**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 StateFree Water Abnormalities0.3([Na+] - 140Chloride abnormalities102 - [Cl]corrHypoproteinemia3(6.5 - [Prottotal]Unmeasured anions make up the balanceTotal, 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.