Biguanides
Metformin
and phenformin are derivatives of guanidine, and are active components of the
French lilac (Galega officinalis).
While both drugs are used widely in India, phenformin has been withdrawn from
several countries in the West because of the serious risk of lactic acidosis
(64 cases/100,000 patient years). Metformin has a risk of only 3 cases/100,000
patient years.
·
Biguanides are used orally in the
management of mild to moderate NIDDM, especially if the patient is elderly and
obese.
·
Metformin is said to be relatively
safe in pregnancy
These
drugs are absorbed from the small intestine, do not bind to plasma proteins,
and are excreted unchanged in the urine. Oral bioavailability is 50 to 60%. The
half-life of metformin varies from 1.3 to 4.5 hours.
·
Biguanides induce increase in
peripheral glucose utilisation, decrease in hepatic gluconeogenesis, and
decrease in intestinal absorption of glucose, vitamin B and bile acids. They
usually do not lower the blood sugar in normal individuals (unless other
hypoglycaemic agents or ethanol has been concomitantly ingested).
·
Diarrhoea, abdominal discomfort,
metallic taste.
·
Lactic
acidosis: While phenformin is associated with a greater risk of
lactic acidosis, the other biguanides can also cause it in the presence of
renal or hepatic impairment, cardiac failure, or chronic hypoxic lung disease.
o
Manifestations: Acute onset of
diarrhoea, vomiting, hyperventilation, and alteration of consciousness.
o
Diagnosis: Anion gap metabolic
acidosis, low serum pH and bicarbonate, elevated serum potassium, normal or
depressed serum chloride, increased blood lactate and lactate/pyruvate.
o
Treatment: Sodium bicarbonate IV (1
to 2 mEq/kg). Upto 50 mEq every 15 minutes may be required. Total dose should
not exceed 400 mEq.
■■ GIT: Nausea,
vomiting, diarrhoea, abdominal cramps, haematemesis.
■■ CNS:
Agitation, confusion, convulsions, coma.
■■ RS: Rapid, deep breathing, pulmonary hypertension.
■■ CVS:
Tachycardia, hypotension.
■■ Others:
Lactic acidosis.
■■Elevation
of lactate/pyruvate ratio.*
■■Elevation
of 3-b-hydroxybutyrate concentration.
■■Blood
glucose may be depressed, normal, or elevated.
■■Leucocytosis,
thrombocytopenia.
■■Elevated
serum creatinine, albuminuria.
■■Lactic
acidosis is characterised by a number of abnormal laboratory values (vide
supra).
■■Stabilisation—Establish
airway, undertake endotracheal
■■intubation,
and perform assisted ventilation (if necessary). Stomach wash, activated
chacoal.
■■Treatment
of hypoglycaemia with 50 ml of 50% glucose IV (0.5 gm/kg/dose in children).
■■Treatment
of acidosis with IV sodium bicarbonate (1 to 2 mEq/kg). Upto 200 to 400 mEq may
be required.
■■Treatment
of hypotension with Trendelenberg position and IV fluids. Pressor amines such
as dopamine must be used with caution, since they can aggravate lactic
acidosis.
■■Haemodialysis.
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