Aging increases upper airway collapsibility in Fischer 344 rats

Exploring hypoglossal nerve stimulation therapy for obstructive sleep apnea: A comprehensive review of clinical and physiological upper airway outcomes

Obstructive sleep apnea (OSA) is a chronic disorder characterized by recurrent episodes of upper airway collapse during sleep, which can lead to serious health issues like cardiovascular disease and neurocognitive impairments. While positive airway pressure serves as the standard treatment, intolerance in some individuals necessitates exploration of alternative therapies. Hypoglossal nerve stimulation (HGNS) promises to mitigate OSA morbidity by stimulating the tongue muscles to maintain airway patency. However, its effectiveness varies, prompting research for optimization. This review summarizes the effects of HGNS on upper airway obstruction from human and animal studies. It examines physiological responses including critical closing pressure, maximal airflow, nasal and upper airway resistance, compliance, stiffness, and geometry. Interactions among these parameters and discrepant findings in animal and human studies are explored. Additionally, the review summarizes the impact of HGNS on established OSA metrics, such as the apnea-hypopnea index, oxygen desaturation index, and sleep arousals. Various therapeutic modalities, including selective unilateral or bilateral HGNS, targeted unilateral HGNS, and whole unilateral or bilateral HGNS, are discussed. This review consolidates our understanding of HGNS mechanisms, fostering exploration of under-investigated outcomes and approaches to drive advancements in HGNS therapy.

An assessment of a simple clinical technique to estimate pharyngeal collapsibility in people with obstructive sleep apnea

Study Objectives

Quantification of upper airway collapsibility in obstructive sleep apnea (OSA) could help inform targeted therapy decisions. However, current techniques are clinically impractical. The primary aim of this study was to assess if a simple, novel technique could be implemented as part of a continuous positive airway pressure (CPAP) titration study to assess pharyngeal collapsibility.

Methods

A total of 35 participants (15 female) with OSA (mean ± SD apnea–hypopnea index = 35 ± 19 events/h) were studied. Participants first completed a simple clinical intervention during a routine CPAP titration, where CPAP was transiently turned off from the therapeutic pressure for ≤5 breaths/efforts on ≥5 occasions during stable non-rapid eye movement (non-REM) sleep for quantitative assessment of airflow responses (%peak inspiratory flow [PIF] from preceding 5 breaths). Participants then underwent an overnight physiology study to determine the pharyngeal critical closing pressure (Pcrit) and repeat transient drops to zero CPAP to assess airflow response reproducibility.

Results

Mean PIF of breaths 3–5 during zero CPAP on the simple clinical intervention versus the physiology night were similar (34 ± 29% vs. 28 ± 30% on therapeutic CPAP, p = 0.2; range 0%–90% vs. 0%–95%). Pcrit was −1.0 ± 2.5 cmH2O (range −6 to 5 cmH2O). Mean PIF during zero CPAP on the simple clinical intervention and the physiology night correlated with Pcrit (r = −0.7 and −0.9, respectively, p < 0.0001). Receiver operating characteristic curve analysis indicated significant diagnostic utility for the simple intervention to predict Pcrit < −2 and < 0 cmH2O (AUC = 0.81 and 0.92), respectively.

Conclusions

A simple CPAP intervention can successfully discriminate between patients with and without mild to moderately collapsible pharyngeal airways. This scalable approach may help select individuals most likely to respond to non-CPAP therapies.

Aging Induced p53/p21 in Genioglossus Muscle Stem Cells and Enhanced Upper Airway Injury

Aging of population brings related social problems, such as muscle attenuation and regeneration barriers with increased aging. Muscle repair and regeneration depend on muscle stem cells (MuSCs). Obstructive sleep apnea (OSA) rises in the aging population. OSA leads to hypoxia and upper airway muscle injury. However, little is known about the effect of increasing age and hypoxia to the upper airway muscle. The genioglossus (GG) is the major dilator muscle to keep the upper airway open. Here, we reported that muscle fiber and MuSC function declined with aging in GG. Increasing age also decreased the migration and proliferation of GG MuSCs. p53 and p21 were high expressions both in muscle tissue and in GG MuSCs. We further found that hypoxia inhibited GG MuSC proliferation and decreased myogenic differentiation. Then, hypoxia enhanced the inhibition effect of aging to proliferation and differentiation. Finally, we investigated that hypoxia and aging interact to form a vicious circle with upregulation of p53 and p21. This vicious hypoxia plus aging damage accelerated upper airway muscle injury. Aging and hypoxia are the major damage elements in OSA patients, and we propose that the damage mechanism of hypoxia and aging in GG MuSCs will help to improve upper airway muscle regeneration.

