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The purpose of this article is to highlight the importance of considering sleep-disordered breathing (SDB) as a potential confounder to rehabilitation research interventions in spinal cord injury (SCI). SDB is highly prevalent in SCI, with increased prevalence in individuals with higher and more severe lesions, and the criterion standard treatment with continuous positive airway pressure remains problematic. Despite its high prevalence, SDB is often untested and untreated in individuals with SCI. In individuals without SCI, SDB is known to negatively affect physical function and many of the physiological systems that negatively affect physical rehabilitation in SCI. Thus, owing to the high prevalence, under testing, low treatment adherence, and known negative effect on the physical function, it is contended that underdiagnosed SDB in SCI may be confounding physical rehabilitation research studies in individuals with SCI. Studies investigating the effect of treating SDB and its effect on physical rehabilitation in SCI were unable to be located. Thus, studies investigating the likely integrated relationship among physical rehabilitation, SDB, and proper treatment of SDB in SCI are needed. Owing to rapid growth in both sleep medicine and physical rehabilitation intervention research in SCI, the authors contend it is the appropriate time to begin the conversations and collaborations between these fields. We discuss a general overview of SDB and physical training modalities, as well as how SDB could be affecting these studies.

Study Objectives Previous studies reported that the apnea–hypopnea index was similar in young adult Black and White participants. However, whether this similarity reflects an analogous combination of apneas and hypopneas is unknown. Likewise, the physiological mechanisms underlying this similarity has not been explored. Methods 60 Black and 48 White males completed the study. After matching for age and body mass index, 41 participants remained in each group. All participants completed a sleep study. Subsequently, standard sleep indices along with loop gain and the arousal threshold were determined. In addition, airway collapsibility (24 of 60 and 14 of 48 participants) and the hypoxic ventilatory response during wakefulness (30 of 60 and 25 of 48 participants) was measured. Results The apnea–hypopnea index was similar in Blacks and Whites (p = .140). However, the index was comprised of more apneas (p = .014) and fewer hypopneas (p = .025) in Black males. These modifications were coupled to a reduced loop gain (p = .0002) and a more collapsible airway (p = .030). These differences were independent of whether or not the groups were matched. For a given hypoxic response, loop gain was reduced in Black compared to White males (p = .023). Conclusions Despite a similar apnea–hypopnea index, more apneas and fewer hypopneas were evident in young adult Black compared to White males. The physiological mechanisms that contribute to these events were also different between groups. Addressing these differences may be important when considering novel therapeutic approaches to eliminate apnea in Black and White participants.

RATIONALE: Daily exposure to mild intermittent hypoxia (MIH) may elicit beneficial cardiovascular outcomes. OBJECTIVES: To determine the effect of 15 days of MIH and in-home continuous positive airway pressure treatment on blood pressure in participants with obstructive sleep apnea and hypertension. METHODS: We administered MIH during wakefulness 5 days/week for 3 weeks. The protocol consisted of twelve 2-minute bouts of hypoxia interspersed with 2 minutes of normoxia. End-tidal carbon dioxide was maintained 2 mm Hg above baseline values throughout the protocol. Control participants were exposed to a sham protocol (i.e., compressed air). All participants were treated with continuous positive airway pressure over the 3-week period. Results are mean ± SD. MEASUREMENTS AND MAIN RESULTS: Sixteen male participants completed the study (experimental n = 10; control n = 6). Systolic blood pressure at rest during wakefulness over 24 hours was reduced after 15 days of MIH (142.9 ± 8.6 vs. 132.0 ± 10.7 mm Hg; P

Objective: The purpose of this study was to examine the effects of overground locomotor training (OLT) on walking endurance and gastrocnemius oxygen extraction in people with chronic cervical motor-incomplete spinal cord injury (SCI). Design: Prospective single-arm pre–post pilot study. Setting: Human Performance Research Laboratory. Participants: Adult men with traumatic chronic cervical SCI (n = 6; age = 30.8 ± 12.5). Intervention: Twenty-four sessions of structured OLT. Outcome measures: Walking endurance was determined during a constant work-rate time-to-exhaustion treadmill test. Normalized perceived fatigability was calculated by dividing subjective ratings of tiredness by walking time. Cardiorespiratory outcomes and muscle oxygen extraction were analyzed using breath-by-breath gas-exchange and near-infrared spectroscopy. Results: OLT resulted in large effects on walking endurance (1232 ± 446 s vs 1645 ± 255 s; d = 1.1; P = 0.045) and normalized perceived fatigability (5.3 ± 1.5 a.u. vs 3.6 ± 0.9 a.u.; d = 1.3; P = 0.033). Small-to-medium effects on absolute (2.8 ± 2.5 a.u. vs 4.2 ± 3.5 a.u.; d = 0.42; P = 0.035) and isotime (2.8 ± 2.5 a.u. vs 3.8 ± 3.0 a.u.; d = 0.33; P = 0.023) muscle oxygen extraction were also observed after OLT. Conclusion: These findings provide preliminary data supporting the potential for improved walking endurance, enhanced muscle O(2) extraction, and reduced perceived fatigability in people with chronic cervical motor-incomplete SCI following the OLT program described in this study.

