Masimo claims Apple's Series six smartwatch infringes five patents for devices that utilise light transmitted through the body to measure oxygen levels in blood.
Masimo develops signal-processing technology for health-care monitors and its spin-off Laboratories Inc first sued Apple in January 2020.
The case is being reviewed by the US Patent and Trademark Office, and Apple argues that the patents didn't cover new inventions.
The trade agency won't delay consideration of the patent complaint and these investigations typically take 15 to 18 months.
'Apple heavily markets that feature of the Series 6 to give the watch the appearance of a medical device Yet, hidden from the millions of purchasers of the Series 6, Apple warns in the fine print that the blood oxygen measurements should not be relied upon for medical purposes,' said Masimo.