Breathing Resistance: Keeping the Requirements Realistic

Summary

Breathing resistance is usually defined by work of breathing per volume respiratory ventilation. It is divided into internal and external resistance and also into an elastic and a resistive component. Total breathing resistance is an important factor, limiting a diver’s physical capacity. In unmanned tests of breathing equipment, only external breathing resistance is measured. The ultimate and ideal requirement for breathing resistance is that the equipment adds no extra resistance to breathing, but instead assists respiratory ventilation so that the density dependent increase of internal resistance is reduced. Realistic requirements, however, must be set so that total cost to fulfil them is justified by gain in safety and efficiency. To determine this, breathing resistance must be weighted against other important properties of the life support equipment such as complexity, fail frequency, maintenance, mobility, entanglement, noise, vision, temperature, pCO₂, pO₂ and humidity. Requirements for breathing resistance are included in different proposed performance goals for breathing equipment. At a workshop held at NUTEC in 1981, Morrison’s performance goals proposed for Department of Energy gained most support. NEDU also presented performance goals which were about equal on the important points. Main differences were: NEDU proposed only recommended, not also acceptance levels like Morrison, NEDU was more stringent for low respiratory ventilations and less stringent for high ventilations, hydrostatic imbalance (HI) was treated differently. The main problem is to interpret ‘recommended’ and ‘acceptable’ levels for breathing resistance. There should be no justification to use equipment not within the acceptance limits. Justifications to use equipment within acceptance levels, but not within recommended levels are: it is impossible to reach recommended levels and at the same time fulfil other requirements, important for safety and efficiency, the depth is shallow and working conditions are specially safe and easy. Today, with improved equipment and much deeper depths, ‘realistic’ requirements have come much closer to ‘recommended’ levels than they were in 1981.