The units custom-made ECMO circuit contains a soft shell reservoir bag to assist with priming and de-airing. During the set-up of an ECMO circuit crystalloid fluid was added to the reservoir bag and the centrifugal head, oxygenator and associated tubing circuit primed passively. After correct positioning of the head in the system driver and applying power to the circuit no flow could be produced. The circuit was checked for obstruction and on close inspection it was found that the outlet of the bag was almost completely adhered to the bag structure. Cavitation was observed in the centrifugal head inlet line. The application of negative pressure to the outlet effectively sealed the bag preventing flow. The seal was pulled apart sufficiently enough to allow flow however slight damage to the bag occurred. The bag is always removed from the circuit prior to the initiation of ECMO and after adequate de-airing this was performed in the usual manner. ECMO was then established in the patient uneventfully.