Interesting post, and I'm sorry I have no answers but it has triggered some curiosity and I hope someone does have some answers. As you know the QRS is when the ventricles depolarize, i.e. contract. I know when the heart is racing, trying to get as much oxygen as possible around to your muscles, it has the ability to pump harder as well as faster. I assume this means a faster contraction of the ventricles which would in turn show a shorter QRS. This must mean the heart has the ability to contract more slowly when relaxed or under beta blockers, making a slightly longer QRS. I'm only thinking about this logically and so will eagerly await for someone else to post a response.
It's a shame you can't attach the lead, jump up and down a few times or run on the spot, quickly lay down and take a reading to see if the QRS shortens.
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