Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and the prolonged sequelae after COVID-19 (>3 months; Long COVID) have similar symptomology, are both associated with autonomic dysfunction, and a growing proportion of Long COVID patients are developing ME/CFS. We aimed to determine an autonomic phenotype of patients with ME/CFS vs Long COVID. We hypothesized that the groups would differ from controls yet be similar to one another. We recruited sedentary controls (n = 10), mild/moderate ME/CFS patients (n = 12), and Long COVID patients (n = 9) to undergo 1) breathing 5 % CO, 2) breathing 10 % O, and 3) 5-minutes of 70° head-up tilt. Respiratory, hemodynamic, and cerebrovascular variables were measured throughout the 3 trials. Resting vascular function and cognitive-motor-integration were also assessed. ME/CFS and Long COVID were similar to the healthy controls and each other with regard to resting vascular function and the hemodynamic responses to hypoxia, hypercapnia, and head-up tilt (p > 0.05). However, in ME/CFS we observed a greater reduction of cerebrovascular resistance (p = 0.041) and impaired autoregulation (p = 0.042) during hypercapnia alongside impaired cognitive-motor integration (p < 0.02), and in Long COVID we observed reduced peripheral and end-tidal oxygen (p < 0.04) and less vagal withdrawal during tilt (p = 0.028). Our findings suggest unique phenotypes when comparing ME/CFS and Long COVID whereby we have shown that Long COVID patients experience hypoxia while upright contributing to less vagal withdrawal, and ME/CFS patients experience impaired cerebrovascular control during hypercapnia potentially leading to reduced cognitive-motor integration. These differences could stem from disease severity/duration or some unique aspect of the COVID-19 virus. 2 2
