1. By Heidi Allen, DVM, Dipl. ACVIM
Acid Base Physiology
By Heidi Allen, DVM, Dipl. ACVIM

2. Why is assessing acid-base status important?
Assessing the tachypnic animal
Low oxygenation vs. blowing off CO2
Assessing vomiting animals
Metabolic alkalosis with high outflow obstruction vs. metabolic acidosis
Diabetic animals
Ketoacidosis vs. ketosis alone

3. Why is assessing acid-base status important?
Renal failure animals
Do you need to supplement NaHCO3?
Dyspnic animals
All though arterial blood gas is best we can make some assessments using venous samples.

4. Acid-Base Physiology pH = -log [H+] Carbonic acid equation ↑H+ ≈ ↓pH
H+ + HCO3  H2CO3  H20 + CO2

5. Defined as an increase in H+
Acid-Base Physiology
Metabolic acidosis
Defined as an increase in H+

6. Metabolic Acidosis Causes Lactic acidosis Ketoacidosis Dehydration
Hypovolemia
Hypoxia
Anemia
Ketoacidosis

7. Metabolic Acidosis Causes Renal failure Gastrointestinal loss of HCO3
Decreased ability to excrete H+
Increased excretion of HCO3
Gastrointestinal loss of HCO3
Renal tubular acidosis

8. Metabolic Acidosis Causes Miscellaneous Aspirin overdose Methanol

9. What effect does an increased H+ have on the carbonic acid equation?
Metabolic Acidosis
What effect does an increased H+ have on the carbonic acid equation?
H+ + HCO3  H2CO3  H20 + CO2 .

10. ↑H+ Causes a mild shift to the right
Metabolic Acidosis
↑H+ Causes a mild shift to the right
↑H+ + HCO3  H2CO3  H20 + CO2
Leading to HCO3, & CO2

11. Metabolic Acidosis
Unfortunately this shift is not enough to overcome the increased hydrogen so H+ is still very elevated.
 H, HCO3, PCO2, &  pH

12. Metabolic Acidosis Compensatory mechanisms
Extracellular buffering (decrease in HCO3)
Intracellular buffering
Hydrogen enters cells in exchange for K, proteins, phosphate, and bone carbonate
Respiratory compensation (takes 1-2 hrs)
Renal hydrogen excretion (takes 2-5 days)

13. Respiratory Compensation
Metabolic Acidosis
Respiratory Compensation
– Blow off CO2
H+ + HCO3 H2CO3  H20 + ↓CO2
Allows equation to be pulled further to the right, decreasing H+ and HCO3

14. Compensated Metabolic Acidosis
H+ + HCO3 H2CO3  H20 + CO2
End result:
Mildly increased H+
Mildly decreased pH
Decreased HCO3
Decreased CO2 (compensation)

15. Acid-Base Physiology
Metabolic alkalosis
Defined as a ↓H+

16. Metabolic Alkalosis Causes High outflow GI obstruction
Loss of hydrogen and chloride
Diuretic therapy
Iatrogenic

17. Metabolic Alkalosis Causes Hypokalemia
Compensation for hypokalemia is to move K out of cells using H-K exchange
Hydrogen goes into cells causing loss of H in blood stream and metabolic alkalosis

18. What effect does decreased H+ have on the carbonic acid equation?
Metabolic Alkalosis
What effect does decreased H+ have on the carbonic acid equation?
H+ + HCO3  H2CO3  H20 + CO2

19. ↓H+ Causes a mild shift to the left
Metabolic Alkalosis
↓H+ Causes a mild shift to the left
↓H+ + HCO3  H2CO3  H20 + CO2
Leading to HCO3 & CO2

20. Metabolic Alkalosis
Unfortunately this shift is not enough to overcome the decreased hydrogen so H+ is still very low.
H+ + HCO3  H2CO3  H20 + CO2
H, HCO3, CO2, & pH

21. Respiratory Compensation
Metabolic Alkalosis
Respiratory Compensation
– Retain CO2
H+ + HCO3  H2CO3  H20 + ↑CO2
Allows equation to be pulled further to the left, increasing H+ but also increasing HCO3

22. Compensated Metabolic Alkalosis
H+ + HCO3  H2CO3  H20 + CO2
End result:
Mildly decreased H+
Mildly increased pH
Increased HCO3
Increased PCO2 (compensation)

23. Respiratory alkalosis
Acid-Base Physiology
Respiratory alkalosis
Defined as a ↓CO2

24. Respiratory Alkalosis
Causes
Hypoxemia
In some cases of respiratory disease oxygen can not get into blood stream but CO2 can get out.
In these cases the body increases respiratory rate in response to hypoxemia which PCO2

25. Respiratory Alkalosis
Causes
Hypoxemia Examples
Mild to moderate pneumonia or CHF
PTE
Interstitial fibrosis

26. Respiratory Alkalosis
Causes
Stimulation of respiratory center
Intracranial disease
Hepatic encephalopathy
Gram negative sepsis
Rapid correction of metabolic acidosis
Over compensation
Last hrs

