Periodic breathing and the pathogenesis of occlusive sleep apneas

Autonomic regulation during sleep and wakefulness: a review with implications for defining the pathophysiology of neurological disorders

Cardiovascular and respiratory parameters change during sleep and wakefulness. This observation underscores an important, albeit incompletely understood, role for the central nervous system in the differential regulation of autonomic functions. Understanding sleep/wake-dependent sympathetic modulations provides insights into diseases involving autonomic dysfunction. The purpose of this review was to define the central nervous system nuclei regulating sleep and cardiovascular function and to identify reciprocal networks that may underlie autonomic symptoms of disorders such as insomnia, sleep apnea, restless leg syndrome, rapid eye movement sleep behavior disorder, and narcolepsy/cataplexy. In this review, we examine the functional and anatomical significance of hypothalamic, pontine, and medullary networks on sleep, cardiovascular function, and breathing.

Cardioventilatory Control in Preterm-born Children and the Risk of Obstructive Sleep Apnea

RATIONALE

The contribution of ventilatory control to the pathogenesis of obstructive sleep apnea (OSA) in preterm-born children is unknown.

OBJECTIVES

To characterize phenotypes of ventilatory control that are associated with the presence of OSA in preterm-born children during early childhood.

METHODS

Preterm- and term-born children without comorbid conditions were enrolled. They were categorized into an OSA group and a non-OSA group on the basis of polysomnography.

MEASUREMENTS AND MAIN RESULTS

Loop gain, controller gain, and plant gain, reflecting ventilatory instability, chemoreceptor sensitivity, and blood gas response to a change in ventilation, respectively, were estimated from spontaneous sighs identified during polysomnography. Cardiorespiratory coupling, a measure of brainstem maturation, was estimated by measuring the interval between inspiration and the preceding electrocardiogram R-wave. Cluster analysis was performed to develop phenotypes based on controller gain, plant gain, cardiorespiratory coupling, and gestational age. The study included 92 children, 63 of whom were born preterm (41% OSA) and 29 of whom were born at term (48% OSA). Three phenotypes of ventilatory control were derived with risks for OSA being 8%, 47%, and 77% in clusters 1, 2, and 3, respectively. There was a stepwise decrease in controller gain and an increase in plant gain from clusters 1 to 3. Children in cluster 1 had significantly higher cardiorespiratory coupling and gestational age than clusters 2 and 3. No difference in loop gain was found between clusters.

CONCLUSIONS

The risk for OSA could be stratified according to controller gain, plant gain, cardiorespiratory coupling, and gestational age. These findings could guide personalized care for children at risk for OSA.

Modest Improvement of Untreated Severe Sleep-Disordered Breathing in the Middle-Aged and Elderly

OBJECTIVE

It has been reported that untreated sleep-disordered breathing (SDB) deteriorates over time, however this remains contentious. The aim of the present study is to evaluate the clinical course of SDB in middle-aged and older SDB patients, and to identify how relevant factors contribute to the change in SDB severity.

METHODS

Baseline and follow-up polysomnographic data of 56 untreated SDB patients (mean age, 61.2±5.71) were obtained retrospectively and the mean interval was 62.4±22.0 months. Subgroup analysis was performed based on the baseline severity, and the factors associated with the course of SDB were analyzed.

RESULTS

At the baseline, 13 subjects were simple snorers, 15 had mild to moderate SDB, and 28 were severe SDB patients. While there was no significant change in apnea-hypopnea index (AHI) as a whole, subgroup analysis showed decrease of AHI in severe SDB patients (43.9±10.6 to 35.6±20.0, p=0.009). The change in supine time percent and baseline AHI were associated with the change in AHI (β=0.387, p=0.003; β=-0.272, p=0.037).

CONCLUSION

Untreated SDB did not deteriorate over time with modest improvement in severe SDB. A proportion of severe SDB patients might expect decrease in SDB severity irrespective of changes in sleep position or body weight.

