Introduction to Acid Base Disorders

 

In order to approach acid base disorders, consider the following equations:

1) Henderson Hasselbalch equationpH = 6.1 + log [HCO3-]/ 0.03 PCO2

where 6.1 is the pKa (negative logarithm of the acid dissociation constant) for carbonic acid (H2CO3) and 0.03, the factor which relates PCO2 to the amount of CO2 dissolved in plasma.

2) Kassirer-Bleich equation [H+] = 24 × PCO2 / [HCO3-]

pH

[H+], nanomol/L

7.80

16

7.70

20

7.60

26

7.50

32

7.40

40

7.30

50

7.20

63

7.10

80

7.00

100

6.90

125

6.80

160

 

Although cumbersome and somewhat difficult to use at the bedside, both equations represent a very important relationship. They predict that the ratio of dissolved CO2 to HCO3- , rather than their actual concentrations, determines hydrogen ion concentration and thus pH.

A drop or rise in PCO2 will result in a drop or rise in [H+] respectively.

[HCO3-] on the other hand is inversely related to H+ concentration whereby a drop in bicarbonate levels result in an increase in H+ concentration while a rise in bicarbonate levels result in a reduction in H+ concentration.

This buffer system is of physiologic importance because both the pulmonary and renal mechanisms for regulating pH work by adjusting this ratio.

The PCO2 can be modified by changes in alveolar ventilation, while plasma [HCO3-] can be altered by regulating its generation and excretion by the kidneys.

Definitions

An acid base disorder is a change in the normal value of extracellular pH that may result when renal or respiratory function is abnormal or when an acid or base load overwhelms excretory capacity.

 Normal acid base values

                                                              pH                   PCO2            HCO3-
                                Range:               7.35- 7.45            36-44            22-26
                                Optimal value          7.40                   40                 24

Acid base status is defined in terms of the plasma pH.

Acidemia - decrease in the blood pH below normal range of 7.35 -7.45
Alkalemia - Elevation in blood pH above the normal range of 7.35 – 7.45

 

Clinical disturbances of acid base metabolism classically are defined in terms of the
HCO3-  /CO2  buffer system.

Acidosis – process that increases [H+] by increasing PCO2 or by reducing [HCO3-]
Alkalosis – process that reduces [H+] by reducing PCO2 or by increasing [HCO3-]

lightbulbIt is important to note that though acidosis and alkalosis usually leads to acidemia and alkalemia respectively, the exception occurs when there is a mixed acid base disorder. In that situation, multiple acid base processes coexisting may lead to a normal pH or a mixed picture. This will be discussed in more detail later.

Since PCO2 is regulated by respiration, abnormalities that primarily alter the PCO2 are referred to as respiratory acidosis (high PCO2) and respiratory alkalosis (low PCO2).

In contrast, [HCO3-] is regulated primarily by renal processes. Abnormalities that primarily alter the [HCO3-] are referred to as metabolic acidosis (low [HCO3-]) and metabolic alkalosis (high [HCO3-]).

Simple acid base disorders : Disorders that are either metabolic or respiratory.

Mixed acid base disoders: More than one acid base disturbance present. pH may be normal or abnormal.