A randomized, double crossover study to investigate the influence of saline infusion on sleep apnea severity in men

STUDY OBJECTIVES

Obstructive sleep apnea (OSA) is commoner in patients with fluid-retaining states than in those without fluid retention, in men than in women, and worsens with aging. In men, OSA severity is related to the amount of fluid shifting out of the legs overnight, but a cause-effect relationship is not established. Our objective was to test the hypothesis that mimicking fluid overload during sleep would increase severity of OSA more in older (≥ 40 years) than in younger men (< 40 years).

DESIGN

Randomized, single-blind, double crossover study.

SETTING

Research sleep laboratory.

PATIENTS OR PARTICIPANTS

Seven older and 10 younger men with non-severe or no sleep apnea, matched for body mass index.

INTERVENTIONS

During the control arm, normal saline was infused to keep the vein open. During intervention, subjects received an intravenous bolus of normal saline (22 mL/kg body weight) after sleep onset while they were wearing compression stockings to prevent fluid accumulation in the legs.

MEASUREMENTS AND RESULTS

Compared to younger men, infusion of similar amounts of saline in older men caused a greater increase in neck circumference (P < 0.05) and in the AHI (32.2 ± 22.1 vs. 2.2 ± 7.1, P = 0.002).

CONCLUSIONS

Older men are more susceptible to the adverse effects of intravenous fluid loading on obstructive sleep apnea severity than younger men. This may be due to age-related differences in the amount of fluid accumulating in the neck or upper airway collapsibility in response to intravenous fluid loading. These possibilities remain to be tested in future studies.

Leptin and the control of pharyngeal patency during sleep in severe obesity

RATIONALE

Obesity imposes mechanical loads on the upper airway, resulting in flow limitation and obstructive sleep apnea (OSA). In previous animal models, leptin has been considered to serve as a stimulant of ventilation and may prevent respiratory depression during sleep. We hypothesized that variations in leptin concentration among similarly obese individuals will predict differences in compensatory responses to upper airway obstruction during sleep.

METHODS

An observational study was conducted in 23 obese women [body mass index (BMI): 46 ± 3 kg/m(2), age: 41 ± 12 yr] and 3 obese men (BMI: 46 ± 3 kg/m(2), age: 43 ± 4 yr). Subjects who were candidates for bariatric surgery were recruited to determine upper airway collapsibility under hypotonic conditions [pharyngeal critical pressure (passive PCRIT)], active neuromuscular responses to upper airway obstruction during sleep, and overnight fasting serum leptin levels. Compensatory responses were defined as the differences in peak inspiratory airflow (ΔVImax), inspired minute ventilation (ΔVI), and pharyngeal critical pressure (ΔPCRIT) between the active and passive conditions.

RESULTS

Leptin concentration was not associated with sleep disordered breathing severity, passive PCRIT, or baseline ventilation. In the women, increases in serum leptin concentrations were significantly associated with increases in ΔVImax (r(2) = 0.44, P < 0.001), ΔVI (r(2) = 0.40, P < 0.001), and ΔPCRIT (r(2) = 0.19, P < 0.04). These responses were independent of BMI, waist-to-hip ratio, neck circumference, or sagittal girth.

CONCLUSION

Leptin may augment neural compensatory mechanisms in response to upper airway obstruction, minimizing upper airway collapse, and/or mitigating potential OSA severity. Variability in leptin concentration among similarly obese individuals may contribute to differences in OSA susceptibility.

Differential effects of targeted tongue exercise and treadmill running on aging tongue muscle structure and contractile properties

Age-associated changes in tongue muscle structure and strength may contribute to dysphagia in elderly people. Tongue exercise is a current treatment option. We hypothesized that targeted tongue exercise and nontargeted exercise that activates tongue muscles as a consequence of increased respiratory drive, such as treadmill running, are associated with different patterns of tongue muscle contraction and genioglossus (GG) muscle biochemistry. Thirty-one young adult, 34 middle-aged, and 37 old Fischer 344/Brown Norway rats received either targeted tongue exercise, treadmill running, or no exercise (5 days/wk for 8 wk). Protrusive tongue muscle contractile properties and myosin heavy chain (MHC) composition in the GG were examined at the end of 8 wk across groups. Significant age effects were found for maximal twitch and tetanic tension (greatest in young adult rats), MHCIIb (highest proportion in young adult rats), MHCIIx (highest proportion in middle-aged and old rats), and MHCI (highest proportion in old rats). The targeted tongue exercise group had the greatest maximal twitch tension and the highest proportion of MHCI. The treadmill running group had the shortest half-decay time, the lowest proportion of MHCIIa, and the highest proportion of MHCIIb. Fatigue was significantly less in the young adult treadmill running group and the old targeted tongue exercise group than in other groups. Thus, tongue muscle structure and contractile properties were affected by both targeted tongue exercise and treadmill running, but in different ways. Studies geared toward optimizing dose and manner of providing targeted and generalized tongue exercise may lead to alternative tongue exercise delivery strategies.

