Interaction of chemical and state effects on ventilation during sleep onset

Central sleep apnea

Central apnea syndrome is a disorder with protean manifestations and concomitant conditions. It can occur as a distinct clinical entity or as part of another clinical syndrome. The pathogenesis of central sleep apnea (CSA) varies depending on the clinical condition. Sleep-related withdrawal of the ventilatory drive to breathe is the common denominator among all cases of central apnea, whereas hypocapnia is the final common pathway leading to apnea in the majority of central apnea. Medical conditions most closely associated with CSA include heart failure, stroke, spinal cord injury, and opioid use, among others. Nocturnal polysomnography is the standard diagnostic method, including measurement of sleep and respiration. The latter includes detection of flow, measurement of oxyhemoglobin saturation and detection of respiratory effort. Management strategy incorporates clinical presentation, associated conditions, and the polysomnographic findings in an individualized manner. The pathophysiologic heterogeneity may explain the protean clinical manifestations and the lack of a single effective therapy for all patients. While research has enhanced our understanding of the pathogenesis of central apnea, treatment options are extrapolated from treatment of obstructive sleep apnea. Co-morbid conditions and concomitant obstructive sleep apnea influence therapeutic approach significantly. Therapeutic options include positive pressure therapy, pharmacologic therapy, and supplemental Oxygen. Continuous positive airway pressure (CPAP) is the initial standard of care, although the utility of other modes of positive pressure therapy, as well as pharmacotherapy and device-based therapies, are currently being investigated.

Hyperoxia enhances slow-wave forebrain states in urethane-anesthetized and naturally sleeping rats

Oxygen (O₂) is a crucial element for physiological functioning in mammals. In particular, brain function is critically dependent on a minimum amount of circulating blood levels of O₂ and both immediate and lasting neural dysfunction can result following anoxic or hypoxic episodes. Although the effects of deficiencies in O₂ levels on the brain have been reasonably well studied, less is known about the influence of elevated levels of O₂ (hyperoxia) in inspired gas under atmospheric pressure. This is of importance due to its typical use in surgical anesthesia, in the treatment of stroke and traumatic brain injury, and even in its recreational or alternative therapeutic use. Using local field potential (EEG) recordings in spontaneously breathing urethane-anesthetized and naturally sleeping rats, we characterized the influence of different levels of O₂ in inspired gases on brain states. While rats were under urethane anesthesia, administration of 100% O₂ elicited a significant and reversible increase in time spent in the deactivated (i.e., slow-wave) state, with concomitant decreases in both heartbeat and respiration rates. Increasing the concentration of carbon dioxide (to 5%) in inspired gas produced the opposite result on EEG states, mainly a decrease in the time spent in the deactivated state. Consistent with this, decreasing concentrations of O₂ (to 15%) in inspired gases decreased time spent in the deactivated state. Further confirmation of the hyperoxic effect was found in naturally sleeping animals where it similarly increased time spent in slow-wave (nonrapid eye movement) states. Thus alterations of O₂ in inspired air appear to directly affect forebrain EEG states, which has implications for brain function, as well as for the regulation of brain states and levels of forebrain arousal during sleep in both normal and pathological conditions. NEW & NOTEWORTHY We show that alterations of oxygen concentration in inspired air biases forebrain EEG state. Hyperoxia increases the prevalence of slow-wave states. Hypoxia and hypercapnia appear to do the opposite. This suggests that oxidative metabolism is an important stimulant for brain state.

Control of Ventilation in Health and Disease

Control of ventilation occurs at different levels of the respiratory system through a negative feedback system that allows precise regulation of levels of arterial carbon dioxide and oxygen. Mechanisms for ventilatory instability leading to sleep-disordered breathing include changes in the genesis of respiratory rhythm and chemoresponsiveness to hypoxia and hypercapnia, cerebrovascular reactivity, abnormal chest wall and airway reflexes, and sleep state oscillations. One can potentially stabilize breathing during sleep and treat sleep-disordered breathing by identifying one or more of these pathophysiological mechanisms. This review describes the current concepts in ventilatory control that pertain to breathing instability during wakefulness and sleep, delineates potential avenues for alternative therapies to stabilize breathing during sleep, and proposes recommendations for future research.

Augmented cardiovascular responses to episodes of repetitive compared with isolated respiratory events in preschool children with sleep-disordered breathing

BACKGROUND

Childhood sleep disordered breathing (SDB) presents as isolated respiratory events or episodes of consecutive repetitive events. We hypothesized that the surge in blood pressure (BP) and heart rate (HR) would be greater at the termination of events during episodes of repetitive events than following isolated events.

