Technical modification of the Metamax II portable metabolic analyser for operation with a breathing apparatus

The O2 uptake of firefighters working in hot and polluted environment is not known. The portable Metamax II might measure the O2 uptake of firefighters using a breathing apparatus. However, the Metamax requires an inspiration signal between two expirations to work properly. When a firefighter inspires from bottles with pressurised air, the inspired air cannot readily be passed through a metabolic analyser. Expired air is on the other hand released to the surroundings and may be sampled for further analyses. In addition, the Metamax II does not tolerate heat, and gases at the scene of fire may damage its delicate sensors. We have modified the Metamax II, producing an artificial inspiration signal after every expiration. We have also protected the instrument from heat at the scene of fire. Expired air was sampled from exercising subjects using a breathing apparatus. The Metamax II instrument was run in the normal and modified modes while the instrumentʼs reported O2 uptake was recorded. Control experiments showed that without an inspiration signal, the instrument did not work reliably. In a typical experiment the reported O2 uptake was only 50% of the true value. In further experiments an artificial inspiration signal was sent to the main unit after every expiration. Then the instrument worked properly although only expired air passed through the Metamaxʼ breathing valve. With proper modifications of the Metamax II, the instrument worked well even if only expired air passed through the breathing valve. The valve may thus be mounted on the outlet of a firefighterʼs breathing mask. The instrument can readily be protected from heat at the scene of fire and thus be used to measure the O2 uptake of smoke divers during realistic exercises in hot and polluted environments