1. Arterial Blood Gas Assessments
Chapter 4
 Arterial Blood Gas Assessments

2. Table 4-1. Normal Blood Gas Values

3. Box 4-1. Acid-Base Disturbance Classifications

4. This Chapter Provides the Following Review
The PCO2/HCO3/pH relationship—an essential cornerstone of ABG interpretations
The six most common acid-base abnormalities seen in the clinical setting
The metabolic acid-base abnormalities
The hazards of oxygen therapy in the patient with chronic ventilatory failure with hypoxemia

5. Figure 4-1. Nomogram of the PCO2/HCO3/pH relationship.

6. Figure 4-2. Acute ventilatory failure is confirmed when the reported PCO2, pH, and HCO3 values all intersect within the red-colored respiratory acidosis bar. For example, when the PCO2 is 60 mm Hg at a time when the pH is 7.28 and the HCO3 is 26 mEq/L, acute ventilatory failure is confirmed. -

7. Figure 4-3. Acute alveolar hyperventilation is confirmed when the reported PCO2, pH, and HCO3 values all intersect within the red-colored respiratory alkalosis bar. For example, when the reported PCO2 is 25 mm Hg at a time when the pH is 7.55 and the HCO3 is 21 mEq/L, acute alveolar hyperventilation is confirmed.

8. A Quick Clinical Calculation for Acute PaCO2 Changes in pH and HCO3

9. Acute Increases in PaCO2 (e.g., Acute Hypoventilation)

10. Using the Normal ABG Values as a Baseline—pH 7
Using the Normal ABG Values as a Baseline—pH 7.40, PaCO2 40, and HCO3 24:
For every 10 mm Hg the PaCO2 increases, the pH will decrease about 0.06 units and the HCO3 will increase about 1 mEq/L.
Or, for every 20 mm Hg the PaCO2 increases, the pH will decrease about 0.12 units and the HCO3 will increase about 2 mEq/L.

11. Using the Normal ABG Values as a Baseline—pH 7
Using the Normal ABG Values as a Baseline—pH 7.40, PaCO2 40, and HCO3 24 (Cont’d)
Thus if the patient’s PaCO2 suddenly were to increase to, say, 60 mm Hg, the expected pH change would be about 7.28 and the HCO3 would be about 26 mEq/L.

12. Using the Normal ABG Values as a Baseline—pH 7
Using the Normal ABG Values as a Baseline—pH 7.40, PaCO2 40, and HCO3 24 (Cont’d)
It should be noted, however, that if the patient’s PaO2 is severely low, lactic acid may also be present.
This results in a combined metabolic and respiratory acidosis.
In such cases the patient’s expected pH and HCO3 values would both be lower than expected for a particular PaCO2 level.

13. Acute Decreases in PaCO2 (e.g., Acute Hyperventilation)

14. Using the Normal ABG Values as a Baseline—pH 7
Using the Normal ABG Values as a Baseline—pH 7.40, PaCO2 40, and HCO3 24
For every 5 mm Hg the PaCO2 decreases, the pH will increase about 0.06 units and the HCO3 will decrease about 1 mEq/L.
Or, for every 10 mm Hg the PaCO2 decreases, the pH will increase about 0.12 units and the HCO3 will decrease about 2 mEq/L.

15. Using the Normal ABG Values as a Baseline— pH 7
Using the Normal ABG Values as a Baseline— pH 7.40, PaCO2 40, and HCO3 24 (Cont’d)
Thus if the patient’s PaCO2 suddenly were to decrease to, say, 30 mm Hg, the expected pH change would be about 7.52 and the HCO3 would be about 22 mEq/L.

16. Using the Normal ABG Values as a Baseline— pH 7
Using the Normal ABG Values as a Baseline— pH 7.40, PaCO2 40, and HCO3 24 (Cont’d)
Again, it should be noted, however, that if the patient’s PaO2 is severely low, lactic acid may also be present.
In such cases the patient’s expected pH and HCO3 values would both be lower than expected for a particular PaCO2 level.