METHODS

% change in HR and pulse transit time (PTT; inverse surrogate of BP) were calculated from the last half of an event to: (i) between successive repetitive events; (ii) termination of the last repetitive event; (iii) event termination for isolated events.

RESULTS

69% of the children exhibiting both isolated and repetitive events had more repetitive than isolated events. %HR change between repetitive events (27 ± 1%) was greater than at event termination for isolated events (17 ± 1%; P < 0.001). %PTT change at the termination of the last repetitive event (-8 ± 2%) was greater than at the termination of isolated events (-2 ± 2%; P < 0.05).

CONCLUSION

Episodes of repetitive respiratory events evoke a greater acute cardiovascular response, including surges in BP and HR between events, than do isolated events. Given that the majority of respiratory events in preschool children occur as repetitive episodes, this finding should be taken into account when assessing the impact of respiratory events for a given child.

Future of Sleep-Disordered Breathing Therapy Using a Mechanistic Approach

Sleep disordered breathing (SDB) is highly prevalent among patients with cardiovascular disease (CVD), and the relationship between SDB and CVD may be bidirectional. However, SDB remains underdiagnosed and undertreated. One of the major barriers identified by cardiologists is lack of satisfaction with SDB therapy. This situation could be the result of the discordance between treatment and the pathophysiological characteristics of SDB. This condition is caused by multiple pathophysiological mechanisms, which could be classified into upper airway anatomic compromise, pharyngeal dilator muscle dysfunction, and ventilatory control instability. However, the effective treatment of SDB remains limited, and positive airway pressure therapy is still the mainstay of the treatment. Therefore, we review the pathophysiological characteristics of SDB in this article, and we propose to provide individualized treatment of SDB based on the underlying mechanism. This approach requires further study but could potentially improve adherence and success of therapy.

EEG spectral analysis of apnoeic events confirms visual scoring in childhood sleep disordered breathing

PURPOSE

This study compared electroencephalogram (EEG) spectral analysis with standard visual scoring to assess the validity of clinical classification of arousals at respiratory event termination in children with obstructive sleep apnoea (OSA).

METHODS

Twenty children (six M/14 F) aged 7-12 years, diagnosed with moderate to severe OSA participated in this study. Overnight polysomnography was performed, and sleep stages and arousals visually scored using clinical paediatric measures. The EEG was spectrally analysed in six 5-s epochs across respiratory events, namely two consecutive 5-s epochs pre-event onset and a 5s epoch post-event onset, 5-s before event termination, and two contiguous 5-s epochs post-event termination. EEG spectral power distribution was compared across respiratory events visually categorised as full cortical arousals, subcortical activations, or non-arousals using specialised software (Sleep Research System 5.0).

RESULTS

There was no difference in power spectra between events in REM and NREM sleep and these were combined. There was a statistically significant fall from pre-arousal baseline values in delta and theta spectral power at respiratory event terminations associated with cortical arousals only. No change in power was detected at respiratory event terminations associated with subcortical activations or non-arousals.

CONCLUSIONS

The lack of significant EEG spectral power changes at respiratory event terminations not associated with visually identified cortical arousals indicates undetected micro-arousals are not present. The results support the validity of clinical classifications of arousals at respiratory event termination.

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.

Central sleep apnea in patients with congestive heart failure

Central apnea during sleep represents a manifestation of breathing instability in many clinical conditions of varied etiologies. Central apnea is the result of transient cessation of ventilatory motor output, which represents that inhibitory influences favoring instability predominate over excitatory influence favoring stable breathing. This article will review the determinants of central apnea, the specific features of CHF-related central apnea, and outline a management approach

Central sleep apnea: Pathophysiology and treatment

Central sleep apnea (CSA) is characterized by a lack of drive to breathe during sleep, resulting in repetitive periods of insufficient ventilation and compromised gas exchange. These nighttime breathing disturbances can lead to important comorbidity and increased risk of adverse cardiovascular outcomes. There are several manifestations of CSA, including high altitude-induced periodic breathing, idiopathic CSA, narcotic-induced central apnea, obesity hypoventilation syndrome, and Cheyne-Stokes breathing. While unstable ventilatory control during sleep is the hallmark of CSA, the pathophysiology and the prevalence of the various forms of CSA vary greatly. This brief review summarizes the underlying physiology and modulating components influencing ventilatory control in CSA, describes the etiology of each of the various forms of CSA, and examines the key factors that may exacerbate apnea severity. The clinical implications of improved CSA pathophysiology knowledge and the potential for novel therapeutic treatment approaches are also discussed.

