Breathing out the waste gas (carbon dioxide) from the lungs is known as expiration. This is usually passive and does not require particularly strong muscles, but coughing does require effective contraction of these expiratory muscles and normal functioning of the upper airway (bulbar) muscles. Scoliosis (a curved spine) reduces lung volume even more, and causes the respiratory muscles to contract inefficiently due to the asymmetrical shape of the chest.
After a period of breathing at low lung volumes, the chest wall tends to become stiff and less compliant making it more difficult for the respiratory muscles to expand and draw enough oxygen into the lungs. Children and adults with low lung capacity are prone to chest infections and these are slow to clear because coughing is ineffective. If the swallowing muscles are weak, food may go down the wrong way into the lungs leading to recurrent infections.
During sleep, inspiratory and upper airway muscles normally relax and each breath becomes smaller so the oxygen level in the body goes down. If these muscles are already very weak then the oxygen level becomes even lower, this is known as under ventilation or hypoventilation. In mild cases, hypoventilation does not cause any symptoms and is only noticeable in rapid eye movement (REM) sleep when we often dream. However, if hypoventilation at night progresses it can lead to low oxygen and high carbon dioxide levels during the day.
If upper airway muscles are particularly weak, short episodes of upper airway obstruction may occur during sleep; this is known as obstructive sleep apnoea. Often hypoventilation and obstructive sleep apnoea coexist.
Spotting the symptoms and acting on them
Symptoms of nocturnal hypoventilation include morning headaches, lethargy, breathlessness, disturbed sleep, sweating at night and poor appetite. Erratic noisy breathing during sleep may be observed. In young children, failure to thrive or gain weight is not uncommon. These symptoms should be reported to your doctor.
Breathing problems can also be picked up during routine clinic visits. Vital capacity and overall respiratory muscle strength can be measured by simple blowing tests. Once vital lung capacity is less than 60% predicted and respiratory muscle strength falls below 30% of normal, hypoventilation becomes a possibility and regular checks should be made. Hypoventilation is unlikely if vital capacity is over 60% of normal, other than at the time of a severe chest infection. If problems with breathing during sleep are suspected a ‘sleep study’ will usually be carried out.
Here oxygen and carbon dioxide levels (and sometimes the pattern of breathing) are monitored overnight using small probes attached painlessly to the surface of the body. The sleep study may show a variety of findings. The most common of these is a fall in oxygen level and rise in carbon dioxide particularly during REM sleep due to under ventilation. In others short episodes of stopping breathing (apnoeas) due to either obstruction of a floppy upper airway (obstructive sleep apnoea) or lack of breathing effort (central sleep apnoea) are seen.