Important Points
in
Acid/Base Interpretation

• The HCO3 concentration rises about 1 - 2 mEq/L for each acute 10 torr increase in PaCO2 above normal value;  maximal change is about 4 mEq/L.

• The HCO3 concentration falls 1 - 2 mEq/L for each acute 10 torr decrease in PaCO2 below normal value;  maximal change is about 6 mEq/L.

• Acute respiratory and metabolic disorders can be distinguished by their PaCO2 and HCO3 values;  HCO3 above 30 or below 15 implies a metabolic component.

• During chronic elevation of the PaCO2, each 10 torr increase in PaCO2 causes a 4 mEq/L increase in HCO3 concentration.

• An acute 10 torr increase in PaCO2 results in a 0.05 unit decrease in pH;   an acute 10 torr decrease in PaCO2 results in a 0.10  unit increase in pH.

• Rapid determination of the predicted respiratory pH

  • Determine the difference between the measured PaCO2 and normal value, then move the decimal 2 places to the left.

  • If the PaCO2 is greater than normal, subtract half the value from 7.40

  • If the PaCO2 is less than normal, move the decimal two places to the left and add the value to 7.40.

  • Examples:

    • pH = 7.01; PaCO2 = 75 (PaCO2 > 30 torr)
      75 - 30 = 45;  0.45 X 0.5 = 0.225
      7.40 - 0.225 = 7.175

    • pH = 7.43; PaCO2 = 23 (PaCO2 < 30 torr)
      30 - 23 = 7; 
      7.40 + 0.7 = 7.47

• Rapid determination of the metabolic component

  • A 10 torr change in HCO3 concentration changes the pH by 0.15 units;  if the pH decimal is moved two places to the right, then a 10:15 or 2/3 relationship exits.

  • The absolute difference between the measured pH and the predicted respiratory pH is the metabolic component of the pH change;  moving the decimal point two places to the right and multiplying by 23 yields an estimate of the mEq/L variation of the buffer baseline (usually assumed to be the HCO3 concentration change)

• Rapid quantitative clinical determination of acid-base changes

  • Determine the predicted respiratory pH

  • Estimate BE or deficit

  • Examples:

    • pH = 7.02; PaCO2 = 75
      Predicted respiratory pH:
          75 - 30 = 45; 0.45 X 0.5 = 0.225
          7.40 - 0.23 = 7.17
      Metabolic component:
          7.17 - 7.02 = 15 X 2/3 = 10 mEq/L base deficit

    • pH = 7.64; PaCO2 = 25
      Predicted respiratory pH:
          30 - 25 = 0.05
          7.40 + 0.05 = 7.45
      Metabolic component:
          7.64 - 7.45 = 19 X 2/3 = 12.67 mEq/L base deficit (BE)

• Quantitative analysis of nonrespiratory acid-base ratio state

  • Enter the observed BE at the bottom of the following table:

  Quantitative Analysis of Nonrespiratory Acid-Base Ration State

  Free Water Abnormalities	0.3([Na+] - 140
  Chloride abnormalities	102 - [Cl]corr
  Hypoproteinemia	3(6.5 - [Prottotal]
  Unmeasured anions make up the balance
  Total, or the observed (reported) BE

                  [Cl]corr =  [Cl]observed X 140/[Na+]
                  For [Alb} instead of [Prottotal] use 3.7(4.5-[Alb]).

  • Calculate and enter the expected contributions of BE of any free water abnormalitity, abnormality of the corrected [Cl-], and abnormal protein concentrations;  be careful about the + and - signs.

  • Sum the values from the statement immediately above;  be careful about the sign (+ or -)

  • Compare the value from the first statement above to the third statement.  The observed BE should never be greater than the summed values of the third statement;  if the observed BE is less, the presence of unmeasured anions (UA-) equal in magnitude to the difference of the summed values in the second statement and BE can be inferred.

  • Example:

    • If:  pH = 7.250, Na+ = 135, PCO2 = 75, Cl- = 79, BE = 2; Proteintotal = +8.4

    • Then: 
          Free Water Abnormality = -1.5
          Chloride Abnormality = +20
          Hypoproteinemia = -5.7
          Observed BE = 2

    • Therefore:
          UA- = 16.3
          [(14.3) + (2) = 16.3]

• Summary of Acid-Base Disorders:

  • Acid-base state is determined by changes in PaCO2, Strong Ion Difference (SID), and Proteintotal.

  • All acid-base disturbances are caused by changes in PaCO2, SID, and Proteintotal.

  • Since protein concentration is not manipulated clinically, all acid-base disturbances are corrected by changes in PaCO2 and SID.