Peripheral chemoreceptor activity in sleeping neonates exposed to warm environments

In neonates, it is often assumed that ventilatory control and heat stress interact. Thus the two factors have been implicated in various pathologies (apnoea, sudden infant death syndrome). However, little is known about the mechanisms of this interaction, and the influence of sleep is still debated. This study aimed at determining the influence of warm exposure on the decrease in ventilation during a hyperoxic test (HT), which is considered to be a measure of peripheral chemoreceptor activity. The test was performed in active (AS) and quiet sleep (QS) in 12 neonates exposed to thermoneutral or warm environments. The HT consisted of 30 s of inspired, 100% O(2). The ventilatory response was assessed in terms of a response time, defined as the time elapsing between HT onset and the first significant change in V(E). Our results show that, in both thermal conditions, the fall in V(E) was higher in AS than in QS. Warm exposure significantly enhanced the ventilatory response in AS (-27.5 +/- 8.7% vs. -38.3 +/- 8.8%, P < 0.01) but not in QS. A thermometabolic drive or inputs from thermoreceptors could be involved in the reinforcement of peripheral chemoreceptor activity in AS in warmer environments, which could contribute to an increasing risk of apnoea in neonates with altered chemoreceptor function. Since hypothalamic structures are involved in thermoregulatory, sleep processes and (probably) in respiratory control, it could well be the principal site where this interaction occurs.

The effect of arousals during sleep onset on estimates of sleep onset latency

It is well established that insomniacs overestimate sleep-onset latency. Furthermore, there is evidence that brief arousals from sleep may occur more frequently in insomnia. This study examined the hypothesis that brief arousals from sleep influence the perception of sleep-onset latency. An average of four sleep onsets was obtained from each of 20 normal subjects on each of two nonconsecutive, counterbalanced, experimental nights. The experimental nights consisted of a control night (control condition) and a condition in which a moderate respiratory load was applied to increase the frequency of microarousals during sleep onset (mask condition). Subjective estimation of sleep-onset latency and indices of sleep quality were assessed by self-report inventory. Objective measures of sleep-onset latency and microarousals were assessed using polysomnography. Results showed that sleep-onset latency estimates were longer in the mask condition than in the control condition, an effect not reflected in objective sleep-stage scoring of sleep-onset latency. Furthermore, an increase in the frequency of brief arousals from sleep was detected in the mask condition, and this is a possible source for the sleep-onset latency increase perceived by the subjects. Findings are consistent with the concept of a physiological basis for sleep misperception in insomnia.

Effect of age on sleep onset-related changes in respiratory pump and upper airway muscle function

In normal young men, there is an abrupt fall in ventilation (VE), a rise in upper airway resistance (UAR), and falls in the activities of the diaphragm (Di), intercostals (IC), genioglossus (GG), and tensor palatini (TP) at sleep onset. On waking, there is an abrupt increase in VE and fall in UAR and an increase in the activities of Di, IC, GG, and TP. The aim of this study was to determine whether these changes are age dependent. Nine men aged 20 to 25 yr were compared with nine men aged 42 to 67 yr. Airflow, UAR, Di, and IC surface electromyograms (EMGs) and the intramuscular EMGs of GG and TP were recorded. It was found that the falls in IC, GG, and TP at the transition from alpha to theta electroencephalogram (EEG) activity were significantly greater in the older than in the younger men (P < 0.05) and that the fall in Di was also greater, although this was only marginally significant (P = 0.15). The rise in GG at theta-to-alpha transitions was also greater in the older than in the younger men, and there was a trend for TP to be higher.

Ventilatory instability during sleep onset in individuals with high peripheral chemosensitivity

Previous work has shown that the magnitude of state-related ventilatory fluctuations is amplified over the sleep-onset period and that this amplification is partly due to peripheral chemoreceptor activity, because it is reduced by hyperoxia (J. Dunai, M. Wilkinson, and J. Trinder. J. Appl. Physiol. 81: 2235-2243, 1996). These data also indicated considerable intersubject variability in the magnitude of amplification. A possible source of this variability is individual differences in peripheral chemoreceptor drive (PCD). We tested this hypothesis by measuring state-related ventilatory fluctuations throughout sleep onset under normoxic and hyperoxic conditions in subjects with high and low PCD. Results demonstrated that high-PCD subjects experienced significantly greater amplification of state-related ventilatory fluctuations than did low-PCD subjects. In addition, hyperoxia significantly reduced the amplification effect in high-PCD subjects but had little effect in low-PCD subjects. These results indicate that individuals with high PCD are likely to experience greater sleep-related ventilatory instability and suggest that peripheral chemoreceptor activity can contribute to sleep-disordered breathing.

