What is the meaning of sinus rhythm with fusion complexes? is this an arrhythmia? is it dangerous?

Mainly not dangerous: fusion complexes or beats usually occurs when there is a premature beat coinciding with a ventricular beat- causing a different configuration. can occur with atrial beats. most importantly, it is not dangerous if they occur infrequently. but, if they occur frequently- it can lead to problems. so, we usually recommend a 24 hour monitor to see how often it happens and then advise or reassure patient