Using the Berlin Questionnaire to identify patients at risk for the sleep apnea syndrome

BACKGROUND

Although sleep apnea is common, it often goes undiagnosed in primary care encounters.

OBJECTIVE

To test the Berlin Questionnaire as a means of identifying patients with sleep apnea.

DESIGN

Survey followed by portable, unattended sleep studies in a subset of patients.

SETTING

Five primary care sites in Cleveland, Ohio.

PATIENTS

744 adults (of 1008 surveyed [74%]), of whom 100 underwent sleep studies.

MEASUREMENTS

Survey items addressed the presence and frequency of snoring behavior, waketime sleepiness or fatigue, and history of obesity or hypertension. Patients with persistent and frequent symptoms in any two of these three domains were considered to be at high risk for sleep apnea. Portable sleep monitoring was conducted to measure the number of respiratory events per hour in bed (respiratory disturbance index [RDI]).

RESULTS

Questions about symptoms demonstrated internal consistency (Cronbach correlations, 0.86 to 0.92). Of the 744 respondents, 279 (37.5%) were in a high-risk group that was defined a priori. For the 100 patients who underwent sleep studies, risk grouping was useful in prediction of the RDI. For example, being in the high-risk group predicted an RDI greater than 5 with a sensitivity of 0.86, a specificity of 0.77, a positive predictive value of 0.89, and a likelihood ratio of 3.79.

CONCLUSION

The Berlin Questionnaire provides a means of identifying patients who are likely to have sleep apnea.

Sympathetic neural mechanisms in obstructive sleep apnea

Blood pressure, heart rate, sympathetic nerve activity, and polysomnography were recorded during wakefulness and sleep in 10 patients with obstructive sleep apnea. Measurements were also obtained after treatment with continuous positive airway pressure (CPAP) in four patients. Awake sympathetic activity was also measured in 10 age- and sex-matched control subjects and in 5 obese subjects without a history of sleep apnea. Patients with sleep apnea had high levels of nerve activity even when awake (P < 0.001). Blood pressure and sympathetic nerve activity did not fall during any stage of sleep. Mean blood pressure was 92 +/- 4.5 mmHg when awake and reached peak levels of 116 +/- 5 and 127 +/- 7 mmHg during stage II sleep (n = 10) and rapid eye movement (REM) sleep (n = 5), respectively (P < 0.001). Sympathetic activity increased during sleep (P = 0.01) especially during stage II (133 +/- 9% above wakefulness; P = 0.006) and REM (141 +/- 13%; P = 0.007). Peak sympathetic activity (measured over the last 10 s of each apneic event) increased to 299 +/- 96% during stage II sleep and to 246 +/- 36% during REM sleep (both P < 0.001). CPAP decreased sympathetic activity and blood pressure during sleep (P < 0.03). We conclude that patients with obstructive sleep apnea have high sympathetic activity when awake, with further increases in blood pressure and sympathetic activity during sleep. These increases are attenuated by treatment with CPAP.

Pathophysiology of sleep apnea

Sleep-induced apnea and disordered breathing refers to intermittent, cyclical cessations or reductions of airflow, with or without obstructions of the upper airway (OSA). In the presence of an anatomically compromised, collapsible airway, the sleep-induced loss of compensatory tonic input to the upper airway dilator muscle motor neurons leads to collapse of the pharyngeal airway. In turn, the ability of the sleeping subject to compensate for this airway obstruction will determine the degree of cycling of these events. Several of the classic neurotransmitters and a growing list of neuromodulators have now been identified that contribute to neurochemical regulation of pharyngeal motor neuron activity and airway patency. Limited progress has been made in developing pharmacotherapies with acceptable specificity for the treatment of sleep-induced airway obstruction. We review three types of major long-term sequelae to severe OSA that have been assessed in humans through use of continuous positive airway pressure (CPAP) treatment and in animal models via long-term intermittent hypoxemia (IH): 1) cardiovascular. The evidence is strongest to support daytime systemic hypertension as a consequence of severe OSA, with less conclusive effects on pulmonary hypertension, stroke, coronary artery disease, and cardiac arrhythmias. The underlying mechanisms mediating hypertension include enhanced chemoreceptor sensitivity causing excessive daytime sympathetic vasoconstrictor activity, combined with overproduction of superoxide ion and inflammatory effects on resistance vessels. 2) Insulin sensitivity and homeostasis of glucose regulation are negatively impacted by both intermittent hypoxemia and sleep disruption, but whether these influences of OSA are sufficient, independent of obesity, to contribute significantly to the "metabolic syndrome" remains unsettled. 3) Neurocognitive effects include daytime sleepiness and impaired memory and concentration. These effects reflect hypoxic-induced "neural injury." We discuss future research into understanding the pathophysiology of sleep apnea as a basis for uncovering newer forms of treatment of both the ventilatory disorder and its multiple sequelae.

