like Monty and Tim, I'm surprised by your doc request.
The main reason is that from most studies, swimming is requesting much less air than biking and running, at same muscle effort level. Because arm muscles are lighter than legs muscles ? Because lack of access to air push anaerobic ?
Apparantly, from research studies, elite swimmers only use 3 liters oxygen per minute at 4mmol/L lactate (1 hour max effort), vs 4,5 liters for bikers.
At 52 years old, with my average lungs and heart and blood, I can't support 4.5 liters for an hour (neither 5mn), but 3 liters is OK.
And with my small swimming muscles (if I do not overuse my legs), 1.5 liter is probably enough.
Consequence is that swimming is putting less pressure on lungs and cardio system than biking and running.
Personally I noticed that on both breathing and BPM.
On the other side :
yes, in tri, I probably swim at a lactate level higher than bike and run, as it is a shorter part.
maybe snorkel giving constant access to air is better for you. Necessary ?
Either you want to follow your doc, and tri "pre DSQ", or you can test normal breathing (maybe limiting leg use) and see another doc with swimming experience ?
Hope you will find solution to overcome the issue. Good luck !
I find this hard to believe. My HR is just as high at end of the swim in an Oly Dist race as it is at end of the run, e.g. about 90-95% of max HR. And, if swimmers were truly using only 2/3 the O2 of BR guys, then how come they burn so many frigging calories??? AFAIK, calorie burn is directly proportional to O2 usage. Phelps allegedly ate 12,000 cal/day, which is no doubt an exaggeration, but OTOH I bet MP and other top swimmers do eat 7000-8000/day when in heavy training. I am suspicious of the validity of the flume-based measurements for swimming b/c it would be almost impossible to swim all out with all those wires and tubes attached to you. I suspect the guys/girls in the studies can go much harder with "no strings attached", so to speak. :)
"Anyone can be who they want to be IF they have the HUNGER and the DRIVE."