27. Respiratory Alkalosis
What effect does ↓CO2 have on the body?
H+ + HCO3  H2CO3  H20 + CO2

28. Respiratory Alkalosis
↓CO2 Causes a shift to the right
H+ + HCO3  H2CO3  H20 + ↓CO2
Leading to ↓H+ & ↓HCO3 & ↑pH

29. Respiratory Alkalosis
See both an acute
and a chronic
Metabolic
compensatory
response.

30. Respiratory Alkalosis
Acute compensation
Use nonbicarbonated buffers
Cl/HCO3 exchange in cell membranes
Similar for both dogs and cats
Occurs with in 15 minutes

31. Respiratory Alkalosis
Chronic compensation
Dogs
Renal adaptation
Increase H+ retention
Increase HCO3 excretion
Takes 2-5 days to reach steady state
Maybe a similar mechanism in cats

32. Compensated Respiratory Alkalosis
Unfortunately this is not enough to override the ↓CO2

33. Compensated Respiratory Alkalosis
↓H+ +  HCO3  H2CO3  H20 +  CO2
End result:
Mildly decreased H+
Mildly increased pH
Decreased PCO2
Decreased HCO3 (Compensation)

34. Acid-Base Physiology
Respiratory acidosis
Defined as a ↑CO2

35. Respiratory Acidosis Causes of Respiratory acidosis
Congestive heart failure
Primary lower airway disease
Upper airway disease
Others

36. What effect does an increased CO2 have on the carbonic acid equation?
Respiratory Acidosis
What effect does an increased CO2 have on the carbonic acid equation?
H+ + HCO3  H2CO3  H20 + CO2

37. ↑CO2 Causes a shift to the left
Respiratory Acidosis
↑CO2 Causes a shift to the left
H+ + HCO3  H2CO3  H20 + ↑CO2
Leading to ↑H+ & ↑HCO3

38. Respiratory Acidosis See both an acute and a chronic Metabolic
compensatory
response.

39. Respiratory Acidosis Acute compensation Can not use HCO3 buffers
Use proteins such as hemoglobin
H2CO3 + Buf HBuf +HCO3
Can cause an increase in HCO3
1Meq/L per 10 mmHg PCO2
Works poorly as a buffer

40. Respiratory Acidosis Chronic compensation Dogs
Renal adaptation
Increase H+ excretion
Increase HCO3 retention
Takes 2-5 days to reach steady state
Cats may not be able to compensate

41. Respiratory Acidosis Metabolic Compensation – Increased HCO3
↓H+ + ↑HCO3  H2CO3  H20 + CO2
-- Allows equation to be pulled back to the right, decreasing H+

42. Compensated Respiratory Acidosis
Unfortunately this is not enough to override the ↑CO2

43. Compensated Respiratory Acidosis
↑H+ +  HCO3  H2CO3  H20 +  CO2
End result:
Mildly increased H+
Mildly decreased pH
Increased PCO2
Increased HCO3 (Compensation)

44. Normal values Venous blood gases Canine Feline pH – 7.397 pH – 7.343
PCO2 – PCO2 – 38.7
HCO3 – HCO3 – 20.6
PO2 – 52.1 (?)

45. Normal values Arterial blood gases Canine Feline pH – 7.407 pH – 7.386
PCO2 – PCO2 – 31.0
HCO3 – HCO3 – 18.0
PO2 – PO2 – 106.8

46. Acid Base Analysis Is the patient acidic or alkalotic?
Does the PCO2 or HCO3 match the pH?
If PCO2 matches then it is respiratory, if HCO3 matches it is metabolic.
The other value measures compensation.

47. Maggie 10-year-old F/S Lab

48. Maggie 10-year-old F/S Lab
History of acute collapse 1 hr ago
Presents with pale gums, tachypnea, tachycardia, poor pulses
PCV/TS/ Venous Blood gas/Glucose

49. Maggie 10-year-old F/S Lab
PCV/TS – 36%/5.8
Venous blood gas-
pH – 7.1
PCO2- 35
HCO3- 14
Blood glucose - 78

50. Maggie 10-year-old F/S Lab
What is her acid/base status?

51. Maggie 10-year-old F/S Lab
pH – 7.1
PCO2- 35
HCO3- 14
Classified as uncompensated metabolic acidosis

52. Maggie 10-year-old F/S Lab
20 minutes post initial presentation obtained arterial blood gas (On O2)
pH – HCO3- 5.0
PCO PO2 – 189

53. Maggie 10-year-old F/S Lab
Now what is her acid/base status?

54. Maggie 10-year-old F/S Lab
pH – HCO3- 5.0
PCO PO2 – 189
Classified as compensated metabolic acidosis

55. Maggie 10-year-old F/S Lab
How do her clinical signs match up?
Tachycardia, pale gums, poor pulses
Tachypnea
Venous blood gas Arterial blood gas
pH – pH – 7.28
PCO PCO2 – 10.1
HCO HCO3 – 5.0
PO2 – 189
Does she need oxygen?

