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The age of patient, degree of damage to heart, current state of first degree heart block
& associated symptoms, if any, and current status of thyroid hormone levels is not mentioned, and they play a significant role in prognosis of this patient.
A heart attack
in a patient with preexisting first degree heart block, need not carry a worse prognosis or relatively more chances of death. However development of new, higher degree of block with heart attack carries more chances of complications and death.
if uncorrected, itself may lead to weakening of heart muscle and reduced pumping.
you are welcome to raise further query with more information.