The structural changes of pharyngeal airway contributing to snoring after orthognathic surgery in skeletal class III patients

BACKGROUND

Two-jaw surgery including mandibular and maxillary backward movement procedures are commonly performed to correct class III malocclusion. Bimaxillary surgery can reposition the maxillofacial bone together with soft tissue, such as the soft palate and the tongue base. We analyzed changes of pharyngeal airway narrowing to ascertain clinical correlations with the prevalence of snoring after two-jaw surgery.

METHODS

A prospective clinical study was designed including a survey on snoring and three-dimensional (3D) computed tomography (CT) in class III malocclusion subjects before and after bimaxillary surgery. We conducted an analysis on changes of the posterior pharyngeal space find out clinical correlations with the prevalence of snoring.

RESULTS

Among 67 subjects, 12 subjects complained about snoring 5 weeks after the surgical correction, and examining the 12 subjects after 6 months, 6 patients complained about the snoring. The current findings demonstrated the attenuation of the largest transverse width (LTW), anteroposterior length (APL), and cross-sectional area (CSA) following bimaxillary surgery given to class III malocclusion patients, particularly at the retropalatal level. The average distance of maxillary posterior movements were measured to be relatively higher (horizontal distance 3.9 mm, vertical distance 2.6 mm) in case of new snorers.

CONCLUSIONS

This study found that bimaxillary surgery could lead to the narrowing of upper airway at the retropalatal or retroglossal level as well as triggering snoring in subjects with class III malocclusion. Based on the current clinical findings, we also found that upper airway narrowing at retropalatal level may contribute to increasing the probability of snoring and that polysonography may need to be performed before orthognathic surgery in subjects with class III malocclusion.

Dynamic loop gain increases upon adopting the supine body position during sleep in patients with obstructive sleep apnoea

BACKGROUND AND OBJECTIVE

Obstructive sleep apnoea (OSA) is typically worse in the supine versus lateral sleeping position. One potential factor driving this observation is a decrease in lung volume in the supine position which is expected by theory to increase a key OSA pathogenic factor: dynamic ventilatory control instability (i.e. loop gain). We aimed to quantify dynamic loop gain in OSA patients in the lateral and supine positions, and to explore the relationship between change in dynamic loop gain and change in lung volume with position.

METHODS

Data from 20 patients enrolled in previous studies on the effect of body position on OSA pathogenesis were retrospectively analysed. Dynamic loop gain was calculated from routinely collected polysomnographic signals using a previously validated mathematical model. Lung volumes were measured in the awake state with a nitrogen washout technique.

RESULTS

Dynamic loop gain was significantly higher in the supine than in the lateral position (0.77 ± 0.15 vs 0.68 ± 0.14, P = 0.012). Supine functional residual capacity (FRC) was significantly lower than lateral FRC (81.0 ± 15.4% vs 87.3 ± 18.4% of the seated FRC, P = 0.021). The reduced FRC we observed on moving to the supine position was predicted by theory to increase loop gain by 10.2 (0.6, 17.1)%, a value similar to the observed increase of 8.4 (-1.5, 31.0)%.

CONCLUSION

Dynamic loop gain increased by a small but statistically significant amount when moving from the lateral to supine position and this may, in part, contribute to the worsening of OSA in the supine sleeping position.

Pathogenesis of central and complex sleep apnoea

Central sleep apnoea (CSA) - the temporary absence or diminution of ventilatory effort during sleep - is seen in a variety of forms including periodic breathing in infancy and healthy adults at altitude and Cheyne-Stokes respiration in heart failure. In most circumstances, the cyclic absence of effort is paradoxically a consequence of hypersensitive ventilatory chemoreflex responses to oppose changes in airflow, that is elevated loop gain, leading to overshoot/undershoot ventilatory oscillations. Considerable evidence illustrates overlap between CSA and obstructive sleep apnoea (OSA), including elevated loop gain in patients with OSA and the presence of pharyngeal narrowing during central apnoeas. Indeed, treatment of OSA, whether via continuous positive airway pressure (CPAP), tracheostomy or oral appliances, can reveal CSA, an occurrence referred to as complex sleep apnoea. Factors influencing loop gain include increased chemosensitivity (increased controller gain), reduced damping of blood gas levels (increased plant gain) and increased lung to chemoreceptor circulatory delay. Sleep-wake transitions and pharyngeal dilator muscle responses effectively raise the controller gain and therefore also contribute to total loop gain and overall instability. In some circumstances, for example apnoea of infancy and central congenital hypoventilation syndrome, central apnoeas are the consequence of ventilatory depression and defective ventilatory responses, that is low loop gain. The efficacy of available treatments for CSA can be explained in terms of their effects on loop gain, for example CPAP improves lung volume (plant gain), stimulants reduce the alveolar-inspired PCO₂ difference and supplemental oxygen lowers chemosensitivity. Understanding the magnitude of loop gain and the mechanisms contributing to instability may facilitate personalized interventions for CSA.

