Search strategy and selection criteria
Our methodology followed the reporting guidelines of Meta-analysis Of Observational Studies in Epidemiology (MOOSE) and Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. We electronically ran a search on CENTRAL, MEDLINE/PubMed, Cochrane Library, and Scopus. On Pubmed the word search used was (COVID OR SARS COV2 OR pandemic) AND (ARDS) OR (acute respiratory distress syndrome) OR (acute lung injury) OR (respiratory failure) OR (respiratory insufficiency) OR (ECMO) OR (extracorporeal membrane oxygenation)).
We contacted authors for some missing data, searched trial registries, included the grey literature, and used studies accepted and ahead of print. We did our search from inception up to May 28, 2020, which was the date of our final search without language restrictions. We used both subject headings and text word terms to search for articles about ECMO or mechanical ventilation with ARDS in COVID-19 patients. There were no language restrictions. Inclusion criteria were (all criteria should be concomitantly met for study inclusion): a) study reporting on 5 or more patients with final outcomes; b) with confirmed COVID 19 infection; c) receiving venovenous ECMO. Exclusion criteria were (one criterion was sufficient for study exclusion): a) inclusion of <5 patients with COVID-19 infection treated ECMO (thereby, any study reporting on fewer than 5 patients or case reports treated with ECMO were excluded); b) duplicate publication (in which case only the most recent report from the same study group was included in the systematic review). Use of a sample size cut-off was chosen to limit the risk of imprecision and publication bias c) Studies in which the main focus was veno-arterial ECMO for treating COVID-19 or d)studies with insufficient data about outcome endpoints (mortality, extubation, weaning, and discharge). AH, TA and HY independently reviewed the titles and abstracts of all citations. Then, they independently reviewed the full text of both definite and potentially eligible studies for inclusion. Disagreements were reviewed by a fourth reviewer HE, who had a deciding vote. The study protocol link is at www.crd.yorl.ac.uk/PROSPERO under registration number CRD42020183861