Primary Versus Mixed Conditions

This page provides two ways to consider acid-base disturbances:
 

Method I.  A rigorous method which involves calculation of the expected compensations.
Method II.  A quick and dirty method to tell from a blood gas if a respiratory condition is simple or compensated.
Method III.  Look on a nomogram.

Also see the discussion of how to interpret the base deficit on a blood gas.

Method I.  Expected compensations

In acid-base disorders, there are expected compensatory mechanisms.  For instance, when bicarbonate is lost the primary process is metabolic alkalosis and the normal response is compensatory respiratory acidosis (retention of CO₂).  If the change in pCO₂ or HCO₃^- is equivalent to the expected compensatory response, the disorder is "simple".  However, if the compensation is outside the normal range, it is a mixed disorder; that is, two primary processes are taking place simultaneously.  The expected compensation is for each condition is defined below:

Metabolic

Acidosis

Expected pCO₂ = 1.5 x [HCO₃^-] + 8 ± 2

Alkalosis

Expected pCO₂ = 6 mmHg per 10 mEq/L  in HCO₃^-

Respiratory

Acidosis

 

Acute	Expected HCO3- =	1 mEq/L for each 10 mm pCO2
Chronic	Expected HCO3- =	3.5mEq/L for each 10 mmHg pCO2

Alkalosis

 

Acute	Expected HCO3- =	2 mEq/L for each 10 mm Hg pCO2
Chronic	Expected HCO3- =	5 mEq/L for each 10 mmHg pCO2

Some examples:

2.  pH=7.08, pCO₂=14, HCO₃^-=4, Na=140, Cl=104:

• Primary disorder is a metabolic acidosis
• The pH is low indicating the primary disorder is acidosis.
• The pCO₂ is low, the expected compensation, trying to "blow off" CO₂.
• The compensation does not fully correct the primary problem.
• The predicted pCO₂ by the above equation is 1.5*4+8 = 14.
• This is the observed pCO₂
• The anion gap is 140 - (104 + 4) = 32, thus elevated.

3.  pH 7.08, pCO₂=14, HCO₃^-=4, Na=140, Cl=124:

• AG = 140 - (124 + 4) = 12
• Same situation as above, but chloride has replaced bicarbonate.

• For the pH to be normal, this must be mixed disorder (respiratory compensation can never fully correct a simple metabolic acidosis).
• The anion gap is 16, thus increased.
• Expected pCO₂ is 1.5*10+8 = 23 (21 at minimum).
• Thus, there must be an element of respiratory alkalosis too.

5.  In a patient with severe BPD, cor pulmonale, and who is on diuretics, the pH=7.42, pCO₂ = 65, HCO₃^-=41, Na 143, K 3.1, Cl 88:

This chronic condition can be approached from either the viewpoint of a a respiratory acidosis, or a metabolic alkalosis -- it doesn't matter which one you start with, the result is the same: this is a mixed condition.  To prove it:

A)  Start with a metabolic alkalosis, the patient has too much bicarbonate...

• The expected compensation would be retention of CO₂, 6 mmHg for each 10 mEq/L HCO₃^-.
• Given a HCO₃^- of 41, with normal of 24: 41-24 = 17
• Therefore, pCO₂ should be 1.7 * 6 + 40 = 50.2 mmHg
• However, pCO₂ is measured at 65
• Thus, there is a respiratory acidosis (due to CO₂ retention) which complicates the metabolic alkalosis which has come about secondary to diurectic use.

• The expected renal compensation would be a 3.5 mEq/L increase for every 10 mmHg increase in pCO₂.
• Thus, 65-40=25
• The HCO₃^- should be 2.5*3.5+24 = 32.75
• However, it is measured as 41
• Thus, there is a metabolic alkalosis (due to diuretic use) which complicates the respiratory acidosis due to CO₂ retention.

Method II.  Estimating by pH and pCO₂

In children

• the normal pH ranges from 7.35 to 7.45
• the normal pCO₂ ranges from 35 to 45

This is a useful way to analyze the situation when all you have is a blood gas, and the bicarbonate value is not directly measured as in the above examples.

Examples:

1.  Given the ABG of 7.5/29/94/25 (pH/pCO₂/pO2/HCO₃^-)

• Since it is alkalemia, start at the upper end of normal, 7.45
• Add 0.008 for each mmHg that the pCO₂ differs from normal
• The lower limit of normal pCO₂ is 35, so
• (35 - 29) = 6 mmHg less than normal pCO₂
• 6  * 0.008 = 0.048
• thus, predicted pH is 7.45 + 0.048 = 7.498
• this jibes well with the measured 7.5
• thus, this is an uncompensated respiratory alkalosis

• Since it is acidemia, start at the lower limit of normal, 7.35
• Subtract 0.008 for each mmHg that the pCO₂ differs from normal
• The upper limit of normal pCO₂ is 45, so
• (64-45) = 19 mmHg more than normal pCO₂
• 19 * 0.008 = 0.152
• thus, predicted pH is 7.35 - 0.152 = 7.198
• this agrees well with the measured 7.2
• thus, this is an uncompensated respiratory acidosis

• Start with 7.35 because it is acidemia
• The pCO₂ differs from the maximum by (72 - 45) = 27
• Expected pH = 7.35 - (0.008 * 27) = 7.13
• Thus, this is not a simple respiratory acidosis; some compensation is present

• The pH is elevated but the pCO₂ is also elevated.  This cannot be a primary respiratory problem, but must be a metabolic alkalosis.  The degree to which each contribute requires additional information, either a serum bicarbonate and the application of method one (above) or interpretation of the base deficit.

Method III.  Acid-Base Nomograms

The following nomogram can be used to classify a condition based on blood gas measurements:

Adapted from: Goldberg, M., Green, S.B., Moss, M.L., et al.  1973. JAMA223:269. Copyright 1973, American Medical Association.