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Acid-Base Physiology

Acid-Base Physiology. 2012. Objectives. Understand normal mechanisms and regulation of acid-base balance Interpret blood gases Understand the effects of acidosis and alkalosis Evaluate and manage acidosis and alkalosis. Normal Physiology.

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Acid-Base Physiology

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  1. Acid-Base Physiology 2012

  2. Objectives • Understand normal mechanisms and regulation of acid-base balance • Interpret blood gases • Understand the effects of acidosis and alkalosis • Evaluate and manage acidosis and alkalosis

  3. Normal Physiology • Acid-base balance maintained by normal pulmonary excretion of CO2 and renal excretion of acid • Organic buffers: HCO3-, HPO4, protein anions, carbonate • 90% of bicarb is reabsorbed by kidney • Renal excretion – H+ combines with urinary titratable acids (phosphates) or ammonia to form ammonium • Henderson-Hasselbach equation: pH = 6.1 + log (HCO3 ÷ (0.03 x PCO2))

  4. Compensatory Mechanisms • pH is determined by ratio of HCO3 and PCO2 • Body responds to changes in pH by attempting to normalize the pH • Buffering • Respiratory – alterations in paCO2 • Renal – alterations in bicarbonate excretion

  5. Compensatory Mechanisms • Compensated metabolic acidosis: • 1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated metabolic alkalosis: • 0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated respiratory acidosis: • Acute- 1 meq/L  for every 10 mmHg  in pCO2 • Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2 • Compensated respiratory alkalosis: • Acute- 2 meq/L  for every 10 mmHg  in pCO2 • Chronic- 4 meq/L  for every 10 mmHg  in pCO2

  6. Blood Gas Interpretation • General guidelines: • 1) Is it acidosis or alkalosis? • Acidosis – pH < 7.38 • Alkalosis – pH > 7.42 • 2) Is it primary respiratory or metabolic? • Evaluate paCO2 and bicarbonate • 3) Is there compensation? • Calculations from previous slides

  7. Blood Gas Interpretation • 4) If respiratory disturbance, is it acute or chronic? • Respiratory acidosis: • Acute decrease in pH = 0.08 x (paCO2-40)/10 • Chronic decrease in pH = 0.03 x (paCO2-40)/10 • Respiratory alkalosis • Acute increase in pH = 0.08 x (40-paCO2)/10 • Chronic increase in pH = 0.03 x (40-paCO2)/10 • 5) If metabolic disturbance, is there an anion gap? • Check serum Na, Cl, HCO3

  8. Case #1 • pH 7.16, pCO2 70, HCO3 24 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical Scenario?

  9. Case #1 • pH 7.16, pCO2 70, HCO3 24 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical Scenario?

  10. Case #1 • pH 7.16, pCO2 70, HCO3 24 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical Scenario?

  11. Case #1 • pH 7.16, pCO2 70, HCO3 24 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • No … likely acute • Clinical Scenario?

  12. Case #1 • pH 7.16, pCO2 70, HCO3 24 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical Scenario? • 2 yo receiving deep sedation by the adult ED attending who gives him 4 mg morphine, respiratory rate is 6

  13. Acute Respiratory Acidosis • Respiratory pathophysiology – airway obstruction, severe pneumonia, chest trauma, pneumothorax • Acute drug intoxication (narcotics, sedatives) • Residual neuromuscular blockade • CNS disease (head trauma, decreased consciousness) • Bicarbonate is often normal, kidneys have not had time to compensate

  14. Case #2 • pH 7.6, pCO2 23, HCO3 22 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  15. Case #2 • pH 7.6, pCO2 23, HCO3 22 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  16. Case #2 • pH 7.6, pCO2 23, HCO3 22 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  17. Case #2 • pH 7.6, pCO2 23, HCO3 22 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • No … likely acute • Clinical scenario?

  18. Case #2 • pH 7.6, pCO2 23, HCO3 22 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario? • 4 mo mechanically ventilated pt who was bagged during transport to CT scan by an overeager intern

  19. Respiratory Alkalosis • Pain, anxiety • Hypoxemia • Interstitial lung disease • Severe congestive heart failure (pulmonary edema) • Pulmonary emboli • Drugs – salicylates, methylxanthines, nicotine • Sepsis, fever • Hepatic failure – encephalopathy • Pregnancy • Thyrotoxicosis • CNS hemorrhage • Overagressive mechanical ventilation

  20. Case #3 • pH 7.29, pCO2 26, HCO3 12 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  21. Case #3 • pH 7.29, pCO2 26, HCO3 12 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  22. Case #3 • pH 7.29, pCO2 26, HCO3 12 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  23. Compensatory Mechanisms • Compensated metabolic acidosis: • 1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated metabolic alkalosis: • 0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated respiratory acidosis: • Acute- 1 meq/L  for every 10 mmHg  in pCO2 • Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2 • Compensated respiratory alkalosis: • Acute- 2 meq/L  for every 10 mmHg  in pCO2 • Chronic- 4 meq/L  for every 10 mmHg  in pCO2

  24. Case #3 • pH 7.29, pCO2 26, HCO3 12 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Yes … 1.2 mmHg decrease in pCO2 for every 1 meq/L decrease in HCO3 • Clinical scenario?

