Using the detrend function

Hey everyone, I am having some issues using the detrend function on matlab, when I try to use it on physio data it works on some of the graphs but not others, the graphs do tend to have pretty varying shapes so I'm not sure if I would be better served using a different function or linear regression, etc. Below is an example of the graph where the detrend function does not seem to work. The code is just plotting the physio data versus time. Any help/advice would be appreciated, thank you!