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Background: Neonatal extracorporeal membrane oxygenation (ECMO) patients are particularly vulnerable to the effects of uncompensated insensible water loss resulting in hypernatraemia. There exists a long-standing relationship between hypernatraemia and varying degrees of cerebral dysfunction. The aim of this study is to explore the degree to which free water loss occurs across a commonly used ECMO oxygenator, the polymethylpentene (PMP) membrane Hilite 2400LT (Medos, Medizintechnik AG, Stolberg, Germany). The secondary aim is to assess to what extent the addition of heat and/or humidity ameliorates this water loss., Methods: An ECMO circuit consisting of a centrifugal pump and a Hilite 2400LT oxygenator was primed with crystalloid and albumin. Each experimental trial was carried out in triplicate, with gas flow rates of 1, 3 and 4.8 L/min being investigated. Fluid loss was assessed at six time points over a 24-hour period., Results: Water loss increased significantly from 1 to 3 L/min gas flow (p=0.05) and from 3 to 4.8 L/min gas flow (p=0.025). The mean water loss differences between the differing gas flow trials per L/min gas flow were non-significant (72.4 ±3.9 ml/24 hrs). The effect of heating the gas to 37 °C did not significantly alter water loss, whereas heat and humidity reduced water loss significantly (p=0.009)., Conclusions: Insensible water loss from a Hilite 2400LT oxygenator is approximately 72 ml/day per L/min gas flow over 24 hrs. Heating and humidifying the gas reduces the fluid loss significantly to approximately 8 ml/L/min gas flow over 24 hrs (p=0.009).