Hypopnea is a partial airway obstruction during sleep, in which there is breathing effort but reduced airflow. There must be at least a 50% reduction in normal airflow for at least 10 seconds and more than five per hour. To be scored as a hypopnea, an event must also be accompainied by a desaturation (a drop in blood oxygen of at least 3%).  Hypopneas also result in arousals and symptoms similar to obstructive sleep apnea. The literature is consistent that obstructive sleep apnea to upper airway resistance syndrome represents a continuum from most severe sequelae to least, and that all are serious in terms of having morbid consequences.
Figure 2
Figure 2

The yellow highlighted area identifies a hypopnea, an eleven second event of 50-80% reduction in airflow but with breathing effort in both chest and abdominal areas.  The blue identifies a desaturation, a 5% drop in blood oxygen level from 96% to 91%, over an eleven second period.  Note that if follows in close proximity to the hypopnea and an 11 second apneic event identified by green highlighting. 
The whole top half of Figure 2 shows an epoch of 60 seconds.  The top line identifies snores.  The numbers under the line shows the volume in decibels.  The fourth line down is a represententation of air flow.  The fifth line shows chest effort and the sixth line shows abdominal effort.  The seventh line down reflects blood oxygen levels and gives digital readings.  The eighth line from the top shows pulse rate changes and the exact digital readings are shown above the line.  The bottom line indicates body position which is also spelled out above the line.
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