17. Table 4-2. General Rule of Thumb for the Paco2/ HCO−3/pH Relationship

18. The Six Most Common Acid-Base Abnormalities Seen in the Clinical Setting
Acute alveolar hyperventilation
Acute ventilatory failure
Chronic ventilatory failure with hypoxemia
Acute alveolar hyperventilation superimposed on chronic ventilatory failure
Acute ventilatory failure superimposed on chronic ventilatory failure

19. *When pulmonary pathology is present
Acute Alveolar Hyperventilation with Hypoxemia (Acute Respiratory Alkalosis)
ABG Changes Example
pH: increased
PaCO2: decreased 29 mm Hg
HCO3: decreased 22 mEq/L
PaO2: decreased 61 mm Hg*
*When pulmonary pathology is present

20. The most common cause of acute alveolar hyperventilation is: Hypoxemia

21. Figure 4-4. Relationship of venous admixture to the stimulation of peripheral chemoreceptors in response to alveolar consolidation.

22. Figure 4-5. The PaO2 and PaCO2 trends during acute alveolar hyperventilation.

23. Box 4-2. Pathophysiologic Mechanisms That Lead to a Reduction in the Paco2

24. Acute Ventilatory Failure with Hypoxemia (Acute Respiratory Acidosis)
ABG Changes Example
pH: decreased 7.21
PaCO2: increased 79 mm Hg
HCO3: increased (slightly) 28 mEq/L
PaO2: decreased 57 mm Hg

25. Chronic Ventilatory Failure with Hypoxemia (Compensated Respiratory Acidosis)
ABG Changes Example
pH: normal 7.38
PaCO2: increased 66 mm Hg
HCO3: increased (significantly) 35 mEq/L
PaO2: decreased 63 mm Hg

26. Box 4-3. Respiratory Diseases Associated with Chronic Ventilatory Failure during the
Advanced Stages

27. Figure 4-6. The PaO2 and PaCO2 trends during acute or chronic ventilatory failure.

28. Acute Ventilatory Changes Superimposed on Chronic Ventilatory Failurez
Acute alveolar hyperventilation superimposed on chronic ventilatory failure
Acute ventilatory failure superimposed on chronic ventilatory failure

29. Acute Alveolar Hyperventilation Superimposed on Chronic Ventilatory Failure (Acute Hyperventilation on Compensated Respiratory Acidosis)
ABG Changes Example
pH: increased 7.53
PaCO2: increased 51 mm Hg
HCO3: increased 37 mEq/L
PaO2: decreased 46 mm Hg

30. Table 4-3. Examples of Acute Changes in Chronic Ventilatory Failure

31. Acute Ventilatory Failure Superimposed on Chronic Ventilatory Failure (Acute Hypoventilation on Compensated Respiratory Acidosis)
ABG Changes Example
pH: decreased 7.21
PaCO2: increased 110 mm Hg
HCO3: increased 43 mEq/L
PaO2: decreased 34 mm Hg

32. Table 4-3. Examples of Acute Changes in Chronic Ventilatory Failure

33. Lactic Acidosis Metabolic Acidosis
Because acute hypoxemia is commonly associated with respiratory disorders, acute metabolic acidosis (caused by lactic acid) often further compromises respiratory acid-base status.

34. Lactic Acidosis Metabolic Acidosis (Cont’d)
ABG Changes Example
pH: decreased
PaCO2: normal or decreased mm Hg
HCO3: decreased mEq/L
PaO2: decreased mm Hg

35. Figure 4-1. Nomogram of the PCO2/HCO3/pH relationship.

36. Metabolic Acid-Base Abnormalities

37. Metabolic Acidosis ABG Changes Example pH: decreased 7.26
PaCO2: normal 37 mm Hg
HCO3: decreased 18 mEq/L
PaO2: normal 94 mm Hg
(or decreased if lactic (or 52 mm Hg if acidosis is present) lactic acidosis is present)

38. Anion Gap
The anion gap is used to determine if a patient’s metabolic acidosis is caused by either:
the accumulation of fixed acids (e.g., lactic acids, keto acids, or salicylate intoxication), or
an excessive loss of HCO3 .

