Need for stress test after catheterization

Hello,
  My 83-old grandfather was advised to complete Isotope stress test in 2009. But he already have completed catheterization in 2005 that showed three-vessel disease, with most arteries having 90% occlusion. His new cardiologist wants to do Isotope testing, his old cardiologist sees no benefit in this testing. And we are in between.
My question is:
   - Should this isotope testing be done after catheterization? What new infromation could be gained from that?
   - Since adenosine or dobutamine be used as stimulants, what kind of danger could it be? I read some articles on PubMed saying that adenosine/dobutamine can cause things from AV block to ventricular fibrillation.

With appreciation for your wisdom

[kenkeith]

The adverse effects experienced are similar to those with use of adenosine. While adverse effects are less frequent with dipyridamole (47% of patients), they tend to be more serious than those associated with adenosine. The most common adverse effects of dipyridamole are chest pain (19%), headache (12%), and hypotension (4.6%). In addition, 12% of patients require aminophylline for reversal of adverse effects.

A stress test provides an insight to the degree of exercise a patient can safely tolerate as many doctors want their patients to exercise as part of the treatment regimen.  Also, based on stress test results enables a doctor/surgeon to evaluate if the patient's condition is able to successfully (benefit/risk) meet surgical stress if an operation is planned or future events make necessary.