In Reply:-The observations of the authors are correct, from the strictly empirical standpoint. I wish to note, however, that interference filters are most effective when the interference source is limited. (Sometimes easier said than done)

The obvious prevention is low impedance patient connection and minimal length cables, ideally placed at random angle (not parallel) to other power and signal cables. This technique minimizes the cross talk between noise sources.

The secondary defense against unwanted interference pick up and amplification is a good front-end amplifier, characterized by high Common Mode rejection characteristics. The 2000A monitor exhibits a 90-100 DB Common Mode rejection. This is the point where approximately 90% of the effective noise immunity is obtained. It is well worth the effort to test, calibrate, or repair this section before postprocessing filters are introduced.

Postprocessing filters, such as the one found in the monitor's Refresh Memory board, will clean up the visible display of the signal, but with substantial side effects. One of the most obvious symptoms is the elimination or attenuation of all signals in the noise source and the physiologic signal within the band width of the filter. In practical terms, the 50-cycle filter will reduce or remove the pacer pulses from the QRS complex display (we have provided a distinctly separate path for pacer detection and display). Due to inherent time constants in the filters, the QRS complex will tend to "wonder" on the screen, extending artifact recovery times.

We have standardized on this filter because, on the whole, the benefits are justifiable. I believe that an understanding of the subject matter and prevention of the root causes will benefit this and other users.

Peter Ronay, Senior Engineer, Technical Support, Datascope Corporation, 580 Winters Avenue, P. O. Box 5, Paramus, New Jersey 07653-0005.

(Accepted for publication August 19, 1996.)