Respiratory Compensation in Metabolic Disorders

OVERVIEW

The extent of respiratory compensation for a metabolic disorder is determined by:

1. the balance between the abnormality in the pH (hence the drive to change)
2. how hard it is to get there (eg. work of respiratory muscles in hyperventilation)

A knowledge of the expected degree of compensation for a given acid base status is necessary to determine the presence of an additional respiratory disorder.

Two traditional methods have been used:

1. use of formulae
2. use of a standardized diagram

The direction of change in the CO2 should be to normalise the pH for the underlying disorder. A normal pH indicates an additional process is present. The commonest relevant formulae to estimate the PaCO2 in use are:

METABOLIC ACIDOSIS

• the expected PaCO2 = the last two digits of the pH (+/- 2-5 mmHg; from pH 7.1 to 7.6)

OR

• the expected PaCO2 = (1.5 X measured bicarbonate) + 8 (+/- 2)
• other approaches: expected change in PaCO2 = Standard Base Excess or 1.2 mmHg fall in PaCO2 for each 1 mmol/L reduction in HCO3.

METABOLIC ALKALOSIS

• the same equation is used, though the reliability may be less than with a metabolic acidosis. Expected PaCO2 = the last two digits of the pH (+/- 2-5; from pH 7.1 to 7.6).
• other approaches: change in PaCO2 = 0.6 X Standard Base Excess, 0.7 mmHg rise in PaCO2 for each 1 mmol/L increase in HCO3.