Automatic sleep/wake identification from wrist activity

The purpose of this study was to develop and validate automatic scoring methods to distinguish sleep from wakefulness based on wrist activity. Forty-one subjects (18 normals and 23 with sleep or psychiatric disorders) wore a wrist actigraph during overnight polysomnography. In a randomly selected subsample of 20 subjects, candidate sleep/wake prediction algorithms were iteratively optimized against standard sleep/wake scores. The optimal algorithms obtained for various data collection epoch lengths were then prospectively tested on the remaining 21 subjects. The final algorithms correctly distinguished sleep from wakefulness approximately 88% of the time. Actigraphic sleep percentage and sleep latency estimates correlated 0.82 and 0.90, respectively, with corresponding parameters scored from the polysomnogram (p < 0.0001). Automatic scoring of wrist activity provides valuable information about sleep and wakefulness that could be useful in both clinical and research applications.

Longitudinal study of moderate weight change and sleep-disordered breathing

CONTEXT

Excess body weight is positively associated with sleep-disordered breathing (SDB), a prevalent condition in the US general population. No large study has been conducted of the longitudinal association between SDB and change in weight.

OBJECTIVE

To measure the independent longitudinal association between weight change and change in SDB severity.

DESIGN

Population-based, prospective cohort study conducted from July 1989 to January 2000.

SETTING AND PARTICIPANTS

Six hundred ninety randomly selected employed Wisconsin residents (mean age at baseline, 46 years; 56% male) who were evaluated twice at 4-year intervals for SDB.

MAIN OUTCOME MEASURES

Percentage change in the apnea-hypopnea index (AHI; apnea events + hypopnea events per hour of sleep) and odds of developing moderate-to-severe SDB (defined by an AHI > or =15 events per hour of sleep), with respect to change in weight.

RESULTS

Relative to stable weight, a 10% weight gain predicted an approximate 32% (95% confidence interval [CI], 20%-45%) increase in the AHI. A 10% weight loss predicted a 26% (95% CI, 18%-34%) decrease in the AHI. A 10% increase in weight predicted a 6-fold (95% CI, 2.2-17.0) increase in the odds of developing moderate-to-severe SDB.

CONCLUSIONS

Our data indicate that clinical and public health programs that result in even modest weight control are likely to be effective in managing SDB and reducing new occurrence of SDB.

Practice Parameters for the Indications for Polysomnography and Related Procedures: An Update for 2005

These practice parameters are an update of the previously-published recommendations regarding the indications for polysomnography and related procedures in the diagnosis of sleep disorders. Diagnostic categories include the following: sleep related breathing disorders, other respiratory disorders, narcolepsy, parasomnias, sleep related seizure disorders, restless legs syndrome, periodic limb movement sleep disorder, depression with insomnia, and circadian rhythm sleep disorders. Polysomnography is routinely indicated for the diagnosis of sleep related breathing disorders; for continuous positive airway pressure (CPAP) titration in patients with sleep related breathing disorders; for the assessment of treatment results in some cases; with a multiple sleep latency test in the evaluation of suspected narcolepsy; in evaluating sleep related behaviors that are violent or otherwise potentially injurious to the patient or others; and in certain atypical or unusual parasomnias. Polysomnography may be indicated in patients with neuromuscular disorders and sleep related symptoms; to assist in the diagnosis of paroxysmal arousals or other sleep disruptions thought to be seizure related; in a presumed parasomnia or sleep related seizure disorder that does not respond to conventional therapy; or when there is a strong clinical suspicion of periodic limb movement sleep disorder. Polysomnography is not routinely indicated to diagnose chronic lung disease; in cases of typical, uncomplicated, and noninjurious parasomnias when the diagnosis is clearly delineated; for patients with seizures who have no specific complaints consistent with a sleep disorder; to diagnose or treat restless legs syndrome; for the diagnosis of circadian rhythm sleep disorders; or to establish a diagnosis of depression.

