Do circadian rhythms in respiratory control contribute to sleep-related breathing disorders?

New and Emerging Approaches to Better Define Sleep Disruption and Its Consequences

Current approaches to quantify and diagnose sleep disorders and circadian rhythm disruption are imprecise, laborious, and often do not relate well to key clinical and health outcomes. Newer emerging approaches that aim to overcome the practical and technical constraints of current sleep metrics have considerable potential to better explain sleep disorder pathophysiology and thus to more precisely align diagnostic, treatment and management approaches to underlying pathology. These include more fine-grained and continuous EEG signal feature detection and novel oxygenation metrics to better encapsulate hypoxia duration, frequency, and magnitude readily possible via more advanced data acquisition and scoring algorithm approaches. Recent technological advances may also soon facilitate simple assessment of circadian rhythm physiology at home to enable sleep disorder diagnostics even for “non-circadian rhythm” sleep disorders, such as chronic insomnia and sleep apnea, which in many cases also include a circadian disruption component. Bringing these novel approaches into the clinic and the home settings should be a priority for the field. Modern sleep tracking technology can also further facilitate the transition of sleep diagnostics from the laboratory to the home, where environmental factors such as noise and light could usefully inform clinical decision-making. The “endpoint” of these new and emerging assessments will be better targeted therapies that directly address underlying sleep disorder pathophysiology via an individualized, precision medicine approach. This review outlines the current state-of-the-art in sleep and circadian monitoring and diagnostics and covers several new and emerging approaches to better define sleep disruption and its consequences.

Bi-directional relationships between co-morbid insomnia and sleep apnea (COMISA)

Insomnia and obstructive sleep apnea (OSA) commonly co-occur. Approximately 30-50% of patients with OSA report clinically significant insomnia symptoms, and 30-40% of patients with chronic insomnia fulfil diagnostic criteria for OSA. Compared to either insomnia or OSA alone, co-morbid insomnia and sleep apnea (COMISA) is associated with greater morbidity for patients, complex diagnostic decisions for clinicians, and reduced response to otherwise effective treatment approaches. Potential bi-directional causal relationships between the mechanisms and manifestations of insomnia and OSA could play an integral role in the development and management of COMISA. A greater understanding of these relationships is required to guide personalized diagnostic and treatment approaches for COMISA. This review summarizes the available evidence of bi-directional relationships between COMISA, including epidemiological research, case studies, single-arm treatment studies, randomized controlled treatment trials, and objective sleep study data. This evidence is integrated into a conceptual model of COMISA to help refine the understanding of potential bi-directional causal relationships between the two disorders. This theoretical framework is essential to help guide future research, improve diagnostic tools, determine novel therapeutic targets, and guide tailored sequenced and multi-faceted treatment approaches for this common, complex, and debilitating condition.

Particularities and Clinical Applicability of Saccharin Transit Time Test

Introduction  The importance of mucociliary clearance (MCC) for the respiratory system homeostasis is clear. Therefore, evaluating this defense mechanism is fundamental in scientific research and in the clinical practice of pulmonology and of associated areas. However, MCC evaluation has not been so usual due to the complexity of methods that use radiolabeled particles. Nevertheless, as an interesting alternative, there is the saccharin transit time (STT) test. This method is reproducible, simple to perform, noninvasive, does not demand high costs, and has been widely used in studies of nasal MCC. Although the STT test is widely used, there is still lack of a detailed description of its realization. Objective  The present literature review aims to provide basic information related to the STT test and to present the findings of the previous studies that used this method, discussing variations in its execution, possible influences on the obtained results and limitations of the method, as well as to relate our experience with the use of STT in researches. Data Synthesis  There are several factors that can alter the results obtained from STT tests, which would raise difficulties with proper interpretation and with the discussion of the results among different studies. Conclusions  Saccharin transit time is a widely used method for the evaluation of nasal MCC, and therefore, the standardization related to the previous and concurrent to test orientations, and also its execution, become essential to improve its accuracy, and allow comparisons among different studies.

