The device that uses an infrared LED and a nearby photosensor to measure blood volume changes by detecting backscattered light is called what?

• A. tcPO2
• B. Impedance plethysmography
• C. Photoplethysmography
• D. Volume plethysmography

Answer: (C)

Measuring pulsatile blood volume using light hinges on how arterial blood absorbs light as it fills and drains with each heartbeat. An infrared LED shines into the skin, and a nearby photodetector picks up the light that is scattered back from the tissue. When arterial volume increases, more light is absorbed and less is scattered back; when the volume decreases, more light returns to the detector. This rhythmical change in backscattered light creates a waveform that mirrors the heartbeat.

That method is Photoplethysmography. It’s the basis for common devices like pulse oximeters and many wearable heart-rate monitors, and it can operate in reflectance (backscattering) or transmission modes.

Why the other options don’t fit: transcutaneous oxygen tension uses a heated sensor to estimate oxygen partial pressure in tissue, not backscattered light from blood volume. Impedance plethysmography detects changes in electrical impedance with skin electrodes to infer volume changes, not light-based signals. Volume plethysmography is not the standard term for this optical method and isn’t described by using an infrared LED with a photosensor detecting backscattered light.