by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology
This page describes the interpretation of the acid-base component of blood gas results. Designing the interactive acid-base diagram necessitated the development of a logical approach. This page converts the logic back into a human process.
In a Perfect World complete information about a patient is available before acid-base values are analyzed. What follows is a logical framework for looking at acid-base values with no patient. Reports may say that the results are "typical of" or "characteristic of" a single clinical problem. However, identical results can also be obtained from a complex combination of clinical problems.
Is the pH normal, acid, or alkaline – critical because it governs all the subsequent thinking. In acute problems the change is usually acidic - a low pH - e.g., 7.2 or 7.1. This is because failure, either respiratory or metabolic, results in the accumulation of acids. The following paragraphs assume the result is acid. However, also look at the Table of Details which follows the paragraphs below.
Step 2: If the respiratory change is also acid (raised PCO2), then the cause is respiratory, unless the metabolic component is also acidic – in which case both are contributing to the acidic pH.
If the PCO2 is not like the pH, i.e., the PCO2 is low (alkaline), then the primary problem must be metabolic and the low PCO2 is compensating for the metabolic acidosis.
Step 3: If the Standard Base Excess (SBE) is acidic (a negative SBE), then the cause is metabolic. The exception, as above, is when the respiratory component is also acid when both are contributing 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.
Step 4: An few adjectives are useful to characterize magnitude and in conversation adjectives are casually. The interactive diagram necessitated a disciplined sequence: None or Negligible, Minimal, Mild, Moderate, Marked, Severe. This is overkill for bedside or clinical comment. In the diagram, these adjectives are applied to both the respiratory and metabolic components and, at pH=7.4, should "balance" with the PCO2 and the SBE being equal and opposite. 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 actually used are shown in the following table:
|Alkalosis||Severe||< 18||> 13|
|Marked||18 to 25||13 to 9|
|Moderate||25 to 30||9 to 6|
|Mild||30 to 34||6 to 4|
|Minimal||34 to 37||4 to 2|
|Normal||Normal||37 to 43||2 to -2|
|Acidosis||Minimal||43 to 46||-2 to -4|
|Mild||46 to 50||-4 to -6|
|Moderate||50 to 55||-6 to -9|
|Marked||55 to 62||-9 to -13|
|Severe||> 62||to < -13|
A Pure. or acute, respiratory disturbance is found close to the SBE=0 line: the change in ventilation has occured too rapidly for metabolic compensation to occur.
A pure metabolic disturbance would lie close to PCO2 = 40 mmHg (5.33 kPa). In practice, however, this is seen rarely because partial repiratory compensation occurs even as the metabolic change develops.
Step 5: Characteristic compensation zones indicate where someone with a prolonged single problem is likely to lie (see the Acid-Base Diagram). These zones of partial compensation lie roughly half way between no compensation and complete compensation.
A respiratory acidosis with a PCO2 of 60 mmHg (raised by 20mmHg) would require for "complete compensation" an SBE = 12 mEq/L (using the 5 to 3 ratio given above). An SBE=0 mEq/L would suggest "no compensation". A value in the middle (SBE = 6 mEq/L) is typical "compensation for chronic respiratory acidosis".
A metabolic acidosis with SBE of -12 (reduced by 12 mEq/L) would require for "complete compensation" a PCO2 = 20 mmHg (using the 5 to 3 ratio given above). A normal PCO2 would indicate "no compensation". A value in the middle (30 mmHg) is typical "compensation for metabolic acidosis".
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.
Six Classical Acid-Base Disturbances
|Acid||Acid||Alk||Resp. Acid. Comp||SBE Half way - Normal Met. Comp.|
|Norm||Resp. Acid. Pure||SBE Normal - No Met. Comp|
|Alk||Acid||Met. Acid. Comp||PCO2 Half way - Normal Resp. Comp.|
|Alk||Alk||Acid||Resp. Alk. Comp||SBE Half way - Normal Met. Comp.|
|Norm||Resp. Alk. Pure||SBE Normal - No Met. Comp|
|Acid||Alk||Met. Alk. Comp||PCO2 Half way - Normal Resp. Comp|
Four Other Acid-Base Disturbances
|Acid||Acid||Acid||Combined Acidosis||Not Applicable - Both Acid|
|Norm||Acid||Met. Acid. Pure||PCO2 Normal - No Resp. Comp|
|Alk||Alk||Alk||Combined Alkalosis||Not Applicable - Both Components Alkaline|
|Norm||Alk||Met. Alk. Pure||PCO2 Normal - No Resp. Comp|
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:
Logical Approach to an Acid pH:
The following 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.
Alan W. Grogono
|Copyright Mar 2018.|
All Rights Reserved