Acid-base disorders

5.1 Introduction

This chapter discusses how to use arterial or venous blood gas results and routine serum electrolytes to identify an acid-base disorder. ^,

5.1.1 Henderson-hasselbalch equation

Interpretations of blood gas findings start with the Henderson-Hasselbalch equation:

pH=pKa+log[Base][Acid]where pKais the negative log of the acid dissociation constantpH=pKa+log[Base][Acid]where pKais the negative log of the acid dissociation constant
pH=pKa+log[Base][Acid]where pKais the negative log of the acid dissociation constant

The blood buffering system uses bicarbonate as the base and carbonic acid as the acid; therefore, this equation can be rewritten as follows:

pH=pKa+log[HCO−3][H2CO3]pH=pKa+log[HCO3−][H2CO3]
pH=pKa+log[HCO3−][H2CO3]

Using a pK _a value of 6.1 for carbonic acid, and a conversion factor of 0.03 to express the acid concentration in terms of partial arterial pressure of CO ₂ (paCO ₂ ), which is measured in arterial blood gases (ABGs), this is finally rewritten as follows:

pH=6.1+log[HCO−3]0.03paCO2pH=6.1+log[HCO3−]0.03paCO2
pH=6.1+log[HCO3−]0.03paCO2

Since this final expression includes a logarithm, which is difficult for quick bedside calculation, several simple approximations may be used, as discussed on the pages that follow.

Note that at a normal pH of 7.4, the concentration of the base [ HCO−3
HCO 3 −
] of about 25 mEq/L is 20 times that of carbonic acid with a concentration of 1.2 mEq/L (or a pCO ₂ of 40 mmHg).

5.2 Acid-base disorder diagnostic algorithm

This algorithm provides an interpretation of ABGs in conjunction with plasma chemistry.

To use this algorithm:

1.

Examine the pH and identify acidemia or alkalemia.
2.

Using the bicarbonate ( HCO−3
HCO 3 −
) concentration obtained from serum electrolytes and the pCO ₂from the ABG, identify whether the primary cause of the disorder is metabolic or respiratory (see ABG algorithm below).
3.

Perform a calculation to examine whether a primary respiratory disorder has appropriate metabolic compensation, or whether a primary metabolic disorder has appropriate respiratory compensation (refer to the compensation table on the next page)

If neither case is true, a second primary disorder—considered to be a “complex” (meaning more than just one) acid-base disorder rather than a “simple” (meaning a single) acid-base disorder—is underlying the observed changes.

Compensation for Respiratory Alkalosis	Compensation for Respiratory Acidosis
Acute	Acute
When acute, expect serum <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − to fall about 2 mEq/L for each 10-mmHg decrease in pCO ₂for normal metabolic compensation.	When acute, expect serum <SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − to rise about 1 mEq/L for each 10-mmHg increase in pCO ₂for normal metabolic compensation.
Chronic	Chronic
When over 24 hours, expect serum <SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − to fall about 5 mEq/L for each 10-mmHg decrease in pCO ₂for normal metabolic compensation.	When over 24 hours, expect serum <SPAN role=presentation tabIndex=0 id=MathJax-Element-9-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − to rise about 3.5 mEq/L for each 10-mmHg increase in pCO ₂for normal metabolic compensation.

Compensation for Metabolic Alkalosis	Compensation for Metabolic Acidosis
There are three common ways to evaluate for normal respiratory compensation response (±2 mmHg): 1. Expect pCO ₂to rise 0.7 mmHg for each 1 mEq/L rise in serum <SPAN role=presentation tabIndex=0 id=MathJax-Element-10-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − for normal respiratory compensation. 2. pCO ₂should be equal to serum <SPAN role=presentation tabIndex=0 id=MathJax-Element-11-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − + 15 mmHg up to a pCO ₂of about 60 when the pCO ₂rises no further. 3. The easy way: pCO ₂should be equal to the last 2 digits of the pH up to pH 7.60.	There are three common ways to evaluate for normal respiratory compensation response (±2 mmHg): 1. Expect pCO ₂to decrease 1.2 mmHg for each 1 mEq/L fall in <SPAN role=presentation tabIndex=0 id=MathJax-Element-12-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − . 2. pCO ₂should be equal to 1.5 times ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-13-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO−3 HCO 3 − ) + 8. 3. The easy way: pCO ₂should be equal to the last 2 digits of the pH down to pH 7.10.

Compensation for Metabolic Alkalosis

Compensation for Metabolic Acidosis

There are three common ways to evaluate for normal respiratory compensation response (±2 mmHg):

1.

Expect pCO ₂to rise 0.7 mmHg for each 1 mEq/L rise in serum HCO−3
HCO 3 −
for normal respiratory compensation.
2.

pCO ₂should be equal to serum HCO−3
HCO 3 −
+ 15 mmHg up to a pCO ₂of about 60 when the pCO ₂rises no further.
3.

The easy way: pCO ₂should be equal to the last 2 digits of the pH up to pH 7.60.

There are three common ways to evaluate for normal respiratory compensation response (±2 mmHg):

1.

Expect pCO ₂to decrease 1.2 mmHg for each 1 mEq/L fall in HCO−3
HCO 3 −
.
2.

pCO ₂should be equal to 1.5 times ( HCO−3
HCO 3 −
) + 8.
3.

The easy way: pCO ₂should be equal to the last 2 digits of the pH down to pH 7.10.