Staff Answer
Sep 08, 2026 - 12:34 PM
Yes — the reason a standard disposable absorption/transmission sensor such as the Medtronic OXISENSOR MAXA can sometimes display an SpO₂ number when placed on the forehead is that the monitor is still receiving an optical pulsatile signal. However, getting a number does not mean the placement is validated or that the number is accurate.
Technical difference
| Feature | Standard absorption / transmission sensor (e.g., MAXA) | Forehead reflectance sensor |
|---|---|---|
| Light arrangement | LED and detector are on opposite sides of the tissue | LED(s) and detector are on the same side |
| Measurement | Measures light transmitted through the tissue | Measures light reflected/scattered back from tissue |
| Typical site | Finger, toe, ear | Forehead |
| Intended tissue geometry | Relatively thin tissue that light can pass through | Thicker tissue where reflected light can be detected |
| Low peripheral perfusion | Can lose signal during vasoconstriction/shock | Often performs better because forehead blood flow is less affected |
| Forehead use | Not intended/validated for this application | Specifically designed and validated for forehead placement |
Medtronic describes its Max-Fast forehead sensor as a reflectance sensor and specifically instructs users not to use digit sensors on the head or ears. Medtronic notes that digit sensors used on the forehead can produce inaccurate, often overestimated SpO₂ readings.
Nellcor™ Max-Fast™ forehead SpO₂ sensor
So why does MAXA still give a number on the forehead?
The MAXA sensor is designed to measure oxygen saturation by comparing the red/infrared light absorption characteristics of pulsatile blood. Even when placed incorrectly on the forehead, some light from the LEDs can interact with superficial blood vessels and tissue and reach the detector through scattering/reflection.
The monitor's algorithm may therefore find a signal that looks sufficiently pulsatile to calculate an SpO₂ value. The important distinction is:
A numerical reading ≠ a validated/accurate reading.
This is particularly important in an emergency. The number may appear plausible while being biased or unstable. Medtronic specifically warns that a digit sensor placed on the forehead can produce inaccurate, often overestimated, values.
Why the forehead sensor is different
A true forehead reflectance sensor deliberately places the emitter and detector next to each other. Instead of requiring light to travel completely through the tissue, it detects back-scattered light. This makes the optical geometry appropriate for the forehead.
The forehead also has relatively good blood supply, including vessels supplied around the supraorbital region, and is generally less affected by peripheral vasoconstriction than the fingers. That's why a dedicated forehead sensor can be particularly useful when finger perfusion is poor
