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(Chest. 2006;130:1362-1368.)
© 2006 American College of Chest Physicians

Autonomic Cardiac Modulation in Obstructive Sleep Apnea*

Effect of an Oral Jaw-Positioning Appliance

Paolo Coruzzi, MD; Massimo Gualerzi, MD; Edoardo Bernkopf, MD; Lorenzo Brambilla, MD; Valerio Brambilla, MD; Vanna Broia, MD; Carolina Lombardi, MD and Gianfranco Parati, MD

* From Fondazione Don C. Gnocchi – ONLUS (Drs. Gualerzi, L. Brambilla, and V. Brambilla), Department of Clinical Sciences (Dr. Coruzzi), University of Parma, Parma; Consultant Orthodontic (Drs. Bernkopf and Broia), Parma; and Department of Clinical Medicine, Prevention and Applied Biotechnologies (Drs. Lombardi and Parati), Milano, Italy.

Correspondence to: Massimo Gualerzi, MD, Fondazione Don C. Gnocchi – ONLUS, Fondazione Don C. Gnocchi, Piazzale dei Servi n° 3, I 43100, Parma, Italy; e-mail: gualerzimassimo{at}hotmail.com

Abstract

Background: Patients with obstructive sleep apnea (OSA) are characterized by deranged cardiovascular variability, a well-established marker of cardiovascular risk. While long-term treatment with continuous positive airway pressure leads to a significant improvement of cardiovascular variability, little is known of the possibility of achieving the same results with other therapeutic approaches. The aim of our study was to investigate the responses of autonomic indexes of neural cardiac control to another type of OSA treatment based on an oral jaw-positioning appliance.

Methods: In 10 otherwise healthy subjects with OSA (OSA+) and in 10 subjects without OSA (OSA–) we measured heart rate, BP, and indices of autonomic cardiac regulation derived from time-domain and spectral analysis of R-R interval (RRI), before and after 3 months of treatment with the oral device. High-frequency (HF) power of RRI was taken as an index of parasympathetic cardiac modulation, and the ratio between low-frequency (LF) and HF RRI powers as an indirect marker of the balance between sympathetic and parasympathetic cardiac modulation.

Results: At baseline, in comparison with OSA– subjects, OSA+ subjects displayed a significantly lower RRI variance (p < 0.02) and reduced HF RRI powers (p < 0.001). After 3 months of treatment with the oral device, the OSA+ group showed a marked reduction in apnea-hypopnea index (p < 0.001), a lengthening in RRI and a significant increase in its variance (p < 0,02), an increased HF RRI power (from 134 ± 26 to 502 ± 48 ms2, p < 0.001), and a reduction in LF/HF RRI power ratio (from 3.11 ± 0.8 to 1.5 ± 0.5). As a result of these changes, after the 3-month treatment there were no more significant differences between the two groups in these parameters. In both OSA+ and OSA– groups, body weight, heart rate, and BP did not change over time.

Conclusions: Three months of treatment with a specific oral jaw-positioning appliance improves cardiac autonomic modulation in otherwise healthy patients with OSA of mild degree.

Key Words: autonomic nervous system • cardiovascular prevention • heart rate variability • obstructive sleep apnea • oral device

Obstructive sleep apnea (OSA) is a common and often neglected disorder1 that is strongly associated with known cardiovascular risk factors, including obesity, insulin resistance, and dyslipidemia. OSA itself may represent an independent risk factor for hypertension,2 heart failure,3 myocardial infarction,4 and stroke.5 The mechanisms underlying the association between OSA and cardiovascular disease are not well defined, even though it has been suggested that a deranged cardiovascular variability may represent a pivotal link in this context.6 An altered cardiovascular variability, reflecting a deranged autonomic cardiovascular regulation, may not only predict morbidity and mortality in patients with diabetes,7 heart failure,8 and myocardial infarction,9 but also represent an independent risk factor in normal and hypertensive subjects.1011 This may be the case also for patients with OSA, who are characterized both at night and during wakefulness by increased plasma levels of norepinephrine,12 elevated muscle sympathetic nerve activity,13 and altered heart rate variability.514 Evidence is available that long-term treatment with continuous positive airway pressure (CPAP) results in a significant improvement of these autonomic indices in OSA patients.151617 This therapeutic approach, however, if often poorly accepted by patients on a long-term basis. The oral jaw-positioning appliance,18 known to induce an enlargement of the upper airway, represents an alternative, useful, and well-tolerated treatment for OSA, leading to a significant reduction of breathing disorders during sleep.1920 In spite of this, the effects of its application on indexes of neural autonomic cardiac control have not been previously investigated. The aims of our study were to fill this gap and to evaluate the changes induced by treatment with a specific oral device on time-domain and frequency domain parameters of heart rate variability.

