mentary oxygen with the Air-Viva lowered the inspired carbon dioxide concentration in a stepwise fashion, although the concentration did not reach zero even with 10 1 added oxygen/min. Comment Resuscitator bags refill with air during the patient's expiratory phase. To prevent rebreathing it is essential that the air intake and expired gases are separated. The valve of the Air-Viva, however, has a common port for both, which leads to expired gas being drawn into the bag during refilling. Supplementary oxygen, by displacement, reduces the amount of expired gas that enters the bag, so that the inspired carbon dioxide concentration falls as the flow of oxygen is increased. Nevertheless, as we found that the Air-Viva valve tended to obstruct when an oxygen flow of above 4 1/min was added it is impracticable to prevent rebreathing by using high flows of oxygen. The use of an International Standards Organisation model lung with added carbon dioxide is a standard method for demonstrating rebreathing in anaesthetic breathing systems, and the results obtained are comparable with clinical events.1 We were able to confirm these findings in two patients during the routine use of the Air-Viva bag. In both cases the inspired carbon dioxide concentration rose to 2% within 20 seconds. Resuscitation bags are used for the resuscitation and during the transport of critically ill patients. In both of these circumstances a raised inspired carbon dioxide concentration may have serious deleterious effects, such as raised intracranial pressure and peripheral vasodilatation. Rebreathing also, of course, implies a reduction in the inspired oxygen concentration. We believe, therefore, that the Air-Viva resuscitation bag should be withdrawn from use. Although it is no longer marketed (having been superceded by the Air-Viva 2), there are many such bags in general use. The Air-Viva 2 is of completely new design and performed satisfactorily in this test.
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