Model-based stability assessment of ventilatory control in overweight adolescents with obstructive sleep apnea during NREM sleep

Obstructive sleep apnea (OSA) involves the interplay of several different factors such as an unfavorable upper airway anatomy, deficiencies in pharyngeal muscle responsiveness, a low arousal threshold, and ventilatory control instability. Although the stability of ventilatory control has been extensively studied in adults, little is known about its characteristics in the pediatric population. In this study, we developed a novel experimental setup that allowed us to perturb the respiratory system during natural non-rapid eye movement (NREM) sleep conditions by manipulating the inspiratory pressure, provided by a bilevel pressure ventilator, to induce sighs after upper airway stabilization. Furthermore, we present a modeling framework that utilizes the noninvasively measured ventilatory responses to the induced sighs and spontaneous breathing data to obtain representations of the processes involved in the chemical regulation of respiration and extract their stability characteristics. After validation with simulated data, the modeling technique was applied to data collected experimentally from 11 OSA and 15 non-OSA overweight adolescents. Statistical analysis of the model-derived stability parameters revealed a significantly higher plant gain and lower controller gain in the OSA group (P = 0.046 and P = 0.007, respectively); however, no differences were found in loop gain (LG) and circulatory time delay between the groups. OSA severity and LG, within the 0.03-0.04-Hz frequency band, were significantly negatively associated (r = -0.434, P = 0.026). Contrary to what has been found in adults, our results suggest that in overweight adolescents, OSA is unlikely to be initiated through ventilatory instability resulting from elevated chemical loop gain.

Altered breathing syndrome in heart failure: newer insights and treatment options

In patients with heart failure (HF), altered breathing patterns, including periodic breathing, Cheyne-Stokes breathing, and oscillatory ventilation, are seen in several situations. Since all forms of altered breathing cause similar detrimental effects on clinical outcomes, they may be considered collectively as an "altered breathing syndrome." Altered breathing syndrome should be recognized as a comorbid condition of HF and as a potential therapeutic target. In this review, we discuss mechanisms and therapeutic options of altered breathing while sleeping, while awake at rest, and during exercise.

Obstructive sleep apnea syndrome: from phenotype to genetic basis

Obstructive sleep apnea syndrome (OSAS) is a complex chronic clinical syndrome, characterized by snoring, periodic apnea, hypoxemia during sleep, and daytime hypersomnolence. It affects 4-5% of the general population. Racial studies and chromosomal mapping, familial studies and twin studies have provided evidence for the possible link between the OSAS and genetic factors and also most of the risk factors involved in the pathogenesis of OSAS are largely genetically determined. A percentage of 35-40% of its variance can be attributed to genetic factors. It is likely that genetic factors associated with craniofacial structure, body fat distribution and neural control of the upper airway muscles interact to produce the OSAS phenotype. Although the role of specific genes that influence the development of OSAS has not yet been identified, current researches, especially in animal model, suggest that several genetic systems may be important. In this chapter, we will first define the OSAS phenotype, the pathogenesis and the risk factors involved in the OSAS that may be inherited, then, we will review the current progress in the genetics of OSAS and suggest a few future perspectives in the development of therapeutic agents for this complex disease entity.

Increased propensity for central apnea in patients with obstructive sleep apnea: effect of nasal continuous positive airway pressure

RATIONALE

There is increasing evidence of increased ventilatory instability in patients with obstructive sleep apnea (OSA), but previous investigations have not studied whether the hypocapnic apneic threshold is altered in this group.

OBJECTIVES

To compare the apneic threshold, CO2 reserve, and controller gain between subjects with and without OSA matched for age, sex, and body mass index.

