No adverse reactions normally occur in patients taking betablockers who undergo dipyridamole–thallium-201 scintigraphyand echocardiography,but case reports suggest that very rarelybradycardia and asystole can occur.
A 71-year-old woman taking nadolol 120mg daily and bendroflumethiazide,with a 3-week history of chest pain, was given a 300mg dose of oral dipyridamole as part of a diagnostic dipyridamole-thallium imaging test for coronary artery disease. She was given thallium-201 intravenously, 50 minutes after dipyridamole, but 3 minutes later, while exercising, she complained of chest pain and then had a cardiac arrest. She was given cardiopulmonary resuscitation and a normal cardiac rhythm was obtained after she was given intravenous aminophylline (See reference number 1).
Adverse interactions occurred in another 2 patients taking beta blockers during diagnostic dipyridamole-thallium stress testing. One patient, who was taking atenolol, developed bradycardia then asystole, which was treated with aminophylline and atropine, and other patient, who was taking metoprolol, developed bradycardia, which resolved after she was given aminophylline (See reference number 2).
These reports need to be set in a broad context. A very extensive study of high-dose dipyridamole echocardiography (10 451 tests in 9 122 patients) noted significant adverse effects in only 96 patients,with major adverse reactions occurring in just 7 patients. Three of 7 developed asystole and two of these patients were taking unnamed beta blockers (See reference number 3).
One possible explanation is that both drugs have negative chronotropic effects on heart
The value and safety of dipyridamole perfusion scintigraphy and echocardiography have been very extensively studied in very large numbers of patients,and reports of bradycardia and asystole, attributed to an interaction between dipyridamole and beta blockers, are sparse. It would therefore appear to be a relatively rare interaction (if such it is).
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