Positive-pressure ventilation is not without its own major problems, in particular the increased risk of pulmonary barotrauma. In an attempt to overcome many of the non-physiological limitations of positive-pressure ventilation, HaYek..§nd..SchonfeId...,(,19.9.0)i developed a lightweight flexible chest enclosure for human use ( Fig 1). It is connected to a high-frequency oscillator which oscillates the chest around a variable subatmospheric (negative) pressure. Both inspiratory and expiratory phases are active and therefore fully controllable (via chamber pressure and variable inspiratory-to expiratory (I:E) ratio). As a result, respiratory rate is not limited and external high-frequency oscillation (EHFO) can be achieved. This concept of negative-pressure ventilatory support equates closely to the natural physiology of spontaneous respiration.
In EHFO both peak inspiratory chamber pressure and end-expiratory chamber pressure (EECP) are controllable. While inspiratory pressures are almost always negative, EECP can be positive, atmospheric, or negative. This allows control over end-expiratory lung volume so that ventilation can proceed below, at, or above functional residual capacity in conditions where lungs are hyperinflated, normal, or underinflated respectively.
As the airway is open to atmospheric pressure, the airway pressure will fluctuate around zero provided that the I:E ratio is 1:1. Altering the I:E ratio will produce predictable variations in both peak inspiratory pressure and EECP. Hayek suggested that a positive EECP is conceptually the same as auto-PEEP; increasing frequency and/or I:E ratio will increase the degree of auto-PEEP present.
Positive end-expiratory chamber pressure (PEECP) will reduce lung volume by compression; the effect of this on oxygenation is balanced by the beneficial effect of the auto-PEEP generated. Another potential advantage of maintaining PEECP is the limiting effect on chronic hyperinflation, further reducing the risk of pulmonary barotrauma. In studies on humans with normal lungs, adequate ventilation was achieved with chamber pressures of +5 to -15 cmH 2O or +10 to -22 cmH2O. At these pressures, mean chamber pressure is always negative (-5 to -10 cmH2O).
Tidal volume is determined by 'span', i.e. the pressure difference between end-expiratory and peak inspiratory pressures, and by frequency. Increasing span increases tidal volume, while increasing frequency decreases it. Since minute ventilation is the product of frequency and tidal volume, minute volume will increase provided that the incremental rise in frequency is greater than any associated decrease in tidal volume. CO 2 elimination increases with increasing minute ventilation up to a frequency of 180 cycles/min. Any further increase in frequency will not increase CO 2 clearance. By increasing peak and mean chamber pressures, good CO2 elimination can be achieved at lower frequencies. Experiments on animals with 'ARDS-like' lungs showed that increasing mean chamber pressure increased lung volume and improved oxygenation.
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