Some characteristics of human inspiratory flow of particular relevance to breathing at raised environmental pressure
The pressure-flow characteristics of inspiration have been studied, with particular reference to breathing dense gas at raised environmental pressure. The inspiratory pressure-flow relationship was determined from maximal and submaximal breaths performed by divers at pressures equivalent to 18, 39 and 56 msw, and compared with the relationship measured at sea level. Inspiratory flow limitation was found at every depth, becoming more severe as environmental pressure was increased. Similar studies performed at sea level in subjects breathing dense gas mixtures or breathing against a fixed resistance also showed some evidence for inspiratory flow limitation. Evidence was also found for sudden, transient flow interruptions breathing dense gas at raised environmental pressure and at sea level. An additional effect of high pressure on inspiration was observed, which leads to a distinctive pattern of oscillatory flow and which may be related to the High Pressure Neurological Syndrome. Flow oscillations developed in divers suffering from HPNS. These were of the same frequency range as muscle tremors associated with the syndrome. A mechanism of dynamic inspiratory airway compression has been proposed to explain these results.