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Acid-Base Disorders Adapted from Haber, R.J.: “A practical Approach to Acid- Base Disorders.” West J. Med 1991 Aug; 155: Allison B. Ludwig, M.D. Site Director, Jacobi Medicine Clerkship

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Goals: Learn to work through acid-base disorders without nomograms or complicated mathematical formulas Be able to recognize and work through multiple offsetting disorders that are coincident in the same patient

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Outline: Data Base, Terms and Definitions Simple-Acid Base Disorders Mixed Acid-Base Disorders

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Data Base Arterial pH Arterial pCO2 Serum HCO3 (best from blood chemistry)

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Terms and Definitions Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis Respiratory compensation for metabolic disorders is rapid Full metabolic compensation for respiratory disturbances requires renal adjustment and takes 3-5 days

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Outline: Data Base, Terms and Definitions Simple-Acid Base Disorders Mixed Acid-Base Disorders

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Simple Acid-Base Disorders Look at the pH in order to determine the primary abnormality Pathophysiologic principle: body does not fully compensate even for chronic acid-base disorders

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Example #1 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis pH7.50pCO229 HCO322 Acute Respiratory Alkalosis

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Causes of Acute Respiratory Alkalosis Anxiety Hypoxia Lung disease with or without hypoxia CNS disease Drug use—salicylates, catecholamines, progesterone Pregnancy Sepsis Hepatic Encephalopathy Mechanical Ventilation

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Example #2 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis pH7.25pCO260 HCO326 Acute Respiratory Acidosis

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Causes of Acute Respiratory Acidosis CNS depression—drugs, CNS event Neuromuscular disorders—myopathies, neuropathies Acute airway obstruction—upper airway, laryngospasm, bronchospasm Severe pneumonia or pulmonary edema Impaired lung motion—hemothorax, pneumothorax Thoracic cage injury—flail chest Ventilator dysfunction

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Example #3 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis pH7.34pCO260 HCO331 Chronic Respiratory Acidosis with Metabolic Compensation

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Causes of Chronic Respiratory Acidosis Chronic lung disease—obstructive or restrictive Chronic neuromuscular disorders Chronic respiratory center depression— central hypoventilation

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Example #4 Variabl e Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis pH7.50pCO248 HCO336

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Causes of Metabolic Alkalosis Vomiting Diuretics Excess mineralocorticoid activity—Cushing’s syndrome, Conn’s syndrome, exogenous steroids, licorice ingestion, increased renin states, Bartter’s syndrome Excess alkali administration Refeeding alkalosis

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Example #5 Variabl e Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis pH7.20pCO221 HCO38 Metabolic Acidosis with Respiratory Compensation

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Causes of Metabolic Acidosis Non-Gap GI HCO3 loss: diarrhea, ureteral diversions Renal HCO3 loss: RTA, aldosterone inhibitors, carbonic anhydrase inhibitors Iatrogenic: normal saline Anion Gap Ketoacidosis: diabetic, alcoholic Renal failure Lactic Acidosis Rhabdomyolysis Toxins: methanol, ethylene glycol, paraldehyde, salicylates

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Outline: Data Base, Terms and Definitions Simple-Acid Base Disorders Mixed Acid-Base Disorders

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The Rules Look at the pH: whichever side of 7.40 the pH is on, the process that caused it to shift to that side is the primary abnormality Principle: the body doesn’t fully compensate for primary acid-base disorders Calculate the anion gap: Na – (Cl + HCO3): if the anion gap is >20, there is a primary metabolic acidosis regardless of pH or HCO3 Principle: the body doesn’t generate a large anion gap to compensate for a primary disorder Calculate the excess anion gap (total anion gap minus the normal anion gap) and add this to the measured HCO3 concentration, if >30, there is underlying metabolic alkalosis; if 30, there is underlying metabolic alkalosis; if <24, there is underlying non-gap metabolic acidosis Principle: 1 mmol of unmeasured acid titrates 1 mmol of bicarbonate

