What would make a cardiologist choose to use an event monitor for 30 days compared to 24hrs?

Various: Symptoms that due not occur every day may not be caught on a monitor that just lasts 24 hours. If you are dealing with a more serious arrhythmia, longer analysis gives your doctor more information.

Frequency of symptom: If the symptoms you have are not predictably occurring on a daily basis, an longer time period of monitoring would be needed to identify the underlying problem. In that case, an event monitor would be most appropriate.

It depends. : If what your experiencing is a daily event a 24 hr holter can pick it up. But if what your experiencing happens only a few times a month you need a 30 day monitor to pick it up.