peridito
He would definitely have had an ECG, when the paddles of the defibrillator are applied it shows a single ECG lead - that's what the HCP using the defib (in emergency cases) uses to decide to shock.
I've used a defib numerous times but always in a hospital setting where the patient was already wired up.
OK other tests they mayor may do depend on what they are looking for as an underlying cause, now my knowledge is pre covid but if they thought he was having an Myocardial infarction (MI) (medical term for a heart attack) then he would have been sent for a cardiac catheter and given 'clot buster' drugs.
I'm not going to go into details unless they decide to do one and you want details.
Now they are looking for other reasons for a cardiac arrest.
OK stay with me for a bit. Your body's circulation is a bit like your central heating, the boiler pumps the water round the system and the boiler resounds to inputs like temperature or timings.
The timings are rom an electrical system.
OK your heart has an electrical system too. A cardiac arrest is when this has stopped working completely or gone so haywire it's like sparks going off everywhere. There is another rare condition where the ECG is normal but the heart isn't beating - that would have been dealt with.
So the HCP now have to try to find out what caused the wiring to misbehave and what they can do to stop it happening in the future.
He will be having daily ECGs and probably be on a monitor, an ECG actually shows you what the electrical activity is doing.
He may also be on a temporary pacemaker.
Obviously I'm just giving general information here, it may be that he will be fitted with a pacemaker or an ICD (this is a defib fitted inside the chest with wires to the heart).
He may be sent for an electrophysiology study.
He may also be given medication.
The important thing now is that he is in the best place and being cared for.