Hypoventilation is a common problem due to weakened respiratory muscles in patients with some types of muscular dystrophies such as Duchenne muscular dystrophy (DMD). These patients require long-term ventilation support either at night or whenever their lung function, measured by forced vital capacity (FVC), is below acceptable levels.
Respiratory failure impacts the survival rate of muscular dystrophy patients but long-term ventilation has been shown to improve survival significantly. For example, a study that evaluated the efficacy of long-term ventilation over 25 years showed that the five-year survival rate is 70%–75% and the 10-year survival rate is 40% in DMD patients, which were significantly higher compared to patients who did not receive ventilation support.
How ventilation works
A ventilator is a device that blows fresh air at high pressure into the airways and then removes it from the lungs. It supplements reduced lung function so that the patient receives adequate oxygen, and excess carbon dioxide is removed from the body. The air pumped by the ventilator is carried into and out of the lungs through a tracheal tube that is attached at one end to the ventilator and the other end to the lungs through a mouthpiece or a hole created by tracheostomy.
Types of ventilation
Ventilation support can be non-invasive or invasive, with three types available for muscular dystrophy patients with respiratory failure.
Nocturnal non-invasive ventilation
Nocturnal non-invasive ventilation is used when FVC is below 30%, and patients encounter sleep problems caused by obstructive sleep apnea. In DMD patients, nocturnal nasal ventilation can be given using a continuous airway pressure (CPAP) or a bilevel airway pressure (BiPAP) generator.
A CPAP machine delivers a constant inhalation and exhalation pressure. A BiPAP device provides higher pressure during inhalation to help push the air in, and a lower pressure during exhalation to help patients breathe air out. Their use improves sleep quality, decreases daytime sleepiness, improves daytime gas exchange (carbon dioxide and oxygen), and leads to a slower rate of decline in lung function, all of which improve quality of life and survival.
Daytime non-invasive ventilation
MD patients require 24-hour ventilation support when their respiratory function deteriorates and their FVC is reduced to 15%–20%, and carbon dioxide levels in their blood exceed 45 mmHg. Several daytime non-invasive devices and techniques are available.
- Mouthpiece ventilation is the most commonly used daytime non-invasive technique. It is well-tolerated and does not interfere with eating or speaking.
- Glossopharyngeal or frog breathing involves incremental inflation of the lungs by gulping air into the oropharynx and forcing the air from the pharynx into the trachea. This technique is used to allow for short periods of mechanical ventilation and is useful in the event of ventilatory failure.
- An abdominal pressure ventilator uses an inflatable bladder placed over the abdomen, which is connected to a conventional portable ventilator. This method generally does not work in obese patients or those with scoliosis.
- Negative-pressure ventilation uses a tank, jacket, or chest cuirass ventilator. It works on the principle of enclosing the chest and abdomen in an airtight rigid chamber from which air is intermittently evacuated. This creates a sub-atmospheric pressure around the lower thorax and abdomen that causes air to be drawn into the lungs. These machines are large, cumbersome, and not portable.
Continuous invasive ventilation
When patients cannot use the non-invasive devices or lack sufficient mouth and/or neck control to use a mouthpiece during the day, a tracheostomy is performed to provide continuous ventilation. Tracheostomy is a surgical procedure in which a hole is created in the front of the neck and a breathing tube (tracheostomy tube) is inserted into the windpipe that directly carries air into the lungs.
The tracheostomy tubes are safer and more comfortable than breathing tubes through the mouth. They also bypass any blockages in the windpipes. Air can be blown through the tracheostomy tubes at higher pressures in patients with lung disease or scoliosis. Moreover, mucus and secretions from the lower airways and trachea that cannot be cleared by coughing can be removed by direct airway tracheostomy suctioning. Otherwise, the accumulation of these secretions can result in respiratory infections.
The disadvantages of tracheostomy include impaired swallowing, difficulties in speaking, increased risk of aspiration, airway occlusion by a mucus plug, and infections. Therefore, it is preferred only when the non-invasive ventilation methods do not work.
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