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Acid-Base Balance Tutorial

by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

Acid-Base Interpretation

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Interpretation Icon"Don't ever over-analyze your results. Don't ever try to find your own secret or the one which you admire. One does not try to catch soap bubbles. One enjoys them in flight and is grateful for their fluid existence." – Ernst Haas.

Index.

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Purpose.

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.

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Constraints of Not Knowing Patient Details:

Globe "Analyzing acid-base data with no patient is a like bidding a bridge hand with no partner: worth doing, but not life-like."

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. A report may invite you to "Consider" a single clinical problem. However, identical results could also be obtained from a complex combination of clinical problems and therapy.

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The pH - the Overall Change:

The pH "The pH is the essential strarting point because it shows the overall trend."

Step 1: Is the pH normal, acid, or alkaline – critical because it governs all the subsequent thinking. In acute problems the change is usually more 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 acidemia. However, also look at the Table of Details which follows the paragraphs below.

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PCO2 - the Respiratory Component:

Respiratory "PCO2 is the obvious next step: Does the PCO2 change match the pH, i.e., are they both acid?"

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.

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Standard Base Excess - the Metabolic Component:

Metabolic "Finally judge the metabolic component does it match the overall change, i.e., are they both acid?"

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.

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Adjectives to Describe Changes:

Metabolic "Judging the magnitude of the change is less important than judging: 'How critically ill is the patient?'"

Step 4: Few adjectives are used to characterize magnitude in casual conversation. The interactive diagram necessitated a disciplined sequence: No, Minimal, Mild, Moderate, Marked, Severe. In the diagram, these adjectives are applied to both the respiratory and metabolic components and, at pH=7.4, they "balance" with the PCO2 and the SBE being equal and opposite. If at pH 7.4 the PCO2 is described as a marked acidosis then the SBE must be a marked alkalosis. The steps actually used are shown in the following table (approximate for SBE):

   
Adjective
PCO2
mmHg
SBE
mEq/L
Alkalosis Severe < 10 > 16.5
Marked 10 to 19 12 to 16.5
Moderate 19 to 26 8 to 12
Mild 26 to 32 4.5 to 8
Minimal 32 to 37 2 to 4.5
Normal Normal 37 to 43 2 to -2
Acidosis Minimal 43 to 48 -2 to -4.5
Mild 48 to 54 -4.5 to -8
Moderate 54 to 61 -8 to -12
Marked 61 to 70 -12 to -16.5
Severe > 70 < -16.5

"Pure" Disturbances:

A pure, or acute, respiratory disturbance is found close to the SBE=0 line: the change in ventilation has occurred 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 rare because partial respiratory compensation occurs even as the metabolic change develops.

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Recognizing Compensation:

QuestionMark "For practical purposes, typical compensation returns the pH about halfway back towards normal."

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. 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. A normal PCO2 would indicate "no compensation". A value in the middle (30 mmHg) is typical "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 occasionally encountered: two pure uncompensated metabolic disturbances and the two combined disturbances.

1. Six Classical Acid-Base Disturbances

pH PCO2 SBE Interpretation Compensation
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
 

2. Four Other Acid-Base Disturbances

pH PCO2 SBE Interpretation Compensation
Acid Norm Acid Met. Acid. Pure PCO2 Normal - No Resp. Comp
Acid Acid Combined Acidosis No Compensation - Both Acid
Alk Norm Alk Met. Alk. Pure PCO2 Normal - No Resp. Comp
Alk Alk Combined Alkalosis No Compensation - Both Alkaline

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 multiple problems and do not, therefore, display a characteristic response to a single condition.
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Examples of Interpretation:

Examples "Try to analyze each example before you read the explanation."  

Logical Approach to an Acid pH: The following section was added in March 2003 at the request of a reader:

The examples allow a logical analysis sequence to be followed:

  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.
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Example A: pH = 7.2, PCO2 = 60 mmHg (8 kPa), 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 (8 kPa), 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 (2.7 kPa) on the acid side of normal 40 mmHg (5.3 kPa). Complete compensation would require 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 ±3mEq/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 (8 kPa), 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 (4 kPa), 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). Complete compensation would require a fall in the PCO2 of 17 mmHg (2.3 kPa) to 23 mmHg (3.1 kPa)
  6. The actual PCO2 is 30 mmHg (4 kPa) which is roughly half way towards normal, i.e., a typical respiratory compensation. The range is about ±5 mmHg (±0.7 kPa) wide - in this example between about 27 mmHg (3.6 kPa) and 37 mmHg (4.9 kPa).
  7. Magnitude: marked metabolic acidosis with mild respiratory compensation.
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Conclusion

Conclusion "Thank you readers!" I appreciate feedback and would enjoy hearing from you and considering your suggestions. Thank you.

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Acid-Base Tutorial
Alan W. Grogono
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All Rights Reserved
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