Introduction: coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 is currently a major public health challenge. COVID-19 infection results in diverse symptoms and morbidity. In severe cases, COVID-19 pathophysiology includes destruction of lung epithelial cells, thrombosis, and vascular leak leading to acute respiratory distress syndrome (ARDS) and subsequent pulmonary fibrosis. Clinical manifestations of COVID-19 can include cardiac involvement with complications, such as myocarditis (including fulminant cases), arrhythmias and rapid-onset heart failure. In children, severe acute infection is less common than in adults. Another COVID-19 related entity is a delayed life-threatening syndrome mimicking incomplete Kawasaki disease (KD) and toxic shock syndrome now designated as pediatric inflammatory multisystem syndrome (PIMS-TS) or MIS-C. Although rare, this syndrome remains an important source of pediatric COVID-19 related morbidity and mortality. The cardiovascular system is one of the major systems affected by this syndrome, manifesting mainly as myocarditis and/or coronary vasculitis. Endothelial dysfunction (ED) is a frequent long-term complication in patients after KD, manifesting as diminished production or availability of nitric oxide (NO) and an imbalance in the relative contribution of endothelium-derived relaxing and contracting factors. Little is known regarding ED post COVID-19 in the pediatric population but there is some evidence that endothelial injury might play a role in the pathogenesis of the disease. Patients who recover from acute COVID-19 infection or post-COVID-19 syndrome can have diverse complaints and symptoms such as chest pain, palpitations, weakness, myalgia and dyspnea. Several "return to play" guidelines exist for athletes, adults and children, due to concerns about long term cardiovascular and respiratory complications. Cardiopulmonary exercise testing (CPET) has become an important clinical tool to evaluate exercise capacity and predict outcome in patients with heart failure and other cardiac conditions. It provides assessment of integrative exercise responses involving the pulmonary, cardiovascular, and skeletal muscle systems, which are not adequately reflected through the measurement of individual organ system function. CPET is increasingly being used in a wide spectrum of clinical applications for evaluation of undiagnosed exercise intolerance and for objective determination of functional capacity and impairment. Few reports are available regarding CPET in post- COVID -19 infection adult patients, most of them after severe acute illness. Those reports demonstrated decreased aerobic capacity, not related to pulmonary limitation.

Peripheral Arterial Tonography (EndoPAT™) is a non-invasive and operator independent assessment of endothelial dysfunction. It evaluates the change in the volume of the pulse, using a volume sensor that is placed on the second fingertip both on the hand that is under evaluation and the free hand, measuring the reactive hyperemia mediated by NO release in response to local ischemia. EndoPAT™ has been in clinical research use for evaluation of endothelial changes in vasculitis, and has been deemed feasible and reproducible in the pediatric population. EndoPAT™, has been validated as a good evaluation tool for various blood-vessel related functions including arterial stiffness. In children, EndoPAT™ feasibility has been demonstrated in various conditions including inflammatory bowel disease (IBD) and Type I diabetes mellitus. We will therefore use this method to evaluate possible endothelial dysfunction in patients who suffered from PIMS. Endothelial dysfunction was previously evaluated by serum levels of endothelin, endothelin is a vasoconstricting substances that was previously used to evaluate endothelial dysfunction and high levels of endothelin were previously proved to be associated with coronary vasoconstriction. We aim to evaluate children, adolescents and adults, who had infection with COVID-19 or were diagnosed with PIMS and compare them to healthy controls, using pulmonary function tests, echocardiography, 6-min walk test (6MWT), CPET ,EndoPAT™ and serum levels of endothelin in order to identify long term cardiac or pulmonary residual function/limitation which might explain the symptoms or the long-term cardiopulmonary sequela of COVID-19 infection. Study design: Pediatric and adult patients recovering from COVID-19 with or without complaints or followed at the PIMS outpatient clinic or referred to the CPET clinic at Rambam Medical Center will be enrolled in the study. Data will be collected retrospectively/ prospectively, approximately 3 months after acute infection or post- COVID PIMS. CPET will be performed as part of the clinical evaluation recommended by the following physician. EndoPAT assesment- as part of the research. and blood endothelin levels- as part of the research. Participants will undergo all or part of the evaluation, and possibly repeated evaluations if needed. For CPET results each patient will be age- and sex-matched with an historical control from our CPET research database containing records of 200 healthy patients. For EndoPAT results- patient recovering from COVID-19 without PIMS will be evaluated as controls and the data will be compared to normal known values for age.