METHODS

Hypocapnia was induced via nasal mechanical ventilation for 3 minutes. Cessation of mechanical ventilation resulted in hypocapnic central hypopnea or apnea depending upon the magnitude of the hypocapnia. The apnea threshold (Pet(CO2)-AT) was defined as the measured Pet(CO2) at which the apnea closest to the last hypopnea occurred. The CO2 reserve was defined as the change in Pet(CO2) between eupneic Pet(CO2) and Pet(CO2)-AT. Controller gain was defined as the ratio of change in Ve between control and hypopnea or apnea to the DeltaPet(CO2).

MEASUREMENTS AND MAIN RESULTS

Eleven pairs of subjects were studied. There was no difference in the Pet(CO2)-AT between the two groups. However, the CO2 reserve was smaller in the subjects with OSA (2.2 +/- 0.6 mm Hg) compared with the control subjects (4.5 +/- 1.4 mm Hg; P < 0.001). The controller gain was increased in the subjects with OSA (3.7 +/- 1.3 L/min/mm Hg) compared with the control subjects (1.6 +/- 0.5 L/min/mm Hg; P < 0.001). Controller gain decreased and CO2 reserve increased in seven subjects restudied after using continuous positive airway pressure for 1 month.

CONCLUSIONS

Ventilatory instability is increased in subjects with OSA and is reversible with the use of continuous positive airway pressure.

Upper airway mechanics

This review discusses the pathophysiological aspects of sleep-disordered breathing, with focus on upper airway mechanics in obstructive and central sleep apnoea, Cheyne-Stokes respiration and obesity hypoventilation syndrome. These disorders constitute the end points of a spectrum with distinct yet interrelated mechanisms that lead to substantial pathology, i.e. increased upper airway collapsibility, control of breathing instability, increased work of breathing, disturbed ventilatory system mechanics and neurohormonal changes. Concepts are changing. Although sleep apnoea is considered more and more to be an increased loop gain disorder, the central type of apnoea is now considered as an obstructive event, because it causes pharyngeal narrowing, associated with prolonged expiration. Although a unifying concept for the pathogenesis is lacking, it seems that these patients are in a vicious circle. Knowledge of common patterns of sleep-disordered breathing may help to identify these patients and guide therapy.

Mechanisms of apnea

This paper focuses on the underlying mechanisms contributing to sleep-disordered breathing. Obstructive sleep apnea (OSA) is the most common sleep-related breathing disorder and is characterized by repetitive narrowing or collapse of the pharyngeal airway during sleep. Conversely, central sleep apnea (CSA), highly prevalent in congestive heart failure, is distinguished by a lack of drive to breathe during sleep, resulting in repetitive periods of insufficient ventilation. Both lead to compromised gas exchange, impaired sleep continuity, and catecholamine surges and are associated with major comorbidities including excessive daytime sleepiness and increased risk of cardiovascular disease. Although OSA and CSA exist on a spectrum of sleep-disordered breathing, the 2 entities may overlap in their underlying pathophysiologies. This brief review summarizes the etiology and current understanding of OSA and CSA pathophysiology and the role that the cardiovascular system may play in contributing to disease pathology and highlights the likely substantial overlap that exists between the various forms of sleep-disordered breathing.

The ventilatory responsiveness to CO(2) below eupnoea as a determinant of ventilatory stability in sleep

