British journal of anaesthesia
-
Tidal ventilation causes within-breath oscillations in alveolar oxygen concentration, with an amplitude which depends on the prevailing ventilator settings. These alveolar oxygen oscillations are transmitted to arterial oxygen tension, PaO2, but with an amplitude which now depends upon the magnitude of venous admixture or true shunt, QS/QT. We investigated the effect of positive end-expiratory pressure (PEEP) on the amplitude of the PaO2 oscillations, using an atelectasis model of shunt. ⋯ Clear oscillations of PaO2 were seen even at the lowest mean PaO2, 9.5 kPa. Conventional respiratory models of venous admixture predict that these PaO2 oscillations will be reduced by the steep part of the oxyhaemoglobin dissociation curve if a constant pulmonary shunt exists throughout the whole respiratory cycle. The facts that the PaO2 oscillations occurred at all mean PaO2 values and that their amplitude increased with increasing PEEP suggest that QS/QT, in the atelectasis model, varies between end-expiration and end-inspiration, having a much lower value during inspiration than during expiration.
-
Anaesthesia systems that minimize the use of volatile anaesthetics to reduce cost and pollution are of interest. Closed circuit anaesthesia is the ideal solution, but requires continuous adjustment of fresh gas flow and composition and thus is demanding in routine practice. We describe an alternative system, the Reflector system, which is open in regard to oxygen, nitrogen and N2O, and semiclosed in regard to volatile anaesthetics. ⋯ Isoflurane consumption using the Reflector system in bench tests and an animal study was compared with that of an open system. In bench tests consumption was reduced by 79% and 82%, at a respiratory frequency of 10 and 20 min-1, respectively. The corresponding mean figures from the animal experiment were 65% and 77%.