As medical technology continues to advance and the drive toward cost savings continues, an increased number of critically ill patients are being discharged from the hospital to the home setting. One of the most dramatic changes in home care is the sharp rise in the number of patients being discharged to their home on portable home ventilators. The home care ventilator patient now covers all age groups, requiring the Respiratory Therapist to have a thorough background in the variety of disease processes and ventilator management techniques, along with a thorough knowledge of the variety of ventilators and associated equipment required to provide ventilatory life support in the home.

The purpose of this article is to review the different portable ventilators available in the home setting. Ventilators, whether hospital or home based, can be divided into two categories: Negative Pressure Ventilators and Positive Pressure (Volume) Ventilators.

The iron lung consists of a large metal cylinder with flexible diaphragm operated by a piston rod at the distal end. This ventilator has the capacity to develop intra-tank pressure of up to -25 to -30 cmH2O and a respiratory rate between 10 and 30 breaths per minute. Although the iron lung is the most effective means of negative pressure ventilatory support, it is quite large, requiring a considerable amount of operating space. It prevents easy accessibility to the patient and is very uncomfortable for many patients.

The cuirass ventilator consists of a rigid shell, some models consist of shells of either reinforced plastic or fiberglass which extend from the symphysis pubis to the suprasternal notch. The cuirass is sealed to the chest and abdominal walls with a flexible rubber diaphragm. Correct fitting of the cuirass is imperative for its proper functioning. The cuirass must be connected to a power source capable of generating -25 to -30 cmH2O pressure. Since the cuirass only covers the chest area the patient can be ventilated in either the sitting or supine position, therefore making the cuirass ventilator the least confining of the negative pressure ventilators. If the cuirass fits too loosely, poor sealing of the cuirass to the body surface will result in delivery of inadequate tidal volumes. However, excessive tightening of the straps or a too small cuirass can result in patient discomfort, restriction of chest wall motion, and skin irritation. Due to the small tidal volumes generated by this unit in patients with normal chest wall and lung compliance, the cuirass is not capable of providing complete ventilatory support in apneic patients.

The body wrap consists of a one-piece plastic suit, a flat rigid plate, and a large plastic grid. The patient lies supine on the plate which extends the length of the thoracic spine. The grid is placed over the patient's thorax and the separate power source is connected to the mid-portion of the suit by a plastic tube. The grid causes the negative pressure generated within the suit to be applied predominately to the thorax since the plastic is sucked tightly against all other body parts. The body wrap is portable, lightweight and much more comfortable than the iron lung. The body wrap does not allow access to the patient without interruption of ventilatory support, and generally only provides modest tidal volumes. It can only be used on patients who are not totally dependent on ventilatory support and can maintain unassisted ventilation for prolonged periods of time.

The major advantage of negative pressure ventilation is that it eliminates the need for tracheal intubation usually required for positive pressure ventilatory supports: therefore decreasing the number of complications and discomforts of tracheal intubation. The patient is also able to verbally communicate with family and medical personnel while being mechanically ventilated.

The disadvantages of negative pressure ventilation will of course depend on the type of negative pressure ventilator being used. Negative pressure ventilation is not suitable for all patients. Negative pressure ventilation is not suitable for all patients. Negative pressure ventilators cannot sustain adequate ventilation for prolonged periods in apneic patients. Other disadvantages include the fact that regulation of inspiratory flow rates and cycle duration is not possible. lncreased thoracic elastance may prevent generation of adequate tidal volumes. The lack of complete accessibility to the patient impedes the performance of adequate chest physiotherapy. The lack of control of the upper airway places patients with disorders of upper airway function at increased risk of aspiration of gastric contents and upper airway obstruction. Therefore it is recommended that negative pressure ventilation only be used on those patients who do not have excessive secretion, markedly increased thoracic elastance, or disorders of the upper airway.

Positive Pressure Ventilation
The spectrum of patients requiring positive pressure ventilation demands that portable volume ventilators have various degrees of sophistication. Needs may range from occasional ventilatory support to complete ventilatory use. The patient may range from a child to an adult. The needs of a ventilator dependent child may vary greatly from the needs of a ventilator dependent adult. Most modern portable and critical care ventilators use positive pressure to inflate the patients lungs. This pressure causes a positive pressure to inflate the patient's lungs. This pressure causes a positive intra thoracic and alveolar pressure during inpiration. As with negative pressure ventilation, expiration is largely passive.

There are a number of home positive pressure ventilators currently available in the home care market. These include the LP-6 Ventilator, the Bear 33, the PLV-1OO, and the PB-28OO, just to name a few. Regardless of the name brand, all portable ventilators should have the blowing features: an internal and external battery source, a way in which to add oxygen to the patient's breathing circuit, a way to measure delivered and exhaled volumes and the ability to alert patient and/or caregivers of low patient pressures.

Portable volume ventilators are often used in locations that lack emergency AC power. All units should, therefore, have an external battery and an internal power source that is capable of running the unit for up to one hour under normal working conditions. The internal battery should charge automatically whenever the ventilator is connected to AC power. The ventilator should not have a separate charging mode, but instead charge while operating in any ventilation mode. The internal power source should activate automatically when the line voltage falls below the unit's operating range. A low battery alarm should sound at least 20 - 45 minutes before operation of the ventilator is affected. A visual indicator should identify when the unit is charging. The external battery is often dependent on the type portable ventilator being used. lf AC power is lost, the ventilator should switch to the external battery automatically. Portable volume ventilators should have some means of providing oxygen-enriched inspired air to the patient. Delivered oxygen concentration should be within + or - 4% of the set level over the ventilator's normal range of operation.

Measurements of delivered and exhaled volumes are usually performed by using a Wright's spirometer to measure the amount of air coming directly from the ventilator during each inspiratory breath. The exhaled volume is measured with a Wright's spirometer by attaching it to the exhalation port on the patients' breathing circuit. These volumes should be measured periodically to assess the patients' condition and to ensure proper ventilator functioning.

Monitors and alarms are essential to establishing and maintaining adequate and safe ventilation. Adjustable alarms should be available for detecting excessive increases and decreases in patient airway pressures. Pressure alarms should be adjustable between a minimum of 5 cmH2O and at least 60 cmH20. Pressure should be monitored just proximal to the patient's airway.

The disadvantages of positive pressure ventilators, are the patient usually requires tracheal intubation (Tracheostomy), therefore increasing the risk for infections and airway obstruction. With the application of positive pressure directly to the lungs the incidence for barotrauma is greatly increased.

The advantages of portable positive ventilators are the fact they are smaller, reliable, and more portable than the hospital based ventilator. Therefore allowing patients who would otherwise be confined to the hospital to be discharged to the home setting. The number of ventilator dependent patients being discharged from the hospital to the home setting is continuing to increase. It is imperative that the respiratory therapists, physicians, and discharge planning nurses are familiar with the different types of portable positive pressure and negative pressure ventilators available in the home setting. Manufacturers are continuing to explore methods of developing ventilators that better meet the needs of the large spectrum of patients requiring mechanical ventilation in the home setting.