Mathematical model for the calculation of oxygen concentrations in a closed circuit oxygen rebreathing apparatus

Background: Closed circuit oxygen rebreathing diving apparatus are used by armed forces in special tasks because of their advantages of long endurance, low noise and minimal gas escape. There is little knowledge about the administered oxygen concentrations in these systems. Closed circuit oxygen rebreathing apparatus are also used as a first aid device for the treatment of severe disorders. Because of similar constructive components, these rebreathing apparatus are comparable to the Drager LAR V model. Hypothesis: This study was conducted to measure the oxygen concentrations in the LAR V and estimate the correlation between oxygen concentration and pre-breathing purges. Method: Subjects were 12 males who performed the pre-breathing procedure. The oxygen concentrations in the breathing loop were measured after each purge. Results: The oxygen concentrations depended on the volume of the apparatus' dead space, the total capacity of the divers breathing system respective to the volume of the breathing purges and the number of pre-breathing purges. The maximum oxygen concentration was reached after eight purges (O-2 = 85%). An equation to estimate the oxygen concentration inside the LAR V and first aid rebreather was derived. Conclusions: The results indicated that the present purging procedure (three purges) before diving is normally not sufficient to remove the nitrogen totally from the deadspace of the LAR V, the divers airways and lungs. Only a small modification (six purges) is necessary to improve the safety of the diver in case of a diving apparatus malfunction during the mission profile.