KEY POINTS Repeated daily mild intermittent hypoxia has been endorsed as a therapy to promote the recovery of respiratory and limb motor dysfunction. One possible side-effect of this therapy is an increase in apnoeic event number and duration, which is particularly relevant to participants with motor disorders coupled with an increased incidence of sleep apnoea. In this study, we report that increases in apnoeic event number and duration, following exposure to daily intermittent hypoxia, are the result of an increase in respiratory loop gain and the arousal threshold, in participants with obstructive sleep apnoea. Daily exposure to mild intermittent hypoxia also led to an increase in the ventilatory response to arousal. Accordingly, individuals with motor disorders receiving mild intermittent hypoxia as a therapy should be screened for the presence of sleep apnoea, and if present, administration of intermittent hypoxia during hours of wakefulness should be combined with continuous positive airway pressure treatment during sleep. ABSTRACT We determined if exposure to mild intermittent hypoxia (MIH) causes an increase in loop gain (LG) and the arousal threshold (AT) during non-rapid eye movement (NREM) sleep. Male participants with obstructive sleep apnoea (apnoea-hypopnoea index > 5 events/h), matched for age, body mass index and race were divided into two groups (n = 13 in each group). Following a baseline sleep study, one group was exposed to twelve 4-min episodes of hypoxia each day for 10 days and the other group to a sham protocol (SP). On Days 1 and 10, a sleep study was completed following exposure to MIH or the SP. For each sleep study, LG and the AT were measured during NREM sleep, using a model-based approach, and expressed as a fraction of baseline measures. LG increased after exposure to MIH (Day 1: 1.11 ± 0.03, P = 0.002, Day 10: 1.17 ± 0.05, P = 0.001), but not after the SP (Day 1: 1.03 ± 0.04, P = 1.0, Day 10: 1.0 ± 0.02, P = 1.0). AT also increased after exposure to MIH (Day 1: 1.13 ± 0.05, P = 0.01, Day 10: 1.19 ± 0.08, P = 0.05) but not after the SP (Day 1: 1.04 ± 0.05, P = 0.6, Day 10: 0.96 ± 0.04, P = 1.0). Our results might account for increases in apnoea frequency and duration previously observed during NREM sleep following exposure to MIH. Our results also have implications for the use of MIH as a therapeutic modality.

This review explores forms of respiratory and autonomic plasticity, and associated outcome measures, that are initiated by exposure to intermittent hypoxia. The review focuses primarily on studies that have been completed in humans and primarily explores the impact of mild intermittent hypoxia on outcome measures. Studies that have explored two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of ventilation and upper airway muscle activity, are initially reviewed. The role these forms of plasticity might have in sleep disordered breathing are also explored. Thereafter, the role of intermittent hypoxia in the initiation of autonomic plasticity is reviewed and the role this form of plasticity has in cardiovascular and hemodynamic responses during and following intermittent hypoxia is addressed. The role of these responses in individuals with sleep disordered breathing and spinal cord injury are subsequently addressed. Ultimately an integrated picture of the respiratory, autonomic and cardiovascular responses to intermittent hypoxia is presented. The goal of the integrated picture is to address the types of responses that one might expect in humans exposed to one-time and repeated daily exposure to mild intermittent hypoxia. This form of intermittent hypoxia is highlighted because of its potential therapeutic impact in promoting functional improvement and recovery in several physiological systems.

STUDY DESIGN: Pre-Post, Repeated Measures. OBJECTIVES: To determine if a warm-up bout of exercise could elicit a phasic ventilatory response to constant work rate (CWR) exercise in individuals with incomplete spinal cord injury (iSCI) during unsupported CWR treadmill walking. Describe the changes in ventilatory kinetics, ventilatory variability and ratings of perceived exertion (RPE) before and after 12 and 24 weeks of overground locomotor training (OLT). Investigate the relationship among minute ventilation (V(E)) variability, RPE, and walking endurance. SETTING: Laboratory. METHODS: A 6-min CWR was used as a warm-up preceding a CWR, at the same walking speed, until volitional fatigue or 30 min. Breath-by-breath ventilatory data were examined during the second CWR using a mono-exponential model. V(E) variability was calculated as the difference between the observed and predicted values. Data were time-matched before and after 12 and 24 weeks of OLT. A Pearson’s correlation was used for V(E) variability, RPE, and walking endurance. RESULTS: A warm-up CWR did elicit a phasic ventilatory response. OLT resulted in faster ventilatory kinetics. Ventilatory variability reduced after 12 weeks of OLT but returned to pre-OLT values after 24 weeks of training. The change in V(E) variability was correlated with the change in RPE throughout the study. 12 and 24 weeks of OLT resulted in significant improvements in treadmill walking time. CONCLUSIONS: SCI patients can achieve a phasic ventilatory response to walking if the exercise bout is preceded by a warm-up. OLT normalizes the ventilatory kinetics and improves walking endurance. The change in V(E) variability is correlated to RPE.