Owlet sells a "smart sock" that measures pulse and O2 levels and alerts you if they drop below a set level.
It's not a medical grade device like the NICUs have, but it lets you rest easier knowing you'll be notified if your baby's pulse or O2 levels drop.
Now, what would be really helpful for small babies is consumer-available version of the 'clicker' breathing monitors they use in NICU wards. Our oldest was in NICU for nearly two weeks due to being a little prem, and when we got them home the lack of that regular 'click... click...' from the monitor was so hard to get used to.