The elderly are particularly vulnerable to “post-hospital syndrome,” some experts believe, and that may be why so many patients return. They believe it underlies the stubbornly high rate of hospital readmissions among older patients. In 2016, about 18 percent of discharged Medicare beneficiaries returned to the hospital within 30 days, according to the federal Centers for Medicare and Medicaid Services. We all know a loved one (or had the misfortune of it being us) who was admitted to a hospital for a serious illness. Even after discharge, the stress and disruptions of hospitalization — interrupted sleep, weight loss, mild delirium, deconditioning caused by days in bed — left us disoriented and weakened, a…