Skin damage can significantly influence Vital Parameter measurements in a PPG wrist-worn medical device as it causes interference with light transmission. A hematoma (blood pooling under the skin) absorbs and scatters light differently than healthy tissue. Since PPG sensors rely on light absorption and reflection to measure blood oxygen levels, the presence of a hematoma can distort the readings. Damaged blood vessels and localized bleeding can also lead to abnormal blood perfusion, affecting the accuracy of the SpO₂ calculation.
The IFU states:
⚠️ DO NOT use this Bracelet if you have severe blood flow problems or blood disorders.
⚠️ DO NOT use this Bracelet if you have damaged or irritated skin.
For this patient, the PPG signal is severely disrupted due to hematomas. The accumulation of blood outside the blood vessels occurs when blood leaks from damaged vessels into the surrounding tissue. The photo clearly shows damage and bruising. The CardioWatch LEDs cannot shine through properly. For patients with hematoma, this causes low PPG green signal and may result in Non-Wearing Alerts. Also SpO₂ measurements are unreliable for the same reason as PPG Red and iR are effected by the damage and bruising.
Example of distrubed signal due to Hematome:
‘Non-Wearing’ Alerts are the result of very low PPG Green values. Unreliable saturation measurements is caused by the extremely unstable signal.
This is what a good PPG looks like:
See also: Wearing your Bracelet and Prevent skin irritation