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.
Richard Robinson, The Gale Group Inc., Gale, Detroit,