Upper airway reflexes and obstructive sleep apnoea.

Reflexes originated from the supralaryngeal airways, larynx and lungs can affect upper airway calibre r 11. Specific receptors at the lnryngeal level have been thought to be involved 12J. N~tsal obstruction in normal men m<\y cause apnoeas allll in addition. more arousals and full awakenings from sleep [3]. The underlying mechanism remains unclear. It was demonstrated that nasal occlusion increases respiratory drive to the upper airways and that obstructive sleep apnoea is. therefore. not e licited by a decreased respiratory drive to upper airway muscles 141. Apnoeas ~•re like ly to occur because increased airway resistance induces more negative airway pressures. It appears that "unknown" reflexes may be involved in the stimulation of upper airways (and in arousals). Arterial chemoreflexes are known to provide a powerful excitatory input m inspiratory und expiratory hypoglossa l motone urons: their stimulation helps in maintaining upper airway patency 15]. 11 has even been postulated thal peripheral chemoreceptor sti mutation induced by hypoxia prefe re ntially influe nces those motoneurons that regulate upper airway muscle activation [6] . However, recent data indicate that chemoreceptors do not affect upper airway muscle activity independently from other stimuli such as airway pressure [7]. Although reflex control may be important, it has also been demonstrated that dilatation of the upper airways is reflected by indices that quantify the central inspiratory drive in both normal subjects and patients with sleep apnoea t8]. This suggests that the activity of the central nervous system also plays uu imponnnr ro le in the (in)stability of the upper airway. Suppression of rhe re1lexes originating in the upper <1irway by I'Opical oropharyngeal anaesthesia leads to an in creased number of obstructive apn oeas and hypopnoeas in normal men [9], confirming the possible clinical significance of these upper airway reflexes in maintaining patency of the upper airway during sleep. In this issue of the journal, ToMORt et al. [JO] have reviewed, in depth, a number of upper airway reflexes in the cat: including the sniff-like aspiration reflex and gasping. They describe the similari ties and discrepancies between these two reflexes. Both reflexes are characterized by a powerful inspiratory effort. A fast and powerful activation of practically all phrenic motoneurons and inspiratory muscles