Risk factors for sleep-disordered breathing in children. Associations with obesity, race, and respiratory problems

This study examined risk factors for sleep-disordered breathing (SDB) in children and adolescents; specifically, quantifying risk associated with obesity, race, and upper and lower respiratory problems. Subjects were participants in a genetic-epidemiologic study of SDB and included 399 children and adolescents 2 to 18 yr of age, recruited as members of families with a member (a proband) with known sleep apnea (31 index families) or as members of neighborhood control families (30 families). SDB was assessed with home overnight multichannel monitoring and SDB was defined based on an apneahypopnea index >/= 10 (moderately affected) or < 5 (unaffected). SDB of moderate level was significantly associated with obesity (odds ratio, 4.59; 95% confidence interval [CI], 1.58 to 13.33) and African-American race (odds ratio, 3.49; 95% CI, 1.56 to 8.32) but not with sex or age. After adjusting for obesity, proband sampling, race and familial clustering, sinus problems and persistent wheeze each independently (of the other) predicted SDB. These data suggest the importance of upper and lower respiratory problems and obesity as risk factors for SDB in children and adolescents. Increased risk in African Americans appears to be independent of the effects of obesity or respiratory problems.

Daytime sleepiness, snoring, and obstructive sleep apnea. The Epworth Sleepiness Scale

The Epworth Sleepiness Scale (ESS) is a simple questionnaire measuring the general level of daytime sleepiness, called here the average sleep propensity. This is a measure of the probability of falling asleep in a variety of situations. The conceptual basis of the ESS involves a four-process model of sleep and wakefulness. The sleep propensity at any particular time is a function of the ratio of the total sleep drive to the total wake drive with which it competes. ESS scores significantly distinguished patients with primary snoring from those with obstructive sleep apnea syndrome (OSAS), and ESS scores increased with the severity of OSAS. Multiple regression analysis showed that ESS scores were more closely related to the frequency of apneas than to the degree of hypoxemia in OSAS. ESS scores give a useful measure of average sleep propensity, comparable to the results of all-day tests such as the multiple sleep latency test.

Association of sleep-disordered breathing and the occurrence of stroke

RATIONALE

Sleep-disordered breathing has been linked to stroke in previous studies. However, these studies either used surrogate markers of sleep-disordered breathing or could not, due to cross-sectional design, address the temporal relationship between sleep-disordered breathing and stroke.

OBJECTIVES

To determine whether sleep-disordered breathing increases the risk for stroke.

METHODS

We performed cross-sectional and longitudinal analyses on 1,475 and 1,189 subjects, respectively, from the general population. Sleep-disordered breathing was defined by the apnea-hypopnea index (frequency of apneas and hypopneas per hour of sleep) obtained by attended polysomnography. The protocol, including polysomnography, risk factors for stroke, and a history of physician-diagnosed stroke, was repeated at 4-yr intervals.

MEASUREMENTS AND MAIN RESULTS

In the cross-sectional analysis, subjects with an apnea-hypopnea index of 20 or greater had increased odds for stroke (odds ratio, 4.33; 95% confidence interval, 1.32-14.24; p = 0.02) compared with those without sleep-disordered breathing (apnea-hypopnea index, <5) after adjustment for known confounding factors. In the prospective analysis, sleep-disordered breathing with an apnea-hypopnea index of 20 or greater was associated with an increased risk of suffering a first-ever stroke over the next 4 yr (unadjusted odds ratio, 4.31; 95% confidence interval, 1.31-14.15; p = 0.02). However, after adjustment for age, sex, and body mass index, the odds ratio was still elevated, but was no longer significant (3.08; 95% confidence interval, 0.74-12.81; p = 0.12).

CONCLUSIONS

These data demonstrate a strong association between moderate to severe sleep-disordered breathing and prevalent stroke, independent of confounding factors. They also provide the first prospective evidence that sleep-disordered breathing precedes stroke and may contribute to the development of stroke.

