![]() ![]() The higher the loop gain, the higher the overventilation or underventilation response. ![]() The loop gain is an engineering term that describes the degree of response, in this case of the respiratory control system, after a ventilatory disturbance. Apneic events are the result of over-response or under-response of the respiratory control system to minimal changes in nocturnal PaCO 2 (high “loop gain”). This is typically characterized by awake PaCO 2 less than 45 mm Hg. In contrast to NREM, REM sleep is less dependent on the PaCO 2-driven chemoreceptor input, likely related to an increase in the central inspiratory neural drive as the main driver of respiratory function. Therefore, breathing instability at sleep onset is commonly observed (Fig. The apnea will endure until the PaCO 2-level increases back to the sleep eucapnic level, with subsequent reinitiation of breathing. During sleep, the previous awake eucapnic PaCO 2 level will become the new apneic threshold (PaCO 2 level below which a central apnea will occur). ![]() When falling asleep, a new eucapnic level (2–6 mm Hg) above awakening PaCO 2 level is set as a result of a physiologic decrease in tidal volume. This transition shows highly sensitive dependence of the respiratory control system on PaCO 2-driven chemoreceptor input during the shift of eucapnic levels from awake to sleep state and vice versa. These are considered physiologic, and they are observed in healthy individuals during the transition from awakening to superficial stages of NREM sleep (e.g., N1 and N2). ![]() When falling asleep, the usual influence of wakefulness on the drive to breathe is lost, facilitating the subsequent development of sleep-onset apneas. ![]()
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