Chemical stress test

A friend of mine had a pacemaker implanted a month ago.
Right after the implantation procedure he began to experience a number
of bad complications like post-op infection, pulmonary edema and
several arrhythmias. Echocardiogram and TEE were negative and now he
is recoverying from all that... but his cardiologist still has some
doubt from the echo and wants him to have also a scintigraphy at rest
and under chemical stress in some weeks.

Coming to the point, my friend is quite worried with the perspective
of the chemical stress test. For example, he'd like to know:
-why are they suggesting a chemical instead of a treadmill test?
-is a chemical test more risky than a treadmill, or viceversa?
-how would they interrupt the procedure in case of a contingence,
given that drugs have already been injected?
-any possible interferpherence of the test with the pacemaker
function?

And, last but not least, could you suggest some "good" links to those
matters?

Thanks in advance
---
Uplbet
.