REVERSE POLARITY – What’s Up (or Down) With That?

Thoracic and abdominal respiratory effort waveforms are used during PSG to differentiate sleep disordered breathing events as being obstructive or central in origin. Out-of-phase chest and abdomen excursions (paradoxical breathing patterns) occurring during a pause in airflow are indicative of obstructive events. In-phase chest and abdomen excursions is indicative of non-obstructive normal breathing.

Some effort belt technologies have been found to suffer from a number of problems including Reverse Polarity artifact. Reverse polarity occurs when either a chest or abdomen effort belt incorrectly reverses its original polarity resulting in false obstructive (out-of-phase) effort waveforms.

Effort belts that incorporate piezo crystal technology have a tendency to experience this artifact. The sensing element on a piezo crystal effort belt is located only on a very small section of the belt’s length.  As such, there are situations when a patient is lying on top of the piezo crystal and the effort signal can be dampened or not detected. This produces erroneous readings or unexplained changes in polarity that look like paradoxical breathing patterns.

Additionally, RIP (Respiratory Inductance Plethysmography) belts have the potential to reverse polarity of the output waveform should the patient move in a manner as to fold the elastic RIP belt a certain way.

Fortunately, Dymedix respiratory effort belts are not susceptible to reverse polarity. All Dymedix respiratory effort belts utilize PVDF (Polyvinylidene Fluoride) technology. The PVDF film incorporated into each belt is polarized with a positive polarity on one side and negative polarity on the other side (similar to the top and bottom of a battery) making polarity reversal physically and electrically impossible to occur. And as PVDF responds to pressure in addition to strain / tension, dampening of the effort signal as experienced with piezo crystal belts does not occur.

The odds of a Dymedix PVDF belt changing polarity during PSG testing would be comparable to the odds of a refrigerator magnet spontaneously changing polarity and falling to the floor.