The first thing to think about is if you need an external video capture
device.
Your laptop likely does not have built in video capture.
With that in mind, do you know what the resolution and so forth are of the
signal from the medical system video output(s) that you are thinking of
using? That may have a major impact on what video capture devices can be
used.
Finally, there may be a quasi legal safety issue with attaching or even
using a non medical rated electronic device in conjunction with a medical
system.
By the way, the medical monitor may actually see a composite video signal
instead of a true digital RGB signal. The basic nature of RGB or composite
video from a digital device or video card can be more like an analog signal
with steps.

I can't speak to current medical equipment, but it was a common past
practice to use non standard video in medical systems, just to lock the
customer into the OEMs accessories for a specific medical system.

Video capture devices
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=10610
04&Sku=P67-1096
S video to RCA adapters
http://www.optimization-world.com/products/catid/7.html