I\ understand the concern. The question is legitimate and both my cardiologist and EP have gone over this with me at my request but feel I am not taking a risk with such momentary peaks. I often swim repeat sets of 50 yd swims with 5-10 reps per set - example -- a set of 8 reps of 50 yds swimming in 40 seconds and repeating on the 60 second mark. We do 3-4 sets of those 8 reps. After the first 2-3 reps , my heart rate at push off starts at 100-110 and goes up to 120-130 by the end of the 50 yards. Later in the set it starts at 120 and goes to 140-150, and at the end of some sets it go as high at 160--168, especially if i am swimming more like 35 seconds per 50 yards for some of the later swims. But all the higher bpm numbers are momentary peaks that quickly drop off to 120 or so as I recover for the next rep. If i am just swimming a longer swim, normally the heart rate is more like 130. In swimming it is easy to go many arm strokes without breathing, especially at the end, and that probably makes some of the difference. I am back to 60-75 bpm in 10 -15 minutes. I cannot ger anywhere close to 150 bpm on a bike. My treadmill stress test was shut off by the techs at 140 bpm (13+ METs) but it was clear that I could have easily put in a burst of speed for 30 seconds that would have raised the results before more full fatigue. . Note that I have cut the swim from 3200 yards per workout to more like 1600 - 2000 -- . . In swimming I have won the US nationals and am currently ranked #3 nationally. I am 81 going on 82, not 90