Ventilatory Assistance Devices
Ventilatory assistance devices
Ventilatory assistance devices are mechanical devices that help a person breathe by replacing some or all of the muscular effort required to inflate the lungs.
Ventilation is the process of inflating and deflating the lungs in order to breathe. Normally, a person uses several sets of muscles to accomplish this—the diaphragm at the base of the lungs, the muscles between the ribs (intercostals), and, to a small extent, the muscles of the lower neck and shoulder area. When these muscles are weakened through disease or injury, the ability to ventilate is impaired. As a result, a person cannot get sufficient oxygen into, and carbon dioxide out of, the lungs in order to maintain appropriate levels in the blood. In addition, weakened ventilation muscles also impair the ability to cough, which is an essential part of clearing lung secretions and preventing infection.
Ventilatory assistance devices may be needed due to:
- muscular dystrophies (progressive muscle weakening disorders)
- amyotrophic lateral sclerosis (ALS), a progressive disease causing muscle weakness
- high spinal cord injury (injury to the spinal cord in the neck)
- Guillain-Barré syndrome (a rapidly progressive but reversible loss of muscle control)
- myasthenia gravis, acute crisis (MG is a muscle-weakening disease, in which patients may experience a "crisis" of rapid and dangerous loss of muscle strength)
- head trauma
- botulism (poisoning by botulinum toxin , usually from improperly preserved food)
- tetanus (poisoning by tetanus bacteria, usually by a deep puncture wound)
Nighttime ventilators are also used for people with obstructive sleep apnea . This is a condition in which breathing is impaired during sleep by obstructions in the airway, most often extra tissue at the rear of the throat.
Ventilatory assistance is not the same as supplying extra oxygen, as is done for people whose lungs are damaged. The person who needs ventilatory assistance generally has normal gas exchange capacity, and simply needs help moving air in and out. Supplemental oxygen can worsen the situation in such cases, as it may depress the normal signals from the brain to stimulate breathing.
A ventilator is a machine that uses a tube to blow air into, and suck it out of, the body. The ventilator may be designed to deliver air at a set volume (volume ventilator) or at a set pressure (pressure ventilator).
Volume ventilator settings may be adjusted to deliver a variable volume of air depending on the patient's needs, and can either cycle automatically or be initiated by the patient's voluntary efforts.
Pressure ventilators come in two major styles. Continuous positive airway pressure (CPAP) delivers air at a steady pressure, which assists the patient while breathing in (inspiration) and resists breathing out (expiration). The purpose of CPAP is not to completely inflate the lungs, but rather to maintain an open airway. This makes it most appropriate for use in sleep apnea, in which a patient's airway closes frequently during sleep. In contrast, bi-level positive airway pressure (BiPAP) delivers a higher pressure on inspiration in order to allow the patient to completely inflate the lungs, and then switches to a low pressure on expiration, to allow easy exhalation. BiPAP is a common choice for patients with neuromuscular disease, whose respiratory muscles are weakened.
There are other rarer devices in use, which surround a patient's chest cavity and abdomen with a rigid shell, and change the pressure within. By lowering the pressure, air rushes into the lungs. The "iron lung" is one such device; smaller and more portable "cuirasses" are occasionally used to similar effect.
The air from a ventilator is delivered to the patient either through a face mask or directly into the lungs through a tracheostomy (trach) tube. Each has its advantages and disadvantages.
A tracheostomy is an opening in the airway in the middle of the throat through which a tube is inserted to deliver air. The properly chosen trach tube will fit comfortably. A widespread misunderstanding about tracheostomy ventilation is that it prevents talking, but this does not have to be so. Trach tubes are available that do not interfere with speech, and patients contemplating tracheostomy should ensure that their respiratory specialist is familiar with them. Trach tubes may provide the patient a greater sense of security and, unlike a face mask, it can easily be hidden from view with a well-placed scarf. Trach tubes do require daily lung hygiene, either by the patient or by a trained caregiver. This involves suctioning out secretions from the lungs, which tend to be increased due to the presence of the tube.
Face masks fit snugly over the mouth and nose, and are held on with a strap. Finding the right mask takes some time, but a well-fitting mask is comfortable and easy to tolerate for many hours per day. The mask usually must be removed to talk, but this does not present a problem for many patients who retain some use of their hands. The mask may also be used at night.
Other noninvasive interfaces are also available, including mouthpieces and "nasal pillows" that fit into one or the other orifice and are smaller than masks. These methods are usually chosen for patients who need fewer hours per day of ventilatory assistance.
Patients with weakened respiratory muscles may be even more in need of cough assistance than they are of ventilatory assistance. Cough assistance may be delivered manually by a caregiver or by a machine (the in-exsufflator or cough assist) that is designed to inflate the lungs and then rapidly withdraw air, as occurs in a normal cough. This clears secretions that would otherwise accumulate and provide a locus for infection, as well as interfere with gas exchange.
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