can you recommend a blood pressure monitor for higher than normal?

Our machine works fine unless your pressure it past a certain level.  My wife's pressure sometimes goes quite high and it won't read.  We don't want to run to the ER every time we think it's too high.  There's a range where it's just high but not anything that doctors will do because it isn't that high. 

We need to know how high it really is though.

And yes, she's been to lot's of doctors, she's on medication.  She does all the things they say should do.  We just need a monitor that will read blood pressure at higher levels.