Sleep apnea and daytime sleepiness and fatigue: relation to visceral obesity, insulin resistance, and hypercytokinemia

Sleep apnea and associated daytime sleepiness and fatigue are common manifestations of mainly obese middle-aged men. The onset of sleep apnea peaks in middle age, and its morbid and mortal sequelae include complications from accidents and cardiovascular events. The pathophysiology of sleep apnea remains obscure. The purpose of this study was to test three separate, albeit closely related, hypotheses. 1) Does sleep apnea contribute to the previously reported changes of plasma cytokine (tumor necrosis factor-alpha and interleukin-6) and leptin levels independently of obesity? 2) Among obese patients, is it generalized or visceral obesity that predisposes to sleep apnea? 3) Is apnea a factor independent from obesity in the development of insulin resistance? Obese middle-aged men with sleep apnea were first compared with nonapneic age- and body mass index (BMI)-matched obese and age-matched lean men. All subjects were monitored in the sleep laboratory for 4 consecutive nights. We obtained simultaneous indexes of sleep, sleep stages, and sleep apnea, including apnea/hypopnea index and percent minimum oxygen saturation. The sleep apneic men had higher plasma concentrations of the adipose tissue-derived hormone, leptin, and of the inflammatory, fatigue-causing, and insulin resistance-producing cytokines tumor necrosis factor-alpha and interleukin-6 than nonapneic obese men, who had intermediate values, or lean men, who had the lowest values. Because these findings suggested that sleep apneics might have a higher degree of insulin resistance than the BMI-matched controls, we studied groups of sleep-apneic obese and age- and BMI-matched nonapneic controls in whom we obtained computed tomographic scan measures of total, sc, and visceral abdominal fat, and additional biochemical indexes of insulin resistance, including fasting plasma glucose and insulin. The sleep apnea patients had a significantly greater amount of visceral fat compared to obese controls (<0.05) and indexes of sleep disordered breathing were positively correlated with visceral fat, but not with BMI or total or sc fat. Furthermore, the biochemical data confirmed a higher degree of insulin resistance in the group of apneics than in BMI-matched nonapneic controls. We conclude that there is a strong independent association among sleep apnea, visceral obesity, insulin resistance and hypercytokinemia, which may contribute to the pathological manifestations and somatic sequelae of this condition.

Sleep-disordered breathing in community-dwelling elderly

These are the final results of a survey of sleep-disordered breathing, which examined objective and subjective information from a large randomly selected elderly sample. We randomly selected 427 elderly people aged 65 yr and over in the city of San Diego, California. Twenty-four percent had an apnea index, AI, greater than or equal to 5 and 62% had a respiratory disturbance index, RDI, greater than or equal to 10. Correlates of sleep-disordered breathing included high relative weight and reports of snoring, breathing cessation at night, nocturnal wandering or confusion, daytime sleepiness and depression. Body mass index, falling asleep at inappropriate times, male gender, no alcohol within 2 hr of bedtime and napping were the best predictors of sleep-disordered breathing. Despite statistical significance, all of the associations between interview variables and apnea indices were small. No combination of demographic variables and symptoms allowed highly reliable prediction of AI or RDI.

Association of sleep apnoea with myocardial infarction in men

To examine the hypothesis that sleep apnoea is a risk factor for ischaemic heart disease, overnight polysomnography was performed in 101 unselected male survivors of acute myocardial infarction (MI) aged less than 66 yr and in 53 male subjects of similar age without evidence of ischaemic heart disease. The apnoea index (AI, number of apnoea episodes per hour of sleep) was 6.9 (SEM 1.2) in the MI patients versus 1.4 (0.3) in the control subjects. After adjustment for age, body mass index, hypertension, smoking, and cholesterol level, multiple logistic regression analysis identified the top quartile of AI (greater than 5.3) as an independent predictor of MI patients. The relative risk for myocardial infarction between the highest and lowest quartiles of AI was 23.3 (95% confidence interval 3.9-139.9).

