1. Acid-Base Disorders Adapted from Haber, R.J.: “A practical Approach to Acid- Base Disorders.” West J. Med 1991 Aug; 155:156-151 Allison B. Ludwig, M.D. Site Director, Jacobi Medicine Clerkship

2. 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

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

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

5. Terms and Definitions Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35 - 7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  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

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

7. 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

8. Example #1 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  Respiratory acidosis HCO3 Metabolic acidosis  22 – 26  Metabolic Alkalosis pH7.50pCO229 HCO322 Acute Respiratory Alkalosis

9. 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

10. Example #2 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  Respiratory acidosis HCO3 Metabolic acidosis  22 – 26  Metabolic Alkalosis pH7.25pCO260 HCO326 Acute Respiratory Acidosis

11. 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

12. Example #3 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  Respiratory acidosis HCO3 Metabolic acidosis  22 – 26  Metabolic Alkalosis pH7.34pCO260 HCO331 Chronic Respiratory Acidosis with Metabolic Compensation

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

14. Example #4 Variabl e Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  Respiratory acidosis HCO3 Metabolic acidosis  22 – 26  Metabolic Alkalosis pH7.50pCO248 HCO336

15. 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

16. Example #5 Variabl e Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  Respiratory acidosis HCO3 Metabolic acidosis  22 – 26  Metabolic Alkalosis pH7.20pCO221 HCO38 Metabolic Acidosis with Respiratory Compensation

17. 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

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

19. 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

20. 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

21. Example #6 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  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

22. What’s the Diagnosis? Salicylate Overdose

23. Example #7 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35- 7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  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

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

25. Example #8 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  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

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

27. Example #9 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  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

28. 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

29. Example #10 Variable Primary Disorder Normal Range, arterial Gas Primary Disorder pHAcidemia  7.35-7.45  Alkalemia pCO2 Respiratory alkalosis  35 - 45  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

30. 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

31. 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!!