Adaptive Mechanisms

During conditions of upper airway collapse ventilation is compromised leading to a decrease in minute ventilation (Vj). Adaptive mechanisms counteracting the decrease in minute ventilation include an increase in mean inspiratory flow rate [tidal volume (VT)/inspiratory time (Tj)j during the inspiratory phase of the respiratory cycle and prolongation of the inspiratory time of the respiratory cycle [also referred to as the duty cycle (Ti/Ttot)] (7-9). Under conditions of internal respiratory loading (increase of upper airway resistance) the Vt/Ti responds until it starts to decline. This is due to mechanical impediment increasing above levels where augmented neural drive reaches a ceiling where it cannot further increase inspira-tory flow rate. At this point a second line of defense may be activated: the duty cycle, that is, prolongation of the inspiratory portion of the respiratory cycle. Previous and recent work has suggested that both compensatory mechanisms are distinct phenotypic traits and therefore genetically predetermined (7,10,11).

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Sleep Apnea

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