Ventilators, Portable

Purpose

Portable ventilators provide long-term ventilatory support for patients who do not require complex criticalcare ventilators. These portable units are commonly used in special extended care facilities, in step-downrespiratory care units, or in the home. They can also be used for short-term transport or in emergencies.

Mechanical ventilators compensate for deficiencies in normal breathing by aiding or augmenting spontaneous breathing or by completely regulating a prescribed breathing pattern for a patient who cannot breathe without assistance. These units provide-positive pressure ventilation (PPV) to produce adequate alveolar gas exchange. Unlike physiologic breathing in which the diaphragm contracts, producing negativepressure in the thoracic cavity to expand the lungs, PPV inflates the lungs by producing positive pressure2 at the airway opening. The resulting pulmonary ventilation maintains normal arterial carbon dioxide (CO2) and oxygen (O2) levels. During the inspiratory cycle of ventilation, the exhalation valve is closed, positive pressure develops in the breathing circuit, and gas is forced into the patient’s lungs. During the expiratorycycle, the exhalation valve is open, and gas is passively released from the patient’s lungs.

Patients who require long-termmechanical ventilation include adults and children who have impaired ortotal loss of ventilatory function resulting from various etiologies — such as neuromuscular diseases, restrictive and chronic obstructive lung diseases, and spinal cord injuries — as well as children born prematurely or with neonatal lung disease.

Advantages of using mechanical ventilation in the home rather than in the hospital include decreased exposure to nosocomial infections, increased mobility, improved nutritional status, resumption of more normal interactions and routines of daily living, and lower healthcare costs. Most portable ventilators are simple to operate and normally do not have the sophisticated controls necessary for patients in intensive care units and neonatal intensive care units, but they typically have more monitors and/or alarms, as well as more mode flexibility, than transport ventilators.

All portable ventilator systems include the appropriate controls for setting operating modes and alarms;somesystems also include special breathing circuits,O2 accumulators, and heated humidifiers or heat/moisture exchangers (HMEs). Power can be supplied through an alternating-current (AC) line, an external battery (e.g.,a car battery), or an internal battery.

Portable ventilators deliver room air or O2-enriched gas to the inspiratory limb of the breathing circuit,where it can be humidified by a heated humidifier or an HME before delivery to the patient. Typically, these ventilators drive air into the breathing circuit with a motor-driven piston or turbine. For O2 enrichment, most portable ventilators use an accumulator, which collects O2 and mixes it with air before it is drawn into the ventilator; alternatively, O2 can be delivered directly into the breathing circuit from a separate source, such as an O2 tank.

Passive one-way valves within the ventilator allow the patient to breathe spontaneously. Placing an H-valve in the inspiratory limb reduces the work required for the patient to breathe spontaneously — called the work of breathing (WOB) — by allowing the patient to inhale room air directly through an open port in the valve instead of through the valves and tubing inside the ventilator.

Flexible tubing is used near the patient connection to reduce the traction on the trachea from the tracheostomy cannula, which is the typical route forventilation gas to enter and leave the lungs. Nasal ventilation delivered through a face mask is sometimesused, usually for patients who require ventilation only at night. The exhalation valve, which occludes the exhalation limb during inhalation to divert gas into the patient’s lungs and opens during exhalation to release the exhaled breath, is located close to the patient connection. This eliminates the need for an exhalation hose and simplifies the breathing circuit.

