Open Pleth™ with Equine Flowmetrics™ software is a modification of whole body plethysmography that avoids the use of an enclosure. Measurement of flow is made at the opening of the nares in the horse. Volume displacements at the body surface are measured directly using elastic sensors instead of box pressure. The elastic sensors have coils running through them that when stretched, reduce their impedance (i.e. reduced inductance) to DC current. For example, during inspiration, the sensors stretch, permitting an increased electrical signal, which is proportional to tidal volume.
The flowmetric system relies on the respiratory system modeled as a bellows. Just as a bellows consists of a nozzle to deliver flow, and the collapsible bellows to general flow, the horse’s respiratory system consists of the nose and chest/abdomen components of the breathing mechanism. Flow at the nose and chest/abdomen are compared, and their differences recorded as a signal proportional to constriction of airways.