Section 6: High-Frequency Ventilators

High-frequency ventilators³⁷ are designed to provide the lowest possible tidal volume by ventilating at frequencies much higher than conventional ventilators. The idea behind this design is to minimize the risk of volutrauma. Unfortunately, after more than 30 years of clinical research, the evidence for the attainment of this goal remains controversial.

In the United States, conventional ventilators are limited to a maximum frequency of 150 breaths/minute, but they are rarely used clinically above about a fourth (adults) to a half (neonates) of that limit. In contrast, high-frequency ventilators are approved for operation at many hundreds of cycles per minute (up to 15 Hz for the 3100 high-frequency oscillator). Many models of high-frequency ventilators have come and gone over the past three decades, and they continue to evolve outside the United States.

Within the United States, there are only two basic designs for delivering low tidal volumes at high frequencies. One design is called a jet ventilator. A jet is a stream of gas forced out of a small-diameter opening (e.g., 2 mm) into a relatively larger-diameter tube (e.g., an endotracheal tube at 8 mm). This pneumatic configuration leads to what is called jet entrainment, whereby the faster-moving gas molecules in the jet stream drag along slower-moving molecules in the surrounding gas. The result is that the total gas flow delivered to the endotracheal tube is higher than the flow from the jet orifice itself. This device becomes a ventilator when the jet orifice is supplied with timed pulses of driving pressure such that the operator can control frequency and duty cycle of the jet pulses.

In simple jet ventilators, the pressure, volume, and flow delivered to the patient for any given ventilator settings are dependent on the impedance of the patient’s respiratory system. The key concept of jet ventilation is that the ventilator forces gas into the lungs, but it depends on the passive recoil of the respiratory system to “exhale” that gas. Jet ventilation is considered a form of intermittent mandatory ventilation because spontaneous breathing is uninhibited and the patient’s inspiratory efforts do not affect the frequency of mandatory breaths set on the ventilator, as with simple IMV on conventional ventilators. The flow pulsations are superimposed on the patient’s spontaneous breaths (if any).

In more sophisticated devices (e.g., the Bunnell Life Pulse ventilator), the airway pressure is adjusted by an automatic feedback control mechanism, allowing the ventilator to deliver a mode classified as pressure control intermittent mandatory ventilation with set-point targeting. The Percussionaire VDR-4 ventilator is also a jet ventilator but uses a much more complex jet mechanism composed of a “sliding venture.” In the case of the VDR-4, airway pressure is not feedback controlled, but it is intermittent mandatory ventilation with set-point targeting.

The other design of high-frequency ventilator available in the United States is known as a high-frequency oscillator. An oscillator is something like a piston or a diaphragm that forces gas both into and out of the airway, in contrast to a jet ventilator, which just forces gas into the airway and relies on passive recoil of the respiratory system to force gas out. As with simple jets, simple oscillators (e.g., the 3100 high-frequency oscillatory ventilator) do not use feedback control to maintain a consistent airway pressure waveform. Pressure, volume, and flow oscillations for a given ventilator setting are all dependent on the impedance of the respiratory system. Spontaneous breathing is generally unimpeded, and hence the mode is classified as intermittent mandatory ventilation with set-point targeting.

Bunnell Life Pulse High-Frequency Ventilator

The Bunnell Life Pulse High-Frequency Ventilator (Figure 10-77) is indicated for use in ventilating critically ill infants with pulmonary interstitial emphysema (PIE). Infants studied ranged in birth weight from 1.65 to 7.8 lb (750–3529 g) and in gestation age from 24 to 41 weeks. The Bunnell Life Pulse High-Frequency Ventilator (HFV) is also indicated for use in ventilating critically ill infants with respiratory distress syndrome (RDS) complicated by pulmonary air leaks that are, in the opinion of their physicians, failing on conventional ventilation. Infants of this description studied ranged in birth weight from 1.32 to 8 lb (600–3660 g) and in gestational age from 24 to 38 weeks. The Bunnell Life Pulse High-Frequency Ventilator is a microprocessor-controlled infant ventilating system capable of delivering and monitoring between 240 and 660 humidified high-frequency “breaths” per minute. All hardware elements in the system, from the initial gas input connection to the integral humidifier to the LifePort adapter, have been specifically designed to convey information back to the controlling software elements. Together, these elements form a fully integrated, self-regulating unit that maximizes both machine efficiency and patient safety (Figure 10-78).

Operator Interface

The operator interface of the Bunnell Life Pulse consists of LED displays and membrane switches (up and down arrows) for adjusting settings (Figure 10-79).

Modes

The Life Pulse is unique among ventilators in that it is designed to be used with a conventional infant ventilator at all times (i.e., the ventilator must provide PC-IMV with time-triggered, time-cycled mandatory breaths, PEEP/CPAP, and continuous bias flow from a separate heated humidifier). The Life Pulse produces pulses (jets) of flow that create breaths that are provided in tandem with PC-IMV of the conventional ventilator. The Life Pulse allows the operator to set the peak inspiratory pressure, the breath rate (frequency), and the jet valve on time or inspiratory time (essentially controlling the duty cycle of the jet pulse). The conventional ventilator is essential for providing gas for the patient’s spontaneous breathing, PEEP, and periodic normally sized breaths (background intermittent mandatory ventilation). The Life Pulse monitors the pressures in the patient’s airway by using a pressure transducer located in the Patient Box. It is recommended that the Life Pulse be connected to the patient and the PIP, PEEP, and mean airway pressure (MAP) values produced by the conventional ventilator be monitored using the pressure monitoring capability of the Life Pulse.

Special Features

The Life Pulse is designed to be used with a special endotracheal tube adapter (the LifePort, Figure 10-80) and a standard endotracheal tube. Unlike most conventional ventilators, the Life Pulse has been programmed to automatically establish high and low alarm limits around servo pressure, mean airway pressure, and peak inspiratory pressure and to automatically check for other alarm conditions.

Manufacturer’s Specifications

Table 10-70

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