Acid – Base Disorders Viyeka Sethi PGY 4 Med-Peds

Four Main Acid-Base Disorders DisorderPrimary Alteration Secondary Response Mechanism of Response Metabolic Acidosis in plasma HCO3 in plasma pCO2 Hyperventilation Metabolic Alkalosis in plasma HCO3 increase in pCO2 Hypoventilation Respiratory Acidosis in plasma pCO2 in plasma HCO3 Increase in acid excretion; increase in reabsorption of HCO3 Respiratory Alkalosis in plasma pCO2 in plasma HCO3 Suppression of acid excretion; decrease in reabsorption of HCO3

Normal Values ArterialVenous pH7.40<7.35 HCO324 pCO240>40 pO2>70<60

Step by Step 1. Determine the primary disturbance:  Acidemia or Alkalemia: look at the pH < 7.40 = acidemia > 7.40 = alkalemia  Respiratory or Metabolic: look at HCO3 and CO2 HCO3 = primary metabolic acidosis pCO2 = primary respiratory acidosis and vice versa for alkalosis

2. Determine acute or chronic for Respiratory Disturbance: o Compensation attempts to normalize pH but can be present with an abnormal pH o Expected change in pCO2 best used for primary metabolic disturbance and expected change in HCO3 for primary respiratory disturbance

Respiratory Disturbance Compensation pCO2 in pH in HCO3 Acute Resp. Acidosis Chronic Resp. Acidosis pCO2 in pH in HCO3 Acute Resp. Alkalosis Chronic Resp. Alkalosis

3. Primary Metabolic Disturbance: o Calculate anion gap : Na – (Cl + HCO3) o Normal = 12 +/- 2 o If gap is >20 then there is primary metabolic acidosis regardless of pH or bicarb. o Helps narrow differential with a anion gap or non-anion gap metabolic acidosis

4. Assess appropriate respiratory compensation for metabolic disorder: o Respiratory compensation is fast o Winters formula: Expected pCO2 = (1.5 * HCO3) + 8 (+/-2) o If measured pCO2 is < expected then co-existing resp. alkalosis > expected then co-existing resp. acidosis

5. Determine if other metabolic disturbances co-exist with AG metabolic acidosis: o Delta gap – accounts for increase in anion gap and shows any variation in HCO3 o If no other disorder is present then the calculation should be 24

Delta Gap Corrected HCO3 = measured HCO3 + (AG - 12) o So if corrected HCO3 >24 then metabolic alkalosis co-exists <24 then non-anion gap metabolic acidosis co- exists

Hints Simple acid base – compensatory response always in direction of primary variable Compensation is more pronounced with chronic disorders Normal pH indicates two or more acid-base disorders If given electrolytes, use them!!!

Pneumonics for pnuemonic lovers Metabolic Acidosis Anion Gap “MUDPILERS” Metabolic Acidosis Non- Gap “HARDUPS” Acute Resp. Acidosis “anything causing hypoventilation” Metabolic Alkalosis “CLEVERPD” Respiratory Alkalosis “CHAMPS” Methanol Uremia DKA/Alcoholic ketoacidosis Paraldehyde Isoniazid Lactic acidosis Ethanol Renal failure/Rhabdo Salicylates Hyperalimentation Acetazolamide Renal Tubular Acidosis Diarrhea Uretero-Pelvic shunt Post-hypocapnia Spironolactone CNS depression Airway obstruction Pulmonary edema Pneumonia Hemo/Pneumo thorax Neuromuscular Contraction Licorice Endocrine (Conn/Cushing /Bartters) Vomiting Excess alkali Refeeding Post- hypercapnia Diuretics CNS disease Hypocapnia Anxiety Mech. Ventilation Progesterone Salicylates Sepsis

Case 1 3 yo boy with diarrhea is evaluated in the ER. Initial ABG shows: ph – 7.23 HCO3 – 10 pCO2 – 23 AG - 13 Alkalemia or acidemia? Low pH = acidemia Primary disturbance resp. or metabolic? Low HCO3 = metabolic acidosis Normal AG so no need for Delta gap equation BUT Is there adequate respiratory compensation? (Use Winter’s formula) Expected pCO2 = (1.5 * HCO3) + 8 +/-2 So, Expected pCO2 = (1.5 * 10) + 8 +/-2 =(15) + 8 +/-2 =23 +/- 2 So, we have a metabolic acidosis with respiratory compensation

Case 2 5 yo boy presents to ED with dyspnea for 3 days. ABG shows the following: pH – 7.35 paCO2 – 60 paO2 – 57 HCO Acidemia or alkalemia? Low pH = Acidemia Primary Resp. or Metabolic disturbance? Respiratory Check yourself….using rules for Primary Resp disturbance For every 10 increase in pCO2 - pH decreases by.08 acutely or.03 chronically HCO3 increases by 1 acutely or 3 chronically So, this is a chronic resp. acidosis

Case 3 15 yo with 4 days of vomiting and fever. pH- 7.50Na- 138 pCO2- 42Cl- 80 pO2- 80HCO3- 34 Acidemia or alkalemia? High ph = alkalemia Respiratory or Metabolic? High HCO3 = metabolic Is Resp. compensation appropriate? Exp. pCO2 = (1.5 * HCO3) + 8 +/-2 =(51) + 8 +/-2 =59 +/- 2 So, NO the CO2 is lower so we have a Co-existent resp.alkalosis Anion Gap? 138 – ( ) = 24 Checking the Delta gap only useful in a primary metabolic acidosis.

Case 4