Heather Stone is the chair of the Medicare Portability Task Force which is working to ensure that all U.S. citizens have access abroad to their earned benefits. 
Citizens who worked in the US, or for US companies, paid into the Medicare system throughout their careers.  Once they begin receiving the Social Security they earned throughout their work lives, their benefit is automatically reduced for the cost of their Medicare.  But US Citizens residing outside the country are unable to use the Medicare they paid for, and continue to pay for, without often-costly travel back to the United States.  Many choose that option.  Others never receive the benefits for which they paid.  This has implications for fairness and cost to the Medicare system.
For more information contact medicare@democratsabroad.org or follow this group on Substack https://themedicarereporter.substack.com/