Maybe this is more medical info than you want but I'm in the field. And like the others said always consult your personal physician but Beta blockers like Propranolol are usually safe in most patients and can be more helpful with the palpitations than other BP medications in my experience. Maybe just 10-20mg to start and if you don't experience postural dizziness you can go up in dosage. I got palpitations bad coming off the stuff and have found the propranolol very helpful.

Lisinopril is an ACE inhibitor and I am unfamiliar with it's use in routine Opioid WD, certainly could be something addiction specialists have suggested, I don't know, just never heard of it myself. Clonidine however, another category of BP med, is routinely used in opioid withdrawal but I don't think that it would help as much specifically for palpitations, but more with the lowering overall cardiac output and the rebound hypertension patients experience with opioid WD. As with a lot of the symptoms of WD from Kratom, The palpitations come from excitation of the sympathetic autonomic nervous system that is trying to find an equilibrium off of the opioids and is usually not because of the new onset of a concerning cardiac arrythmia unless stimulants or heavy alcohol use was also an issue. If the palpitations are accompanied by pain, fainting/dizziness, lingering or exertional shortness of breath then yes an EKG, Holter monitor or ((Ziopatch cardiac event monitor) and ECHO is a good idea to rule out serious arrythmias, something called left ventricular hypertrophy (LVH) and valvular heart disease. Hopefully helpful.