Waleed Talal Alotaibi MBBS

objectives Definitions How to approach? Differential diagnosis Anion gap VS. non-anion gap metabolic acidosis Treatment of severe metabolic acidosis

Definitions Normal pH 7.36 to 7.44 HCO3 22 to 28 meq/LPCO2 36 to 44 mmHg Acidemia Alkalemia Acidosis Alkalosis Metabolic acidosis Metabolic alkalosis Respiratory acidosis Respiratory alkalosis Simple acid-base disorder Mixed acid-base disorder

How to approach? In simple 4 steps: First: determine the primary diagnosis Second: assess the degree of compensation Third: calculate anion gap (if elevated, caculate delta delta ratio) Fourth: clinical diagnosis

Primary diagnosis Metabolic acidosis: Low arterial pH, and low serum HCO3

Compensation Respiratory compensation begins within 30 min and complete within 12 to 24 hrs Expected compensation: Decrease in PCO2 = 1.2 ×Δ HCO3 PCO2 = 1.5 × HCO3 + 8 ± 2 PCO2 = serum HCO PCO2 = decimal digits of pH

Anion gap Why anion gap? Normal serum AG 7 to 13 meq/L Serum AG = measured cations – measured anions Serum AG = (Na + k) – (Cl + HCO3) Serum AG = unmeasured anions – unmeasured cations

Anion gap If AG is elevated then calculate delta delta ( Δ AG/ Δ HCO3) why? (calculated AG-expected AG)/(24-HCO3) Expected AG = 2.5 × albumin (g/dL)

Clinical diagnosis AG metabolic acidosis VS. non-AG metabolic acidosis

AG metabolic acidosis MUDPILERS Methanol Uremia DKA Paraldehyde INH Lactic acidosis Ethanol/ethylene glycol Rhabdomyolysis, renal failure Salicylate

AG metabolic acidosis Work up for ketonuria, if negative do renal function, lactate and osmolal gap (OG) OG = measured osmoles – calculated osmoles Calculated osmoles = (2 × Na) + (glucose/18) + (BUN/2.8) If OG > ingestion? Do toxin screen

Non-AG metabolic acidosis HARD UPS Hyperalimentation Acetazolamide Renal Tubular Acidosis Diarrhea Uretero-Pelvic Shunt Post-Hypocapnia Spironolactone

Non-AG metabolic acidosis Workup: urine AG = (U Na + U K) – U Cl If negative: appropriate renal response to acidemia If positive: the kidneys failed to secrete NH4

Treatment Usually according to the underlying cause Severe metabolic acidosis (pH < 7.2): The goal is to achive pH >7.20 and HCO3 >8 NaHCO3 8- HCO3 × wt. × 0.5 = mmol of HCO3

References Uptodate The Massachusetts General hospital Handbook of Internal Medicine, 4 th edition