Chapter 10: High Frequency Ventilation

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.

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.

At the rates provided with HFJV and HFOV, tidal volumes are often smaller than those with conventional ventilation. Generally speaking, the higher the respiratory rate, the lower the tidal volume. At moderate to high rates (≥ 8 Hz), tidal volumes can be less than anatomic dead space.

Although numerous mechanisms are active during HFV to establish gas exchange (Figure 10-3), normal convection and molecular diffusion are the primary mechanisms affecting gas exchange. Other principles enhance molecule diffusion and the dispersion of gases in the airway. With HFPV, tidal volume is also dependent on the pressure control setting.

Frequency, I:E ratio, and pressure amplitude are the three variables affecting ventilation during HFOV. Tidal volume during HFOV is also affected by rate. As rate is decreased, tidal volume increases and vice versa. Pressure amplitude (the pressure developed as the oscillator forces gas into the airway) and a longer inspiratory time also increase the tidal volume (Table 10-2). Neonates are generally ventilated at high rates (10-15 Hz) and low-pressure amplitudes (20-30 cm H₂O), which generate very small tidal volumes. Adults are ventilated at lower rates (3-8 Hz) and higher pressure amplitudes (60-90 cm H₂O). With neonates, the bias flow in the circuit is about 10 L/min, whereas with adults the bias flow is about 30 L/min range. In adults, tidal volumes approaching those delivered by conventional mechanical ventilation (3-4 mL/kg) are possible at 3 Hz and 90 cm H₂O pressure amplitude. During HFOV, it is not possible for the patient to trigger the ventilator; heavy sedation, and sometimes paralysis, is needed to ensure patient-ventilator synchrony. In the setting of a high Paco₂ and maximal ventilator settings, the cuff may be deflated, which clears CO₂ from the central airway and endotracheal tube. Oxygenation with HFOV is determined by Fio₂ and mean airway pressure (P̄aw). P̄aw is similar to positive end-expiratory pressure (PEEP) in its effect on oxygenation, since at the alveolar level there is minimal pressure change during HFOV (particularly at high rates).

It is estimated that as little as 15% of the inspiratory pressure amplitude is transmitted to the alveolar level through an 8-mm internal diameter endotracheal tube at 8 Hz. With smaller tubes and more rapid rates, less pressure is transmitted. With a pressure amplitude of 60 cm H₂O (30 cm H₂O above and 30 cm H₂O below P̄aw during inhalation and exhalation, respectively) at 8 Hz with an 8-mm internal diameter tube, the alveolar pressure would fluctuates 4.5 cm H₂O above and below the P̄aw. A lung recruitment maneuver is often performed when HFOV is initiated, by increasing P̄aw for a short time and then decreasing it to the lowest level that maintains oxygenation. In adults, P̄aw during HFO are generally set at 25 and 35 cm H₂O.

With HFJV the tidal volume is affected by the gas that is entrained via the conventional ventilator. Estimation of tidal is currently not possible during HFOV and HFPV. With HFJV and HFPV, oxygenation is controlled in the same manner as during conventional ventilation. PEEP and Fio₂ are set to achieve the desired oxygenation status.

With HFOV, overdistending peak airway pressures and P̄aw airway pressure applied tends to be much higher than what is used during conventional ventilation. In addition, if low respiratory rates are used (3-6 Hz), the attenuation of the pressure amplitude at the alveolus may not be as great as expected and alveolar pressures may greatly exceed 30 cm H₂O, particularly if the P̄aw is ≥ 30 cm H₂O. With HFJV and HFPV, it is difficult to estimate the peak alveolar pressure. During jet ventilation, the higher the pressure generating the jet flow, the higher is the potential peak airway pressure. HFPV has been proposed as a mechanism to move secretions more effectively along the tracheobronchial tree.

HFOV is used in adults with severe acute respiratory distress syndrome (ARDS) and refractory hypoxemia. However, evidence is lacking for the superiority of HFOV to conventional ventilation, and there is some evidence that survival with HFOV may be worse than with conventional ventilation. Thus, the improved oxygenation with HFOV does not translate to better outcomes. Issues with the use of HFOV are the high cost of current devices, that patients receiving HFOV must be heavily sedated and sometimes paralyzed, and that most clinicians lack familiarity with HFOV. In addition, during HFOV, there is no mechanism to monitor peak alveolar pressure or tidal volume. The available devices in the United States may require a significantly greater number of arterial blood gases than with conventional ventilation. Other than the treatment of severe ARDS, there is no role for HFOV in the management of adults.

In adults, HFJV has been reserved for intraoperative use and for percutaneous transtracheal ventilation when an airway cannot be established. HFJV can be used as a method of providing ventilatory support during tracheal or bronchial surgery. HFJV may also be used in a trauma setting where tracheal intubation is difficult or impossible. The use of transtracheal jet ventilation via the cricothyroid membrane is part of the difficult airway algorithm.

HFPV has found a niche in some burn centers for the management of patients with ARDS. Some believe the addition of oscillations on top of the pressure control facilitates movement of secretions to the larger airways, allowing for their suctioning. However, there is no convincing evidence that the use of HFPV improves outcome over conventional ventilation.