Chapter 10: High Frequency Ventilation

Introduction

High frequency ventilation (HFV) has been available since the late 1960s, but was not an accepted approach to ventilatory support in adults until the late 20th century. The primary reason is a lack of definitive evidence indicating HFV's superiority over conventional ventilation. Although there have been many animal studies demonstrating a physiologic benefit for HFV, evidence is lacking for a survival benefit in adults with high frequency oscillation (HFO). Equivalence has become even more pronounced with the use of lung protective approaches to conventional ventilatory support. Recent evidence indicates that HFV may be detrimental when used with high airway pressures, similar to established concerns with conventional ventilation.

Approaches Available

Conventional mechanical ventilation is provided at respiratory rates < 1 Hz (1 Hz = 60 breaths/min). HFV uses respiratory rates at 2 to 15 Hz. The frequency range is determined by the specific technique and the size of the patient. Regardless of technique, adults are generally ventilated toward the lower end of the respiratory rate spectrum and neonates toward the high end of the spectrum.

There are four techniques for HFV: high frequency positive pressure ventilation (HFPPV), high frequency jet ventilation (HFJV), high frequency oscillatory ventilation (HFOV), and high frequency percussive ventilation (HFPV) (Table 10-1). With HFPPV, conventional ventilators are used to provide rates at the low end of the HFV spectrum. With HFPPV rates slightly above, conventional rates are used and gas flow is provided by the same gas delivery mechanism as in conventional ventilation. This form of HFV is not commonly used. During HFJV, gas under high pressure is injected into the airway while a secondary gas source is entrained to provide the tidal volume. This approach uses respiratory rates in the low to middle part of the HFV rate spectrum. With HFJV, a jet ventilator in tandem with a conventional ventilator may be needed. HFOV has an active inspiratory and expiratory phase. HFOV establishes gas flow into the airway by the rapid movement of a diaphragm or piston (Figure 10-1). With HFOV, respiratory rates at 3 to 8 Hz are used with adults, at the highest rate that allows an adequate Paco₂. The most commonly used high frequency technique is HFOV. With HFPV, small tidal volume oscillations are superimposed on conventional pressure-controlled ventilation (Figure 10-2) at rates in the 2 to 8 Hz range.