High Frequency Ventilation
A High frequency ventilation (HFV) is a form of mechanical ventilation that uses small tidal volumes (Vts approaching deadspace volume or less) and high respiratory rates (>120 breaths/min).
B Basically four different types of high frequency approaches to ventilation have been used.
1. High frequency positive-pressure ventilation (HFPPV): This approach uses conventional ventilators to provide HFV.
a. Respiratory rate ranges from 120 to 240 breaths/min.
c. Inspiratory-to-expiratory (I:E) ratios are set at approximately 1:2.
d. Positive end-expiratory pressure (PEEP) can be set at any level.
e. Technically HFPPV is applied via conventional ventilators or pneumatic valve systems.
2. High frequency jet ventilation (HFJV): This approach normally combines the use of a jet ventilator and a conventional ventilator.
a. Respiratory frequency ranges from 120 to 1200 breaths/min or 2 Hz to 20 Hz. One hertz (Hz) equals 60 breaths/min.
b. Vt is estimated to be approximately 2 to 5 ml/kg.
c. The I:E ratio is set at approximately 1:2.
d. PEEP can be set at any level.
e. All systems use a ventilator specifically designed for HFJV and a secondary gas source, commonly a conventional ventilator.
f. Each ventilation system uses a 14- to 18-gauge small-bore injector in which gas is periodically introduced at high pressure (15 to 50 pounds per square inch) into a specially designed endotracheal tube adapter (Figure 42-1). However, the small-bore injector can also be inserted directly into the cricothyroid membrane.
g. Gas is also entrained from a secondary source, mixing with the injector flow to establish Vt (see Figure 42-1).
h. Exhaled gas leaves via a separate route (see Figure 42-1).
i. Gas flow through the injector is interrupted at clinician-set intervals by pneumatic, fluidic, or electronically controlled solenoid valves.
j. With all systems exhalation is passive.
k. The jet flow is humidified by water dripped in front of the injector. The entrained gas is normally humidified.
3. High frequency oscillation (HFO)
a. The most common approach to HFV used today is HFO.
b. Requires a unique self-contained system to provide ventilation
c. Basically a bias flow of gas passes across the airway, exiting through a variable resistor.
d. A mechanism to oscillate the flow is included perpendicular to the bias flow (Figure 42-2).
e. The oscillation causes the inspiratory phase to be positive pressure and the expiratory phase to be negative pressure.
f. The force of the oscillation is referred to as the delta pressure: The total pressure change from inspiration to expiration. If the delta pressure is 60 cm H2O, +30 cm H2O is developed during inspiration and −30 cm H2O is developed during expiration. This is above and below the mean airway pressure.
g. The resistance to the bias flow and the total bias flow controls the mean airway pressure.
h. With some systems the I:E ratio can be adjusted from 1:1 to 1:2.
i. Frequency can vary with most HFO systems from 3 Hz to 20 Hz.
j. As with other ventilatory systems, FIO2 can be adjusted from 0.21 to 1.0.
k. PEEP is not set during HFO. However, during HFO especially at high frequency settings, the mean airway pressure is essentially equal to the PEEP level because the delta pressure is markedly decreased by the time it reaches the alveolar level.
l. High frequency flow interruption is a form of HFO that does not have a negative expiratory phase. The bias flow is simply interrupted to allow passive exhalation to occur.
4. High frequency percussive ventilation (HFPV)
a. This approach to HFV is truly a combination of conventional ventilation and HFV.
b. As noted in Figure 42-3 gas is delivered as a pressure-limited conventional breath with oscillations superimposed on the breath.
c. Oscillations can be delivered either during inspiration only or during inspiration and expiration. Throughout the ventilatory cycle patients may breathe spontaneously in association with the conventional and high frequency breaths.
d. Thus the conventional gas delivery and high frequency approaches to ventilation can be adjusted.
e. HFPV operates by using a high frequency injector that injects gas flow to operate a sliding Venturi valve used to entrain a secondary humidified flow of gas (Figure 42-4).