56. Tom 9-year-old M/C DMH

57. Tom 9-year-old M/C DMH Presented for acute GI signs
Developed CHF post abdominal exploratory
Treated with Lasix

58. Tom 9-year-old M/C DMH
48 hrs post Lasix therapy Venous blood gas was performed
pH – 7.553
PCO
HCO

59. What is his acid/base status?
Tom 9-year-old M/C DMH
What is his acid/base status?

60. Classified as a metabolic alkalosis with respiratory compensation
Tom 9-year-old M/C DMH
pH – 7.553
PCO
HCO
Classified as a metabolic alkalosis with respiratory compensation

61. Missy – 12-year-old F/S Bichon

62. Missy – 12-year-old F/S Bichon
History of acute onset respiratory distress
Presents with grade IV/VI heart murmur, crackles bilaterally, cyanosis
Thoracic radiographs, venous blood gas

63. Missy – 12-year-old F/S Bichon
Thoracic radiographs – cardiomegally, diffuse alveolar pattern
Venous blood gas-
pH – 7.15
PCO2 – 56
HCO3 - 20

64. Missy – 12-year-old F/S Bichon
What is her acid/base status?

65. Missy – 12-year-old F/S Bichon
pH – 7.15
PCO2 – 56
HCO3 - 20
Classified as uncompensated respiratory acidosis

66. Toby 12-year-old M/C Westie

67. Toby 12-year-old M/C Westie
History of chronic cough x 2 years
Recent worsening of cough
Decreased appetite, lethargy x 1 week
Increased respiratory rate, unwilling to walk this evening

68. Toby 12-year-old M/C Westie
Physical examination
Depressed, pale pink mm, 5% dehydrated
Tachypnic and slightly dyspnic
Thoracic auscultation – fine crackles bilaterally, harsh BV sounds
Thoracic radiographs, venous blood gas

69. Toby 12-year-old M/C Westie
Thoracic radiographs – diffuse bronchointerstitial pattern
Venous blood gas –
pH 7.35
PCO2 – 43
HCO3 - 30

70. Toby 12-year-old M/C Westie
What is his acid/base status?

71. Toby 12-year-old M/C Westie
pH 7.35
PCO2 – 43
HCO3 - 30
Classified as a compensated respiratory acidosis

72. Questions

73. Therapy for Acid Base Disorders

74. Metabolic Acidosis Detrimental effects of Acidosis
Decreased myocardial contractility when pH < 7.2
Predispose heart to VPCs
Peripheral insulin resistance
Obtunded state or coma

75. Metabolic Acidosis Treatment
IV fluids to address dehydration or hypovolemia –
Use pH balanced fluids such as LRS or Norm R

76. Metabolic Acidosis Treatment
Blood transfusions for anemia

77. Metabolic Acidosis Treatment
NaHCO3 supplement
Use only when dehydration, hypovolemia, and anemia have been addressed
pH is < 7.2
Do not use when body has not shown compensation with a low PCO2

78. Metabolic Acidosis Treatment
NaHCO3 supplement
.3 x [Wt(Kg)] x BE
BE = Normal HCO3 (24) – HCO3 of patient
If pH < 7.1 then bolus 25%
Give 50% over 12 hrs.

79. Metabolic Acidosis Treatment
NaHCO3 supplement
Monitor acid-base status q 6 hrs
Stop supplement when pH is 7.2
Recheck acid-base status 6 hrs later to make sure further supplementation is not needed.

80. Metabolic Alkalosis Renal physiology
Normally expect the kidney to excrete HCO3 and conserve H+
Normal human patients given 1,000 mEq NaHCO3 /day for 2 weeks excreted virtually all of the HCO3
Metabolic alkalosis has significantly less HCO3 load
- Burton David Rose, 1994

81. Metabolic Alkalosis Renal physiology Hypochloremic metabolic alkalosis
Decreased Cl- to Macula Densa
Increased Renin excretion
Increases distal tubule H+ secretion

82. Metabolic Alkalosis Renal physiology Hypochloremic metabolic alkalosis
H+-ATPase pump in collecting tubule.

83. Metabolic Alkalosis Renal physiology Hypokalemia
Increased H+/K+ exchange leading to influx of H+ into cells. This leads to H+ secretion in renal tubules.
Severe hypokalemia causes renal excretion of Cl-

84. Metabolic Alkalosis Treatment IV fluids - .9% NaCl K+ supplementation
Replenishes Cl-
Acidic fluid
With out Cl- you can not encourage H+ retention and HCO3 excretion
K+ supplementation
Correct primary problem

85. Respiratory Acidosis and Alkalosis
Treatment
No specific treatment necessary
Therapy directed towards the primary problem.

86. Acid Base Physiology