Respiratory-related evoked potentials during the transition from alpha to theta EEG activity in stage 1 NREM sleep

It has been argued previously that evoked potential components during Stage 1 sleep in response to both auditory and respiratory stimuli are intermediate between those of wakefulness and Stage 2 sleep. However, state fluctuations in the ECG between alpha and theta during Stage 1 sleep have been linked to changes in a number of respiratory functions including ventilation, upper airway resistance and chemical drive. It was therefore hypothesized that if respiratory related evoked potentials (RREP) were averaged separately for alpha and theta EEG periods during Stage 1 sleep, the alpha RREP would resemble wakefulness and the theta RREP would resemble Stage 2 sleep. RREPs were produced by 250 ms occlusions in 10 subjects. EEG was recorded from 29 scalp sites, referenced to linked ears, together with EOG and EMG. The N1 component was not specifically associated with alpha vs. theta activity, but appeared to be sensitive to any decrease in arousal level, suggesting that it was more related to attention than to changes in the EEG. The late N2 and P300 components were present during wake and Stage 1 alpha. However, in Stage 1 theta, different late components emerged (N300 and P450) that differed in latency, amplitude or topographical distribution from those seen in wakefulness. The P2 proved difficult to interpret, whereas the N550 did not appear until Stage 2 sleep, and as such, was not dependent on alpha/theta state. The results indicate that RREP components are differentially affected by the transition into sleep.

Variability of vigilance and ventilation: studies on the control of respiration during sleep

Ventilation is under metabolic as well as under behavioural control. This causes a complex interaction between states of 'vigilance' and respiration. This paper briefly summarizes sleep-related changes of respiration and presents an experimental study on the course of respiratory CO2-sensitivity during a whole night's sleep in ten healthy volunteers. The feedback control of breathing was challenged by continuous step changes of inspired CO2 every 7 min, resulting in 60, 3-step steady-state hypercapnic ventilatory responses (HCVR) per night in each subject. We analysed the variability of baseline ventilation and the effects of hypercapnia on ventilation with respect to sleep stages. There were only small differences in baseline PCO2 and ventilation between sleep stages, but a high variability of the slope of the CO2-response curves in the course of the night, ranging from 0.5 to 3.0 L min(-1) Torr(-1). The HCVR was significantly lower during REM sleep than during all stages of NREM sleep. Due to a compensatory left shift of the flattened CO2-response curves, however, ventilation at baseline CO2 as well as during slight hypercapnia varied much less than would be expected from the high variability of slopes. We conclude that the characteristics of the CO2-sensitive feedback control system of respiration, are highly variable during sleep, but due to offsetting effects, PCO2 and ventilation remain quite stable in the physiological range.

Activity of respiratory pump and upper airway muscles during sleep onset

Ventilation decreases at sleep onset. This change is initiated abruptly at alpha-theta electroencephalographic transitions. The aim of this study was to determine the contributions of reduced activity in respiratory pump muscles and upper airway dilator muscles to this change. Surface electromyograms over the diaphragm (Di) and intercostal muscles and fine-wire intramuscular electrodes in genioglossus (GG) and tensor palatini (TP) muscles were recorded in nine healthy young men. It was shown that phasic Di and both phasic and tonic TP activities were lower during theta than during alpha activity. Breath-by-breath analysis of the changes at alpha-theta transitions during the sleep-onset period showed a number of changes. At alpha-theta transitions, phasic activity of Di, intercostal, and GG muscles fell and rose again, and phasic and tonic activities of TP fell and remained at low levels during theta. With a state transition from theta to alpha, the phasic and tonic activities of the Di, GG, and TP increased dramatically. It is now clear that the fall in ventilation that occurs with sleep is related to a fall in activities of both upper airway dilator muscles and respiratory pump muscles.

Load compensation as a function of state during sleep onset

Ventilation decreases and airway resistance increases with the loss of electroencephalogram alpha activity at sleep onset. The aim of this study was to determine whether reflexive load compensation is lost immediately on the loss of alpha activity. Six healthy male subjects were studied under two conditions (load and control-no load), in three states (continuous alpha, continuous theta, and immediately after a transition from alpha to theta), and in two phases (early and late sleep onset). Ventilation and respiratory timing were measured. A comparison of loaded with control conditions indicated that loading had no effect on inspiratory minute ventilation during continuous alpha (differential effect of 0.00 l/min) and only a small, nonsignificant effect in theta immediately after phase 2 transitions (0.31 l/min), indicating a preservation of load compensation at these times. However, there were significant decreases in inspiratory minute ventilation on loaded trials during continuous theta in phase 2 (0.77 l/min) and phase 3 (1.15 l/min) and during theta immediately after a transition in phase 3 (0.87 l/min), indicating a lack of reflexive load compensation. The results indicate that, because reflex load compensation is state dependent, state-related changes in airway resistance contribute to state-related changes in ventilation during sleep onset. However, this effect was slightly delayed with transitions into theta early in sleep.