Time of day affects chemoreflex sensitivity and the carbon dioxide reserve during NREM sleep in participants with sleep apnea

Our investigation was designed to determine whether the time of day affects the carbon dioxide reserve and chemoreflex sensitivity during non-rapid eye movement (NREM) sleep. Ten healthy men with obstructive sleep apnea completed a constant routine protocol that consisted of sleep sessions in the evening (10 PM to 1 AM), morning (6 AM to 9 AM), and afternoon (2 PM to 5 PM). Between sleep sessions, the participants were awake. During each sleep session, core body temperature, baseline levels of carbon dioxide (PET(CO2)) and minute ventilation, as well as the PET(CO2) that demarcated the apneic threshold and hypocapnic ventilatory response, were measured. The nadir of core body temperature during sleep occurred in the morning and was accompanied by reductions in minute ventilation and PetCO2 compared with the evening and afternoon (minute ventilation: 5.3 ± 0.3 vs. 6.2 ± 0.2 vs. 6.1 ± 0.2 l/min, P < 0.02; PET(CO2): 39.7 ± 0.4 vs. 41.4 ± 0.6 vs. 40.4 ± 0.6 Torr, P < 0.02). The carbon dioxide reserve was reduced, and the hypocapnic ventilatory response increased in the morning compared with the evening and afternoon (carbon dioxide reserve: 2.1 ± 0.3 vs. 3.6 ± 0.5 vs. 3.5 ± 0.3 Torr, P < 0.002; hypocapnic ventilatory response: 2.3 ± 0.3 vs. 1.6 ± 0.2 vs. 1.8 ± 0.2 l·min(-1)·mmHg(-1), P < 0.001). We conclude that time of day affects chemoreflex properties during sleep, which may contribute to increases in breathing instability in the morning compared with other periods throughout the day/night cycle in individuals with sleep apnea.

Brown Norway and Zucker Lean rats demonstrate circadian variation in ventilation and sleep apnea

STUDY OBJECTIVES

Circadian rhythms influence many biological systems, but there is limited information about circadian and diurnal variation in sleep related breathing disorder. We examined circadian and diurnal patterns in sleep apnea and ventilatory patterns in two rat strains, one with high sleep apnea propensity (Brown Norway [BN]) and the other with low sleep apnea propensity (Zucker Lean [ZL]).

DESIGN/SETTING

Chronically instrumented rats were randomized to breathe room air (control) or 100% oxygen (hyperoxia), and we performed 20-h polysomnography beginning at Zeitgeber time 4 (ZT 4; ZT 0 = lights on, ZT12 = lights off). We examined the effect of strain and inspired gas (twoway analysis of variance) and analyzed circadian and diurnal variability.

MEASUREMENTS AND RESULTS

Strain and inspired gas-dependent differences in apnea index (AI; apneas/h) were particularly prominent during the light phase. AI in BN rats (control, 16.9 ± 0.9; hyperoxia, 34.0 ± 5.8) was greater than in ZL rats (control, 8.5 ± 1.0; hyperoxia, 15.4 ± 1.1, [strain effect, P < 0.001; gas effect, P = 0.001]). Hyperoxia reduced respiratory frequency in both strains, and all respiratory pattern variables demonstrated circadian variability. BN rats exposed to hyperoxia demonstrated the largest circadian fluctuation in AI (amplitude = 17.9 ± 3.7 apneas/h [strain effect, P = 0.01; gas effect, P < 0.001; interaction, P = 0.02]; acrophase = 13.9 ± 0.7 h; r (2) = 0.8 ± 1.4).

CONCLUSIONS

Inherited, environmental, and circadian factors all are important elements of underlying sleep related breathing disorder. Our method to examine sleep related breathing disorder phenotypes in rats may have implications for understanding vulnerability for sleep related breathing disorder in humans.

