Thanks for your concerns. Most likely he was given a substance called adenoseine, which is given through an IV to simulate exercise response in the heart; one of the side effects is transient heart block
with slow heart rates, which is probably what he exprienced and made him feel bad with those symptoms; it is just a side effect of the medication, which can be more common with old adults.
Dr Brenes-Salazar MD
Mayo Clinic MN