Apple’s long-running patent battle with health technology company Masimo has become even more expensive, with a federal judge ordering the iPhone maker to pay an additional US$184 million (A$264 million) in interest.

The latest payment comes on top of the US$634 million in damages Apple was ordered to pay last year after it was found to have infringed a Masimo patent covering low-power pulse oximeter technology used in the Apple Watch.

The original damages were calculated based on the licensing fees Apple should have paid Masimo for using the patented blood oxygen technology. Apple challenged the addition of interest, arguing that it would effectively provide Masimo with a windfall.

US District Judge James V. Selna rejected that argument, ruling that interest was necessary to compensate Masimo for royalty payments it should have received earlier.

There was at least some relief for Apple over how that interest was calculated. Masimo had pushed for a 7 per cent interest rate, but the judge instead determined that the rate should reflect Masimo’s own borrowing costs.

Even using the lower rate, annual compounding resulted in another US$184 million being added to Apple’s bill, taking the combined damages and interest in the case to US$818 million.

The lawsuit is one part of a wider legal fight between Apple and Masimo over the blood oxygen technology used in the Apple Watch.

In a separate case, the US International Trade Commission ruled that Apple had infringed Masimo patents, resulting in restrictions on the sale and import of certain Apple Watch models with blood oxygen functionality in the US.

Apple subsequently redesigned how its Blood Oxygen feature works on affected devices in an effort to comply with the restrictions.

The latest ruling adds another significant financial penalty to a dispute that has already forced Apple to make changes to one of the Apple Watch’s major health features.