A measure of ventilatory variability at wake-sleep transition predicts sleep apnea severity

Mid-cervical spinal cord contusion causes robust deficits in respiratory parameters and pattern variability

Mid-cervical spinal cord contusion disrupts both the pathways and motoneurons vital to the activity of inspiratory muscles. The present study was designed to determine if a rat contusion model could result in a measurable deficit to both ventilatory and respiratory motor function under “normal” breathing conditions at acute to chronic stages post trauma. Through whole body plethysmography and electromyography we assessed respiratory output from three days to twelve weeks after a cervical level 3 (C3) contusion. Contused animals showed significant deficits in both tidal and minute volumes which were sustained from acute to chronic time points. We also examined the degree to which the contusion injury impacted ventilatory pattern variability through assessment of Mutual Information and Sample Entropy. Mid-cervical contusion significantly and robustly decreased the variability of ventilatory patterns. The enduring deficit to the respiratory motor system caused by contusion was further confirmed through electromyography recordings in multiple respiratory muscles. When isolated via a lesion, these contused pathways were insufficient to maintain respiratory activity at all time points post injury. Collectively these data illustrate that, counter to the prevailing literature, a profound and lasting ventilatory and respiratory motor deficit may be modelled and measured through multiple physiological assessments at all time points after cervical contusion injury.

Isoflurane and ketamine anesthesia have different effects on ventilatory pattern variability in rats

We hypothesize that isoflurane and ketamine impact ventilatory pattern variability (VPV) differently. Adult Sprague-Dawley rats were recorded in a whole-body plethysmograph before, during and after deep anesthesia. VPV was quantified from 60-s epochs using a complementary set of analytic techniques that included constructing surrogate data sets that preserved the linear structure but disrupted nonlinear deterministic properties of the original data. Even though isoflurane decreased and ketamine increased respiratory rate, VPV as quantified by the coefficient of variation decreased for both anesthetics. Further, mutual information increased and sample entropy decreased and the nonlinear complexity index (NLCI) increased during anesthesia despite qualitative differences in the shape and period of the waveform. Surprisingly mutual information and sample entropy did not change in the surrogate sets constructed from isoflurane data, but in those constructed from ketamine data, mutual information increased and sample entropy decreased significantly in the surrogate segments constructed from anesthetized relative to unanesthetized epochs. These data suggest that separate mechanisms modulate linear and nonlinear variability of breathing.

Lung and brainstem cytokine levels are associated with breathing pattern changes in a rodent model of acute lung injury

Acute lung injury evokes a pulmonary inflammatory response and changes in the breathing pattern. The inflammatory response has a centrally mediated component which depends on the vagi. We hypothesize that the central inflammatory response, complimentary to the pulmonary inflammatory response, is expressed in the nuclei tractus solitarii (nTS) and that the expression of cytokines in the nTS is associated with breathing pattern changes. Adult, male Sprague-Dawley rats (n=12) received intratracheal instillation of either bleomycin (3units in 120μl of saline) or saline (120μl). Respiratory pattern changed by 24h. At 48h, bronchoalveolar lavage fluid and lung tissue had increased IL-1β and TNF-α levels, but not IL-6. No changes in these cytokines were noted in serum. Immunocytochemical analysis of the brainstem indicated increased expression of IL-1β in the nTS commissural subnucleus that was localized to neurons. We conclude that breathing pattern changes in acute lung injury were associated with increased levels of IL-1β in brainstem areas which integrate cardio-respiratory sensory input.

Obstructive sleep apnea syndrome: natural history, diagnosis, and emerging treatment options

Sleep apnea is an entity characterized by repetitive upper airway obstruction resulting in nocturnal hypoxia and sleep fragmentation. It is estimated that 2%-4% of the middle-aged population has sleep apnea with a predilection in men relative to women. Risk factors of sleep apnea include obesity, gender, age, menopause, familial factors, craniofacial abnormalities, and alcohol. Sleep apnea has been increasingly recognized as a major health burden associated with hypertension and increased risk of cardiovascular disease and death. Increased airway collapsibility and derangement in ventilatory control responses are the major pathological features of this disorder. Polysomnography (PSG) is the gold-standard method for diagnosis of sleep apnea and assessment of sleep apnea severity; however, portable sleep monitoring has a diagnostic role in the setting of high pretest probability sleep apnea in the absence of significant comorbidity. Positive pressure therapy is the mainstay therapy of sleep apnea. Other treatment modalities, such as upper airway surgery or oral appliances, may be used for the treatment of sleep apnea in select cases. In this review, we focus on describing the sleep apnea definition, risk factor profile, underlying pathophysiologic mechanisms, associated adverse consequences, diagnostic modalities, and treatment strategies.

Ventilatory patterning in a mouse model of stroke

Cheyne-Stokes respiration (CSR) is a breathing pattern characterized by waxing and waning of breath volume and frequency, and is often recognized following stroke, when causal pathways are often obscure. We used an animal model to address the hypothesis that cerebral infarction is a mechanism for producing breathing instability. Fourteen male A/J mice underwent either stroke (n=7) or sham (n=7) procedure. Ventilation was measured using whole body plethysmography. Respiratory rate (RR), tidal volume (V(T)) and minute ventilation (V(e)) mean values and coefficient of variation were computed for ventilation and oscillatory behaviors. In addition, the ventilatory data were computationally fit to models to quantify autocorrelation, mutual information, sample entropy and a nonlinear complexity index. At the same time post-procedure, stroke when compared to sham animal breathing consisted of a lower RR and autocorrelation, higher coefficient of variation for V(T) and higher coefficient of variation for V(e). Mutual information and the nonlinear complexity index were higher in breathing following stroke which also demonstrated a waxing/waning pattern. The absence of stroke in the sham animals was verified anatomically. We conclude that ventilatory pattern following cerebral infarction demonstrated increased variability with increased nonlinear patterning and a waxing/waning pattern, consistent with CSR.

