Effect of episodic hypoxia on the susceptibility to hypocapnic central apnea during NREM sleep

Experimental protocols and preparations to study respiratory long term facilitation

Respiratory long-term facilitation is a form of neuronal plasticity that is induced following exposure to intermittent hypoxia. Long-term facilitation is characterized by a progressive increase in respiratory motor output during normoxic periods that separate hypoxic episodes and by a sustained elevation in respiratory activity for up to 90min after exposure to intermittent hypoxia. This phenomenon is associated with increases in phrenic, hypoglossal or carotid sinus nerve inspiratory-modulated discharge. The examination of long-term facilitation has been steadily ongoing for approximately 3 decades. During this period of time a variety of animal models (e.g. cats, rats and humans), experimental preparations and intermittent hypoxia protocols have been used to study long-term facilitation. This review is designed to summarize the strengths and weaknesses of the models, preparations and protocols that have been used to study LTF over the past 30 years. The review is divided into two primary sections. Initially, the models and protocols used to study LTF in animals other than humans will be discussed, followed by a section specifically focused on human studies. Each section will begin with a discussion of various factors that must be considered when selecting an experimental preparation and intermittent hypoxia protocol to examine LTF. Model and protocol design recommendations will follow, with the goal of presenting a prevailing model and protocol that will ultimately ensure standardized comparisons across studies.

Sustained hyperoxia stabilizes breathing in healthy individuals during NREM sleep

The present study was designed to determine whether hyperoxia would lower the hypocapnic apneic threshold (AT) during non-rapid eye movement (NREM) sleep. Nasal noninvasive mechanical ventilation was used to induce hypocapnia and subsequent central apnea in healthy subjects during stable NREM sleep. Mechanical ventilation trials were conducted under normoxic (room air) and hyperoxic conditions (inspired PO(2) > 250 Torr) in a random order. The CO(2) reserve was defined as the minimal change in end-tidal PCO(2) (PET(CO(2))) between eupnea and hypocapnic central apnea. The PET(CO(2)) of the apnea closest to eupnea was designated as the AT. The hypocapnic ventilatory response was calculated as the change in ventilation below eupnea for a given change in PET(CO(2)). In nine participants, compared with room air, exposure to hyperoxia was associated with a significant decrease in eupneic PET(CO(2)) (37.5 ± 0.6 vs. 41.1 ± 0.6 Torr, P = 0.001), widening of the CO(2) reserve (-3.8 ± 0.8 vs. -2.0 ± 0.3 Torr, P = 0.03), and a subsequent decline in AT (33.3 ± 1.2 vs. 39.0 ± 0.7 Torr; P = 001). The hypocapnic ventilatory response was also decreased with hyperoxia. In conclusion, 1) hyperoxia was associated with a decreased AT and an increase in the magnitude of hypocapnia required for the development of central apnea. 2) Thus hyperoxia may mitigate the effects of hypocapnia on ventilatory motor output by lowering the hypocapnic ventilatory response and lowering the resting eupneic PET(CO(2)), thereby decreasing plant gain.

Sleep state dependence of ventilatory long-term facilitation following acute intermittent hypoxia in Lewis rats

Ventilatory long-term facilitation (vLTF) is a form of respiratory plasticity induced by acute intermittent hypoxia (AIH). Although vLTF has been reported in unanesthetized animals, little is known concerning the effects of vigilance state on vLTF expression. We hypothesized that AIH-induced vLTF is preferentially expressed in sleeping vs. awake male Lewis rats. Vigilance state was assessed in unanesthetized rats with chronically implanted EEG and nuchal EMG electrodes, while tidal volume, frequency, minute ventilation (Ve), and CO(2) production were measured via plethysmography, before, during, and after AIH (five 5-min episodes of 10.5% O(2) separated by 5-min normoxic intervals), acute sustained hypoxia (25 min of 10.5% O(2)), or a sham protocol without hypoxia. Vigilance state was classified as quiet wakefulness (QW), light and deep non-rapid eye movement (NREM) sleep (l-NREM and d-NREM sleep, respectively), or rapid eye movement sleep. Ventilatory variables were normalized to pretreatment baseline values in the same vigilance state. During d-NREM sleep, vLTF was observed as a progressive increase in Ve post-AIH (27 + or - 5% average, 30-60 min post-AIH). In association, Ve/Vco(2) (36 + or - 2%), tidal volume (14 + or - 2%), and frequency (7 + or - 2%) were increased 30-60 min post-AIH during d-NREM sleep. vLTF was significant but less robust during l-NREM sleep, was minimal during QW, and was not observed following acute sustained hypoxia or sham protocols in any vigilance state. Thus, vLTF is state-dependent and pattern-sensitive in unanesthetized Lewis rats, with the greatest effects during d-NREM sleep. Although the physiological significance of vLTF is not clear, its greatest significance to ventilatory control is most likely during sleep.