Mechanical ventilation is the use of a mechanical device (machine) to inflate and deflate the lungs.
Purpose
Mechanical ventilation provides the force needed to deliver air to the lungs in a patient whose own ventilatory abilities are diminished or lost.
Description
Breathing requires the movement of air into and out of the lungs. This is normally accomplished by the diaphragm and chest muscles. A variety of medical conditions can impair the ability of these muscles to accomplish this task, including:
Mechanical ventilation may also be used when the airway is obstructed, especially at night in sleep apnea.
Mechanical ventilation may be required only at night, during limited daytime hours, or around the clock, depending on the patient's condition. Some patients require mechanical ventilation only for a short period, during recovery from traumatic nerve injury, for instance. Others require it chronically, and may increase the number of hours required over time as their disease progresses.
Mechanical ventilation is not synonymous with the use of an oxygen tank. Supplemental oxygen is used in patients whose gas exchange capacity has diminished, either through lung damage or obstruction of a major airway. For these patients, the muscles that deliver air work well, but too little oxygen can be exchanged in the remaining lung, and so a higher concentration is supplied with each breath. By the same token, many patients who require mechanical ventilation do not need supplemental oxygen. Their gas exchange capacity is normal, but they cannot adequately move air into and out of the lungs. In fact, excess oxygen may be dangerous, since it can suppress the normal increased respiration response to excess carbon dioxide in the lungs.
Mechanical ventilation systems come in a variety of forms. Almost all systems use a machine called a ventilator that pushes air through a tube for delivery to the patient's airways. The air may be delivered through a nasal or face mask, or through an opening in the trachea (windpipe), called a tracheostomy. Much rarer are systems that rhythmically change the pressure around a patient's chest when the pressure is low, air flows into the lungs, and when it increases, air flows out.
Ventilators
Ventilators can either deliver a set volume with each cycle, or can be set to a specific pressure regimen. Both are in common use. Volume ventilator settings are adjustable for total volume delivered, timing of delivery, and whether the delivery is mandatory or determined by the patient's initial inspiratory effort.
Pressure ventilators deliver one of two major pressure regimens. Continuous positive airway pressure (CPAP) delivers a steady pressure of air, which assists the patient's inspiration (breathing in) and resists expiration (breathing out). The pressure of CPAP is not sufficient to completely inflate the lungs; instead its purpose is to maintain an open airway, and for this reason it is used in sleep apnea, in which a patient's airway closes frequently during sleep.
Bilevel positive airway pressure (BiPAP) delivers a higher pressure on inspiration, helping the patient obtain a full breath, and a low pressure on expiration, allowing the patient to exhale easily. BiPAP is a common choice for neuromuscular disease.
The choice of ventilator type is partly determined by the knowledge and preferences of the treating physician. Settings are adjusted to maintain patient comfort and appropriate levels of oxygen and carbon dioxide in the blood.
Delivery of air from a ventilator may be either through a mask firmly held to the face, or through a tube inserted into the trachea toward the bottom of the throat. A mask interface is called noninvasive ventilation, while a tracheostomy tube is called invasive ventilation.
Until the mid-1990s, invasive ventilation was the option used by virtually all patients requiring long-term mechanical ventilation. For some patients, tracheostomy continues to be a preferred option. It is commonly used when 24-hour ventilation assistance is required, and may be preferred by patients who find masks uncomfortable or unsightly. Some patients feel ventilation through a "trach tube" is more reassuring. Tracheostomy is also the preferred option for most patients with swallowing difficulties. The potential to choke and suffocate on improperly swallowed food is avoided with a tracheostomy.
Tracheostomies may require more frequent suctioning of airway secretions, produced in response to the presence of the tube and the inflatable cuff that some patients require to hold it in place. The risk of infection is higher, and air must be carefully humidified and cleaned, since these functions are not being served by the nasal passages. Tracheostomies do not prevent speech, despite misinformation to the contrary that even some doctors believe. Speech requires passage of air around the trach tube, which can occur either with an uncuffed tube, or with the presence of a special valve that allows air passage past the cuff.
Noninvasive interfaces come in a variety of forms. A simple mouthpiece may be used, which a patient bites down on to seal the lips around the tube as the pressure cycle delivers a breath. Most masks are individually fitted to the patient's face, and held in place with straps. A tight fit is essential, since the pressure must be delivered to the patient's lungs, and not be allowed to blow out the sides of the mask. Masks may be used around the clock. Nasal masks do not prevent speech, though the tone may change. Oral or full-face masks do interfere with speech, and are typically used at night or intermittently throughout the day, for patients who do not need continuous ventilation assistance.
Other alternatives
The iron lung was an early mechanical ventilation device, and is still in use in some hospitals. The patient's head remains outside of it, while the interior depressurizes. This allows air to push in to the lungs. Repressurizing deflates the lungs again.
A device that works on the same principle is the chest shell (something like a turtle's shell swung around to the front). The pneumobelt applies pressure to deflate, and relaxes it to allow inflation. A rocking bed is used for nighttime ventilation. Tilting the head of the bed down deflates the lungs by allowing the abdominal contents to press against the diaphragm. Reversing the angle reverses the process, allowing inflation.
Preparation
Patients with diseases in which mechanical ventilation may be required are advised to learn as much as possible about treatment options before they become necessary. In particular, it is important to learn about and make decisions about invasive vs. noninvasive ventilation before the time comes. Many patients who begin ventilation with emergency tracheostomy have a difficult time switching to noninvasive ventilation later on (though it is certainly possible).
It is often a good idea to try out different masks and other interfaces before their need arises, and to have these fitted in preparation for a planned transition to the ventilator. Patients can find support groups and other sources of information to learn more about the options and the features of each means of ventilation. Patients may have to help educate their doctors if they are not familiar with noninvasive options.
Patients with neuromuscular disease may have as much or more need for a deep cough as they do for ventilatory assistance, and many patients who undergo emergency tracheostomy do so because their airways have become clogged with mucus build up. Physical therapy cough assistance and a cough assist device are important options for full respiratory health.
Normal results
Mechanical ventilation is a life saver, and provides comfort and confidence to patients who require it. Proper ventilation restores levels of oxygen and carbon dioxide in the blood, improving sleep at night and increasing the ability to engage in activities during the day. When combined with proper respiratory hygiene, it can prolong life considerably. Patients with progressive diseases such as ALS may wish to consider end-of-life decisions before commencing mechanical ventilation, or before the ability to communicate is lost.
BOOKS
Bach, John R. Noninvasive Mechanical Ventilation. Hanley and Belfus, 2002.
Kinnear, W. J. M. Assisted Ventilation at Home: A practicalGuide. Oxford: Oxford Medical Publications, 1994.
