Emergency Medicine

Acid–Base, Electrolyte and Renal Emergencies 125

ACID–BASE DISTURBANCES

(d) assuming 100% humidity at sea level, the A-a gradient can be

calculated by:

A-a gradient = PAO 2 –PaO 2

where PAO 2 = (FiO 2  (760–47))–(PaCO 2 /0.8)

(e) normal A-a gradient is <10 torr (mmHg), or approximately

< (age/4) + 4.

(ii) Review the pH or hydrogen ion status:

(a) normal range pH 7.35–7.45 (H+ 35–45 nmol/L)

(b) acidaemia is a pH <7.35 (H+ >45 nmol/L)

(c) alkalaemia is a pH >7.45 (H+ <35 nmol/L).

(iii) Determine the respiratory component (PaCO 2 ):

(a) normal range 35–45 mmHg (4.7–6.0 kPa)

(b) PaCO 2 >45 mmHg (6.0 kPa):

• acidaemia indicates a primary respiratory acidosis


• alkalaemia indicates respiratory compensation for a
  metabolic alkalosis
  (c) PaCO 2 <35 mmHg (4.7 kPa):


• alkalaemia indicates a primary respiratory alkalosis


• acidaemia indicates respiratory compensation for a
  metabolic acidosis
  (iv) Determine the metabolic component (bicarbonate, HCO 3 ):
  (a) HCO 3 normal range 22–26 mmol/L
  (b) HCO 3 <22 mmol/L:


• acidaemia indicates a primary metabolic acidosis


• alkalaemia indicates renal compensation for a respiratory
  alkalosis
  (c) HCO 3 >26 mmol/L:


• alkalaemia indicates a primary metabolic alkalosis


• acidaemia indicates renal compensation for a respiratory
  acidosis.

3 This approach will determine most primary acid–base disturbances and
their associated renal or respiratory compensatory changes.

4 Remember:
(i) Renal or respiratory compensation is always a secondary process
and should really not then be described in terms of an ‘acidosis’
or ‘alkalosis’:
(a) rather, in the presence of metabolic acidaemia, think of the
respiratory compensation as ‘compensatory hyperventilation’
rather than a ‘secondary respiratory alkalosis’.
(ii) Chronic compensation returns the pH value towards normal, but
overcompensation never occurs.