Influence of arousal threshold and depth of sleep on respiratory stability in man: analysis using a mathematical model

The effect of pharyngeal structures on the severity of obstructive sleep apnea

BACKGROUND/OBJECTIVE

Obstructive sleep apnea (OSA) is characterized by complete or partial cessation of breathing during sleep. The tongue is suggested as a possible anatomical site causing airway obstruction. However, the role of other pharyngeal structures in the development of OSA remains unclear. We designed a study using both the apnea-hypopnea index (AHI) and the oxygen saturation measurements to assess the severity of OSA. We aimed to identify critical anatomical structures of the upper airway that correlate with the severity of OSA and to evaluate the utility of magnetic resonance imaging (MRI) markers to detect possible OSA in patients without overt symptoms.

MATERIALS AND METHODS

The study included participants referred to the neurology outpatient clinic from the check-up unit. Participants were grouped as controls, mild, moderate, or severe OSA according to the AHI. A cranial MRI with a field of view (FOV) encompassing the upper airway structures was obtained from all participants. The areas of the tongue and the uvula were measured on the sagittal images by drawing the boundaries of the tissues manually. The posterior air space (PAS) area was evaluated from regions of interest in five parallel planes.

RESULTS

Of 105 participants, 30 were controls, 27 had mild, 25 had moderate, and 23 had severe OSA. The moderate and severe OSA groups did not differ in oxygen saturation levels during sleep. Therefore, patients with moderate and severe OSA were combined into one group (moderate/severe OSA). The area of the tongue was significantly larger in the moderate/severe OSA group compared to the control group. Both the tongue and the uvula areas showed a significant positive correlation with the AHI.

CONCLUSION

Our findings suggest that the tongue and uvula have prominent roles in the severity of OSAS. It may be useful to measure these structures with MRI to screen for at-risk individuals without overt OSA symptoms.

The importance of obstructive sleep apnoea and hypopnea pathophysiology for customized therapy

The objective of this study is to highlight the importance of anatomical and not-anatomical factors' identification for customized therapy in OSAHS patients. The data sources are: MEDLINE, The Cochrane Library and EMBASE. A systematic review was performed to identify studies that analyze the role of multiple interacting factors involved in the OSAHS pathophysiology. 85 out of 1242 abstracts were selected for full-text review. A variable combinations pathophysiological factors contribute to realize differentiated OSAHS phenotypes: a small pharyngeal airway with a low resistance to collapse (increased critical closing pressure), an inadequate responses of pharyngeal dilator muscles (wakefulness drive to breathe), an unstable ventilator responsiveness to hypercapnia (high loop gain), and an increased propensity to wake related to upper airway obstruction (low arousal threshold). Identifying if the anatomical or not-anatomical factors are predominant in each OSAHS patient represents the current challenge in clinical practice, moreover for the treatment decision-making. In the future, if a reliable and accurate pathophysiological pattern for each OSAHS patient can be identified, a customized therapy will be feasible, with a significant improvement of surgical success in sleep surgery and a better understanding of surgical failure.

The predictive value of Muller's maneuvre for CPAP titration in OSAHS patients

To investigate the role of awake upper airways (UA) endoscopy assessment as a parameter of prediction for CPAP titration in OSAHS patient therapy. Retrospective analysis of UA endoscopic assessment with Mueller's maneuvre and the application of the nose oropharynx hypopharynx score (NOHs) was conducted to obtain a numeric score representing the grade of severity of UA obstruction. Other commonly used predictive parameters for CPAP titration were also included in the study: anthropometric [BMI, neck circumference (NC)] and polysomnographic parameters (AHI, ODI). 3 groups of patients were identified: (1) 67/90 patients requiring intermediate CPAP values, (2) 13/90 patients requiring high CPAP values, and (3) 10/90 patients requiring low pressure values. BMI (p = 0.0013) was the only monitored parameter to show significant statistical value as a CPAP titration predictor. However, higher values of anthropometric parameters (NOHs ≥9, BMI >35, NC >45) showed a sensitivity of 69.2% as a single parameter and 76.9% as combined parameters, and specificity between 66.2 and 72.7% as a single parameter and 43.4% as combined parameters, unequivocally identifying patients requiring high therapeutic CPAP value. A lower cut-off of anthropometric parameters (NOHs ≤6, BMI ≤29, NC <42) showed sensitivity between 40 and 60% as a single parameter and of 90% as combined parameters, and specificity between 68.7 and 80.2% as a single parameter which increased to 93.7% as combined parameters, identifying patients requiring a low therapeutic CPAP value. The results show that anthropometric and polygraphic parameters have no significant independent predictive value for CPAP titration, with the exception of BMI. However, anthropometric parameters showed good levels of sensitivity and specificity in OSAHS patients requiring high or low levels of CPAP therapy.

Termination of respiratory events with and without cortical arousal in obstructive sleep apnea

RATIONALE

A total of 20-30% of respiratory events in obstructive sleep apnea are terminated without clear arousal. Arousals are thought to predispose to further events by promoting hyperventilation, hypocapnia, and upper-airway dilator muscle hypotonia. Therefore, events terminated without arousal may promote stable breathing.

OBJECTIVES

To compare physiologic changes at respiratory event termination with American Sleep Disorders Association (ASDA) Arousal to No Arousal, and determine whether secondary respiratory events are less common and have higher dilator muscle activity after No Arousal compared with ASDA Arousal.

METHODS

Patients with obstructive sleep apnea wore sleep staging, genioglossus (EMG(GG)), and tensor palatini (EMG(TP)) electrodes plus a nasal mask and pneumotachograph. During stable sleep, continuous positive airway pressure (CPAP) was lowered for 3-minute periods to induce respiratory events. Physiologic variables were compared between events terminated with (1) ASDA Arousal, (2) No Arousal, or (3) sudden CPAP increase (CPAPinc, control).

MEASUREMENTS AND MAIN RESULTS

Sixteen subjects had adequate data. EMG(GG), EMG(TP), and heart rate increased after ASDA Arousal (340 ± 57%, 215 ± 28%, and 110.7 ± 2.3%) and No Arousal (185 ± 32%, 167 ± 15%, and 108.5 ± 1.6%) but not CPAPinc (90 ± 10%, 94 ± 11%, and 102.1 ± 1%). Ventilation increased more after ASDA Arousal than No Arousal and CPAPinc, but not after accounting for the severity of respiratory event. Fewer No Arousals were followed by secondary events than ASDA Arousals. However, low dilator muscle activity did not occur after ASDA Arousal or No Arousal (EMG(GG) rose from 75 ± 5 to 125 ± 7%) and secondary events were less severe than initial events (ventilation rose 4 ± 0.4 to 5.5 ± 0.51 L/min).

CONCLUSIONS

Respiratory events that were terminated with ASDA Arousal were more severely flow-limited, had enhanced hyperventilation after event termination, and were more often followed by secondary events than No arousal. However, secondary events were not associated with low dilator muscle activity and airflow was improved after both No Arousal and ASDA Arousal.