A Holter monitor will show a proper ECG if the leads are properly in place and all the leads necessary have been applied. Sometimes they detach, whether while sleeping or exercising, especially if you sweat a lot...which I do, and it was a mess. Even though some leads detached on my first Holter (I have since worn one at least three times), they told me they had enough to determine what was going on.

Note that I was asked to take in a normal day, including any exercise to see if the monitor would pick up something in any one of the typical daily circumstances. So, this was during a hard bicycle ride where I was getting my heart rate up to 150 and higher.

All that to say, yes, the Holter should....'should'... detect any arrythmia if one takes place while you are connected to the device. It should be worn for a minimum of 12 hours. All of mine were issued mid-afternoon, and I was to turn it off, remove the leads, package it up with packaging provided, and return it to the hospital or issuing lab by about noon the next day, They need ot sterilize them, insert new batteries, and be ready for new patients who have been asked to come mid-afternoon.