Upper airway patency during apnoea of prematurity.

Twenty four preterm infants (median birth weight 1120 g and gestation 29 weeks) were studied on 83 occasions by measuring upper airway airflow. Airway patency was detected by the transmission of cardiac impulse up the airway and airway closure by its absence. A total of 309 apnoeas of at least five seconds' duration were recorded. One hundred and eighty (58.0%) were central, 109 (35.5%) mixed, and 20 (6.5%) obstructive. Airway closure was noted in 47% of apparently central apnoeas. Airway closure occurred as apnoea lengthened; the airway remained patent in 38% of apnoeas of 5-9 seconds, 17% of those 10-14 seconds, and 11% of those 15-19 seconds' duration. Airway closure occurred in every apnoea of greater than or equal to 20 seconds. As a consequence, closed apnoeas were longer than open apnoeas (mean 9.7 v 6.6 seconds). In 72% of mixed apnoeas, airway closure was recorded during the central element and this usually preceded obstructive breaths. In 20% of mixed apnoeas and 15.5% of the total group the airways closed, having previously been patent. This occurred after a mean of 3.5 seconds (range 1-17). Mixed apnoea produced a significantly greater drop in arterial oxygen saturation than central apnoea, but only because of the greater duration of mixed apnoea. Airway closure occurs in both central and mixed apnoea and appears to be important in the pathophysiology of mixed apnoea. Central and mixed apnoea are part of a continuum of airway closure and not separate entities.