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Acid-Base Interpretation

Index.

Introduction.

Purpose: This page provides an outline of how to approach the interpretation of the acid-base component of blood gas results. The process of designing the acid-base diagram program necessitated the development of a rigorously logical approach. This page converts this logic back into a human process.

Constraints of Not Knowing Patient Details:

In a Perfect World complete information about a patient is available before you try to analyze the results. A website is not a perfect world. What follows is an approach to understanding naked numbers.

The analysis provides you with a logical framework for looking at acid-base results. Reports may say that the results are "typical of" or "characteristic of" a single clinical problem. Caution: identical results can also be obtained from a complex combination of clinical problems.

The pH - the Overall Change:

Step 1:   The pH. Is it normal, acid, or alkaline. This first step is crucial, because it governs all the subsequent thinking. In most acute problems the change will be acid - a low pH - e.g., 7.2 or 7.1. This is because failure, either respiratory or metabolic, results in the accumulation of acids. In the following paragraphs we will assume the result is acid. However, also look at the Table of Details which follows the paragraphs below.

PCO₂ - the Respiratory Component:

Step 2:   If the respiratory change is like the pH, i.e., both acid, then the cause is respiratory. The exception is when the metabolic component is also acid. Then, both are contributing to the acid pH.

If the PCO₂ is not like the pH, i.e., the PCO₂ is low (alkaline), then the primary problem is not respiratory; the low PCO₂ is a compensation for the metabolic acidosis.

Standard Base Excess - the Metabolic Component:

Step 3:   If the Standard Base Excess (SBE) is the component which is like the pH, i.e., both acid (a negative base excess), then the cause is metabolic. The exception, as above, is when the respiratory component is also acid. Then both contribute to the acid pH.

If the SBE is not like the pH, i.e., the SBE is alkaline, then the primary problem is not metabolic; the high SBE is a compensation for the respiratory acidosis.

Magnitude of Changes:

Step 4:   To describe acid-base disturbances, it is helpful to have a few adjectives to characterize magnitude. In conversation adjectives may be used casually but, for the interactive diagrams, I had to be disciplined and chose this sequence: None or Negligible, Minimal, Mild, Moderate, Marked, Severe. This is overkill for bedside or clinical comment. Just try to judge minor, moderate, or major. The following details are taken from the interactive diagram and are included here for interest only.

These adjectives are applied to both the respiratory and metabolic components and should, therefore, "balance". What does that mean? Whenever the pH is normal, i.e., pH = 7.4. then the PCO₂ and the SBE are equal and opposite. In such circumstances, if the PCO₂ is described as a marked acidosis then logically the SBE must be the exact opposite, a marked alkalosis. Fortunately, the slope for BE/PCO₂ when ph = 7.4 gives us this ratio: three units of change in the SBE is equivalent to a five mmHg change in the PCO2. The steps I actually used are shown in the following table:

Recognizing Compensation:

Step 5:   Characteristic compensation zones indicate where it is most likely that you will find someone with a prolonged single problem, e.g., respiratory or metabolic acidosis. These zones are, not surprisingly, most easily visualized on a diagram. Fortunately there is an easy method of roughly gauging typical compensation. That is because the zones lie roughly half way between no compensation and complete compensation.

If a patient with a respiratory problem has a high PCO₂, e.g., 60 mmHg (raised by 20mmHg as in the above example) then for "complete compensation" the SBE would have to be about 12 (using the 5 to 3 ratio given above). If the SBE were zero it would suggest "no compensation" - typical of an acute process of recent onset. Finally - and the most likely - the patient is somewhere in the middle (SBE = 6 mEq/L) which is typical for "compensation for chronic respiratory acidosis".

To take the inverse example, if a patient with a metabolic problem has a low SBE, e.g., -12, then the PCO₂ would have to be reduced by hyperventilation to about 20 mmHg to achieve "complete compensation". If the PCO₂ were still normal then there would be "no compensation". Again, far the most likely, the patient is somewhere in the middle (30 mmHg) which is typical for "compensation for metabolic acidosis".

