Primary respiratory acid-base disorders

The primary acid-base disorders include respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. Furthermore, each of these can exist as either acute or chronic conditions. Here we will discuss the primary respiratory acid-base disorders; later we will discuss the metabolic disorders.

Hypercapnia arising from alveolar hypoventilation leads to respiratory acidosis. Consider a Davenport diagram-based depiction of the changes associated with a rise in arterial carbon dioxide tension from 40 to 60 mmHg.

Starting at an arterial carbon dioxide tension of 40, serum bicarbonate of 24, and a pH of 7.4, an immediate rise in arterial carbon dioxide tension to 60 results in a pH drop to about 7.3 (Point A to Point B).

The resulting acid-base disorder is known as acute respiratory acidosis.

When arterial carbon dioxide tension is chronically elevated, renal compensation results in increased serum bicarbonate and incomplete correction of pH. As bicarbonate increases, movement along the arterial carbon dioxide tension isobar to Point C occurs. The elevation in bicarbonate from 24 to 38 mmol as a result of renal compensation constitutes a base excess of 14 mmol.

The disorder is referred to as a compensated respiratory acidosis or chronic respiratory acidosis.

Hypocapnia arising from alveolar hyperventilation leads to respiratory alkalosis. Consider the changes associated with a decrease in arterial carbon dioxide tension from 40 to 20 mmHg.

Starting at an arterial carbon dioxide tension of 40, serum bicarbonate of 24, and pH of 7.4, an immediate decrease in arterial carbon dioxide tension to 20 results in a pH rise to about 7.6 (Point A to Point B).

The resulting acid-base disorder is known as acute respiratory alkalosis.

When arterial carbon dioxide tension is chronically decreased, the kidneys compensate by excreting more bicarbonate, correcting the pH. The correction may be nearly complete. The decline in bicarbonate from 24 to 12 mmol as a result of renal compensation constitutes a base deficit of 12 mmol / L.

The disorder is referred to as a compensated respiratory alkalosis or chronic respiratory alkalosis.

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