A six (6) month old child is diagnosed with a gastric outlet obstruction. Investigations reveal a metabolic alkalosis and a urine pH of 5. Describe the physiological basis of these results.

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College Answer

Candidates were expected to identify what compounds are lost during vomiting associated
with gastric outlet obstruction and that intravascular volume depletion is likely. Furthermore
the candidate was then expected to explain that overall the child’s body will defend volume –
then tonicity – then acid base status, in this order. An outline of the major physiological
defences of volume status (e.g. the renin-angiotensin-aldosterone system), and how these
will perpetuate a metabolic alkalosis (prevent the kidney from clearing the excess
bicarbonate in the form of an alkaline urine) was then expected.
A common omission was to not appreciate that the preservation of volume (sodium or
chloride) by the kidneys conflicts with the physiological mechanisms that would allow
bicarbonate loss through the kidneys’ tubules. Restoration of normovolaemia would allow an
‘alkaline tide’ or bicarbonate loss by the kidneys (alkaline urine production).
Syllabus - G12d G2a
References: Power and Kam. P247-266