Technically, any ventilator that operates from a battery and either an internal gas source or a compressed gas cylinder could be considered a transport ventilator. Because of the demands of patient transport, however, these simple criteria are inadequate. Using performance to discriminate transport ventilators yields three categories: automatic resuscitators, pneumatically powered transport ventilators, and sophisticated transport ventilators.
Automatic resuscitators provide ventilation at a set pressure. The user has limited control over respiratory parameters, and some do not allow any control of respiratory rate or tidal volume. The ability to apply PEEP is limited or nonexistent. As a function of pressure cycling, the respiratory rate and tidal volume vary with lung compliance. When compliance is low, tidal volume is small and frequency rapid; when compliance is high, tidal volume is high and frequency slow. These devices are powered pneumatically (requiring no electricity), and have only a mechanical, audible high-pressure alarm, and disposable pressure manometers. Spontaneous ventilation requires entrainment of ambient air, which reduces FIO2. Spontaneous respirations will cause dyssynchrony and increased work of breathing because of limited inspiratory flow rate and difficulty triggering. Automatic resuscitators are designed for use in the prehospital setting by personnel with limited expertise in mechanical ventilation.
The Vortran and Oxylator EM-100 are inexpensive examples of automatic resuscitators (Fig. 27-1). The Vortran has proven unreliable when the orientation is changed, failing to cycle.65,66 In the presence of a leak, if the set peak pressure cannot be reached, both devices will remain stuck in inspiration. Despite these limitations, successful use of the Vortran in intubated, closely monitored subjects has been reported.67 Our experience with the Vortran, however, suggests it is unsuitable for transport of patients with even minor lung disease.
Examples of automatic resuscitators, the Vortran (left) and Oxylator (right).
Simple Transport Ventilator
The simplest transport ventilators provide mechanical ventilation at a specified rate and volume and include a pressure-relief valve with an audible mechanical alarm. Most are powered and controlled pneumatically. In some instances, a battery allows for simple low-presure and high-pressure alarms, as well as monitoring and display of airway pressure. Simple transport ventilators also are used primarily in prehospital settings by personnel with some training in mechanical ventilation. More complex than automatic resuscitators, they offer respiratory rate and tidal volume adjustment and can be used with spontaneously breathing patients.
Examples of Simple Transport Ventilators
The AutoVent 2000 (Fig. 27-2) and 3000 are pneumatically powered transport ventilators operating in the intermittent mandatory ventilation mode. FIO2 is 100% and cannot be adjusted, and PEEP is not available. Mandatory breaths are time-triggered, flow-limited or pressure-limited, and time-cycled, while spontaneous breaths are pressure-triggered, pressure-limited, and pressure-cycled. During spontaneous breaths, the patient breathes from the demand valve at 48 L/min. If patient demand exceeds 48 L/min, ambient air is drawn into the valve, diluting the FIO2. There is no monitoring. An audible alarm sounds if airway pressure exceeds 50 cm H2O.
The AutoVent 2000 simple transport ventilator.
The AutoVent 2000 is designed for adults and allows control of breath rate and tidal volume. The AutoVent 3000 is designed for adults and children and allows the user to control inspiratory time. The AutoVent 4000 offers additional alarms, manometer, adjustable airway pressure relief valve, and continuous positive airway pressure (CPAP) mode; it allows selection of FIO2 0.65 or 1. The ability of the AutoVent 4000 to provide a consistent FIO2 is altered by reduced compliance. As the lung becomes stiffer, the venture mechanism reduces air entrainment and FIO2 increases while tidal volume falls.
The AutoVent ventilators consume approximately 0.5 L/min of gas to operate the logic. The higher the frequency setting, the higher is the gas consumption. The inspiratory flow is fixed at 48 L/min. In patients with an active respiratory drive, the flow capabilities of the AutoVent devices may result in flow starvation, increased work of breathing, and asynchrony.
The Uni-Vent 706 (Fig. 27-3) is a pneumatically powered, electronically controlled ventilator delivering an FIO2 of 1. Controls set inspiratory flow and a series of rate, inspiratory time, and inspiratory-to-expiratory timing (I:E) ratio combinations appropriate for adult and pediatric ventilation. Inspiratory flow is limited from 0 to 90 L/min, and a high-pressure limit can be set (60 or 80 cm H2O) at the patient valve, which activates an audible alarm when exceeded. Prehospital use of the Uni-Vent 706 by paramedics during cardiopulmonary resuscitation of intubated victims has proven successful.68
The Uni-Vent 706 simple transport ventilator.