Respiratory control and sternohyoid muscle structure and function in aged male rats: decreased susceptibility to chronic intermittent hypoxia

Obstructive sleep apnoea syndrome (OSAS) is a common respiratory disorder characterized by chronic intermittent hypoxia (CIH). We have shown that CIH causes upper airway muscle dysfunction in the rat due to oxidative stress. Ageing is an independent risk factor for the development of OSAS perhaps due to respiratory muscle remodelling and increased susceptibility to hypoxia. We sought to examine the effects of CIH on breathing and pharyngeal dilator muscle structure and function in aged rats. Aged (18-20 months), male Wistar rats were exposed to alternating cycles of normoxia and hypoxia (90 s each; F(I)O(2)=5% O(2) at nadir) or sham treatment for 8h/day for 9 days. Following CIH exposure, breathing was assessed by whole-body plethysmography. In addition, sternohyoid muscle contractile and endurance properties were examined in vitro. Muscle fibre type and cross-sectional area, and the activity of key oxidative and glycolytic enzymes were determined. CIH had no effect on basal breathing or ventilatory responses to hypoxia or hypercapnia. CIH did not alter succinate dehydrogenase or glycerol phosphate dehydrogenase enzyme activities, myosin heavy chain fibre areal density or cross-sectional area. Sternohyoid muscle force and endurance were unaffected by CIH exposure. Since we have established that this CIH paradigm causes sternohyoid muscle weakness in adult male rats, we conclude that aged rats have decreased susceptibility to CIH-induced stress. We suggest that structural remodelling with improved hypoxic tolerance in upper airway muscles may partly compensate for impaired neural regulation of the upper airway and increased propensity for airway collapse in aged mammals.

Structural and functional properties of an upper airway dilator muscle in aged obese male rats

BACKGROUND

Age, obesity and male sex are risk factors for the development of obstructive sleep apnoea syndrome.

OBJECTIVE

We examined structural and functional properties of the sternohyoid muscle in young lean and aged obese male rats. We hypothesized that the aged muscle would be vulnerable to oxidative stress (hypoxia).

METHODS

Isometric contractile and endurance properties of the sternohyoid muscle were assessed in vitro with or without the superoxide scavenger Tempol (10 mM). Muscle fibre size and density were determined by myosin heavy chain immunofluorescence. Succinate dehydrogenase (SDH) and glycerol-3- phosphate dehydrogenase (GPDH) enzyme activities were determined.

RESULTS

Fibre hypertrophy, increased fast twitch (type 2X) fibre density, decreased SDH activity and increased GPDH activity, together with increased force and fatigue, were observed in aged obese muscles compared to young lean muscles. Tempol treatment increased strength and sensitivity to stimulation. Hypoxic depression of force was ameliorated by antioxidant treatment with equivalent effects in young lean and aged obese muscle.

CONCLUSIONS

We conclude that the rat sternohyoid exhibits indefinite growth and is protected from oxidative stress as the animal ages.

Effect of age and weight on upper airway function in a mouse model

Defects in pharyngeal mechanical and neuromuscular control are required for the development of obstructive sleep apnea. Obesity and age are known sleep apnea risk factors, leading us to hypothesize that specific defects in upper airway neuromechanical control are associated with weight and age in a mouse model. In anesthetized, spontaneously breathing young and old wild-type C57BL/6J mice, genioglossus electromyographic activity (EMG(GG)) was monitored and upper airway pressure-flow dynamics were characterized during ramp decreases in nasal pressure (Pn, cmH₂O). Specific body weights were targeted by controlling caloric intake. The passive critical pressure (Pcrit) was derived from pressure-flow relationships during expiration. The Pn threshold at which inspiratory flow limitation (IFL) developed and tonic and phasic EMG(GG) activity during IFL were quantified to assess the phasic modulation of pharyngeal patency. The passive Pcrit increased progressively with increasing body weight and increased more in the old than young mice. Tonic EMG(GG) decreased and phasic EMG(GG) increased significantly with obesity. During ramp decreases in Pn, IFL developed at a higher (less negative) Pn threshold in the obese than lean mice, although the frequency of IFL decreased with age and weight. The findings suggest that weight imposes mechanical loads on the upper airway that are greater in the old than young mice. The susceptibility to upper airway obstruction increases with age and weight as tonic neuromuscular activity falls. IFL can elicit phasic responses in normal mice that mitigate or eliminate the obstruction altogether.