What's the difference between SpO2, perfusion index, and a plethysmograph?

SpO2 is your blood oxygen saturation percentage. Perfusion index (PI) shows how strong the pulse signal is at the fingertip — useful for knowing whether a low reading is real or just poor circulation. The plethysmograph is the waveform; a clean, regular wave means the reading is trustworthy. Mid- and higher-tier units like the Innovo iP900AP show all three; budget units typically show only SpO2 and pulse.