Early on in the lockdowns I was advising older relatives to take note of their heartrates and that if they saw high BPMs at rest they should be concerned. As we know, the inflammatory stage of COVID wrecks the lungs and people essentially suffocate. I want to go further into this. Below is a chart of resting heart rates and how fit you are (or are not).
As we age past 30 or so our maximum heart rate drops. The usual calculation is 220 minus your age. For the most vulnerable group for COVID, 65+ that would mean a max heart rate of 155. And that assumes maximal fitness. If you're 80 or above, that max is 140 and again that assumes you're in max health. Now the 220 figure is not dead on accurate. As with everything else there is variance. I regularly top my alleged max heart rate so you should view that number as a guide. No way I make 220 or even 200. Those days are gone,
In addition to this is the maximum aerobic function AKA; MAF. This is the point at which your body goes into anaerobic respiration (using something other than oxygen).You get this number by subtracting your age from 180. So for our 80 year old, that's 100 BPM. For a 65 year old that's 115 bpm.
Here's the point of all this. If you're past your maximum aerobic function AND have a low max heart rate you have a very small window of operating before the stress on your system becomes critical.
So someone with a resting heart rate of 55 starts to get into oxygen problems at 100 BPM. That could be simply walking up some stairs. Now imagine that same person with a 77 resting heart rate with a 100 max. Any stress is likely to put them into oxygen problems. Add a co-morbidity and losing lung capacity and it's curtains.
This is why, IMO, COVID takes out so many elderly people. They simply do not have the aerobic capacity and reserve to deal with losing lung capacity. It is also why rolling them over to the prone position helped.
Younger people and those who are exceptionally fit can "afford" to lose lung capacity in the short term and hence have better outcomes if they have more severe cases. Where you see young and/or fit people with very severe cases you can almost be certain there is some other underlying cause and don't count on the media to ask.
The good thing is that you can increase your MAF at any age. Get a watch or chest strap to monitor your heartrate. Set it to sound an alarm when you hit the magic number and go for a jog (or walk). Any time you hear the alarm you slow to a walk (if you're jogging or running) until your heart rate drops back down. It will take many months if you are unfit OR have been training above your MAF habitually.
Oh, for the record, my resting heart rate is ~55 and slips into the high 40's when I am knocked out asleep. I'm nowhere near 18 years old.