The Fooproof 5-Step Approach to Acid-Base

The Fooproof 5-Step Approach to Acid-Base

THE FOOLPROOF 5STEP APPROACH TO ACID-BASE The Approach That Never Fails Robert C Hollander, M.D. PGY-30 Gainesville VA Five Steps 4. Acidemia v. Alkalemia Metabolic v. Respiratory ?Anion Gap Does the AG = HCOAG = AG = HCOHCO3 ? 5. Is there appropriate compensation? 1. 2. 3. Five Steps Step #1 1. Acidemia v. Alkalemia 4. Metabolic v. Respiratory ?Anion Gap Does the AG = HCOAG = AG = HCOHCO3 ? 5. Is there appropriate compensation? 2. 3. Pre-supposes you have an ABG Accurate conclusions cannot be drawn from HCO3 alone

Normal range: 7.35-7.45 If there is an abnormal pCO2, HCO3 or AG, then 7.4 is the dividing line Five Steps Step #2 1. Acidemia v. Alkalemia 2. Metabolic v. Respiratory 4. ?Anion Gap Does the AG = HCOAG = AG = HCOHCO3 ? 5. Is there appropriate compensation? 3. Ask yourself out loud (softly if others are around) What explains the acidemia? Or What explains the alkalemia? If HCO3 Metabolic If pCO2 If both, pick one and the Foolproof Approach will catch the other later.

Respiratory Five Steps Step #3 2. Acidemia v. Alkalemia Metabolic v. Respiratory 3. ?Anion Gap 1. Does the AG = HCOAG = AG = HCOHCO3 ? Is there appropriate compensation? 4. 5. AG = Na (Cl + HCO3) AG = Unmeasured Anions Unmeasured Cations AG= an artifact of laboratory measurement AG allows inferences about unmeasured anions Albumin excepted, the Unmeasured Anions are salts of organic acids Therefore, AG elevations Metabolic Acidosis Exceptions exists

Anion Gap Methanol Uremia DKA Propylene glycol (not paraldehyde) INH (impaired hepatic clearance of lactate) Lactic acidosis Ethanol/Ethylene Glycol Salicylates Non-Anion Gap Metabolic Acidosis Baseline Abnormal Sodium 136 136 Chloride 102 112 Bicarbonate 24 14 Anion Gap

10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.29 pCO2 40 29 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? Diarrhea, RTA, carbonic anhydrase inhibitors, ureteral diversions Dilutional acidosis, post hypocapnic Five Steps - #5 4. Acidemia v. Alkalemia Metabolic v. Respiratory ?Anion Gap Does the AG = HCOAG = AG = HCOHCO3 ? 5. Is there appropriate compensation? 1.

2. 3. Compensation will return the pH towards normal Compensation is either: Appropriate, or If not, indicative of another acid-base disturbance If Metabolic Acidosis prevails then the Winter Formula applies, predicting the ventilatory response (know this formula!) pCO = 1.5(HCO ) + 8 2 Anion Gap Metabolic Acidosis Baseline Abnormal Sodium 136 136 Chloride 102 102 Bicarbonate

24 14 Anion Gap 10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.29 pCO2 40 29 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? Five Steps - #4 Acidemia v. Alkalemia Metabolic v. Respiratory ?Anion Gap 1. 2. 3.

4. Does the AG = HCOAG = AG = HCOHCO3 ? 5. Is there appropriate compensation? If AG = HCOAG = AG = HCOHCO3 disturbance If AG = HCOAG HCOAG = HCOHCO3 one METABOLIC >1 METABOLIC disturbance Rationale: X meq acid will titrate X meq HCO3 HCO3 will fall by x, AG will rise by x If AG = HCOAG HCOAG = HCOHCO3, then another metabolic disturbance accounts for the difference AG Metabolic Acidosis + Metabolic Alkalosis Baseline Abnormal Sodium 136

136 Chloride 102 92 Bicarbonate 24 14 Anion Gap 10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.29 pCO2 40 29 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation?

DKA + vomiting, AKA + vomiting, Sepsis + vomiting, Sepsis + NG suction AG Met Acidosis + Resp Acidosis Baseline Abnormal Sodium 136 136 Chloride 102 102 Bicarbonate 24 14 Anion Gap 10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.22 pCO2

40 23 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? DKA with respiratory failure (from any cause), Sepsis with respiratory failure (pneumonia + sepsis) Respiratory Alkalosis Baseline Abnormal Sodium 136 136 Chloride 102 102 Bicarbonate 24 24 Anion Gap 10 AG = HCO Anion Gap -

AG = HCO Bicarbonate - pH 7.40 7.50 pCO2 40 32 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? Hypoxia (from any cause), pain, sepsis/endotoxemia, ASA toxicity, anxiety (diagnosis of exclusion) Chronic Respiratory Acidosis-Compensated Baseline Abnormal Sodium 136 136 Chloride 102 92 Bicarbonate

24 31 Anion Gap 10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.36 pCO2 40 56 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? Severe COPD, OSA, Advanced neuromuscular disease Metabolic Alkalosis + Resp. Alkalosis Acute Resp. Alkalosis on Chronic Respiratory acidosis Baseline Abnormal Abnormal

Baseline Sodium 136 136 136 Chloride 102 92 92 Bicarbonate 24 31 31 Anion Gap 10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.50 7.38

pCO2 40 42 55 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? Loop diuretics or vomiting with any primary Respiratory Alkalosis The Chronic CO2 retainer who stops retaining from either pain, hypoxia, sepsis, acute PE or any other acute Respiratory Alkalosis. Learn to recognize the patient who starts from an abnormal baseline. Mixed Disturbance Baseline Admission #1 A month later Sodium 137 139 130 Chloride 105 99 93 Bicarbonate

24 21 13 Anion Gap 10 AG = HCO Anion Gap - AG = HCO Bicarbonate - pH 7.40 7.46 7.38 pCO2 40 21 16 1] Acidemia v. Alkalemia 2] Metabolic v. Respiratory 3] Anion Gap? 4] Anion Gap 5] Compensation? Two interpretations, one unifying diagnosis

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