HFJV in ARDSmaintaining the mean airway pressure

Ventilatory strategies in ARDS have evolved considerably over the last decade. Lung-protective ventilatory strategies, in which high peak inspiratory pressures and alveolar overdistension are avoided, are now universally recommended. In ARDS specifically, lung volume must be maintained and end-expiratory collapse avoided, not least because low levels of end-expiratory pressure may be associated with ventilator-induced lung damage. Because of its small pulse volumes and short cycle times, HFV theoretically provides an ideal method of achieving effective ventilation while avoiding overdistension and maintaining alveolar volume.

HFJV was first recommended in 1982 as the ideal available ventilatory mode for patients with ARDS. In 1983 a prospective randomized cross-over comparison of HFJV versus conventional ventilation in 309 patients with acute lung injury reported that, while safe, predictable, and effective, HFJV appeared to offer no particular advantage (Carion.efa/ 1983). This frequently quoted study considerably dampened enthusiasm for HFJV. However, interpretation at the time did not take into account the lack of lung volume recruitment techniques or attempts to maintain lung volume in either treatment arm.

In 1986 a new HFJV device was developed specifically for ARDS with much greater ventilatory capacity to overcome the inability of older technology to achieve adequate gas exchange in poorly compliant lungs. Features included a rapid-response low-inertia solenoid switching valve which generated an effective square wave as described above. The T-piece entrainment chamber was placed at the proximal end of the endotracheal tube, providing an entrainment ratio of up to 200 per cent, thus greatly enhancing ventilatory capacity as well as allowing effective humidification ( Fig 1). Animal experiments indicated that the amplitude of airway pressure changes was rapidly attenuated along the divisions of the respiratory tree, and that, at the 2-mm airway, airway pressure fluctuations were minimal around a value equivalent to the centrally measured mean airway pressure.

The response to this device in 90 patients with severe ARDS was reported by GJuck. efa/ (19.9.3.).. Lung volume, and hence oxygenation, was maintained by application of the appropriate mean airway pressure in each patient. Significant improvements in arterial oxygenation and reductions in peak and mean airway pressures were observed in the context of excellent CO2 clearance. The study was a non-randomized prospective assessment in which each patient served as his or her control prior to switching to HFJV. Survival data in such a study are difficult to interpret, but in this group, considered as failing conventional support, survival was 58 per cent. With the addition of further patients, it was observed that 19 of 25 patients (76 per cent) who were switched to HFJV within 48 h of ventilation survived. This suggests that early intervention with HFJV may favorably influence outcome. A recent report in 29 pediatric patients with ARDS also noted that survivors (69 per cent) spent significantly less time on conventional ventilation, again suggesting that early introduction of HFJV may limit ventilator-induced lung damage ( Smith et al. 1993).

This potent HFJV ventilator now has market clearance from the United States Food and Drugs Administration regulatory body for use in ARDS and is commercially available (Adult Star 1010, Nellcor Puritan Bennett, California, USA). It is currently used enthusiastically in some centers; with wider availability, its use in severe ARDS is likely to increase.

The use of an effective HFJV device whereby mean airway pressure is maintained is based on similar principles to that of pressure-controlled inverse ratio ventilation

(PC-IRV). Both modes harness the lung volume recruitment capabilities of intrinsic PEEP while operating at different ends of the frequency spectrum ( Fig 2). Peak pressure limitation is inherent in pressure-controlled inverse ratio ventilation and results indirectly from equilibration of directional gas flows with HFJV. Theoretical advantages which can be proposed for HFJV over pressure-controlled inverse ratio ventilation include the following.

1. The minimum alveolar pressure for a given central airway pressure is higher, hence enhancing arterial oxygenation.

2. The peak alveolar pressure for a given airway pressure is lower, hence limiting alveolar volutrauma (in pressure-controlled inverse ratio ventilation the alveolus is exposed to the central limiting peak pressure, whereas in HFJV the peak inspiratory pressure is considerably attenuated at alveolar level).

3. With HFJV, physiotherapy and airway toilet procedures can be performed without disconnection from ventilation and subsequent lung volume de-recruitment (relevant in many patients in whom optimal lung volume recruitment may take many hours, or in those returned to supine following prone repositioning as an alveolar recruitment maneuver).

Healthy Fat Loss For A Longer Life

Healthy Fat Loss For A Longer Life

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