Sleep unmasks a highly sensitive hypocapnia-induced apnoeic threshold, whereby apnoea is initiated by small transient reductions in arterial CO(2) pressure (P(aCO(2))) below eupnoea and respiratory rhythm is not restored until P(aCO(2)) has risen significantly above eupnoeic levels. We propose that the 'CO(2) reserve' (i.e. the difference in P(aCO(2)) between eupnoea and the apnoeic threshold (AT)), when combined with 'plant gain' (or the ventilatory increase required for a given reduction in P(aCO(2))) and 'controller gain' (ventilatory responsiveness to CO(2) above eupnoea) are the key determinants of breathing instability in sleep. The CO(2) reserve varies inversely with both plant gain and the slope of the ventilatory response to reduced CO(2) below eupnoea; it is highly labile in non-random eye movement (NREM) sleep. With many types of increases or decreases in background ventilatory drive and P(aCO(2)), the slope of the ventilatory response to reduced P(aCO(2)) below eupnoea remains unchanged from control. Thus, the CO(2) reserve varies inversely with plant gain, i.e. it is widened with hyperventilation and narrowed with hypoventilation, regardless of the stimulus and whether it acts primarily at the peripheral or central chemoreceptors. However, there are notable exceptions, such as hypoxia, heart failure, or increased pulmonary vascular pressures, which all increase the slope of the CO(2) response below eupnoea and narrow the CO(2) reserve despite an accompanying hyperventilation and reduced plant gain. Finally, we review growing evidence that chemoreceptor-induced instability in respiratory motor output during sleep contributes significantly to the major clinical problem of cyclical obstructive sleep apnoea.

Natural evolution of moderate sleep apnoea syndrome: significant progression over a mean of 17 months

BACKGROUND

Obstructive sleep apnoea (OSA) is associated with increased morbidity and mortality. It has remained unclear whether or not it is progressive. The evolution of OSA was examined in a retrospective case note study of 55 unselected patients of mean (SD) age 55.8 (10) years with mild to moderate disease untreated by interventional methods such as continuous positive airway pressure (CPAP) or surgery. Correlations between clinical and functional variables, upper airway anatomy, and change in disease severity were also investigated.

METHODS

Patients underwent full polysomnography on two occasions (T0 and Tx) at a mean interval of 77 (50) weeks (range 17-229). In addition, upper airway imaging with computed tomographic scanning or cephalometry had been performed in 43 patients at T0. Morbidity before, during, and after the study period was assessed by questionnaire, as was smoking history and alcohol and sedative intake.

RESULTS

The apnoea hypopnoea index (AHI) for the group as a whole increased from 21.8 (11.5) to 33.4 (21.3) (p = 0.0001). Using a 25% change in AHI to divide patients into worsened, stable, and improved groups showed that, although most of the patients deteriorated, 25 patients improved or remained stable. The change in AHI was not correlated with body mass index which remained stable at 29.7 (5.4) kg/m2 versus 29.7 (5.6) kg/m2. There was a trend for apnoea duration to increase. No patient reported increased alcohol consumption and only one patient reported increased use of sedatives between T0 and Tx. No correlation was found between change in AHI and age, time between recordings, anatomical measurements of the upper airway, respiratory function, oximetry, or arterial blood gas tensions. Total cardiovascular and cerebrovascular morbidity was high: hypertension (26 patients, 46%), cardiac arrhythmia (17 patients, 33%), angina (12 patients, 23%), myocardial infarction (10 patients, 19%), and stroke (10 patients, 19%). Twenty nine patients (52%) were prescribed CPAP after Tx, two of whom went on to have maxillofacial surgery. These 29 treated patients had significantly higher values of AHI at T0 and Tx and greater change in AHI than the untreated patients.

CONCLUSIONS

This study shows that mild to moderate OSA has a tendency to worsen in the absence of significant weight gain and that upper airway anatomy and clinical variables do not appear to be useful in predicting progression. It follows that mild to moderate OSA justifies systematic follow up. Deterioration in AHI over a mean of 17 months led to interventional treatment in over 50% of patients in the study.

Detection of sleep associated dysfunctional pharyngeal obstruction in infants

Six infants were referred with symptoms and clinical signs suggesting airway obstruction during sleep. In each case, overnight recordings of arterial oxygen saturation, respiratory movements and end tidal expired carbon dioxide (ETCO2) showed the presence of abnormal episodes of hypoxaemia related to partial or complete airway obstruction and associated with a specific pattern of the inspiratory movement waveforms. These events and patterns were not found on recordings from 20 age-matched healthy infants and young children. ETCO2 levels were also abnormally elevated in all six patients when asleep. Fiber-optic upper airway endoscopy excluded structural abnormalities, including significant tonsillar or adenoidal enlargement, but showed an intermittent dysfunctional inspiratory obstruction in the pharynx. Continuous positive airways pressure and tracheostomy were effective in treating this obstruction.