Screening of obstructive sleep apnea using Hilbert-Huang decomposition of oronasal airway pressure recordings

Polysomnographic signals are usually recorded from patients exhibiting symptoms related to sleep disorders such as obstructive sleep apnea (OSA). Analysis of polysomnographic data allows for the determination of the type and severity of sleep apnea or other sleep-related disorders by a specialist or technician. The usual procedure entails an overnight recording several hours long. This paper presents a methodology to help with the screening of OSA using a 5-min oronasal airway pressure signal emanating from a polysomnographic recording during the awake period, eschewing the need for an overnight recording. The clinical sample consisted of a total of 41 subjects, 20 non-OSA individuals and 21 individuals with OSA. A signal analysis technique based on the Hilbert-Huang transform was used to extract intrinsic oscillatory modes from the signals. The frequency distribution of both the first mode and second mode and their sum were shown to differ significantly between non-OSA subjects and OSA patients. An index measure based on the distribution frequencies of the oscillatory modes yielded a sensitivity of 81.0% (for 95% specificity) for the detection of OSA. Two other index measures based on the relation between the area and the maximum of the 1st and 2nd halves of the frequency histogram both yielded a sensitivity of 76.2% (for 95% specificity). Although further tests will be needed to test the reproducibility of these results, the proposed measures seem to provide a fast method to screen OSA patients, thus reducing the costs and the waiting time for diagnosis.

Neonatal maternal separation disrupts regulation of sleep and breathing in adult male rats

STUDY OBJECTIVES

Neonatal maternal separation (NMS) disrupts development of cardiorespiratory regulation. Adult male rats previously subjected to NMS are hypertensive and show a hypoxic ventilatory response greater than that of controls. These results have been obtained in awake or anesthetised animals, and the consequences of NMS on respiratory control during normal sleep are unknown. This study tested the following.

HYPOTHESES

NMS augments respiratory variability across sleep-wake states, and NMS-related enhancement of the hypoxic ventilatory response occurs during sleep.

METHODS

Two groups of adult rats were used: controls (no treatment) and rats subjected to NMS. Ventilatory activity, coefficient of variation, and hypoxic ventilatory response were compared between groups and across sleep-wake states.

SUBJECTS

Male Sprague Dawley rats-NMS: n=11; controls: n=10. Pups subjected to NMS were isolated from their mother for 3 hours per day from postnatal days 3 to 12. Controls were undisturbed.

MEASUREMENTS AND RESULTS

At adulthood, sleep-wake states were monitored by telemetry, and ventilatory activity was measured using whole-body plethysmography. Sleep and breathing were measured for 2.5 hours (in the morning) while the rats were breathing room air. Data were analysed in 20-second epochs. Rats were then exposed to a brief (90-sec) hypoxic episode (nadir = 12% O2) to measure the hypoxic ventilatory response. The coefficient of variability for tidal volume and breathing frequency decreased during sleep but remained more elevated in NMS rats than in controls. During non-rapid eye movement sleep, the breathing-frequency response to hypoxia of NMS rats was significantly greater than that of controls.

CONCLUSION

Neonatal maternal separation results in persistent disruption of respiratory control during sleep.

Current world literature

This bibliography is compiled by clinicians from the journals listed at the end of this publication. It is based on literature entered into our database between 1 February 2008 and 31 January 2009 (articles are generally added to the database about two and a half months after publication). In addition, the bibliography contains every paper annotated by reviewers; these references were obtained from a variety of bibliographic databases and published between the beginning of the review period and the time of going to press. The bibliography has been grouped into topics that relate to the reviews in this issue.

Upper airway function in the pathogenesis of obstructive sleep apnea: a review of the current literature

PURPOSE OF REVIEW

Obstructive sleep apnea is an increasingly prevalent disease, with a considerable societal burden. The disease is defined by recurrent intermittent collapse of the upper airway. Understanding of and treatment for the disease is largely confined to relief of the mechanical obstruction of the upper airway by application of continuous positive airway pressure, and less commonly weight loss or surgery. However, recent work has focused on the function, rather than structure alone, of the upper airway.

RECENT FINDINGS

The following contributors to upper airway structure and function have been studied: traditional fixed anatomical abnormalities, dynamic anatomical changes, upper airway dilator muscle dysfunction, lung volumes, and instability in control of breathing. In each patient with obstructive sleep apnea, the relative contribution of each of these components may be quite variable. The studies reviewed here describe methods to evaluate these factors, and some attempts at treatment.

SUMMARY

Ongoing studies are attempting to classify patients on the basis of the underlying pathophysiology. This work suggests that obstructive sleep apnea is a heterogeneous disease with multiple root causes. Ultimately, such a classification may allow more individualized treatment, not only relying on mechanical relief of the upper airway obstruction.