Sleep studies of respiratory function and home respiratory supportBMJ 1994; 309 doi: https://doi.org/10.1136/bmj.309.6946.35 (Published 02 July 1994) Cite this as: BMJ 1994;309:35
- A K Simonds
- Royal Brompton Hospital, Sydney Street, London SW3 6NP.
- Accepted 10 January 1994
Breathing differs when we are awake and asleep. Cortical stimulation of the respiratory centres wanes with the onset of sleep, resulting in a decrease in chemosensitivity to carbon dioxide and oxygen tensions; a reduction in upper airway, intercostal, and accessory muscle tone, particularly in rapid eye movement sleep; altered responses to elastic and resistive respiratory loads; and a reduction in metabolic rate.1 Failure to adapt to these physiological processes in predisposed subjects sets the scene for the development of sleep disordered breathing, in the form of the sleep apnoea syndromes and nocturnal hypoventilation (fig 1). Night time and daytime respiratory function is linked in that although obstructive apnoeas occur only during sleep, these can ultimately lead to diurnal abnormalities of breathing. Nocturnal hypoventilation either accompanies or heralds the development of diurnal respiratory failure.
Sleep disordered breathing Obstructive sleep apnoea/hypopnoea syndrome
This syndrome is characterised by snoring and repeated episodes of complete or partial obstruction of the upper airway which cause sleep fragmentation, fluctuations in arterial oxygen saturation, and daytime somnolence. There is no satisfactory definition of the syndrome, as a spectrum exists which ranges from normal breathing through snoring to severe effects. It is also clear that some snorers, in the absence of apnoeic episodes, may also suffer from disrupted sleep (the upper airway resistance syndrome).2 For diagnostic purposes the occurrence of 15 or more episodes of apnoea or hypopnoea per hour (apnoea/hypopnoea index) is usually considered abnormal. In each person the severity of the condition can be expressed by the apnoea/hypopnoea index, but it is the consequences of these events in terms of degree of desaturation, sleep fragmentation, and cardiovascular abnormalities such as hypertension and dysrhythmias which are crucial. The effects of untreated obstructive sleep apnoea/hypopnoea syndrome are listed in the box.