Intermittent hypoxia, respiratory plasticity and sleep apnea in humans: present knowledge and future investigations

This review examines the role that respiratory plasticity has in the maintenance of breathing stability during sleep in individuals with sleep apnea. The initial portion of the review considers the manner in which repetitive breathing events may be initiated in individuals with sleep apnea. Thereafter, the role that two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of upper airway and respiratory muscle activity, might have in modifying breathing events in humans is examined. In this context, present knowledge regarding the initiation of respiratory plasticity in humans during wakefulness and sleep is addressed. Also, published findings which reveal that exposure to intermittent hypoxia promotes breathing instability, at least in part, because of progressive augmentation of the hypoxic ventilatory response and the absence of long-term facilitation, are considered. Next, future directions are presented and are focused on the manner in which forms of plasticity that stabilize breathing might be promoted while diminishing destabilizing forms, concurrently. These future directions will consider the potential role of circadian rhythms in the promotion of respiratory plasticity and the role of respiratory plasticity in enhancing established treatments for sleep apnea.

Circadian rhythms and sleep-related breathing disorders

Recent studies have provided evidence that the human circadian timing system has an influence on respiration and respiratory control. Both sleep and circadian mechanisms combine to mediate the rise in lower airway resistance in nocturnal asthma. In rats, circadian rhythms in minute ventilation are present in both wakefulness and sleep, implying that circadian and sleep mechanisms also combine to influence the control of breathing. The circadian timing system causes a nocturnal increase in the chemoreflex threshold and it is suggested that this may increase the propensity for nocturnal sleep apnea. This hypothesis is supported by a model analysis of human chemoreflex control, and relevant published data are reviewed. The clinical implications of this putative circadian contribution to sleep apnea are potentially very significant, but relevant data are scarce and directions for future research are discussed.

The effect of time of day on apnoea index in the sleeping rat

This study tested the hypothesis that apnoea index would be greater during daytime sleep than nighttime sleep in the rat. Electroencephalogram and electromyogram were monitored via biotelemetry implant and respiration was measured using whole body plethysmography in six male rats in two separate 34h recording sessions per animal. Apnoeas were classified as "spontaneous" or "post-sigh". Daily average spontaneous apnoea index was 35 times greater (p<0.0001) during rapid eye movement (REM) sleep than in non-REM (NREM) sleep. In contrast, daily average post-sigh apnoea index was not significantly greater in REM sleep than in non-REM (NREM) sleep (p=0.39). There was a greater post-sigh apnoea index during daytime REM than during nighttime REM (p=0.043) but REM-related spontaneous apnoea index was unaffected by time of day. There was no day to night difference in spontaneous apnoea index or post-sigh apnoea index during NREM sleep. Respiratory variability (coefficient of variation for breath duration and tidal volume) was not affected by time of day in REM or NREM sleep. We conclude that the circadian timing system has no effect on apnoea index during NREM sleep in the rat, but it may influence the propensity for post-sigh apnoea during REM sleep.

Overnight changes of chemoreflex control in obstructive sleep apnoea patients

We hypothesized that the numerous episodes of hypoxia, hypercapnia and arousal experienced by obstructive sleep apnoea (OSA) patients induce overnight changes in respiratory chemoreflexes. A modification of the Read rebreathing technique assessed chemoreflex characteristics in the evening and the morning of patients undergoing diagnostic assessment for OSA in a clinical sleep laboratory. Two groups were studied: those with apnoea-hypopnoea indices (AHI) greater than 30 composed the OSA group (n = 12), and those with AHI indices less than 10 composed the non-OSA group (n = 12). There was a significant (approximately 30%) overnight increase in chemoreflex sensitivities, without changes in thresholds, in the OSA group. In the non-OSA group there was a significant overnight reduction in chemoreflex thresholds (approximately 5%), without changes in sensitivities. We suggest that these changes affect the stability of the chemoreflex control system in opposite ways as the night proceeds: destabilizing breathing for patients in the OSA group, and stabilising breathing for patients in the non-OSA group.