Materials and Methods

Subjects
We considered for our study 13 consecutive subjects referred to our laboratory because of symptoms of OSA syndrome (OSAS) for diagnostic polysomnography. All subjects were normotensive, free of any other known disease, and receiving no medications. No subjects smoked cigarettes or chewed tobacco at the time of the study and during the previous 6 months. All subjects were used to drinking no more than two cups of expresso coffee per day, this being the case both at baseline and during treatment. On recruitment, all subjects underwent an orthodontic assessment of jaw posture on the three spatial planes to detect possible jaw deviation from normal occlusion. Three subjects were excluded from the study because they did not have any evidence of significant occlusion defects. As a control group, we included 10 healthy subjects matched for age and body mass index, in which occult OSA was excluded using polysomnography.

Measurements and Protocol
Diagnostic polysomnography was performed with a polysomnographic recorder (StarDust; Respironics; Murrysville, PA). Heart rate (ECG), airflow at the mouth and nose, respiratory effort, and arterial oxygen saturation (SaO2) were monitored during the night. The polysomnogram was scored manually according to standard criteria.21 An apnea was defined as cessation of inspiratory airflow for > 10 s with or without oxyhemoglobin desaturation. An obstructive apnea was defined as the absence of airflow in the presence of rib cage and/or abdominal excursion. A central apnea was defined as the absence of rib cage and abdominal excursions with absence of airflow. Hypopnea was defined as a discernible reduction of > 50% airflow and a decrease of oxyhemoglobin saturation ≥ 4%. To diagnose OSA, a threshold of five events per hour was set.22

Measurements of ECG, heart rate, and BP were obtained while subjects were awake, during 20 min of undisturbed supine rest in the same room from 10:00 to 11:00 AM; during the recording, the subjects were asked to avoid sudden changes in respiratory rate, which, in all subjects, remained approximately 12 breaths/min. Studies were conducted 3 h after last meal, and the patients voided before the recordings. These recordings were repeated after a 3-month follow-up in both groups, during which subjects with OSA (OSA+) made use of the oral appliance while no intervention was planned in subjects without OSA (OSA–).

Oral Appliance
An individualized acrylic oral splint was applied to each OSA+ subject (Fig 1 ). The device (Bernkopf-Bertarini) is endowed with orthodontic hooks that fix it to the teeth of the lower dental arch and can be easily removed by the patient. The device consists of an occlusal bite plane set between the two antagonist dental arcades, which modifies the vertical dimension, and of a "repositioning wall" that works over the vestibular surfaces of the upper incisors and canines. This repositioning wall modifies both the sagittal and the lateral relationships between the maxilla and the mandible. Each appliance also had a lingual "target" made by an acrylic ring that stimulates the patient’s tongue toward its proper position on the palatine folds immediately behind the upper incisors. The above described device is prepared after the dentist has repositioned the mandible, by means of adequate manipulation, into the posture regarded as the correct one when compared with the posture usually assumed by the patient. Patients were required to wear the appliance continuously during sleep, and they were assessed monthly to monitor its proper functioning.


Figure 1
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  		Figure 1. Photograph of the oral jaw-positioning appliance (C) and its relation to the upper dental arch (B) and lower dental arch (A).

 
Data Analysis
All the data on cardiac autonomic modulation were obtained during daytime with the patients awake, and were analyzed off-line with a personal computer (MARS 5000; Marquette Hellige; Freiburg, Germany). ECG and respiratory activity were derived, and the resulting tachogram was displayed on a computer screen. All suitable segments from the tachogram, free of ectopic beats and artifacts, were selected for further analysis. A 512-beat length was the minimum period length accepted for analysis. The mean R-R interval (RRI) [measured in milliseconds] and its total variance (milliseconds squared) were calculated. Power spectral analysis was then performed to estimate the powers in the low-frequency (LF) range (0.04 to 0.14 Hz) and in the high-frequency (HF) range (0.15 to 0.50 Hz). To this aim, each RRI series was interpolated and resampled at 2.5 Hz to obtain evenly sampled data. The average RRI spectral power was then integrated in the LF and HF, and the LF/HF ratio was computed accordingly to obtain an indirect marker of the balance between sympathetic and parasympathetic cardiac modulation.2324 The LF and HF components were expressed in absolute units (milliseconds squared). BP was measured by an automatic oscillometric sphygmomanometer (SpaceLabs 90207; OSI Systems; Hawthorne, CA). Differences in all parameters between baseline and follow-up were computed.

