Although several HFV devices have been described, the only form currently relevant to adult practice is high-frequency jet ventilation (HFJV) in which short pulses of gas are accelerated through a jet nozzle into the airway ( Fig, 1). A negative lateral pressure, which enhances the gas pulse volume, is generated around the jet. Such ventilators operate at frequencies of 60 to 600 breaths/min and employ a passive expiratory phase. High-frequency oscillation, which employs an active expiratory phase and is widely applied in the infant respiratory distress syndrome, has not been effectively translated from pediatric to adult practice.
Fig. 1 The jet nozzle sits within the entrainment chamber at the proximal end of the endotracheal tube (ETT), providing an entrainment ratio of up to 200 per cent (in contrast, the entrainment ratio with distal positioning is as low as 25 per cent). The bias gas provides both humidification and variation in fractional inspired oxygen (Fio2). (Reproduced with permission from Eyans,iand,iH§slettI1986).)
Different HFJV devices vary markedly in their performance; small design modifications may result in major changes in ventilatory capacity. This variation, coupled with varying, and sometimes inappropriate, HFJV ventilatory philosophies, have led to inconsistent reports of efficacy. In addition, while less powerful devices may prove effective in mildly diseased lungs, they cannot be expected to support patients with severe disease and markedly reduced pulmonary compliance.
New HFJV devices employ very rapid response solenoid valves so that the gas pulse is delivered with a square pressure wave, allowing optimal entrainment and maintenance of mean airway pressure if required. Earlier models, which generate pseudosinusoidal pressure waves, are unable to provide the ventilatory capacity now available. New machines provide continuous analysis of ventilator function and safety as well as airway pressure profiles which provide clinically relevant assessments of patient-ventilator interaction.
Controls on HFJV devices are frequency, inspiratory time, and driving pressure. Initial settings depend on the device applied, although general principles exist. Airway pressure and arterial oxygenation are influenced by inspiratory time and driving pressure. Inspiratory time has little effect on CO 2 clearance which is most influenced by driving pressure and frequency. Algorithms for intervention when gas exchange is unacceptable are easily constructed, based on an understanding of the influence of these control parameters. Typical settings when ventilating normal lungs are frequency 60 to 120 breaths/min, driving pressure 1 to 1.5 atm (15-22 psi), and inspiratory time 15 to 20 per cent. In contrast, in acute respiratory distress syndrome (ARDS) frequency is set from 250 to 350 breaths/min, driving pressure 2 to 3 atm (30-40 psi), and inspiratory time 36 to 40 per cent. The considerable difference in these initial settings reflects the need for volume maintenance in the poorly compliant lung. In hypoxic patients, external positive end-expiratory pressure (PEEP) valves are necessary with older devices, but newer more powerful technology relies on the effective generation of intrinsic PEEP. Weaning from HFJV can be smoothly achieved by a gradual reduction in all three controls. The availability of high fresh gas flow in an open system, combined with inspiratory support, should favorably influence the work of breathing.
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