The medical requirement was as a result of the military being left in charge of a civillian programme which evolved into the bastardised bureaucratic monster it is today.
Many WW1 pilots suffered from "flying sickness D" which was an ignorant term derived from the debilitating effects of castor oil fumes they breathed. That later became know as Tuberculosis.
It is most probable that Jack Hazlitt had indigestion from dealing with these bludging parasites and was misdiagnosed with a bad heart.
A quick search of this forum will find some very interesting stories.