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 = log (HCO3 ÷ (0.03 x PCO2))
6.1 = pKa (Ka is dissociation constant for the reaction)
0.03 = solubility constant for CO2 in extracellular fluid

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
Winter’s formula
paCO2 increase = (1.5 x bicarb) + 8 (+-2)

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?
Normal bicarb – uncompensated, likely acute respiratory acidosis

9. Case #1 pH 7.16, pCO2 70, HCO3 24 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical Scenario?
Normal bicarb – uncompensated, likely acute respiratory acidosis

10. Case #1 pH 7.16, pCO2 70, HCO3 24 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical Scenario?
Normal bicarb – uncompensated, likely acute respiratory acidosis

11. Case #1 pH 7.16, pCO2 70, HCO3 24 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
No … likely acute
Clinical Scenario?
Normal bicarb – uncompensated, likely acute respiratory acidosis

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
Normal bicarb – uncompensated, likely acute respiratory acidosis

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
Hypoxemia- decreased O2 delivery increases minute ventilation, leading to respiratory alkalosis
Pulmonary disorders (interstitial lung dz, pulm edema, emboli) – affect PO2 more than PCO2, leads to hypoxemia and hyperventilation

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
MUDPILES: methanol, uremia, DKA, paraldehyde, isopropyl alcohol, lactic acidosis, ethylene glycol/ethanol, salicylates

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

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 yo s/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
Decreased extracellular volume and low serum chloride levels
Contraction alkalosis – extracellular volume contracts around fixed quantity of bicarb (total body bicarb is the same)

37. Metabolic Alkalosis Chloride-resistant Mineralocorticoid excess
Renal chloride wasting (Bartter syndrome)
Exogenous alkali (milk-alkali syndrome, massive blood transfusion)
Hypokalemia
Mineralocorticoid excess – increased acid secretion and hypokalemia
Excess steroid admin  cushing’s
Milk alkali syndrome – ingestion of large amounts of calcium with absorbable alkali hypercalcemia
Massive blood transfusion – citrate converts to bicarbonate
Hypokalemia – increased ammonium production, hydrogen secretion, and bicarb reabsorption

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
Severe restrictive disease – i.e. chest wall deformity, scoliosis

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

51. Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?

52. Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?

53. Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?

54. Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Well … 2 meq/L decrease in HCO3 for every 10 mmHg decrease in CO2
Combined respiratory alkalosis & metabolic acidosis
Clinical scenario?
Predicts bicarb of 22, metabolic acidosis also present

55. Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?
Anxious 5 yo who is hyperventilating and has a history of RTA

56. Case #8 pH 7.45, pCO2 54, HCO3 36 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?

57. Case #8 pH 7.45, pCO2 54, HCO3 36 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?

58. Case #8 pH 7.45, pCO2 54, HCO3 36 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?

59. Case #8 pH 7.45, pCO2 54, HCO3 36 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
No … 0.7 mmHg increase for every 1 meq/L predicts pCO2 of 48
Combined metabolic alkalosis and respiratory acidosis
Clinical scenario?

60. Case #8 pH 7.45, pCO2 54, HCO3 36 Acidosis or alkalosis?
Respiratory or metabolic?
Compensated?
Clinical scenario?
1 yo with vomiting for 3 days who presents to the ED with lethargy and decreased arousal, hypoventilating

61. Physiologic Effects of Acidosis
Shifts the oxygen-hemoglobin dissociation curve to the right
Decreased affinity for O2

62. Physiologic Effects of Acidosis
Pulmonary effects – vasoconstriction decreases pulmonary blood flow
Cardiac effects – depressed contractility
Neurologic effects – increased cerebral blood flow, increased ICP
Extracellular shift of K+  hyperkalemia
Sympathetic overactivity, resistance to catecholamines

63. Physiologic Effects of Alkalosis
Shifts the oxygen-hemoglobin dissociation curve to the left
Stronger bond between Hb and O2
Decreased O2 delivery to tissues

64. Physiologic Effects of Alkalosis
Cardiac arrhythmias
Lungs – vasodilation increases pulmonary blood flow
Neurologic effects – headache, seizures, altered mental status
Decreased cerebral blood flow from vasoconstriction
Decreased levels of ionized Ca++
Intracellular shift of potassium  severe hypokalemia

65. Management - Respiratory Acidosis
Treat the underlying disorder
Assist or increase ventilation
Secure airway if necessary
Increase tidal volume or respiratory rate if mechanically ventilated
Noninvasive ventilation
Bronchodilators
Reverse sedative medications

66. Management – Respiratory Alkalosis
Treat the underlying disorder
Decrease ventilation
Decrease respiratory rate
Decrease tidal volume
Sedation and pain control
Reassurance for anxious patients

67. Management - Metabolic Disorders
Acidosis
Treat the underlying disorder
Consider bicarb administration depending on etiology
Dialysis in the setting of renal failure
Alkalosis
Chloride-responsive: replete chloride (NaCl, KCl, ArgCl)
Carbonic-anydrase inhibitors if diuresis needed

68. Conclusion
The body has compensatory mechanisms to maintain acid-base balance.
Blood gases should be interpreted in a systematic way to determine the etiology of the acid-base disturbance.
Acidosis causes pulmonary vasoconstriction, cardiac depression, arterial vasodilation, & decreased O2 affinity.
Alkalosis causes pulmonary vasodilation, arterial vasoconstriction, & increased O2 affinity.
Management of acid-base disorders primarily involves treatment of the underlying disorder.

69. Everyone always has slides of their kids …

70. QUESTIONS?

71. References www.uptodate.com www.emedicine.com
Morganroth ML. Six steps to acid-base analysis: clinical applications. The Journal of Critical Illness. 1990;5:
Morganroth ML. An analytic approach to diagnosing acid- base disorders. The Journal of Critical Illness. 1990;5: