From Openwaterpedia

noun - PVi® or Plethysmograph Variability Index is the variation in perfusion index over the breathing cycle, which may indicate changes in hydration, breathing effort, perfusion, or other factors. The Plethsymographic Waveform displays the real-time pulse pressure waveform. To properly measure PVi®, an individuals should lay down relaxed in a horizontal position and take it at the same time of the day in the same position.

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