MATHEMATICAL-MODEL FOR THE CALCULATION OF OXYGEN CONCENTRATIONS IN A CLOSED-CIRCUIT OXYGEN REBREATHING APPARATUS

Background: Closed circuit oxygen rebreathing diving apparatus are use d by armed forces in special tasks because of their advantages of long endurance, low noise and minimal gas escape. There is little knowledg e about the administered oxygen concentrations in these systems. Close d circuit oxygen rebreathing apparatus are also used as a first aid de vice for the treatment of severe disorders. Because of similar constru ctive components, these rebreathing apparatus are comparable to the Dr ager LAR V model. Hypothesis: This study was conducted to measure the oxygen concentrations in the LAR V and estimate the correlation betwee n oxygen concentration and pre-breathing purges. Method: Subjects were 12 males who performed the pre-breathing procedure. The oxygen concen trations in the breathing loop were measured after each purge. Results : The oxygen concentrations depended on the volume of the apparatus' d ead space, the total capacity of the divers breathing system respectiv e to the volume of the breathing purges and the number of pre-breathin g purges. The maximum oxygen concentration was reached after eight pur ges (O-2 = 85%). An equation to estimate the oxygen concentration insi de the LAR V and first aid rebreather was derived. Conclusions: The re sults indicated that the present purging procedure (three purges) befo re diving is normally not sufficient to remove the nitrogen totally fr om the deadspace of the LAR V, the divers airways and lungs. Only a sm all modification (six purges) is necessary to improve the safety of th e diver in case of a diving apparatus malfunction during the mission p rofile.