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INTERN BASICS Acid-Base August 9, 2005 Jeremy Marcus MD

INTERN BASICS Acid-Base August 9, 2005 Jeremy Marcus MD. ACID BASE – Why it’s “hard” It’s Math Everyone does it differently Everyone thinks they know the “best” way to do it and/or teach it Emphasis on numbers instead of clinical correlation. A word about “internal consistency”

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INTERN BASICS Acid-Base August 9, 2005 Jeremy Marcus MD

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  1. INTERN BASICS Acid-Base August 9, 2005 Jeremy Marcus MD

  2. ACID BASE – Why it’s “hard” • It’s Math • Everyone does it differently • Everyone thinks they know the “best” way to do it and/or teach it • Emphasis on numbers instead of clinical correlation

  3. A word about “internal consistency” • pH = 6.1 + log ([HCO3]/0.03 x pCO2) • [H+] = 24 x pCO2/[HCO3] • What is the equation getting at? • e.g. Pt with COPD, acute-on-chronic tachypnea & dyspnea; team got ABG: • 7.48/87/56, arterial HCO3 = 63

  4. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders

  5. Acidosis or alkalosis pH < 7.38 Acidosis pH > 7.42 Alkalosis

  6. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders

  7. Primary disorder respiratory or metabolic? In respiratory acidosis, pCO2 and HCO3 both go up In metabolic acidosis, pCO2 and HCO3 both go down In respiratory alkalosis, pCO2 and HCO3 both go down In metabolic alkalosis, pCO2 and HCO3 both go up Examples: 7.32/28/83 HCO3 14 7.31/70/75 HCO3 34 7.10/50/55 HCO3 15

  8. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders

  9. If respiratory, is it acute or chronic? Two numbers to remember: 0.08 and 0.03 (works for respiratory acidosis or alkalosis) Acute resp acidosis: for every 10 incr pCO2, pH decr 0.08 Chronic resp acidosis: for every 10 incr pCO2, pH decr 0.03 Acute resp alkalosis: for every 10 decr pCO2, pH incr 0.08 Chronic resp alkalosis: for every 10 decr pCO2, pH incr 0.03 Example: 7.31/70/78 HCO3 34

  10. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders

  11. Appropriate compensation • Metabolic acidosis: Winter’s formula • pCO2 = 1.5 [HCO3] + 8  2 • Alternative: For every HCO3 decr 1, pCO2 decr 1 • Respiratory acidosis/alkalosis: • Acute resp acidosis: for every 10 incr pCO2, HCO3 incr 1 • Chronic resp acidosis: for every 10 decr pCO2, HCO3 incr 4 • Acute resp alkalosis: for every 10 incr pCO2, HCO3 decr 2 • Chronic resp alkalosis: for every 10 decr pCO2, HCO3 incr 5 • Metabolic alkalosis: • pCO2 should never be >55 • If pCO2 is elevated, pH should be alkalemic • For every HCO3 incr 10, pCO2 should incr 7

  12. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders

  13. Calculate the anion gap … every time! • Unmeasured AnionsUnmeasured Cations • Proteins (albumin) 15 mEq/L   Calcium 5 mEq/L Organic acids 5 mEq/L Potassium 4.5 mEq/L Phosphates 2 mEq/L Magnesium 1.5 mEq/L Sulfates 1 mEq/L   • Totals: 23 mEq/L 11 mEq/L • Difference = 12 mEq/L = normal anion gap • Correct for albumin (2.5 for every drop of 1 below 3.0) • - If anion gap > 20 with metabolic alkalosis, there’s an additional acidosis

  14. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders

  15. Assess for complex (triple) disorders Corrected bicarbonate = (AG-12) + HCO3 If < 24, suggests concurrent acidosis If > 24, suggests concurrent alkalosis (Yes, this is the same as “delta delta.”)

  16. Stepwise approach • Acidosis or alkalosis • Primary disorder respiratory or metabolic • If respiratory, is it acute or chronic? • Appropriate compensation? • Calculate anion gap • Assess for complex (triple) disorders • CORRELATE CLINICALLY!

