My 82 year old mother up until now has had good health for most of her life. She does have a pacemaker now.
Over the past 2 weeks she has been feeling exhausted after doing something as simple as making a cup of tea. She has seen a specialist as her iron levels were lowish but apparently not low enough to cause exhaustion. She is on iron tablets though. She has seen a blood specialist as her gamma globulin levels were low but this was also of no real concern apparently. So the mystery remains!! Her pace maker has lots of battery life left in it so this shouldn't be a problem. The main symptoms are:
sudden onset of exhaustion from doing very little
very tired most of the time even after a good night's sleep
can hear her heart beating in her ears sometimes when she lies down
pain across her upper back like she has done something strenuous