Breathing in CMD
Cure CMD would like to thank Dr. Hemant Sawnani from Cincinnati Children’s Hospital for his guidance on this article and tireless support of our community.
One of the primary complications that inhibits the quality of life in people with CMD is the compromise of breathing function and the retention of carbon dioxide (called hypoventilation). Hypoventilation occurs first in sleep, when respiratory effort is least, and gradually spills over into wakefulness, with greater frequency of symptoms in the day.
Breathing? But this is a muscle condition...
People with CMD have problems with ventilation because:
Expansion of the chest is limited by weakness in the diaphragm and chest wall muscles, contractures (tightness) in the joints between the ribs and vertebral bodies, and scoliosis (abnormal curvature of the spine);
These limitations reduce the amount of air that can be inhaled in each breath;
The decreased amount of air inhaled and exhaled causes carbon dioxide levels to buildup.
The Basics of Breathing
When we inhale, we oxygenate; the oxygen moves into the lungs and is absorbed into the circulatory system. When we exhale, we ventilate; carbon dioxide is transported out of the blood, into the lungs, and out the body. Proper ventilation requires enough muscle strength to expand the lungs during inhalation. Depending on how weak the affected individual is, they may or not be able to take a large enough breath in, which means they have a smaller amount of air to breathe out. These smaller breaths are not enough to expel the carbon dioxide, and
it slowly starts to build up over time.
Another often overlooked reality is that adequate airflow is extremely important in driving normally produced airway mucus up to the back of the throat where it would be normally swallowed. However, with shallow breathing, there is not enough expiratory airflow, and there is also a tendency to therefore retain airway mucus, predisposing to mucus plugging of airways. Finally, shallower breathing leads to reduction of the size of alveolar units – the smallest units of the lung where gases (oxygen and carbon dioxide) cross over into and out of circulation, respectively. Collectively across many lung regions, this will lead to reduced total lung volumes. For a variety of reasons, this makes breathing less efficient, and individuals begin to breathe
shallower and faster, making for undue calorie expenditure. When lung volumes reach certain lower thresholds, lung units begin to collapse (called atelectasis).
Key Point: People with CMD do not usually have a problem with oxygenation; they are more likely to experience hypercapnia (elevated carbon dioxide levels)
Hypoxemia & Hypercapnia
Hypo = low
Hyper = high
The act of breathing allows 2 processes to occur. Breathing in facilitates oxygenation, while breathing out facilitates ventilation (getting rid of stale
air). Imbalances in these two major functions lead to the development of low oxygen levels (hypoxemia) and/or elevated carbon dioxide levels (hypercapnia).
Hypoxemia is usually due to problems with the lung tissue, such as pneumonia or asthma, or as a result of mucous plugging where lung secretions cannot be cleared (atelectasis). These issues affect the actual lung airways or lung tissue and prevent inhaled oxygen from getting through the lung tissue into the blood stream. People with CMD do not usually have a problem with oxygenation unless the issues described here are also present.
However, they are more likely to experience hypercapnia, which, if severe enough, can itself lead to associated hypoxemia.
Night Time Ventilation
People with CMD often first develop problems with ventilation in sleep. We all have slightly shallower breaths and a slower breathing rate while sleeping. Our oxygen level also slightly decreases, in part, because effort reduces, lung volumes reduce in the recumbent position, and tissues in the back of the throat relax and may somewhat obstruct breathing. In an unaffected individual, these changes go largely unnoticed, and are well-t
olerated. However, in people with CMD, these changes can significantly impact ventilation during sleep and experience hypoventilation.
In the supine position (lying down on the back), chest wall expansion and diaphragm movement is more difficult as compared to sitting or standing upright. As a result, shallower breathing occurs, and this can cause carbon dioxide levels to rise. Eventually, oxygen levels may also fall.
Hypoventilation is treated not with oxygen, but with non-invasive ventilation to increase the amount of air (volume) exchanged during breathing.
Signs of Hypoventilation
There are a number of symptoms associated with the onset of breathing problems in people with CMD, including:
Difficulty sleeping – feeling “breathless” while lying down
Non-refreshing (non-restorative) overnight sleep
Loss of Appetite
Altered mental status
Any of these symptoms in a person with CMD may indicate the need for assisted ventilation at night and even during the day.
Another important function associated with the loss of respiratory function is the ability to cough and clear airway secretions efficiently.
The lining of our airways produces small amounts of mucous that trap dirt and bacteria – all of these are normally expelled during a cough. A cough begins with a deep breath in. The glottis closes (i.e. the vocal cords completely come together to create a seal), allowing pressure to build up in the lungs as the muscle of the chest and abdomen begin to contract. The respiratory muscles continue to contract and the glottis opens suddenly, forcing air out of the lungs/airways in a quick burst.
