Having looked after a few LZs for ECMO retrievals over the years, the medical team usually spends an hour or so at the hospital, preparing the baby for the transfer. The team plus (sedated) baby then return to the site and the aircraft departs once the incubator has been hoisted into the cab using Fire Service/Coastguard/RAF crew muscle power (I have pics of one such retrieval if anyone really wants them).
Only a handful of hospitals have ECMO machines (Great Ormond St., Leicester and Glasgow, from memory) - they're not something that can come to the patient, regardless of the size of the helicopter. Over the years, we've worked with crews from Leconfield, Wattisham and even Boulmer on these jobs (I'm in south Wales, before anyone asks). Most tend to occur around 3am.....
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