How do I stop patients from leaving the hospital

Thanks all, I changed the hospital around so I have four main rooms that have one bed, scanner, treadmil, annyliser (sp), and computer in it, to save on walking the whole time, so perhaps I will make another room and just chuck a couple of beds in there for spare, my sim never gets to see that many sims anyways, so she can just tend to the ones in the special rooms I created. Thanks once again for all the replies, they are appreciated