SN4 SHORT NOTE 5M ★ High-yield

Describe the scientific basis, composition and use of oral rehydration therapy.

Pharmacology · 2nd Professional — Para-Clinical

Scientific Basis

Oral rehydration therapy corrects the dehydration of acute diarrhoea using an oral solution of glucose and electrolytes, exploiting the sodium-glucose cotransporter (SGLT1) of the intestinal brush border.

In secretory diarrhoea (cholera, enterotoxigenic E. Coli), the toxin drives chloride and water secretion from crypt cells → But the SGLT1 cotransporter on villous cells remains intact → glucose and sodium are absorbed together through this carrier, in a fixed 1:1 relationship → Sodium absorption creates an osmotic gradient → water follows passively → Absorption therefore continues despite ongoing secretion, and the net balance is restored

CLINICAL PEARL

The whole method rests on one fact: the absorptive pathway survives the illness that destroys the secretory balance. Toxin-mediated diarrhoea switches on secretion but leaves the glucose-coupled sodium carrier working — so giving glucose with salt turns that carrier on and pulls water back in. Salt water alone will not do it, and sugar water alone will not do it; the two together will. It has been called one of the most important medical advances of the twentieth century, and it costs almost nothing.

Composition

Constituent	WHO reduced-osmolarity ORS	Purpose
Sodium	75 mmol/L	Replaces the chief cation lost
Glucose	75 mmol/L	Drives sodium absorption through SGLT1 — the equimolar ratio matters, and more glucose is not better
Potassium	20 mmol/L	Replaces potassium losses, which are proportionally large in children and cause ileus and weakness if uncorrected
Chloride	65 mmol/L	Anion
Citrate	10 mmol/L	Corrects the metabolic acidosis; replaced bicarbonate because it is more stable in storage
Total osmolarity	245 mOsm/L	Reduced from the original 311 — the lower osmolarity reduces stool output, vomiting and the need for unscheduled intravenous fluid

Use and the Rest of Management

Give ORS after each loose stool — roughly 50 to 100 mL in children under 2 and 100 to 200 mL in older children, and as much as the patient will drink in adults
Continue feeding throughout, including breastfeeding — withholding food prolongs the illness and worsens nutrition; this reversed decades of contrary advice
Zinc 10 to 20 mg daily for 14 days in children reduces the duration and severity of the episode and the incidence of further episodes over the next 2 to 3 months
Intravenous fluid (Ringer lactate) is needed only for severe dehydration, shock, persistent vomiting or ileus; oral therapy suffices for the great majority
Antibiotics only for specific indications — cholera, dysentery, typhoid, giardiasis, amoebiasis; most acute diarrhoea is viral
Antimotility drugs have NO place in childhood diarrhoea

Applied Aspects

Demonstrate the preparation rather than describing it — one sachet dissolved in the stated volume of clean water. A more concentrated solution is dangerous, causing hypernatraemia and seizures, and is a common error when families assume "stronger is better"
Home fluids are better than nothing — salted rice water, salted buttermilk, coconut water — but ORS is preferred where available
Teach the danger signs that require return: sunken eyes, lethargy, inability to drink, reduced urine, blood in the stool, persistent vomiting, or fever
ORS and zinc are supplied free through India's national programme, and the limiting factor is awareness rather than availability — ORS use remains well below what is achievable
Diarrhoea remains a leading cause of under-five death in India, and almost all of it is preventable by ORS, zinc, rotavirus vaccination, safe water, sanitation and handwashing — none of which requires a hospital
Do not give unnecessary antibiotics and antimotility drugs; they cause harm, cost money and displace the treatment that works

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