ANSWER 89
This man has signs of dehydration and the high urea with a normal creatinine is consistent
with this. He is acidotic. The blood glucose level is not given but the picture is likely to rep-
resenthyperglycaemic ketoacidotic coma.The key clinical features on examination are
dehydration and hyperventilation, and the triggering problem with the infection in the
foot. A persistently high sugar level induced by his infected foot ulcer causes heavy glyco-
suria triggering an osmotic diuresis. This leads to hypovolaemia and reduced renal blood
flow causing prerenal uraemia. The extracellular hyperosmolality causes severe cellular
dehydration, and loss of water from his brain cells is the cause of his coma. Decreased
insulin activity with intracellular glucose deficiency stimulates lipolysis and the production
of ketoacids. He has a high anion gap metabolic acidosis due to accumulation of ketoacids
(acetoacetate and 3-hydroxybutyrate). The anion gap is calculated from the equation:
[Na']'[K']([Cl]'[HCO* 3 ])
and is normally 10–18 mmol/L; in this case it is 31.5 mmol/L. Ketones cause a character-
istically sickly sweet smell on the breath of patients with diabetic ketoacidosis (about 20
per cent of the population cannot smell the ketones). The metabolic acidosis stimulates the
respiratory centre leading to an increase in the rate and depth of respiration (Kussmaul
breathing) producing the reduction in paCO 2 as respiratory compensation for the acidosis.
In older diabetic patients there is often evidence of infection precipitating these metabolic
abnormalities, e.g. bronchopneumonia, infected foot ulcer.
The differential diagnosis of coma in diabetics includes non-ketotic hyperglycaemic coma,
particularly in elderly diabetics, lactic acidosis especially in patients on metformin, pro-
found hypoglycaemia, and non-metabolic causes for coma, e.g. cerebrovascular attacks
and drug overdose. Salicylate poisoning may cause hyperglycaemia, hyperventilation and
coma, but the metabolic picture is usually one of a dominant respiratory alkalosis and
mild metabolic acidosis.
The aims of management are to correct the massive fluid and electrolyte losses, hypergly-
caemia and metabolic acidosis. Rapid fluid replacement with intravenous normal saline
and potassium supplements should be started. In patients with cardiac or renal disease, a
central venous pressure (CVP) line is mandatory to control fluid balance. Regular moni-
toring of plasma potassium is essential, as it may fall very rapidly as glucose enters cells.
Insulin therapy is given by intravenous infusion adjusted according to blood glucose levels.
A nasogastric tube is essential to prevent aspiration of gastric contents, and a bladder catheter
to measure urine production. Antibiotics and local wound care should be given to treat
this man’s foot ulcer. In the longer-term it is important that this patient and his wife are
educated about his diabetes and that he has regular access to diabetes services. His smok-
ing and alcohol consumption will also need to be addressed. There may be social issues to
be considered in relation to his unemployment.
- Dehydration, tachypnoea and ketosis are the key clinical signs of diabetic ketoacidosis.
- Twenty per cent of the population (and therefore doctors) cannot smell ketones.