Question 17

Outline   your   approach   to  determining   the   appropriate   magnitude   of  respiratory compensation for a metabolic acidosis and a metabolic alkalosis.

The  extent of  respiratory compensation for  a  metabolic disorder is  determined by the balance between the abnormality in the pH (hence the drive to change), and 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: use of formulae, and the 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:

For a metabolic acidosis, the expected PaCO2 = the last two digits of the pH (+/- 2-5 mmHg; from pH 7.1 to 7.6; Narins 1980), or the expected PaCO2 = (1.5 X measured bicarbonate) + 8 (+/- 2).  The measured bicarbonate, not the standard bicarbonate, must be used.   Other approachs include: expected change in PaCO2 = Standard Base Excess (Schlichtig R et al Crit Care Med 1998); 1.2 mmHg fall in PaCO2 for each 1 mmol/L reduction in HCO3.

For a 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 approachs: change in PaCO2 = 0.6 X Standard Base Excess (Schlichtig R et al Crit Care Med 1998); 0.7 mmHg rise in PaCO2 for each 1 mmol/L increase in HCO3.

Discussion

This question regards the routine bedside tests for adequacy of compensation.

Specifically it is the magnitude of change of CO2 in response to the changes in pH.

One recalls several rules. None are especially precise. This whole issue is discussed at great length in the chapter on the assessment of compensation by the Boston and Copenhagen methods.

However, this question specifically asks for Winter's Rule:

1) In metabolic acidosis, PCO2 = (1.5 x HCO3) + 8 ... within a range of plus-minus 2mmHg

2) In metabolic alkalosis, PCO2 = (0.7 x HCO3) + 20 ... within a range of plus-minus 5mmHg

References

Roberts, Kathleen E., et al. "Evaluation of respiratory compensation in metabolic alkalosis." Journal of Clinical Investigation 35.2 (1956): 261.