic acidosis simultaneously)
- Clinical cause: Cardiac arrest (hypoperfusion + respiratory failure)
STEP 3: Is the Respiratory Disorder Acute or Chronic?
Formula:
ΔpH = 0.08 × ΔPaCO2 / 10 Expected pH = 7.4 - ΔpH
- If pH in ABG = Expected pH → Acute change (kidneys haven't had time to compensate)
- If pH in ABG > Expected pH → Chronic change (kidneys have already compensated)
Live Example:
PaCO2 = 70 (normal 40), so ΔPaCO2 = 30 ΔpH = 0.08 × (30/10) = 0.08 × 3 = 0.24 Expected pH = 7.4 - 0.24 = 7.16
- If actual pH = 7.16 → Acute respiratory acidosis (COPD sudden exacerbation)
- If actual pH = 7.30 → Chronic respiratory acidosis (long-standing COPD with renal compensation)
STEP 3 (cont.): Expected Compensation Formulas
| Disorder | Expected Compensation Formula |
|---|---|
| Metabolic Acidosis | PaCO2 = (1.5 × HCO3) + 8 ± 2 (Winters' formula) |
| Metabolic Alkalosis | PaCO2 increases by 7 mmHg per 10 mEq rise in HCO3 |
| Resp. Acidosis Acute | HCO3 increases by 1 per 10 mmHg rise in PaCO2 |
| Resp. Acidosis Chronic | HCO3 increases by 3.5 per 10 mmHg rise in PaCO2 |
| Resp. Alkalosis Acute | HCO3 falls by 2 per 10 mmHg fall in PaCO2 |
| Resp. Alkalosis Chronic | HCO3 falls by 4 per 10 mmHg fall in PaCO2 |
Live Example (Winters' formula):
Patient has metabolic acidosis. HCO3 = 14 Expected PaCO2 = (1.5 × 14) + 8 = 21 + 8 = 29 ± 2 (i.e., 27-31) Measured PaCO2 = 29 → Simple metabolic acidosis with appropriate compensation If measured PaCO2 = 45 → too high! → Concurrent respiratory acidosis (mixed disorder) If measured PaCO2 = 18 → too low! → Concurrent respiratory alkalosis (e.g., sepsis + lactic acidosis)
STEP 4: Anion Gap (when metabolic acidosis is present)
Formula:
AG = Na - (Cl + HCO3) | Normal = 10-12 mEq/L
If albumin is low (<4 g/dL), correct it:
Corrected AG = [2.5 × (4 - serum albumin)] + measured AG
Why it matters:
The anion gap tells you what is causing the metabolic acidosis.
| AG Type | Causes | Mnemonic |
|---|---|---|
| Wide AG (>12) | Unmeasured anions accumulating | MUDPILES |
| Normal AG | Bicarbonate lost, Cl replaces it | ACCRUED |
MUDPILES (Wide AG causes):
- M - Methanol
- U - Uremia (renal failure)
- D - DKA (Diabetic Ketoacidosis)
- P - Paraldehyde
- I - IEM (Inborn Errors of Metabolism)
- L - Lactic acidosis (sepsis, cardiac arrest)
- E - Ethylene glycol (antifreeze poisoning)
- S - Salicylates (aspirin overdose)
ACCRUED (Normal AG causes):
- A - Acid ingestion (HCl, NH4Cl)
- C - Compensation for resp. alkalosis
- C - Carbonic anhydrase inhibitors (acetazolamide)
- R - RTA (Renal Tubular Acidosis)
- U - Ureteral diversion
- E - Extra alimentation (TPN)
- D - Diarrhea (loses HCO3)
Live Example:
pH = 7.18, PaCO2 = 22, HCO3 = 8, Na = 140, Cl = 102 AG = 140 - (102 + 8) = 30 (wide gap!) Wide AG Metabolic Acidosis → think DKA, lactic acidosis, etc. Clinical context: diabetic patient, glucose 550 → DKA confirmed
STEP 5: Delta Ratio (when AG is elevated)
Checks whether the elevated anion gap fully accounts for the drop in HCO3, or if there's a hidden concurrent disorder.
Delta Ratio = ΔAG / ΔHCO3 = (AG - 12) / (24 - HCO3)
| Delta Ratio | Interpretation |
|---|---|
| < 0.4 | Hyperchloremic normal anion gap acidosis |
| < 1 | High AG + normal AG acidosis (mixed) |
| 1 - 2 | Pure anion gap acidosis (lactic acidosis ~1.6, DKA closer to 1) |
| > 2 | AG acidosis + concurrent metabolic alkalosis (or pre-existing compensated resp. acidosis) |
Live Example - Delta Ratio:
AG = 28, HCO3 = 16 ΔAG = 28 - 12 = 16 ΔHCO3 = 24 - 16 = 8 Delta Ratio = 16/8 = 2.0
- This is >2 → the AG rise is much larger than the HCO3 fall
- Meaning: HCO3 is not as low as expected → there must be a concurrent metabolic alkalosis hiding underneath
- Clinical example: A vomiting patient (metabolic alkalosis) who develops lactic acidosis from sepsis. The pH ends up in the "normal" range, but the delta ratio unmasks the dual disorder.
Full Worked Case - Putting It All Together
Case: 55-year-old man, known diabetic, comes in with altered consciousness and rapid breathing
ABG: pH = 7.18 | PaCO2 = 22 | HCO3 = 8 | Na = 138 | Cl = 100 | Albumin = 4
Step 1: pH < 7.35 (acidosis); HCO3 is low → Metabolic Acidosis
Step 2: PaCO2 is also low (same direction as HCO3) → compensation present, NOT a mixed disorder
Step 3 (Winters'): Expected PaCO2 = (1.5 × 8) + 8 = 20 ± 2 → Measured = 22 → appropriate compensation ✓ (simple disorder)
Step 4 (AG): AG = 138 - (100 + 8) = 30 → Wide AG metabolic acidosis → MUDPILES → DKA most likely
Step 5 (Delta Ratio): ΔAG = 30-12 = 18; ΔHCO3 = 24-8 = 16; Delta = 18/16 = 1.1 → Pure anion gap acidosis, appropriate for DKA ✓
Diagnosis: DKA with appropriate respiratory compensation. No hidden mixed disorder.
This 5-step systematic method - as referenced in Barash's Clinical Anesthesia, ROSEN's Emergency Medicine, and the Srikamatchi chart - ensures no disorder is missed, including the subtle "masked" mixed disorders that the delta ratio uncovers. Always apply all 5 steps in sequence rather than stopping at Step 1.
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