ie,One of 2 engines out in flight is a threat to the hull therefore a Mayday (even if you can continue on one engine since loss of the remaining engine could be a bad thing! just ask Ryanair) a heart attack passenger is regretable but no threat to the hull therefore a Pan Pan call if you want to off load them at the nearest airport.
Interesting to hear it from a different (nautical) point of view. In fact, the SOP is generally that an engine
fire would be a "Mayday" and an engine
failure (such as a run down in cruise) would be a "Pan Pan". It isn't a hard and fast rule but even on a twin jet, you would give consideration to downgrading to a "Pan" if you'd put the fire out. As you say, "Pan Pan" would be for a lesser event (a heart attack being one), although in certain parts of the world you would still need to say the M-word if you wanted assistance.