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Understanding the anion gap Each millimolar decrease in HCO3 is accompanied by a millimolar increase in the anion gap, the sum of the new (excess) anion gap and the remaining (measured) HCO3 value should be equal to a normal bicarbonate concentration

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Example #6 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis Respiratory Alkalosis and Anion Gap Metabolic Acidosis 1.Look at the pH to determine the primary process. 2.Calculate the anion gap: Na – (Cl + HCO3) 3.Calculate the excess anion gap (total anion gap minus the normal anion gap) and add this to the measured HCO3 concentration, if >30, there is underlying metabolic alkalosis; if 30, there is underlying metabolic alkalosis; if <24, there is underlying non-gap metabolic acidosispH7.50pCO220 HCO315 Na140 Cl103

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What’s the Diagnosis? Salicylate Overdose

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Example #7 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis Anion Gap Metabolic Acidosis and Metabolic AlkalosispH7.40pCO240 HCO324 Na145 Cl100 1.Look at the pH to determine the primary process. 2.Calculate the anion gap: Na – (Cl + HCO3) 3.Calculate the excess anion gap (total anion gap minus the normal anion gap) and add this to the measured HCO3 concentration, if >30, there is underlying metabolic alkalosis; if 30, there is underlying metabolic alkalosis; if <24, there is underlying non-gap metabolic acidosis

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What’s the Diagnosis? Chronic renal failure in a patient with vomiting as his uremia worsened.

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Example #8 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis Respiratory alkalosis, Anion Gap Metabolic Acidosis and Metabolic AlkalosispH7.50pCO220 HCO315 Na145 Cl100 1.Look at the pH to determine the primary process. 2.Calculate the anion gap: Na – (Cl + HCO3) 3.Calculate the excess anion gap (total anion gap minus the normal anion gap) and add this to the measured HCO3 concentration, if >30, there is underlying metabolic alkalosis; if 30, there is underlying metabolic alkalosis; if <24, there is underlying non-gap metabolic acidosis

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What’s the Diagnosis? History of vomiting (metabolic alkalosis), alcoholic ketoacidosis (metabolic acidosis), and bacterial pneumonia (respiratory alkalosis)

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Example #9 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis Respiratory Acidosis, Anion gap Metabolic Acidosis, Metabolic Alkalosis 1.Look at the pH to determine the primary process. 2.Calculate the anion gap: Na – (Cl + HCO3) 3.Calculate the excess anion gap (total anion gap minus the normal anion gap) and add this to the measured HCO3 concentration, if >30, there is underlying metabolic alkalosis; if 30, there is underlying metabolic alkalosis; if <24, there is underlying non-gap metabolic acidosispH7.10pCO250 HCO315 Na145 Cl100

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What’s the Diagnosis? Patient presented in an obtunded state (respiratory acidosis), history of vomiting (metabolic alkalosis), DKA (anion gap metabolic acidosis) Or Chronic respiratory acidosis and metabolic compensation in whom an acute anion gap metabolic acidosis developed

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Example #10 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia Alkalemia pCO2 Respiratory alkalosis Respiratory acidosis HCO3 Metabolic acidosis 22 – 26 Metabolic Alkalosis Anion Gap and Non-Anion Gap Metabolic Acidoses 1.Look at the pH to determine the primary process. 2.Calculate the anion gap: Na – (Cl + HCO3) 3.Calculate the excess anion gap (total anion gap minus the normal anion gap) and add this to the measured HCO3 concentration, if >30, there is underlying metabolic alkalosis; if 30, there is underlying metabolic alkalosis; if <24, there is underlying non-gap metabolic acidosispH7.15pCO215 HCO35 Na140 Cl110

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What’s the Diagnosis? DKA with non-gap acidosis during recovery phase of DKA due to failure to regenerate HCO3 from keto-acids lost in the urine

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Conclusions Acid-base disturbances are easy to analyze if approached systematically Determine primary abnormalities based on pH Calculate the anion gap Calculate the delta gap and add to the measured HCO3 Calculate an anion gap on EVERY chemistry you see If there is an elevated anion gap, remember to get an ABG!!

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