Praise be to Allah.
What is required when doing ghusl is to make the water reach all parts of the body. If a person is wearing a Holter to monitor heart activity and will be harmed if he takes it off – because he has to wear it for a specific time period – and the monitor will be damaged if water gets onto it:
then the right way to deal with that is to wrap the area where the Holter is with something waterproof, if that is possible, then wash the rest of the body, and do tayammum for the part that water did not reach.
This is by analogy with what the jurists mentioned regarding one who has a wound and is not able to wash the wound or wipe over it, and there is no plaster cast over it. He should do tayammum for the part where the wound is, and wash the rest of his body.
It says in Kashshaf al-Qina‘ (1/165): If part of his body is injured and the like, such as if he has a sore, that will be harmed if he washes it or wipes over it with water, then he should do tayammum for that wound or whatever, because of what is noted above, and he should also do tayammum for the part that could be harmed by washing, namely the area near the wound or whatever, because it comes under the same ruling. End quote.
If it is not possible to prevent water reaching the Holter, and it is not possible to prevent damaging it, he may do tayammum for the part where the Holter is, where he cannot make water reach it, and wash the rest of his body.
It says in Zad al-Mustaqni‘: One who is wounded may do tayammum for the site of the wound, and wash the rest of his body. End quote.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: If there is a wound in one of the parts of the body that are to be washed when purifying oneself, then wounds are of different levels:
1. If the wound is uncovered and will not be harmed by washing. In this case, it must be washed if it is in a part of the body that is to be washed when purifying oneself.
2. If it is uncovered but would be harmed by washing, but not by wiping, then in this case it must be wiped over and not washed.
3. If it is covered and will be harmed by washing and wiping, then in this case tayammum must be done.
4. If it is covered with a dressing or the like that is necessary, in this case the dressing may be wiped over; that will suffice instead of washing that part, and tayammum is not to be done."(Arkan al-Islam, p. 234).
Based on that, doing tayammum only was not correct, and you must repeat your ghusl. To be on the safe side, you should also repeat any prayers that you offered after doing that tayammum.
And Allah knows best.