39. Anion Gap (Cont’d)
The law of electroneutrality states that the total number of plasma positively charged ions (cations) must equal the total number of plasma negatively charged ions (anions) in the body fluids.
To calculate the anion gap, the most commonly measured cations are sodium (Na+) ions.

40. Anion Gap (Cont’d)
The most commonly measured anions are chloride (Cl−) ions and bicarbonate (HCO3) ions.
The normal plasma concentrations of these cations and anions are as follows:
Na+: mEq/L
Cl−: mEq/L
HCO3: mEq/L

41. Anion Gap (Cont’d)
Mathematically, the anion gap is the calculated difference between the Na+ ions and the sum of the HCO3 and Cl− ions:
Anion gap = [Na+] − ([Cl−] + [HCO3])
= 140 −
= 140 − 129
= 11 mEq/L

42. Anion Gap (Cont’d)
The normal range for the anion gap is 9 to 14 mEq/L.
An anion gap greater than 14 mEq/L represents metabolic acidosis.

43. Anion Gap (Cont’d)
An elevated anion gap is frequently caused by the accumulation of fixed acids—for example:
Lactic acids
Keto acids
Salicylate intoxication

44. Anion Gap (Cont’d)
This is because the H+ ions that are generated by the fixed acids chemically react with—and are buffered by—the plasma HCO3
This action causes
The HCO3 concentration to decrease and
The anion gap to increase

45. Anion Gap (Cont’d)
Clinically, when the patient exhibits both metabolic acidosis and an increased anion gap, the respiratory care practitioner must investigate further to determine the source of the fixed acids.
This needs to be done in order to appropriately treat the patient.

46. Anion Gap (Cont’d) For example, metabolic acidosis caused by:
Lactic acids justifies the need for oxygen therapy—to reverse the accumulation of the lactic acids, or
Ketone acids justifies the need for insulin—to reverse the accumulation of the ketone acids.

47. Anion Gap (Cont’d)
It is interesting that metabolic acidosis caused by an excessive loss of HCO3 does not cause the anion gap to increase.
For example, in renal disease or severe diarrhea

48. Anion Gap (Cont’d)
This is because as the HCO3 concentration decreases, the Cl− concentration usually increases to maintain electroneutrality.
In short, for each HCO3 ion that is lost, a Cl− anion takes its place
Law of electroneutrality

49. Anion Gap (Cont’d) This action maintains a normal anion gap.
Metabolic acidosis caused by a decreased HCO3 level is commonly called hyperchloremic metabolic acidosis.

50. Summary
When metabolic acidosis is accompanied by an increased anion gap, the most likely cause of the acidosis is fixed acids.
Lactic acids
Keto acids
Salicylate intoxication

51. Summary (Cont’d)
Or, when metabolic acidosis is seen with a normal anion gap, the most likely cause of the acidosis is an excessive loss of HCO3
For example, caused by renal disease or severe diarrhea

52. Metabolic Alkalosis ABG Changes Example pH: increased 7.56
PaCO2: normal 44 mm Hg
HCO3: decreased 27 mEq/L
PaO2: normal 94 mm Hg

53. Figure 4-1. Nomogram of the PCO2/HCO3/pH relationship.

54. Box 4-4. Common Causes of Metabolic Acid-Base Abnormalities

55. The Hazards of Oxygen Therapy in Patients with Chronic Ventilatory Failure with Hypoxemia
High oxygen concentrations may suppress the patient’s so-called hypoxic drive to breathe.