INTRODUCTION

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Sleep-disordered breathing (SDB) has been shown to aggregate significantly within families (1). Family studies have suggested that the risk of SDB may be from 2- to 4-fold greater in relatives of patients with SDB than in controls (1), and that nearly 40% of the variance in the respiratory disturbance index (RDI) of persons with SDB may be explained by familial factors (1). Estimates of heritability of SDB have not been appreciably influenced by adjustments for body mass index (BMI) (1). Also, increased apneic activity has been demonstrated in relatives of nonobese patients with sleep apnea (2). Thus, a familial basis for SDB does not appear to be fully explained by familial similarities in body mass. Inherited craniofacial features that influence upper-airway size have also been postulated to be important in the pathogenesis of SDB. However, differences in craniofacial morphology between relatives of patients and controls have been relatively small (1, 2), suggesting that other factors that influence airway patency may be important in familial SDB.

In addition to being influenced by anatomic factors, airway patency is influenced dynamically by a variety of complex processes associated with the control of both chest-wall and upper-airway neuromuscular function (4). Some of these processes may be closely related to responses to respiratory chemical and mechanical loading (5, 6). Previous studies have suggested that ventilatory responses to chemical stimuli, especially hypoxic responses, have a familial basis (7, 8). A previous study of family members of SDB patients demonstrated that the magnitudes of their ventilatory responses to hypercapnia and hypoxia were significantly correlated (9). However, control subjects were not studied to determine whether these responses were abnormal. We previously reported a depression of the hypoxic ventilatory response, but no differences in hypercapnic responses, among members of a single family having multiple members with SDB (10). This suggested that a genetic predisposition to SDB in this family might have been related to an inherited abnormality in ventilatory control. In the present study, we further evaluated the role of potentially inherited respiratory control mechanisms in familial SDB by assessing respiratory responses to hypoxia, hypercapnia, and ventilatory loading in subjects from well-defined families in which multiple members have SDB, and in a control sample derived from families having no member with SDB.

	METHODS

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Subjects

Subjects were selected from a cohort developed in Cleveland, Ohio, for genetic-epidemiologic studies of SDB (1). The cohort, described in detail previously (1), consisted (at the time of writing of this report of 1,447 members of 141 families recruited through a proband with SDB identified through a sleep laboratory (index families), and of 71 control families, identified as neighbors and/or friends of the probands. In-home sleep studies were performed on probands and all available first-degree relatives of both index and control families (4.9 relatives/family were studied), enabling characterization of the affection status of each family member and the degree to which SDB aggregated within each family. Of the families identified through an affected proband, 53 were characterized as "simplex" (only one individual affected with SDB; see subsequent definition of SDB) and 88 as "multiplex" (two or more affected relatives). Of the control families, 57 were found to have no member with SDB (unaffected families). For the studies described herein, which were designed to examine the relationship of ventilatory-control abnormalities to familial sleep apnea, subjects were selected only from the multiplex families and the unaffected control families in order to achieve three clearly defined groups: (1) subjects with SDB from multiplex families (AffectedFamilial SDB); (2) subjects without SDB from multiplex families (UnaffectedFamilial SDB); and (3) Normal Controls, who had no evidence of or first-degree relative with SDB. Each control was matched by age (± 2 yr) and gender with one of the subjects (in either Group 1 or 2) in the experimental (Familial SDB) families. Because ventilatory-control studies may be difficult to interpret in settings of extreme obesity and pulmonary function impairment, subjects were eligible for these studies only if their BMI was less than 32 kg/m² and their FEV₁ and FVC were > 65% of the predicted value for age, sex, and height. Also, because chronic hypoxemia and/or sleep fragmentation may alter ventilatory-control findings (11), subjects with SDB were eligible for study only if their RDI was < 20, or, if their RDI was > 20 they were under treatment with continuous positive airway pressure (CPAP) (n = 4). Studies were restricted to subjects between 15 and 50 yr of age and to subjects without poorly controlled asthma, cardiac disease, sickle-cell anemia, seizures, migraine headaches, and pregnancy. Analyses were restricted to Caucasians, since insufficient data were available for African-Americans to allow race-specific analyses.

