Practice Question # 788.
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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
− = 24 mEq/L
- 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.
- 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)
- 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.”)
- 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.”)