Sophisticated Transport Ventilator
A sophisticated transport ventilator is capable of performance comparable to an ICU ventilator. These devices may have built-in compressors or turbines to generate positive pressure without compressed gas and contain an air–oxygen blender. Most offer extensive control of ventilator parameters and comprehensive alarms. Sophisticated transport ventilators are intended for interhospital or intrahospital transport of critically ill patients. These devices should be capable of ventilating the sickest patients.
Examples of Sophisticated Transport Ventilators
The IC-2A is a flow controller that can be triggered by pressure or time or manually; it is also pressure- or flow-limited and time-cycled (Fig. 27-4). It requires a compressed-gas source for delivery to the patient, as well as for the fluidic logic circuit. The IC-2A delivers an FIO2 of 1. Controls include the mode of ventilation, inspiratory and expiratory time, inspiratory flow, sensitivity, and PEEP/CPAP level. Airway pressure is displayed on an aneroid pressure gauge. The IC-2A is often used for ventilation in the magnetic resonance imaging scanner because it has no ferrous components. The excessive gas consumption should be considered when gas supplies are limited. The IC-2A operation during synchronized intermittent mandatory ventilation limits the flow delivered to the set flow on the ventilator, potentially increasing the work of breathing. Our experience with this ventilator suggests most patients have to be sedated to tolerate the operational limitations.
The MRI compatible IC-2A.
The LTV 1200 offers the performance of a critical care ventilator at a size and weight appropriate for a transport ventilator (Fig. 27-5). It provides synchronized intermittent mandatory ventilation, controlled mechanical ventilation, pressure support, and CPAP modes. Pressure-controlled and volume-controlled breaths are possible. The integral turbine permits mechanical ventilation without a compressed gas source. Oxygen can be supplied from a low-flow or compressed-gas source. The user has extensive control of ventilator parameters. Airway pressure, total breath rate, exhaled tidal volume, total minute volume, I:E ratio, calculated peak flow, and patient effort are monitored and displayed. The bias flow during exhalation (10 L/min) increases gas consumption and should be considered when planning transports. The LTV 1200 offers additional features, including ventilator presets, and an oxygen conservation feature to decrease gas consumption during transport. The LTV 1200 also utilizes an electronic control of PEEP, compared to the a spring-loaded valve used with the LTV 1000. This allows the LTV 1200 to apply pressure breaths relative to baseline pressure.
The LTV 1200 sophisticated transport ventilator.
The Uni-Vent 754 is an electrically powered flow or pressure controller offering controlled mechanical ventilation, synchronized intermittent mandatory ventilation, and CPAP modes of ventilation. The internal compressor allows use without a compressed gas source. The operator can set mode of ventilation, breath rate, inspiratory time, tidal volume, sensitivity, PEEP, FIO2, and peak inspiratory pressure limit. Airway pressure is displayed, as well as the airway pressure waveform, ventilator breath rate, inspiratory time, I:E ratio, tidal volume, FIO2, mean airway pressure, and baseline airway pressure. A full set of alarms is present. Inspiratory flow is limited to 60 L/min, which can cause dyssynchrony in spontaneously breathing patients. The inability to provide pressure-limited breaths is a minor limitation. The Uni-Vent 754 has a proven track record in transporting ventilated military casualties by U.S. Air Force critical care, air transport teams.
The Uni-Vent 731 transport ventilator (Fig. 27-6) is the next generation of Impact ventilators and is an electrically powered device that offers multiple improvements over the Uni-Vent 754. Most notable is the ability to provide pressure-limited breaths, integral pulse oximeter, and a 10-hour battery life. The peak flow in volume ventilation has also been increased to 100 L/min. The 731 has the capability to provide closed-loop control of inspired oxygen concentration using the pulse oximeter input. The target arterial oxyhemoglobin saturation (SpO2) can be set between 92% and 99%. If SpO2 falls below 88%, the FIO2 is increased rapidly to alleviate hypoxemia. This system has potential important advantages during transport when physiologic changes occur quickly and access to the patient may be limited. At the time of writing, this system is not available in the United States.
The Impact 731 sophisticated transport ventilator.