NPPB and myalgic encephalomeyelitis/chronic fatigue syndrome: On the other hand, a study on long COVID showed that N-terminal pro-B-type natriuretic peptide (NT-proBNP), a biomarker that is commonly associated with heart failure and is produced in response to cardiac muscle cell stretching, was not related to impaired cardiac function but was related to hand-grip strength in long COVID patients with ME/CFS [21].