Ergonomics of Breathing Apparatus, with Special Reference to Work of Breathing, Dead Space and Breathing Resistance
Abstract: A breathing apparatus is a very useful device. It allows people to breathe relatively freely while entering environments that otherwise would be impenetrable such as extreme altitudes, smoke filled buildings, ocean depths and the surface of the moon.However, a breathing apparatus does have some disadvantages. These include breathing resistance, elastance, inertia and also positive or negative pressure breathing. A breathing apparatus may also increase the ventilatory demand due to, for instance, its dead space. These loads will increase the work of breathing. There are two ways to react to added respiratory loads: either the effort is increased so that the ventilation remains adequate or it is not which will cause increased levels of CO2 (hypercapnia). Increased respiratory effort can cause respiratory muscle fatigue and/or dyspnea. Effects of hypercapnia range from headaches and nausea to impaired performance and mental function including loss of consciousness. The aims of this thesis are to investigate how much breathing resistance that is physiologically and subjectively acceptable, to study how the dead space in a breathing apparatus may interact with the user, to study how some breathing gases that are lighter than air may affect the users' respiratory capacities and to review and modify the traditional way that work of breathing is calculated. Based on experimental data it has been shown that the added work of breathing imposed by symmetrical breathing resistance should not exceed 1.5 to 1.8 J/l for the ventilation range 30 to 75 l/min BTPS. Dyspnea and hypercapnia appear to be exclusive reactions, at least in a diving situation, because increased in one were typically not paralleled with increases in the other. The dead space in a breathing apparatus may vary with the user's level of ventilation. Experimental data were compared with data from the literature and it was found that dead space increases the ventilation by about 60% per liter of dead space. The gas mixture Hydrox have respiratory advantages over the gas mixture Heliox. The traditional way of calculating the work of breathing is incomplete. After demonstrating how this omission should be corrected it was demonstrated that the traditional method may have an error of up to 40%. Acceptable levels of elastance and acceptable levels of combined loads need to be determined. The range of acceptable breathing resistance could probably be narrowed down if the studies were performed with more subjects.
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