ABG INTERPRETATION By: Dr. Ashraf Al Tayar, MD,MRCP(I), Assistant Consultant Critical Care, KKNGH.

ABG Interpretation: Comprises 4 steps. Determine the process Determine the primary disorder Sharpen the diagnosis Determine compensation

I. Step 1: Use pH to determine Acidosis - < 7.35 Alkalosis - > 7.45

- Metabolic acidosis pH PCO2 HCO3 - Metabolic alkalosis II. Determine metabolic vs. Respiratory Metabolic disorders: - pH changes in same direction as PCO2 and HCO3 - Metabolic acidosis pH PCO2 HCO3 - Metabolic alkalosis

Respiratory disorders: - pH change in opposite direction of HCO3 and PCO2 - Respiratory acidosis pH PCO2 HCO3 - Respiratory alkalosis

Sharpen the Diagnosis - Calculate the AG - Calculate Osm. Gap - Calculate Urinary AG

AG: Na – ( CL + HCO3) normal 10 (+/-) 2

AG: - Paraproteinaemia - Hyperchloraemia - Hypoalbumenaemia ( AG 2.5 / 1 gm /dL in albumin) - Hypermagnesaemia - Hyponatraemia

AG : Metabolic acidosis (DD) (MUDPILERS) Methanol Uremia DKA Paraldehyde Isopropyl alcohol – Isoniazide Lactic acid Ethylene glycol Rhabdomyolosis Salycylate

Normal AG Metabolic acidosis (Hyperchloremia) A. Hypokalemia - Diarrhea - Urethral diversion - RTA - Mineralocorticorticoid deficiency - CAI: Acetazolamide B. Hyperkalemia - Renal failure (Early) - Renal disease: SLE Amylodosis Sickle cell - Sulphur toxicity

Urine AG - < - 10 extra renal causes - > + 10 Renal causes

Osmolal Gap - measured serum osm. – calc. osm. normal (10 – 15) OG DD: ( MEDIE ) Methanol Ethylene glycol Diuretic : Mannitol Isoprophyl alcohol Ethanol

Determine Compensation Metabolic acidosis PCO2 = 40 + BE Metabolic alkalosis PCO2 = 40 + 0.6 X BE

Respiratory acidosis Acute :HCO3 = PCO2 – 40 / 10 + 24 Chronic : HCO3 = PCO2 – 40 / 3 + 24 Respiratory alkalosis: Acute : 40 – PCO2 / 5 + 24 Chronic: 40 – PCO2 / 2 + 24

Example 1: pH: 7.07 PCO2: 28 HCO3: 8 SBE: - 20

Example 2: pH : 7.33 PCO2 : 20 HCO3 : 12 BE : 12

Example 3 pH : 7.48 PCO2 : 28 HCO3: 22 BE : 2

Effects of Acid base change Acidosis alkalosis CVS Inotropy Inotropy Conduction- defect Altered coronary flow Arterial VD Digoxin toxicity Venous VD Oxygen Delivery O2 Hb binding Affinity 2, 3 DPG 2, 3 DPG Neuromuscular Resp. dep NM excitability Sensorium Encephalopathy Seizures Resp. dep Electrolytes Hyper K+ Hypo K+ Hyper Ca+ Hypo Ca+ Hyperuricaemia Hypophosphatemia

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