High-frequency ventilation (active) — HFV-A is notable for the active exhalation mechanic included. Active exhalation means a negative pressure is applied to force volume out of the lungs. The CareFusion 3100A and 3100B are similar in all aspects except the target patient size. The 3100A is designed for use on patients up to 35 kilograms and the 3100B is designed for use on patients larger than 35 kilograms.
CareFusion 3100A and 3100B High-frequency oscillatory ventilation was first described in 1972 and is used in neonates and adult patient populations to reduce lung injury, or to prevent further lung injury. HFOV is characterized by high respiratory rates between 3.5 and 15
hertz (210 - 900 breaths per minute) and having both inhalation and exhalation maintained by active pressures. The rates used vary widely depending upon patient size, age, and disease process. In HFOV the pressure oscillates around the constant distending pressure (equivalent to mean airway pressure [MAP]) which in effect is the same as
positive end-expiratory pressure (PEEP). Thus gas is pushed into the lung during inspiration, and then pulled out during expiration. HFOV generates very low tidal volumes that are generally less than the dead space of the lung. Tidal volume is dependent on endotracheal tube size, power and frequency. Different mechanisms (direct bulk flow - convective, Taylorian dispersion,
Pendelluft effect, asymmetrical velocity profiles, cardiogenic mixing and molecular diffusion) of gas transfer are believed to come into play in HFOV compared to normal mechanical ventilation. It is often used in patients who have refractory hypoxemia that cannot be corrected by normal mechanical ventilation such as is the case in the following disease processes: severe ARDS, ALI and other oxygenation diffusion issues. In some neonatal patients HFOV may be used as the first-line ventilator due to the high susceptibility of the premature infant to lung injury from conventional ventilation.
Breath delivery The vibrations are created by an electromagnetic valve that controls a piston. The resulting vibrations are similar to those produced by a stereo speaker. The height of the vibrational wave is the amplitude. Higher amplitudes create greater pressure fluctuations which move more gas with each vibration. The number of vibrations per minute is the frequency. One Hertz equals 60 cycles per minute. The higher amplitudes at lower frequencies will cause the greatest fluctuation in pressure and move the most gas. Altering the % inspiratory time (T%i) changes the proportion of the time in which the vibration or sound wave is above the baseline versus below it. Increasing the % Inspiratory Time will also increase the volume of gas moved or tidal volume. Decreasing the frequency, increasing the amplitude, and increasing the % inspiratory time will all increase tidal volume and eliminate CO2. Increasing the tidal volume will also tend to increase the mean airway pressure.
Settings and measurements Bias flow The bias flow controls and indicates the rate of continuous flow of humidified blended gas through the patient circuit. The control knob is a 15-turn pneumatic valve which increases flow as it is turned.
Mean pressure adjust The mean pressure adjust setting adjusts the mean airway pressure (PAW) by controlling the resistance of the airway pressure control valve. The mean airway pressure will change and requires the mean pressure adjust to be adjusted when the following settings are changed: • Frequency (Hertz) • % Inspiratory time • Power and Δp change • Piston centering During high-frequency oscillatory ventilation (HFOV), PAW is the primary variable affecting oxygenation and is set independent of other variables on the oscillator. Because distal airway pressure changes during HFOV are minimal, the PAW during HFOV can be viewed in a manner similar to the
PEEP level in conventional ventilation. The optimal PAW can be considered as a compromise between maximal lung recruitment and minimal overdistention.
Mean pressure limit The mean pressure limit controls the limit above which proximal PAW cannot be increased by setting the control pressure of the pressure limit valve. The mean pressure limit range is 10-45 cmH2O.
ΔP and amplitude The power setting is set as amplitude to establish a measured change of pressure (ΔP). Amplitude/Power is a setting which determines the amount of power that is driving the oscillator piston forward and backward resulting in an air volume (
tidal volume) displacement. The effect of the amplitude on the ΔP that it is changed by the displacement of the oscillator piston and hence the oscillatory pressure (ΔP). The power setting interacts with PAW conditions existing within the patient circuit to produce the resulting ΔP.
% Inspiratory time The percent of inspiratory time is a setting which determines the percent of cycle time the piston is traveling toward (or at its final inspiratory position). The inspiratory percent range is 30—50%.
Frequency The frequency setting is measured in hertz (hz). The control knob is a 10-turn clockwise-increasing potentiometer covering a range of 3 Hz to 15 Hz. The set frequency is displayed on a digital meter on the face of the ventilator. One Hertz is (-/+5%) equal to 1 breath per second, or 60 breaths per minute (e.g., 10 Hz = 600 breaths per minute). Changes in frequency are inversely proportional to the amplitude and thus delivered
tidal volume. ;Breaths per minute (f): :f = Hz \cdot 60_{seconds}
Oscillation trough pressure Oscillation trough pressure is the instantaneous pressure within the HFOV circuit following the oscillating piston reaching its complete negative deflection. :OTP = MAP - (AMP/3) ==Transtracheal jet ventilation==