Emergency Medicine

126 Acid–Base, Electrolyte and Renal Emergencies

ACID–BASE DISTURBANCES

(iii) The presence of a normal pH with abnormal HCO 3 and PaCO 2

suggests both a primary respiratory and a primary metabolic

process are present:

(a) pH normal: PaCO 2 >45 mmHg (6.0 kPa), HCO 3 >26 mmol/L

• dual primary process involving a primary respiratory
  acidosis and a primary metabolic alkalosis
  (b) pH normal: PaCO 2 <35 mmHg (4.7 kPa), HCO 3 <22 mmol/L


• dual primary process involving a primary respiratory
  alkalosis and a primary metabolic acidosis.
  5 Alternatively, for simplicity, use an acid–base nomogram to plot and read off
  the interpretation of the arterial blood gas abnormality (see Fig. 3.1)!

Metabolic acidosis

METABOLIC ACIDOSIS

DIAGNOSIS

1 An abnormal process or condition leading to the increase of fixed acids in

the blood, best determined by a fall in plasma bicarbonate to less than

22 mmol/L.

2 Metabolic acidosis may be associated with a high, normal, or low anion gap.

(i) The anion gap is calculated from the equation [Na+] – ([Cl–] +

[HCO 3 – ]) with all units in mmol/L.

(ii) A normal anion gap is 8–16.

acidosismetabolic

acute respiratory acidosis

chronic respiratoryacidosis

alkalosismetabolic

0246 8101214 kPa

0 20 40 60 80 100

PaCO 2

HCO 3 – (mmol/L)

(^510152025)
30
40
100
80
60
40
20
7.00
7.10
7.20
7.30
7.40
7.50
7.60
(mmHg)
NR
NR
NR
pH
H+
(nmol/L)
NR
acute respiratory
alkalosis
Figure 3.1 Acid–base nomogram for plotting interpretation of the arterial blood gas
(NR is normal range)