Waking genioglossal electromyogram in sleep apnea patients versus normal controls (a neuromuscular compensatory mechanism)

Pharyngeal collapse in obstructive sleep apnea patients is likely a product of a sleep-related decrement in pharyngeal dilator muscle activity superimposed upon abnormal airway anatomy. We postulate that during wakefulness, increased pharyngeal dilator muscle activity in apnea patients compensates for diminished airway size thus maintaining patency. We studied the waking genioglossus (GG) electromyogram (EMG) activity in 11 OSA patients and 14 age-matched controls to determine if GG activity is higher in the awake state in apnea patients than controls. To make this determination, we developed a reproducible methodology whereby true maximal GG EMG could be defined and thus basal activity quantitated as a percentage of this maximal value. Therefore, direct comparisons of basal activity between individuals was possible. We observed apnea patients to have significantly greater basal genioglossal activity compared to controls (40.6 +/- 5.6% vs. 12.7 +/- 1.7% of maximum). This difference persisted when size-matched subsets were compared. This augmented GG activity in apnea patients could be reduced with positive airway pressure. We speculate that this neuromuscular compensation present during wakefulness in apnea patients may be lost during sleep leading to airway collapse.

Prevalence and risk factors for sleep-disordered breathing in 8- to 11-year-old children: association with race and prematurity

OBJECTIVES

To evaluate the extent to which sleep-disordered breathing (SDB) varies with putative demographic and medical risk factors and to estimate the prevalence of undiagnosed SDB.

STUDY DESIGN

Prospective, cross-sectional study in a population-based cohort of 850 children (41% black, 46% preterm), 8 to 11 years of age. Participants' caretakers completed questionnaires about health and sleep. Children underwent overnight in-home cardiorespiratory recordings of airflow, respiratory effort, oximetry, and electrocardiography. SDB was identified by respiratory disturbance indices commonly applied in clinical practice. Risk factors were estimated by logistic regression. Prevalence was derived from cohort-specific estimates with birth weights from US live births data.

RESULTS

Using the most inclusive definition, SDB was detected in 40 (4.7%) participants, with prevalence varying widely across population subsets. Depending on the definition used, SDB was 4 to 6 times more likely in black children compared with white children and almost 3 to 5 times more likely in former preterm compared with term children. The estimated population prevalence of SDB was 2.2% (95% CI, 1.2%, 3.2%).

CONCLUSIONS

SDB is a relatively common condition in 8- to 11-year-old children. Potentially vulnerable subgroups, black children, and former preterm infants, are at increased risk.

Excess weight and sleep-disordered breathing

Excess weight is a well-established predictor of sleep-disordered breathing (SDB). Clinical observations and population studies throughout the United States, Europe, Asia, and Australia have consistently shown a graded increase in the prevalence of SDB as body mass index, neck girth, or other measures of body habitus increases. Clinical studies of weight loss and longitudinal population studies provide strong support for a causal association. The role of excess body weight, a modifiable risk factor, with SDB raises many questions relevant to clinical practice and public health. The topic takes on added importance with the alarming rate of weight gain in children as well as adults in industrialized nations. Among adults ages 30-69 yr, averaging over the estimated United States 2003 age, sex, and BMI distributions, we estimate that approximately 17% of adults have mild or worse SDB (apnea-hypopnea index > or = 5) and that 41% of those adults have SDB "attributable" to having a body mass index of > or =25 kg/m(2). Similarly, we estimate that approximately 5.7% of adults have moderate or worse SDB (apnea-hypopnea index > or = 15) and that 58% of those adults have SDB attributable to excess weight. Clearly, if the expanding epidemic of obesity seen in the United States continues, the prevalence of SDB will almost certainly increase, along with the proportion of SDB attributable to obesity.

Prognostic value of nocturnal Cheyne-Stokes respiration in chronic heart failure

BACKGROUND

Nocturnal Cheyne-Stokes respiration (CSR) occurs frequently in patients with chronic heart failure (CHF), and it may be associated with sympathetic activation. The aim of the present study was to evaluate whether CSR could affect prognosis in patients with CHF.