Statistics
Values are given as means ± SEM, except for demographics of the subjects (means ± SD). Statistical analysis consisted of repeated-measures analysis of variance with time as the within-factor parameter and the difference OSA+/OSA– as the between-factor parameter. Differences were considered to be significant at a level of p < 0.05. All subjects were included after giving informed consent. The study was approved by the Ethics Committee of our institution.

Results

The demographic characteristics of the two groups are presented in Table 1 .


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  		Table 1. Demographics of OSA+ and OSA– Subjects*

 
Baseline
In our OSA+ subjects, mean apnea-hypopnea index (AHI) was 18.2 ± 2/h. As shown in Table 2 , RRI was significantly shorter in OSA+ subjects (820 ± 30 ms) than in OSA– subjects (930 ± 50 ms; p < 0.02). The variance of RRI was significantly reduced in OSA+ subjects compared to OSA– subjects (980 ± 300 ms2 vs 3,800 ± 700 ms2, respectively; p < 0.02). In comparison with OSA– subjects, OSA+ subjects showed a decrease in HF RRI powers (134 ± 26 ms vs 470 ± 50 ms, p < 0.001) and a tendency toward an increased LF/HF ratio of RRI variability (3.11 ± 0.8 vs 1.27 ± 0.6, p = not significant).


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  		Table 2. RRI Mean Values and Variances and RRI Spectral Parameters Derived by Spectral Analysis Through an Autoregressive Model in OSA– and OSA+ Groups*

 
After 3-Month Follow-up
The OSA+ group showed a marked reduction in AHI (from 18 ± 2 to 4 ± 1/h; p < 0.001). Polysomnographic indexes quantifying the severity of OSA (such as AHI, SaO2, and lowest oxygen saturation [low O2]) are shown in Table 3 . Data obtained at baseline and after 3 months of treatment with the oral appliance are separately shown in OSAS subjects. The corresponding parameters obtained at baseline in normal subjects are also shown. After treatment, RRI and its overall variance were no more significantly different between the two groups. More specifically, the OSA+ group showed an increased HF power of the RRI variability (from 134 ± 26 to 502 ± 48 ms2, p < 0.001) and a tendency to a nonsignificant decrease of LF/HF ratio (from 3.11 ± 0.8 to 1.5 ± 0.5) [Table 4 ]. In both groups, body weight and arterial BP did not change over time. ECG, heart rate, and BP of the control group did not change during the follow-up period.


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  		Table 3. Polysomnographic Data in OSAS and Control Groups at Baseline and After 3 Months of Therapy

 

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  		Table 4. RRI, Its Variance, and Power Spectral Analysis of RRI Variability in OSA+ and OSA– Groups*

 
Discussion

The novel finding of our study is that treatment with an oral jaw-positioning appliance may improve cardiac autonomic modulation during wakefulness in healthy patients with mild degree of OSA. It has been suggested that cardiovascular variability is altered in patients with moderate-to-severe sleep apnea (AHI > 20/h), even in the absence of arterial hypertension, heart failure, or other overt cardiovascular problems.625 Our data seem to suggest that such an alteration is detectable also in a mild degree of OSA (OSA+, AHI ≤ 18/h). This finding may be relevant, since previous studies1026 also in normal subjects have demonstrated that impaired cardiovascular variability is involved in determining patient’s cardiovascular risk, being associated with insulin resistance condition, left ventricular hypertrophy, and new-onset hypertension. The ability of CPAP treatment17 not only to improve sleep-related breathing disorders but to normalize alterations in sympathetic activity and cardiovascular variability, has been previously demonstrated in patients with moderate-to-severe sleep apnea, even though, because of frequent side effects, < 50% of patients who are prescribed CPAP treatment are regularly compliant with treatment.27 Consequently, the availability of other effective, noninvasive, and well-tolerated therapeutic approaches would be highly desirable, particularly in patients with milder forms of OSA. Polysomnography often confirms the beneficial effects of this approach by demonstrating a decrease of snoring frequency, AHI or respiratory disturbance index, and oxygen desaturation frequency and intensity in as many as 50 to 80% of patients.2829 Therefore, polysomnography should always be conducted in patients treated with an oral appliance, because an increased AHI after therapy with oral devices has been reported in approximately 13% of patients; while in other cases, a gradual decline in treatment effect either in the short and long term, or an unsatisfactory change in the number of breathing events in patients otherwise reporting a subjective improvement, have been described.303132 Although no guidelines on the applicability of oral devices are available, it has been suggested that this therapy might be less effective in severe OSAS, even if other studies3334 postulate that severity should not be a reason to exclude patients from oral appliance treatment. Magnetic resonance index and cephalometric variables are also debatable indices to guide a therapeutic choice; indices apparently associated with favorable response to oral devices include a cranial position of the hyoid bone, a smaller mandibular plane angle, a reduced lower anterior face height, a longer anterior cranial base, an increased maxillary length, a shorter soft palate, and a "normal" airway diameter.3536 Given that mandibular advancement is the most commonly acknowledged factor responsible for a symptomatic improvement with oral devices, this approach may be particularly indicated for those patients with a retruded mandibular posture, belonging to class II in the orthodontic classification proposed by Angle,37 even though treatment success has also been associated with a lower body mass index, smaller neck circumference, and younger age, and it is more likely reported in patients with posture (supine)-dependent airway obstruction.38

