Mechanical Ventilation Health Article

Definition

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:

• muscular dystrophies
• motor neuron disease, including ALS
• damage to the brain's respiratory centers
• polio
• myasthenia gravis
• myopathies affecting the respiratory muscles
• scoliosis

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.

Masks vs. tracheostomy

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.