Without the ability to cough effectively, there is an increased chance of respiratory infection or mucus plugging and collapse of lung segments. People with CMD often have a weak cough due to the same issues that cause breathing problems (weak muscles in the chest wall and diaphragm, contractures between the ribs, scoliosis).
Surveillance and Intervention
Surveillance of breathing function is vitally important and includes:
Pulmonary Function Testing (PFT)
Pulmonary Function Tests measure the efficiency of the respiratory system. Some important measures are Forced Vital Capacity (FVC), respiratory muscle strength and Cough Peak Flow (CPF). FVC measures the amount of air expelled during a forceful exhalation after filling the lungs with a much air as possible. Respiratory muscle strength tests measure the maximal inspiratory or expiratory pressures that an individual can generate. CPF measures the strength of an individual’s cough. These measures provide most useful information when tracked over time for any individual, allowing better determination of the course of progression, and allowing anticipation of future difficulties.
It is ideal to perform PFTs both sitting and lying down, although not all centers can readily accommodate recumbent testing. The involvement of the diaphragm will be indicated in the difference in FVC between the sitting and supine positions. This test should be repeated annually to evaluate the rate of respiratory decline. Pulmonary Function Testing should begin by age 5, even if no signs of breathing dysfunction are obvious.
If the FVC is less than 80% of the expected value, the frequency of PFTs should be increased to every 6 months to more closely monitor breathing function. If the FVC is less than 60% of the expected value, or there is a greater than 20% decrease in breathing function between sitting and lying down, a polysomnogram should be ordered to evaluate breathing function while sleeping. This may also indicate the need for non-invasive ventilation.
If the FVC is less than 60% of the expected value, or there is a greater than 20% decrease in breathing function between sitting and lying down, a polysomnogram should be ordered to evaluate breathing function while sleeping. This may also indicate the need for non-invasive ventilation.
Sleep Study (Polysomnography)
Because most symptoms of breathing dysfunction occur initially at night, it is important to assess for signs of hypoventilation while sleeping, through polysomnography, or sleep studies. Sleep studies measure oxygen and carbon dioxide levels during sleep, and evaluate if there are any periods of apnea (pauses in breathing). These may indicate the need for non-invasive ventilation (BiPAP/ventilator) while sleeping and possibly during waking hours. This test should be repeated annually to evaluate the rate of respiratory decline and to adjust the settings of the BiPAP or ventilator.
If the PFT and Sleep Study indicate there has been a possible long term, untreated breathing problem, an Echocardiogram should be ordered to ensure there is no right-side heart strain as a result of the prolonged respiratory insufficiency.
Cough Assist Machine
Regular use of a Cough Assist Machine or mechanical insufflation-exsufflation device can help expel mucous when the person with CMD cannot effectively do soon their own. As the name suggests, a Cough Assist machine enhances or augments a natural cough. The machine delivers a large volume of air (positive pressure) to maximally inflate the lungs. After achieving this, the machine reverses flow rapidly and briefly, simulating the rapid expiratory flow of a cough. While the settings can be finely adjusted for comfort, they should be determined by a skilled therapist or physician. Daily use of a Cough Assist machine can help keep airways clear and reduce the chance of respiratory infection. Some people with CMD who use this machine daily often report that it feels easier to breathe following treatment. The machine’s use is generally well tolerated, and complications related to its (safe) use have rarely occurred.
When someone with CMD develops a respiratory illness or infection, their ability to expel mucous can be compromised and their chances for developing atelectasis (collapsed lung tissue) or pneumonia are increased. If the oxygen saturation level is below 93-95%, it is important in these instances to deliver positive air pressure via a cough assist machine or any ventilator to increase the volume of air delivered to the lungs. The instinct is often to deliver oxygen, but oxygen alone cannot overcome the issue. The goal is to enhance the depth of breathing i.e. the amount of air inhaled in each breath. A small device called a pulse oximeter can be obtained at your local drug store or online without a prescription for under $25. It is
important for everyone affected with CMD to own one so that oxygen saturation, particularly when ill with a respiratory virus or infection, can be monitored.
Individuals affected with any neuromuscular disorder as well as those they come into frequent contact with should get an annual flu shot. The injection version that contains a dead virus rather than the mist version that contains a live virus is recommended for both the affected individual and their families/care givers. The flu shot can take several weeks to take effect so it is important to obtain your flu shot as soon as possible, at the beginning of flu season.
Respiratory Function in Neuromuscular Conditions:
Proactive Respiratory Care in Congenital Muscle Disease
Surgeries and Hospitalizations