Extracorporeal membrane oxygenation (ECMO) is a form of cardiopulmonary life-support for critically ill patients where blood is extracted from the vascular system and circulated by a mechanical pump while it is oxygenated and reinfused into the patient's circulation. It is well known that critically ill patients may experience alterations in antibiotic pharmacokinetics, and as a result, dosing modifications are generally required. There is a need to understand how ECMO circuits affect the pharmacokinetics and disposition of drugs. This study is designed to assess the pharmacokinetics of the new broad-spectrum echinocandin, Rezafungin, in critically ill patients receiving ECMO