CLASSIFICATION OF VENTILATORS
Several types of mechanical ventilators exist; they are classified ac-cording to the manner in which they support ventilation. The two general categories are negative-pressure and positive-pressure ven-tilators. The most common category in use today is the positive-pressure ventilator.
Negative-pressure ventilators exert a negative pressure on the external chest. Decreasing the intrathoracic pressure during in-spiration allows air to flow into the lung, filling its volume. Phys-iologically, this type of assisted ventilation is similar to spontaneous ventilation. It is used mainly in chronic respiratory failure associ-ated with neuromuscular conditions, such as poliomyelitis, mus-cular dystrophy, amyotrophic lateral sclerosis, and myasthenia gravis. It is inappropriate for the unstable or complex patient or the patient whose condition requires frequent ventilatory changes. Negative-pressure ventilators are simple to use and do not require intubation of the airway; consequently, they are especially adapt-able for home use.
There are several types of negative-pressure ventilators: iron lung, body wrap, and chest cuirass.
The iron lung is a negative-pressure chamber used for ventilation. It was used extensively during polio epidemics in the past and currently is used by polio survivors and patients with other neuro-muscular disorders.
Both of these portable devices require a rigid cage or shell to cre-ate a negative-pressure chamber around the thorax and abdomen.Because of problems with proper fit and system leaks, these types of ventilators are used only with carefully selected patients.
Positive-pressure ventilators inflate the lungs by exerting positive pressure on the airway, similar to a bellows mechanism, forcing the alveoli to expand during inspiration. Expiration occurs pas-sively. Endotracheal intubation or tracheostomy is necessary in most cases. These ventilators are widely used in the hospital set-ting and are increasingly used in the home for patients with pri-mary lung disease. There are three types of positive-pressure ventilators, which are classified by the method of ending the in-spiratory phase of respiration: pressure-cycled, time-cycled, and volume-cycled. Another type of positive-pressure ventilator used for selected patients is noninvasive positive-pressure ventilation.
The pressure-cycled ventilator ends inspiration when a preset pressure has been reached. In other words, the ventilator cycles on, delivers a flow of air until it reaches a predetermined pressure, then cycles off. Its major limitation is that the volume of air or oxygen can vary as the patient’s airway resistance or compliance changes. As a result, the tidal volume delivered may be inconsis-tent, possibly compromising ventilation. Consequently, in adults, pressure-cycled ventilators are intended only for short-term use. The most common type is the IPPB machine (see previous dis-cussion of IPPB).
Time-cycled ventilators terminate or control inspiration after a preset time. The volume of air the patient receives is regulated by the length of inspiration and the flow rate of the air. Most venti-lators have a rate control that determines the respiratory rate, but pure time-cycling is rarely used for adults. These ventilators are used in newborns and infants.
Volume-cycled ventilators are by far the most commonly used positive-pressure ventilators today (Fig. 25-6). With this type of ventilator, the volume of air to be delivered with each inspiration is preset. Once this preset volume is delivered to the patient, the ventilator cycles off and exhalation occurs passively. From breath to breath, the volume of air delivered by the ventilator is relatively constant, ensuring consistent, adequate breaths despite varying airway pressures.
Positive-pressure ventilation can be given via facemasks that cover the nose and mouth, nasal masks, or other nasal devices. This eliminates the need for endotracheal intubation or tracheostomy and decreases the risk for nosocomial infections such as pneu-monia. The most comfortable mode for the patient is pressure-controlled ventilation with pressure support. This eases the work of breathing and enhances gas exchange. The ventilator can be set with a minimum backup rate for patients with periods of apnea.
Patients are considered candidates for noninvasive ventilation if they have acute or chronic respiratory failure, acute pulmonary edema, COPD, or chronic heart failure with a sleep-related breathing disorder. The device also may be used at home to im-prove tissue oxygenation and to rest the respiratory muscles while the patient sleeps at night. It is contraindicated for those who have experienced respiratory arrest, serious dysrhythmias, cogni-tive impairment, or head or facial trauma. Noninvasive ventila-tion may also be used for patients at the end of life and those who do not want endotracheal intubation but may need short- or long-term ventilatory support (Scanlan, Wilkins & Stoller, 1999).
Bilevel positive airway pressure (bi-PAP) ventilation offers in-dependent control of inspiratory and expiratory pressures while providing pressure support ventilation.
It delivers two levels of positive airway pressure provided via a nasal or oral mask, nasal pillow, or mouthpiece with a tight seal and a portable ventilator. Each inspiration can be initiated either by the patient or by the machine if it is programmed with a backup rate. The backup rate ensures that the patient will receive a set number of breaths per minute (Perkins & Shortall, 2000). Bi-PAP is most often used for patients who require ventilatory assistance at night, such as those with severe COPD or sleep apnea. Tolerance is variable; bi-PAP is usually most successful with highly motivated patients.