Irvine-based Masimo, the maker of pulse oximetry and other medical monitoring devices, has been sued for patent infringement by competitor Essential Medical Devices, Inc.. According to a filing in the Delaware District Court Friday, Essential Medical Devices alleges that Masimo is infringing on a patent owned by the company. The lawsuit also names Masimo spinoff Cercacor Laboratories in the lawsuit. Essential Medical Devices Inc. v. Masimo Corporation et al was filed on Friday.