  17. Differential diagnosis: metabolic acidosis Anion gap acidosis Non-anion gap acidosis K etoacidosis U reterosignoidostomy U remia S aline S alicylates E arly renal failure M ethanol D iarrhea E thanol, ethylene glycol C arbonic anhydrase inhibitors L actate A mino acids R enal tubular acidosis S upplements (TPN) P ancreatic fistula USUALLY saline or diarrhea

  18. Differential diagnosis: metabolic alkalosis • Volume contraction (vomiting, overdiuresis, ascites) • Hypokalemia • Alkali ingestion (bicarbonate) • Excess gluco- or mineralocorticoids • Bartter's syndrome • USUALLY vomiting or overdiuresis

  19. Differential diagnosis: respiratory acidosis • Central Nervous System Depression (Sedatives, CNS disease, Obesity Hypoventilation syndrome) • Pleural Disease (Pneumothorax) • Lung Disease (COPD, pneumonia) • Musculoskelatal disorders (Kyphoscoliosis, Guillain-Barre, Myasthenia Gravis, Polio) • Practically, think about “tiring” (even a little respiratory acidosis in asthma is often a harbinger of badness)

  20. Differential diagnosis: respiratory alkalosis • Catastrophic CNS event (CNS hemorrhage) • Drugs (salicylates, progesterone) • Pregnancy (especially the 3rd trimester) • Decreased lung compliance (interstitial lung disease) • Liver cirrhosis • Anxiety/Pain

  21. Examples 21 yo woman presents with confusion, fever, flank pain, “breathing heavy” 7.32/28 140 104 14

  22. Examples 21 yo woman presents with confusion, fever, flank pain, “breathing heavy” 7.32/28 140 104 14 Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 29 (ok) AG = 22; expected HCO3 = 10 + 14 = 24 (ok)

  23. Examples 21 yo woman presents with confusion, fever, flank pain, “breathing heavy” 7.32/28 140 104 14 Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 29 (ok) AG = 22; expected HCO3 = 10 + 14 = 24 (ok) Anion gap metabolic acidosis DKA with pyelonephritis

  24. Examples 58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol. 7.31/10 123 99 5

  25. Examples 58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol. 7.31/10 123 99 5 Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 15, so concurrent respiratory alkalosis AG = 19; expected HCO3 = 7 + 5 = 12, so non-anion gap metabolic acidosis

  26. Examples 58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol. 7.31/10 123 99 5 Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 15, so concurrent respiratory alkalosis AG = 19; expected HCO3 = 5 + 5 = 10, so non-anion gap metabolic acidosis Anion gap metabolic acidosis, non-anion gap metabolic acidosis, respiratory alkalosis Alcoholic ketoacidosis, diarrhea, pneumonia

  27. Examples 56 yo man found vomiting on the street 7.40/40 145 100 24

  28. Examples 56 yo man found vomiting on the street 7.40/40 145 100 24 Can’t tell primary disorder by pH… but AG = 21 Expected HCO3 = 9 + 24 = 33 so concurrent metabolic alkalosis

  29. Examples 56 yo man found vomiting on the street 7.40/40 145 100 192 3.6 24 9.1 Can’t tell primary disorder by pH… but AG = 21 Expected HCO3 = 9 + 24 = 33 so concurrent metabolic alkalosis Metabolic alkalosis and metabolic acidosis Vomiting in the setting of worsening uremia due to CKD

  30. Examples 58 yo man with 4d cough, vomiting, altered mental status 7.50/20 145 100 15

  31. Examples 58 yo man with 4d cough, vomiting, altered mental status 7.50/20 145 100 15 Respiratory alkalosis AG = 30, so concurrent anion gap metabolic acidosis Expected HCO3 = 18 + 15 = 33 so concurrent metabolic alkalosis

  32. Examples 58 yo man with 4d cough, vomiting, altered mental status 7.50/20 145 100 15 Respiratory alkalosis AG = 30, so concurrent anion gap metabolic acidosis Expected HCO3 = 18 + 15 = 33 so concurrent metabolic alkalosis Respiratory alkalosis, anion gap metabolic acidosis, metabolic alkalosis Pneumonia, alcoholic ketoacidosis, vomiting

  33. Examples 35 yo woman presents obtunded 7.10/50 145 100 15

  34. Examples 35 yo woman presents obtunded 7.10/50 145 100 15 Primary respiratory acidosis AG = 30 so concurrent primary metabolic acidosis Expected HCO3 = 18 + 15 = 33 so concurrent metabolic alkalosis

  35. Examples 35 yo woman presents obtunded 7.10/50 145 100 15 Primary respiratory acidosis AG = 30 so concurrent primary metabolic acidosis Expected HCO3 = 18 + 15 = 33 so concurrent metabolic alkalosis Same as last patient but obtunded so hypoventilating! Hypoventilation due to altered mental status, DKA, vomiting

  36. Landing safely Do the exercise on every ABG for practice

  37. Landing safely Do the exercise on every ABG for practice Do the exercise on every ABG for practice

  38. Landing safely Do the exercise on every ABG for practice Do the exercise on every ABG for practice Ask questions

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