Emergency Medicine

Acid–Base, Electrolyte and Renal Emergencies 127

ACID–BASE DISTURBANCES

3 Causes of a high anion gap metabolic acidosis (anion gap >16) include:

(i) Increased acid production:

(a) ketoacidosis, e.g. diabetic, alcoholic, starvation

(b) lactic acidosis (serum lactate >2.5 mmol/L):

• type A: impaired tissue perfusion in cardiac arrest, shock,
  hypoxia, sepsis


• type B: impaired carbohydrate metabolism in hepatic or
  renal failure, lymphoma, pancreatitis and drugs such as
  metformin.
  (ii) Decreased acid excretion, as in renal failure.
  (iii) Exogenous acid ingestion:
  (a) methanol, ethylene glycol, iron, cyanide and salicylates.
  4 Causes of a normal anion gap metabolic acidosis (anion gap 8–16) include:
  (i) Renal:
  (a) renal tubular acidosis
  (b) carbonic anhydrase inhibitors.
  (ii) Gastrointestinal:
  (a) severe diarrhoea
  (b) small bowel fistula
  (c) drainage of pancreatic or biliary secretions.
  (iii) Other:
  (a) administration of synthetic amino acid solutions, ammonium
  chloride
  (b) recovery from ketoacidosis.
  5 The body compensates to reduce the acid load by hyperventilation.
  The expected compensatory reduction in PaCO 2 may be calculated (see
  Table 3.2):

Table 3.2 Predicting the expected compensatory changes in PaCO 2 and HCO 3

Metabolic acidosis Metabolic alkalosis

Predicted

PaCO 2

(kPa)

0.2  [HCO 3 ] + 1 kPa (+/– 0.25) [HCO 3 ]

+ 2.5 kPa (+/– 0.7)

10

Predicted

PaCO 2

(mmHg)

1.5  [HCO 3 ] + 8 mmHg (+/– 2) 0.7  [HCO 3 ] + 20 mmHg (+/– 5)

Respiratory acidosis Respiratory alkalosis

Predicted

HCO 3

(kPa)

Acute

24 + (PaCO 2 –

5.33)  0.75

Chronic

24 + (PaCO 2 –

5.33)  3

Acute

24 – (5.33 –

PaCO 2 )  1.5

Chronic

24 – (5.33 –

PaCO 2 )  3.75

Predicted

HCO 3

(mmHg)

24 + PaCO 2 – 40

10 24 +(

PaCO 2 – 40

10 )^ ^4 24 – (

40 – PaCO 2

10 )^ ^2 24 – (

40 – PaCO 2

10 )^ ^5

Table 3.2.indd 1 15/03/2011 15:02