Most of the devices used until now in adults may induce their effect by increasing the anteroposterior pharyngeal diameter, resulting in a forced nonphysiologic dental malocclusion.39 A large number of complications such as jaw joint pain, facial muscle pain, tooth pain, and xerostomia may consequently follow.40 On the contrary, the oral appliance used in our study allows a physiologic jaw repositioning toward a lingual target, which stimulates the tongue to keep it in its proper position on palatine folds immediately behind the upper incisors. A repositioning wall, on the vestibular surface of the upper canine teeth, obliged the patient to modify the habitual jaw position in favor of a more appropriate one. In this way, we obtained an enlarging of the retrolingual space and a change from oral to nasal breathing. Our jaw-repositioning appliance was effective in inducing a significant reduction of nightly apneas and an improvement of the altered RRI variability. The precise mechanisms underlying the improvement in autonomic cardiac modulation after 3 months of treatment with our oral appliance were not explored in our study, even though it is likely that an attenuation of the apneic events leading to a decreased chemoreflex sensitivity with a consequent decrease in sympathetic activity played a role.41 Increased production of nitric oxide and systemic release of other vasoactive substances, mediated by restoration of a correct nasal ventilation, may also be involved.42 A number of clinical studies4344 have assessed the possible association between the application of treatment in OSAS and a prolonged arterial BP reduction; in several cases, they have provided conflicting evidence: CPAP being reported either to contribute to the BP reduction or to have no effect at all; unfortunately, these studies were affected by methodologic problems, the most important ones being the lack of proper control groups with placebo and inadequate BP measurement techniques. In contrast with the discouraging results of studies using oral preparations as placebo, in two analyses4546 in which CPAP at subtherapeutic pressures was used as a physical placebo in control groups, significant BP reductions have been demonstrated in active treatment groups, also when considering daytime BP; the average reductions of BP induced by effective CPAP in one of these studies,46 characterized by a particularly long treatment time (9 weeks), were 10.0 mm Hg at daytime and 10.3 mm Hg at nighttime; similarly, controversies have been reported also on the BP-lowering effects of OSAS treatment by an oral device, mainly in relation with the duration of follow-up.47, 48 Thus, we cannot exclude that also in our study, in which no BP reduction was seen after 3 months, some BP effect might become evident over a longer follow-up period. A few favorable aspects of our study deserve to be emphasized. First, since our subjects were free of any disease other than OSA and were receiving no medications, possible confounding effects of concomitant drug treatments on cardiac autonomic modulation could be excluded. Second, OSA patients and control subjects were matched by age and body mass index, thereby ruling out any potential influence of differences in these important confounders on the parameters under evaluation. Third, while treatment-induced changes of respiratory parameters in OSA are usually evaluated during sleep, this being the case also for cardiovascular variability in a few articles,49 our data were obtained during daytime with the patients awake in the absence of either apneas, hypopneas, or hypoxemia. This allowed us to obtain information on the persisting effects of OSA and of its treatment by an oral appliance on autonomic cardiac control also during wakefulness. In conclusion, our study shows that a personalized oral jaw-positioning appliance is not only well tolerated but is also effective in reducing sleep-related respiratory disorders and in inducing an improvement of cardiac autonomic modulation, as reflected by changes in heart rate variability. This effect may have favorable implications for the prevention of cardiovascular disease even in subjects with milder degrees of OSA, an issue that deserves to be explored by longitudinal studies.

Footnotes

Abbreviations: AHI = apnea-hypopnea index; CPAP = continuous positive airway pressure; HF = high frequency; LF = low frequency; low O2 = lowest oxygen saturation; OSA = obstructive sleep apnea; OSA+ = with obstructive sleep apnea; OSA– = without obstructive sleep apnea; OSAS = obstructive sleep apnea syndrome; RRI = R-R interval; SaO2 = arterial oxygen saturation

All of the authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Received for publication February 14, 2006. Accepted for publication April 26, 2006.

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