 Those with either definite pulmonary AVMs or an abnormal contrast echocardiogram with no clearly visible lesions are deemed to be at risk from brain emboli . They are therefore counselled to avoid scuba diving , during which small air bubbles may form in the <unk> that may migrate to the brain and cause stroke . Similarly , antimicrobial prophylaxis is advised during procedures in which bacteria may enter the bloodstream , such as dental work , and avoidance of air bubbles during intravenous therapy . 
