I wouldn't have thought it was 400 Hz either....but

I went to Manage my booking.
Went to the ‘On Board’ tab below the flights
Clicked on the ‘Learn more’ link under the ‘In-Flight Health’ heading.
Then clicked on ‘Fit to Fly’, then went almost to the bottom under the red heading ‘Passengers Travelling with Electronic Medical Devices’ and clicked the link below it.
In the text of the page ( http://www.cathaypacific.com/cpa/en_...0011d21c39____ ) in about the second paragraph in point C it says:
o Cathay Pacific's aircraft power supply is 115Volts at 400 Hertz. However, most medical devices are designed to operate on AC power with voltages from 110 volts to 250 volts and only at 50 or 60 hertz frequency. Medical devices that use motors do not work on a wide range of frequencies. Using the machine on another frequency may damage the power pack attached to the machine. Please confirm with the OEM whether your device can be used with a power supply of 115Volts at 400 Hertz. Cathay Pacific will not be responsible for damages or injuries caused by using the aircraft power supply to operate your device on board.

I have sent the manufacturer an email asking if 400 Hz will work/cause damage, but I've not heard back from them yet, so I was wondering if anyone else has had trouble.