METHODS AND RESULTS

Sixty-two CHF patients with left ventricular ejection fraction </=35%, in NYHA class II to III, underwent clinical evaluation, Doppler echocardiography, ergospirometry, phenylephrine test, Holter recording, and a sleep study to evaluate the occurrence of CSR, expressed as percentage of periodic breathing, and apnea/hypopnea index (AHI) (ie, the number of apneas and hypopneas per hour of recording). During a mean follow-up of 28+/-13 months, 15 patients died of cardiac causes. Nonsurvivors were in a higher NYHA functional class than survivors (P<0.001) and had a more depressed left ventricular ejection fraction (P<0.03), a shorter deceleration time of early filling (P<0. 05), larger left and right atria (P<0.05 and P<0.02, respectively) and a lower peak V(O2) (P<0.05). Nonsurvivors also spent a greater percentage of the night in periodic breathing (P<0.01) with a greater AHI (P<0.03) and showed lower values of diurnal baroreflex sensitivity (P<0.05) and of heart rate variability (sdNN: P<0.01). Multivariate analysis revealed the AHI (chi2, 10.4; P<0.01), followed by left atrial area (chi2, 5.7; P<0.01), as the only independent and additional predictors of subsequent cardiac death. Patients at very high risk for fatal outcome could be identified by an AHI >/=30/h and left atria >/=25 cm2.

CONCLUSIONS

The AHI is a powerful independent predictor of poor prognosis in clinically stable patients with CHF. The presence of an AHI >/=30/h adds prognostic information compared with other clinical, echocardiographic, and autonomic data and identifies patients at very high risk for subsequent cardiac death.

Augmented resting sympathetic activity in awake patients with obstructive sleep apnea

Muscle nerve sympathetic activity (MSA) was recorded during wakefulness in 11 patients with obstructive sleep apnea (OSA) and in 9 sex- and age-matched healthy control subjects. Plasma levels of norepinephrine (NE) and neuropeptide Y were analyzed. Five patients had established hypertension (resting supine systolic BP/diastolic BP > or = 160/95 mm Hg). The investigation was performed after a minimum of 3 weeks' washout period of antihypertensive medication. Muscle sympathetic activity during supine rest was higher in patients compared with controls (p < 0.01) with no difference between normotensive and hypertensive patients. However, systolic, but not diastolic, BP was positively related to resting MSA (n = 20, p < 0.01). There was no significant correlation between body mass index and MSA. Resting MSA was unrelated to disease severity expressed as apnea frequency or minimum SaO2 during the overnight recording. Both the arterial and venous plasma norepinephrine was higher in patients compared with controls (p < 0.05). Plasma levels of NE correlated to resting MSA (p < 0.01) in the whole study group (patients and controls) but not within the respective subgroups. No significant correlation, however, was found between plasma NE (arterial and venous) and BP. Plasma neuropeptide Y-like immunoreactivity was similar in patients and controls. However, one patient with hypertension had approximately twice this level in repeated samples. It is concluded that neurogenic sympathetic activity as well as circulating plasma NE is increased in patients with OSA. This increased sympathetic activity during awake supine rest may reflect a pathophysiologic adaptation to hypoxia and hemodynamic changes occurring at repetitive apneas during sleep. The correlation between MSA and systolic BP implies that this mechanism may be directly or indirectly involved in the development of cardiovascular complications in OSA.

A diagnostic approach to suspected obstructive sleep apnea in children

Most children with obstructive sleep apnea will benefit from tonsillectomy and adenoidectomy. Although polygraphic monitoring remains the definitive diagnostic technique, we wondered if all children suspected of having OSA require such evaluation. We therefore administered a standardized questionnaire to the parents of 23 children with polygraphically proved OSA related to adenotonsillar hypertrophy, 46 age- and sex-matched normal children, and 23 children subsequently referred because of possible OSA. Significantly increased frequencies of the following symptoms were found in the OSA group compared with the control group: difficulty breathing during sleep, 96% vs 2%; apnea observed by the parents, 78% vs 5%; snoring, 96% vs 9%; restless sleep, 78% vs 23%; chronic rhinorrhea, 61% vs 11%; and mouth breathing when awake, 87% vs 18%. Using discriminant analysis, an OSA score was derived that correctly classified all control subjects and 22 of 23 patients with OSA. Considering the data from all groups, we found that (1) OSA scores greater than 3.5 were highly predictive of OSA requiring adenotonsillectomy; (2) no child with an OSA score less than -1 had OSA; and (3) in children with OSA scores between -1 and 3.5, polygraphic monitoring was required to determine the severity of sleep-related airway obstruction and the need for surgical treatment. Use of the OSA score should decrease the need for polygraphic monitoring and facilitate selection of children for tonsillectomy and adenoidectomy.

