ACID-BASE FACTORS
Metabolic Acidosis
The necessity for treating a metabolic acidosis is judged largely
on clinical grounds, with the metabolic acid level base excess (BE) used to determine
the correct dose of bicarbonate (see Chapter
41
). This prediction is based on the size of the treatable space and the
magnitude of the problem:
Dose (mEq) = 0.3 × Weight (kg) × BE (mEq/L)
The dose calculated is sufficient to return the metabolic disturbance almost to zero.
However, bicarbonate therapy is reserved for emergencies and situations in which
indications for therapy are compelling. It is usual to give a reduced dose (about
one half) for several reasons:
- The bicarbonate is injected initially into the plasma volume (about 3 L)
instead of into the calculated treatable space (21 L).
- When bicarbonate is added to acid, it "fizzes." Fortunately, this does
not occur literally in the blood. Nevertheless, most bicarbonate is converted to
carbon dioxide and has to be eliminated. For each 100 mEq that is converted, about
2.24 L of carbon dioxide has to be exhaled, equivalent to 10 minutes of normal production.
- The carbon dioxide that is produced enters the cells freely, unlike the
bicarbonate ions that have been administered. The inside of the cell may initially
become even more acid. However, direct studies with nuclear magnetic resonance have
not confirmed this (Severinghaus, personal communication, 1986).
- The bicarbonate is accompanied by sodium ions that are responsible for
a residual increase in the osmolality of the ECF. In combination with other therapy,
such as intravenous glucose, the hyperosmolality may be critical and cause coma.
In neonates, rapid infusion of bicarbonate may cause intracranial hemorrhage.
- If the body deals with its metabolic acidosis, there is a residual metabolic
alkalosis.
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