Control of ventilation

To closely simulate physiologic breathing and to give clinicians more flexibility in ventilating the patient,portable ventilators offer several ventilation modes. The control mode provides full support to patientswho cannot breathe for themselves. In this mode, the ventilator provides mandatory breaths at preset time intervals, and the patient cannot breathe spontaneously. This mode requires the patient to be paralyzed, unconscious, or sedated to prevent spontaneous attempts to breathe against the ventilator. Assist/control modes also provide full support by delivering an assisted breath whenever the ventilator senses a patient’s inspiratory effort and by delivering mandatory breaths at preset time intervals when the assisted-breath rate falls below a preset ventilatory rate. Mandatory breaths can be synchronized with the start of a patient’s inspiratory efforts using the synchronized intermittent mandatory ventilation (SIMV) mode. SIMV delivers controlled breaths at a set frequency and also allows the patient to breathe spontaneously with no assistance during the periods between the controlled breaths. The mandatory breaths in this mode can be volume controlled and are synchronized with a spontaneous breathing effort if that effort occurs sufficiently close to the time the mandatory breath would have been produced. This reduces the possibility of overinflation, which could result from stacking amandatory breath on a spontaneous breath. As the patient’s spontaneous minute volume increases, the clinician can reduce the set mandatory breath rate and gradually wean the patient from PPV. Portable ventilators use various methods of cycling from inspiration into exhalation. In volume-cycled ventilation, the ventilator cycles into exhalation when a preset tidal volume (the volume of a single breath) has been delivered. In time-cycled ventilation, the ventilator cycles into exhalation after a preset time. A variant of volume- or time-cycled ventilation is volume- or time-cycled pressure-limited ventilation. A mechanical pressure-relief valve that vents and hence limits excess pressure is used on the inspiratory side. When the inspiratory pressure reaches the set level, excess gas is vented, and the inspiratory pressure remains at the set level until the end of inspiration. This is sometimes useful for patients with an uncuffed tracheostomy tube because the volume can be set at a level that compensates for leakage around the tubewithout generating an excessive pressure. At least three variables must be set independently to produce a prescribed breathing pattern; they can be appropriately chosen from the following: the length of the inspiratory or expiratory phase, the ratio of inspiratory time to expiratory time (I:E ratio), the respiratoryrate, the tidal volume, the minute volume, and the average inspiratory flow. For example, the selectedtidal volume, rate, and inspiratory time determine minute volume, flow, expiratory time, and I:E ratio.Many ventilators require the user to set at least the tidal volume, rate, and peak inspiratory flow. Applying constant positive end-expiratory pressure (PEEP), a positive rather than an ambient pressure at the end of exhalation, raises the functional residual capacity, or the lung volume at rest. A separate PEEP valve can be attached to the exhalation valve for controlled breaths. PEEP enhances gas exchange in the lungs and decreases right-to-left pulmonary shunting by preventing alveolar collapse and increasing lung volumes.

Monitors and alarms

Portable ventilators monitor airway pressure and have adjustable low- and high-pressure alarm limits.Airway pressures are measured at the patient connection of the breathing circuit; measuring at the patientconnection produces measurements more reliable than those obtained at other points in the breathing circuit.The high-pressure alarm can warn of peak inspiratory pressure increases caused by decreases in patient compliance, breathing circuit occlusion, or increases in airway resistance (e.g., resulting from a buildup ofsecretions in the airway). The low-pressure or lowminute- volume alarm can warn of a breathing-circuitdisconnection, a leak, or a failure to deliver a breath. Delivery of an inappropriately high tidal volume orocclusion of the breathing circuit’s expiratory limb can result in overdistention of alveoli, which can causepulmonary volutrauma (damage produced by exposing the lungs to excessively high pressure differences).Airway pressure should not exceed the pressure-relief or high-pressure setting, which should be properlyadjusted to protect the lungs. The ventilator should, preferably, relieve pressure in the breathing circuit toambient pressure. Additional monitors are available in case the pressure alarms fail to operate. An exhaled-volume monitor, which can be connected to the exhalation valve, is used to verify minute volume and warn of circuit disconnections or leaks. Many units have remote alarms that are used at distant locations to relayventilator alarms to a caregiver. Some ventilators now include exhaled-volume monitors and apnea alarms.Battery powerSwitchover to battery operation (either internal or external) is usually automatic and is signaled by analarm. A large-capacity external battery (e.g., 12 VDC) is used to supply power during portable operation or extended AC power loss. An internal battery allows temporary backup (usually up to one hour) in the event of external battery depletion and, if the external battery is disconnected, can be used briefly as a power sourcewhile the ventilator user moves to another area,such as the lavatory.

Source: ECRI