The Diagnosis of Acid-Base Disturbances Depends on Interpretation of Measurements of Arterial Blood pH and Carbon Dioxide Tension, from Which Bicarbonate Concentration and Total Buffer Base Are Calculated
Arterial samples must be used to determine the respiratory component of an acid-base abnormality, and samples must be obtained anaerobically to prevent the loss of CO2 from the blood.
The Paco2 and pH are measured with specific electrodes in an arterial blood gas (ABG) analyzer. The plasma IHCO3'] and total buffer base are determined from nomograms or frequently from a built-in program in the ABG analyzer.When ABG data are analyzed, it is useful to ask the following questions:
1. Is the sample acidotic (pH 7.4)?
2. What is the respiratory component (is Paco2 high, low, or normal), and will it explain the pH?
3. What is the metabolic component (is there a base excess or deficit), and will it explain the pH?
4. How can items 1,2, and 3 be combined to explain the data, in view of the fact that compensations rarely return the pH toward normal?
Examples are provided in Table 52-1.
Table 52-1
Examples of Blood Gas Abnormalities
| pH | Paco2 | HCO3 | Base excess | Base deficit | Diagnosis |
| 7.4 | 40 | 24 | 0 | 0 | Normal |
| 7.26 | 60 | 27 | 0 | 0 | Uncompensated respiratory acidosis |
| 7.38 | 60 | 36 | 9 | 0 | Partially compensated respiratory acidosis |
| 7.2 | 40 | 15 | 0 | 12 | Uncompensated metabolic acidosis |
| 7.35 | 22 | 11 | 0 | 12 | Partially compensated metabolic acidosis |
| 7.45 | 20 | 13 | 0 | 11 | Partially compensated respiratory alkalosis |
| 7.55 | 40 | 34 | 11 | 0 | Uncompensated metabolic alkalosis |
| 7.2 | 50 | 19 | 0 | 9 | Combined metabolic and respiratory acidosis |
| 7.6 | 20 | 20 | 0 | 0 | Uncompensated respiratory alkalosis |
| 7.3 | 20 | 9 | 0 | 15 | Partially compensated metabolic acidosis |
Paco2, Arterial carbon dioxide tension (mm Hg); HCO3, bicarbonate (r∩Eq∕L).
Over the Years, Many Terms Have Been Used to Explain Acid-Base Balance
The terms used to explain acid-base balance include the following:
Anion gap: In the blood the total cation concentration (concentration of Na+ + K4 + Mg2+ + Ca2+) should approximately equal the total anion concentration (concentration of HCO3- + CΓ).
Usually, the total cations exceed the total anions; the difference is called the union gap. This gap results from the presence of unaccounted-for anions from fixed acids, such as lactate. In metabolic acidosis the anion gap increases because of increased production of fixed acids.Standard bicarbonate: The plasma [ HCO3-J when Pco2 = 40 mm Hg is known as “standard” bicarbonate. The plasma [HCO3-] can change as a result of respiratory and metabolic disturbances. An increase or decrease in [HCO3-], measured when Pco2 is normal (i.e., 40 mm Hg), results only from metabolic disturbances.
Total carbon dioxide (Teo2): Carbon dioxide is present in the blood in solution and as carbamino compounds, but largely as HCO3-. Teo2 can be measured by adding an acid to the blood and collecting the evolved CO2, which comes primarily from HCO3-. Changes in Tco2 should be interpreted as changes in plasma [HCO3-].