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