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Arterial Blood Gases

The following steps are recommended to evaluate arterial blood gas values. They are based on the assumption that the average values are:

PH = 7.4

PaCO2 = 40 mm Hg

HCO3− = 24 mEq/L

1. First, note the pH. It can be high, low, or normal, as follows:

PH > 7.4 (alkalosis)

PH < 7.4 (acidosis)

PH = 7.4 (normal)

A normal pH may indicate perfectly normal blood gases, or it may be an indication of a compensated imbalance. A compensated imbalance is one in which the body has been able to correct the pH by either respiratory or metabolic changes (depending on the primary problem). For example, a patient with primary metabolic acidosis starts out with a low bicarbonate level but a normal CO2 level. 

Soon afterward, the lungs try to compensate for the imbalance by exhaling large amounts of CO2 (hyperventilation). As another example, a patient with primary respiratory acidosis starts out with a high CO2 level; soon afterward, the kidneys attempt to compensate by retaining bicarbonate. 

If the compensatory mechanism is able to restore the bicarbonate to carbonic acid ratio back to 20:1, full compensation (and thus normal pH) will be achieved.

2. The next step is to determine the primary cause of the disturbance.

This is done by evaluating the PaCO2 and HCO3− in relation to the PH.

Example: pH > 7.4 (alkalosis):

a. If the PaCO2 is < 40 mm Hg, the primary disturbance is respiratory alkalosis. (This situation occurs when a patient hyperventilates and “blows off” too much CO2. Recall that CO2 dissolved in water becomes carbonic acid, the acid side of the “carbonic acid–bicarbonate buffer system.”)

b. If the HCO3− is >24 mEq/L, the primary disturbance is metabolic alkalosis. (This situation occurs when the body gains too much bicarbonate, an alkaline substance. Bicarbonate is the basic or alkaline side of the “carbonic acid bicarbonate buffer system.”)

Example: pH < 7.4 (acidosis)

a. If the PaCO2 is >40 mm Hg, the primary disturbance is respiratory acidosis. (This situation occurs when a patient hypoventilates and thus retains too much CO2, an acidic substance.)

b. If the HCO3− is <24 mEq/L, the primary disturbance is metabolic acidosis. (This situation occurs when the body’s bicarbonate level drops, either because of direct bicarbonate loss or because of gains of acids such as lactic acid or ketones.)

3. The next step involves determining if compensation has begun.

This is done by looking at the value other than the primary disorder. If it is moving in the same direction as the primary value, compensation is underway. Consider the following gases:


(1) 7.20 60 mm Hg 24 mEq/L

(2) 7.40 60 mm Hg 37 mEq/L

The first set (1) indicates acute respiratory acidosis without compensation (the PaCO2 is high, the HCO3− is normal). The second set (2) indicates chronic respiratory acidosis. 

Note that compensation has take place; that is, the HCO3 − has elevated to an appropriate level to balance the high PaCO2 and produce a normal pH.

4. Two distinct acid–base disturbances may occur simultaneously.

These can be identified when the pH does not explain one of the changes.

Example: Metabolic and respiratory acidosis

a. PH 7.21 decreased acid

b. PaCO2 52 increased acid

c. HCO3 13 decreased acid

This is an example of metabolic and respiratory acidosis.