Venovenous extracorporeal membrane oxygenation (VV-ECMO) is a supportive therapy, indicated in case of severe, possibly reversible pulmonary failure, refractory to conventional therapies. Despite advances, morbidity and mortality remain high. Severe neurological complications can occur during ECMO, but their exact etiology is not well understood. It is hypothesized that fast correction of severe hypercapnia, a common indication for venovenous ECMO, may be detrimental for the brain. The supposed mechanism is that fast correction of hypercapnia may result in massive cerebral vasoconstriction and impaired cerebral blood flow (CBF). In this prospective, observational study the aim is to quantify change in CBF during routine initial correction of severe hypercapnia during VV-ECMO. Furthermore, the investigators will record any other hemodynamic changes during VV-ECMO. The hypothesis is that a larger decline in PaCO2 will result in a larger decline of CBF.

Inclusion Criteria: in order to be eligible to participate in this study, a subject must meet all of the following criteria:

• High suspicion of an indication for VV-ECMO;
• Arterial line present to enable blood sampling;
• Older than 18 years.

Exclusion Criteria:

• Subjects will be excluded if the cerebral blood flow cannot be measured using the transcranial Doppler
• VA-ECMO or ECPR;
• No possibility for neuromonitoring measurements due to technical difficulties, e.g. post-craniotomy, unsuitable transcranial window;
• No possibility for neuromonitoring measurements due to circumstantial difficulties, e.g. ECPR;
• Contraindications for ECMO;
• ECMO is initiated in another center.

Critically ill patients with a suspected indication for VV-ECMO. ECMO will be provided according to local guidelines. In the Amsterdam UMC, location AMC, next to VV-ECMO, veno-arterial (VA-ECMO) is also provided. For this study, only patients on VV-ECMO will be included.