We had exactly the same problem on our Spacelabs Monitors - but the interference was worst on the pressure waveform where you would get very large, very sharp spikes, which couldn't possibly have been real pressure variations. When the filtration unit had been running for a bit, you could sometimes watch the spikes gradually reduce and then dissappear. But sometime they didn't - and we couldn't get rid of them with earth straps or by moving the machine. The interference was definately being picked up from the patient and not direct in the monitor, because if you disconnected the patient, you lost the inteference !
No one suggested static as a cause - but that sounds very plausible.
