Arterial Blood Gas Assessments Chapter 4  Arterial Blood Gas Assessments

Table 4-1. Normal Blood Gas Values

Box 4-1. Acid-Base Disturbance Classifications

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

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

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. -

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.

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

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

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.

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.

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.

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

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.

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.

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.

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

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

*When pulmonary pathology is present Acute Alveolar Hyperventilation with Hypoxemia (Acute Respiratory Alkalosis) ABG Changes Example pH: increased 7.55 PaCO2: decreased 29 mm Hg HCO3: decreased 22 mEq/L PaO2: decreased 61 mm Hg* *When pulmonary pathology is present

The most common cause of acute alveolar hyperventilation is: Hypoxemia

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

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

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

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

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

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

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

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

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

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

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

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

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.

Lactic Acidosis Metabolic Acidosis (Cont’d) ABG Changes Example pH: decreased 7.21 PaCO2: normal or decreased 35 mm Hg HCO3: decreased 19 mEq/L PaO2: decreased 34 mm Hg

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

Metabolic Acid-Base Abnormalities

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)

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 .

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.

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+: 140 mEq/L Cl−: 105 mEq/L HCO3: 24 mEq/L

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 − 105 + 24 = 140 − 129 = 11 mEq/L

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.

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

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

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.

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.

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

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

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.

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

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

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

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

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

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.