We randomly assigned 572 patients 55 years of age or older , without known atrial fibrillation , who had had a cryptogenic ischemic stroke or TIA within the previous 6 months ( cause undetermined after standard tests , including 24-hour electrocardiography [ ECG ] ) , to undergo additional noninvasive ambulatory ECG monitoring with either a 30-day event-triggered recorder ( intervention group ) or a conventional 24-hour monitor ( control group ) .