ABG Analysis Dr. Katrina Romualdez ED Registrar Royal Adelaide Hospital

What We’ll Try to Cover (AKA How to Plan your Bathroom Breaks/Sneaky Escapes) Stepwise interpretation

The Great ABG - VBG Debate ABG Pros gold standard for determining arterial metabolic milieu can determine PaO2 ABG Cons Pain for patients (and doctors) Complications (bleeding/pseudo-aneurysm/fistula) Serial sampling VBG values generally have good correlation with some notable exceptions

When is an ABG necessary To accurately determine PaCO2 in severe shock To accurately determine PaCO2 if hypercapnic (PaCO2 > 45 mmHg) To accurately determine arterial lactate (Venous lactate > 2 mmol/L)

Case D.V., 64/M, with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG. RESULTS: pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8

Step 1: Look at pH “-aemia” vs. “-osis”

Step 2: Look at pCO2 same direction as pH: primary metabolic disorder opposite direction to pH: primary respiratory disorder

Case 64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG. RESULTS: Acidaemia Metabolic acidosis pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8

Step 3: Calculate compensatory response Metabolic acidosis Expected pCO2 = 1.5 (HCO3-) + 8 Metabolic alkalosis Expected pCO2 = 40 + [0.7 (HCO3 - 24)] Respiratory Acidosis acute: expected HCO3 = 24 + (pCO2 - 40/10) chronic: expected HCO3 = 24 + 4(pCO2 - 40/10) Respiratory Alkalosis acute: expected HCO3 = 24 - 2(40 - pCO2/10) chronic: expected HCO3 = 24 - 5(40 - pCO2/10)

Step 3: Calculate compensatory response Respiratory Acidosis = HCO3 increases acute: HCO3 increases by 1 mEq/L for every 10 mmHg increase in pCO2 above 40 chronic: HCO3 increases by 4 mEq/L for every 10 mmHg increase in pCO2 above 40 Respiratory Alkalosis = HCO3 decreases acute: HCO3 decreases by 2 mEq/L for every 10 mmHg decrease in pCO2 below 40 chronic: HCO3 decreases by 5 mEq/L for every 10 mmHg decrease in pCO2 below 40

A word about compensation/response Speed/rate of compensation Respiratory compensation: 1-24 hours Metabolic compensation: 2-5 days Respiratory compensation for metabolic alkalosis hypoxia blunts body’s ability to hypo ventilate

Case 64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG. RESULTS: Metabolic Acidosis Expected pCO2 = 24.5 +/- 2 Concomitant respiratory alkalosis pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8

Step 4: Gaps and Biochemistry Anion Gap Delta Ratio** only refers to metabolic processes NOT respiratory assumes 1:1 buffering of unmeasured acids by HCO3 (there are other buffers) overdiagnoses metabolic alkalosis Lactate Ketones Other

Biochemistry Corrections Corrected K Corrected Na Corrected Anion Gap

Case pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8 64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG. RESULTS: Metabolic Acidosis +Concomitant respiratory alkalosis Anion Gap = 33 — Elevated AG metabolic acidosis Delta Ratio = 33-12/18-11 = 3 — 3rd disorder: Metabolic alkalosis pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8

Formal bloods come back… MBA 20 Urea 22 Creatinine 327 Albumin normal Hb 110

Elevated anion gap acidosis Lactic acidosis from hypo-perfusion 64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. Elevated anion gap acidosis Lactic acidosis from hypo-perfusion Systemic: sepsis Regional: ischaemic bowel Renal failure: acute vs chronic

Respiratory Alkalosis Hypoxemia: acute: pneumonia 64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. Respiratory Alkalosis Hypoxemia: acute: pneumonia chronic: chronic pulmonary disease

Step 5: Oxygenation (A Gen Med favorite) A-a gradient = PAO2 - PaO2 Alveolar pO2 (PAO2) = FiO2 (Patm - PH20) - (PaCO2/RQ) Normal range: (Age/4) + 4 Caveat: Varies with age Issues: FiO2 Clinical usefulness

Case pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8 64/M with fever, abdominal pain, and hypotension. He is currently on 6 litres of oxygen in Area A. You get an ABG. RESULTS: Elevated anion gap metabolic acidosis Respiratory alkalosis Metabolic alkalosis FiO2 = 45% A-a gradient = 240 mmHg pH 7.008 Na 142 pCO2 18 K 5.0 pO2 58 Cl 98 HCO3 11 Lactate 6.8

Causes of Increased A-a Gradient Impaired diffusion V/Q mismatch Shunt

Putting It All Together Elevated anion gap acidosis Lactic acidosis from hypo-perfusion Systemic: sepsis Regional: ischaemic bowel Renal failure: acute vs chronic Respiratory Alkalosis with abnormal A-a gradient Hypoxemia: acute: pneumonia chronic: chronic pulmonary disease Metabolic alkalosis Dehydration: contraction alkalosis

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