  25. Case #3 • pH 7.29, pCO2 26, HCO3 12 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario? • 10 yo dev delayed pt admitted with diarrhea, med list reveals mom has been giving Miralax every 4 hours

  26. Metabolic Acidosis • Anion gap

  27. Metabolic Acidosis • Anion gap • Lactic acidosis • DKA • Toxic ingestions (salicylates, ethylene glycol, ethanol, isopropyl alcohol, paraldehyde, methanol) • Renal failure – uremia

  28. Metabolic Acidosis • Nonanion gap

  29. Metabolic Acidosis • Nonanion gap • RTA • Diarrhea • Hypoaldosteronism • Potassium-sparing diuretics • Pancreatic loss of bicarbonate • Ureteral diversion • Carbonic anhydrase inhibitors • Acid administration (ArgCl, NaCl)

  30. Case #4 • pH 7.47, pCO2 46, HCO3 32 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  31. Case #4 • pH 7.47, pCO2 46, HCO3 32 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  32. Case #4 • pH 7.47, pCO2 46, HCO3 32 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  33. Compensatory Mechanisms • Compensated metabolic acidosis: • 1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated metabolic alkalosis: • 0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated respiratory acidosis: • Acute- 1 meq/L  for every 10 mmHg  in pCO2 • Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2 • Compensated respiratory alkalosis: • Acute- 2 meq/L  for every 10 mmHg  in pCO2 • Chronic- 4 meq/L  for every 10 mmHg  in pCO2

  34. Case #4 • pH 7.47, pCO2 46, HCO3 32 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Yes … 0.7 mmHg increase in pCO2 for every 1 meq/L increase in HCO3 • Clinical scenario?

  35. Case #4 • pH 7.47, pCO2 46, HCO3 32 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario? • 5 yos/p appendectomy with NG tube left to suction on 7CH for 5 days

  36. Metabolic Alkalosis • Chloride-responsive (urine Cl < 10 meq/L) • Gastric acid loss (vomiting, NG suction) • Contraction alkalosis (often due to loop or thiazide diuretics) • Posthypercapnia syndrome

  37. Metabolic Alkalosis • Chloride-resistant • Mineralocorticoid excess • Renal chloride wasting (Bartter syndrome) • Exogenous alkali (milk-alkali syndrome, massive blood transfusion) • Hypokalemia

  38. Case #5 • pH 7.30, pCO2 89, HCO3 42 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clincial scenario?

  39. Case #5 • pH 7.30, pCO2 89, HCO3 42 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clincial scenario?

  40. Case #5 • pH 7.30, pCO2 89, HCO3 42 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clincial scenario?

  41. Compensatory Mechanisms • Compensated metabolic acidosis: • 1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated metabolic alkalosis: • 0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3 • Compensated respiratory acidosis: • Acute- 1 meq/L  for every 10 mmHg  in pCO2 • Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2 • Compensated respiratory alkalosis: • Acute- 2 meq/L  for every 10 mmHg  in pCO2 • Chronic- 4 meq/L  for every 10 mmHg  in pCO2

  42. Case #5 • pH 7.30, pCO2 89, HCO3 42 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Yes … 3.5 meq/L increase in HCO3 for every 10 mmHg increase in CO2 • Clincial scenario?

  43. Case #5 • pH 7.30, pCO2 89, HCO3 42 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clincial scenario? • 35 yo CF patient on the Peds floor with end-stage lung disease

  44. Chronic Respiratory Acidosis • Chronic lung diseases (BPD, CF) • Neuromuscular disorders • Severe restrictive lung disease • Severe obesity

  45. Case #6 • pH 6.84, pCO2 82, HCO3 14 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  46. Case #6 • pH 6.84, pCO2 82, HCO3 14 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  47. Blood Gas Interpretation • 4) If respiratory acidosis or alkalosis, is it acute or chronic? • Respiratory acidosis: • Acute decrease in pH = 0.08 x (paCO2-40)/10 • Chronic decrease in pH = 0.03 x (paCO2-40)/10 • Respiratory alkalosis • Acute increase in pH = 0.08 x (40-paCO2)/10 • Chronic increase in pH = 0.03 x (40-paCO2)/10

  48. Case #6 • pH 6.84, pCO2 82, HCO3 14 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario?

  49. Case #6 • pH 6.84, pCO2 82, HCO3 14 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • No … • Combined acidosis • Clinical scenario?

  50. Case #6 • pH 6.84, pCO2 82, HCO3 14 • Acidosis or alkalosis? • Respiratory or metabolic? • Compensated? • Clinical scenario? • 2 mo found down at home, hypoperfusion leading to lactic acidosis, hypoventilation leading to respiratory acidosis

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