Scaling behaviour of heartbeat intervals obtained by wavelet-based time-series analysis

Biological time-series analysis is used to identify hidden dynamical patterns which could yield important insights into underlying physiological mechanisms. Such analysis is complicated by the fact that biological signals are typically both highly irregular and non-stationary, that is, their statistical character changes slowly or intermittently as a result of variations in background influences. Previous statistical analyses of heartbeat dynamics have identified long-range correlations and power-law scaling in the normal heartbeat, but not the phase interactions between the different frequency components of the signal. Here we introduce a new approach, based on the wavelet transform and an analytic signal approach, which can characterize non-stationary behaviour and elucidate such phase interactions. We find that, when suitably rescaled, the distributions of the variations in the beat-to-beat intervals for all healthy subjects are described by a single function stable over a wide range of timescales. However, a similar scaling function does not exist for a group with cardiopulmonary instability caused by sleep apnoea. We attribute the functional form of the scaling observed in the healthy subjects to underlying nonlinear dynamics, which seem to be essential to normal heart function. The approach introduced here should be useful in the analysis of other nonstationary biological signals.

Racial differences in sleep-disordered breathing in African-Americans and Caucasians

In this case-control family study of sleep-disordered breathing (SDB), we describe the distributions of SDB and SDB risk factors in African-Americans and Caucasians. A total of 225 African-Americans and 622 Caucasians, ages 2 to 86 yr, recruited as members of families with an individual with known sleep apnea (85 index families) or as members of neighborhood control families (63 families) were studied with an overnight home sleep-study, questionnaires, and physical measurements. A subsample underwent cephalometry. Outcome measures were the respiratory disturbance index (RDI) and a binary variable indicating the presence of increased apneic activity (IAA). In both races, a strong relationship was demonstrated between the (log transformed) RDI and age and age2. African-Americans with SDB were younger than Caucasians with SDB (37.2 +/- 19.5 versus 45.6 +/- 18.7 yr, p < 0.01). In subjects < or = 25 yr, RDI level and IAA prevalence were higher in African-Americans (odds ratio, adjusted for obesity, sex, proband sampling, and familial clustering, 1.88, 1.03 to 3.52, 95% CI). In this age group, racial differences also were observed in the relationship between RDI and age (p < 0.001 for the RDI-age interaction). This suggests that young African-Americans may be at increased risk for sleep apnea.

Sleep apnea and hypertension. A population-based study

OBJECTIVE

To measure the independent association of sleep-disordered breathing (sleep apnea and habitual snoring) and hypertension in a healthy adult population.

DESIGN

A cross-sectional study of blood pressure during wakefulness and sleep among participants with and without sleep-disordered breathing.

SETTING

Community-based study.

PARTICIPANTS

147 men and women, aged 30 to 60 years, selected from Wisconsin State employees enrolled in the Wisconsin Sleep Cohort Study, an ongoing, prospective, epidemiologic study of sleep-disordered breathing.

MEASUREMENTS

Sleep and medical history interview, nocturnal polysomnography, and 24-hour ambulatory blood pressure monitoring in all participants.

RESULTS

Mean blood pressures were significantly higher among participants with sleep apnea (> or = 5 apneas or hypopneas per hour of sleep) compared with those without (131/80 +/- 1.7/1.1 mm Hg compared with 122/75 +/- 1.9/1.2 mm Hg during wakefulness and 113/66 +/- 1.8/1.1 mm Hg compared with 104/62 +/- 2/1.3 mm Hg during sleep, respectively; P < 0.05). The variability of the blood pressure during sleep was significantly greater in participants with sleep apnea or a history of snoring compared with those without (P < 0.05). After controlling for obesity, age, and sex, sleep apnea was significantly associated with hypertension in a dose-response fashion, with odds ratios ranging from 2.0 for 5 apneic or hypopneic episodes per hour of sleep to 5.0 for 25 apneic or hypopneic episodes.

CONCLUSIONS

Our data indicate an association between hypertension and sleep apnea independent of obesity, age, and sex in a nonselected, community-based adult population.