Ensuring Patient Access to Critical Breakthrough Products Act of 2021 This bill provides for Medicare coverage of medical devices that are approved under the Food and Drug Administration (FDA) Breakthrough Devices Program. (Under the program, manufacturers work with the FDA to expedite the review and approval of certain medical devices that provide for more effective treatment or diagnosis of life-threatening or irreversibly debilitating human diseases or conditions.) The bill allows designated medical breakthrough devices to be temporarily covered under Medicare during a four-year transitional period. The Centers for Medicare & Medicaid Services (CMS) must assign payment codes for such devices within three months of FDA approval. The CMS must also establish a process to allow for continued coverage after the transitional period has expired, taking into account any additional evidence or data the CMS deems necessary. The CMS must also provide for temporary and, where appropriate, permanent Medicare coverage of breakthrough devices for which there is no existing benefit category (i.e., classification).