Table of Details:

The first table below summarizes the six classical acid-base disturbances to be recognized.

The second table shows four combinations that are logically possible and occasionally encountered: the two uncompensated metabolic disturbances and the two combined disturbances.

Recognize typical zones: The pink and blue zones represent the characteristic zones where patients with a single problem are typically found. The grey colored zones are less important for recognition: 1) although pure metabolic acidosis and alkalosis are logically possible, people normally compensate by adjusting their ventilation; 2) people with combined acidosis or alkalosis have mutiple problems and do not, therefore, display a characteristic response to a single condition.

Examples of Interpretation:

Logical Approach to an Acid pH  (for an alkaline pH - substitute alkaline for acid below):

1. Are the pH and the PCO₂ both acid? If so the PCO₂ contributes to the condition.
2. If not (i.e., PCO₂ is alkaline) then the metabolic component is the cause and the PCO₂ is compensatory.
3. Is either PCO₂ or SBE normal? Because, if so, there is no compensation and you have a pure acidosis: pure respiratory acidosis occurs fairly frequently, metabolic rarely as explained above.
4. To be typical the compensation must lie roughly half way between no compensation and complete compensation - use the rule 3 mEq/L = 5 mmHg to work out complete compensation.
5. If both components are acid, you don't have a typical single condition, you have a combined metabolic and respiratory acidosis.

Example A: pH = 7.2, PCO₂ = 60 mmHg, SBE = 0 mEq/L

1. Overall change is acid.
2. Respiratory change is also acid - therefore contributing to the acidosis.
3. SBE is normal - no metabolic compensation. Therefore, pure respiratory acidosis.
4. Typical of acute respiratory depression. Magnitude: marked respiratory acidosis

Example B: pH = 7.35, PCO₂ = 60 mmHg, SBE = 7 mEq/L

1. Overall change is slightly acid.
2. Respiratory change is also acid - therefore contributing to the acidosis.
3. Metabolic change is alkaline - therefore compensatory.
4. The respiratory acidosis is 20 mmHg on the acid side of normal (40). To completely balance plus 20 would require 20 * 3 / 5 = 12 mEq/L SBE
5. The actual SBE is 7 eEq/L, which is roughly half way between 0 and 12, i.e., a typical metabolic compensation. The range is about 6mEq/L wide - in this example between about 3 and 9 mEq/L.
6. Magnitude: marked respiratory acidosis with moderate metabolic compensation

Example C: pH = 7.15, PCO₂ = 60 mmHg, SBE = -6 mEq/L

1. Overall change is acid.
2. Respiratory change is acid - therefore contributing to the acidosis.
3. Metabolic change is also acid - therefore combined acidosis.
4. The components are pulling in same direction - neither can be compensating for the other
5. Magnitude: marked respiratory acidosis and mild metabolic acidosis

Example D: pH = 7.30, PCO₂ = 30 mmHg, SBE = -10 mEq/L

1. Overall change is acid.
2. Respiratory change is alkaline - therefore NOT contributing to the acidosis.
3. Metabolic change is acid - therefore responsible for the acidosis.
4. The components are pulling in opposite directions. SBE is the acid component so it is primarily a metabolic problem with some respiratory compensation
5. The metabolic acidosis is 10 mEq/L on the acid side of normal (0). To completely balance 10 would require 10 * 5 / 3 = 17 mmHg respiratory alkalosis (= 23 mmHg)
6. The actual PCO₂ is 30 eEq/L which is roughly half way between 23 and 40, i.e., a typical respiratory compensation. The range is about 10 mmHg wide - in this example between about 27 and 37 mmHg.
7. Magnitude: marked metabolic acidosis with mild respiratory compensation.

Conclusion

The above section was added in March 2003 at the request of a reader. I appreciate feedback and would enjoy hearing from you and considering your suggestions. Thank you.

Acid-Base Tutorial Alan W. Grogono

Copyright Nov 2009. All Rights Reserved