Definition of SDB

"Affection" status was defined on the basis of age-specific cutoff values for the RDI that identified approximately the highest 10th percentile of subjects without symptoms of SDB (habitual snoring, observed apneas, snorting, or gasping) within age-specific categories (12). Thus, SDB was considered present for an RDI > 5 for ages < 25 yr, an RDI > 10 for ages 26 to 40 yr, and an RDI > 15 for ages 41 to 50 yr.

Assessment of Ventilatory Responses

Responses to hyperoxic hypercapnia, eucapnic hypoxia, and resistive loading were assessed during four breathing trials during wakefulness, each 20 min apart, using rebreathing techniques (modifications of the techniques of Read and colleagues [13] and Kronenberg and associates [14]). Studies were performed at least 2 h after eating and 12 h after last drinking caffeinated beverages. Trial 1 consisted of hyperoxic hypercapnic rebreathing without added inspiratory resistive loading; Trials 2 and 3 consisted of eucapnic hypoxic breathing; and Trial 4 consisted of hyperoxic hypercapnic rebreathing with an added inspiratory resistance (6.5 cm H₂O/L/s). Prior to the rebreathing trials, subjects breathed air through a low-resistance circuit for 3 to 5 min until the breathing pattern stabilized. Following this, the breathing circuit was closed to include a 7 L Neoprene bag to which either room air (1 L more than the subjects' FVC) (for hypoxic studies), or a 7% CO₂/ balanced oxygen mixture (for hypercapnic studies) was added. During hypoxic breathing, end-tidal CO₂ (PET_CO2) was maintained at the subject's baseline level by use of a variable CO₂ bypass. During hypercapnic trials, a Hans-Rudolph (Kansas City, MO) single-balloon occlusion valve on the inspiratory circuit was closed randomly at three- to eight-breath intervals, causing the subject to inspire against a closed circuit at the beginning of the subsequent inspiration. Hypoxic trials were terminated when oxygen saturation fell to 78 to 82%, and hypercapnic trials were terminated when PET_CO2 increased to 60 mm Hg or the subject could no longer tolerate rebreathing. PET_CO2 was measured continuously near the mouth, using an infrared analyzer (Model LB-2; Beckman Instruments, Mountain View, CA); inspiratory flow was measured with a pneumotachograph (Fleisch No. 3); oxygen saturation (Sa_O2) was measured with a finger pulse oximeter (Model 3740; Ohmeda Biox, Austell, GA); and airway pressure at the mouth was measured with a transducer (MP 45-1-1-871; Validyne, Northridge, CA). Flow was integrated with Gould Integrator (11-4113-01; Valley View, OH) to produce a volume signal.

Analyses

Measures of flow, volume (integrated from the flow signal), breathing frequency, PET_CO2, Sa_O2, mouth pressure, VT/TI (inspiratory flow), and mouth occlusion pressure during the first 100 ms of inspiration against an occluded airway (P₁₀₀) were acquired and analyzed with a Grass Model 7E Polygraph and a breath-by-breath acquisition and analysis program (ACQUANAL; Case Western Reserve University, Cleveland, OH; John Butts, 1989). The response to hypoxia was assessed from the slope of the regression equation describing the relationship between VE versus Sa_O2 (mean: Trials 2 and 3). The response to hypercapnia was assessed from the slopes of the regression equations describing the relationship between instantaneous minute ventilation calculated on a breath-by-breath basis (VE) and PET_CO2 (Trial 1), and the relationship between P₁₀₀ and PET_CO2 (Trial 1).

The effects of resistive loading were evaluated by calculating the difference between the slopes of the regression equations for P₁₀₀ versus PET_CO2 with and without ventilatory loading (Trial 4 and Trial 1, respectively), and VE versus PET_CO2 (Trial 4 and Trial 1, respectively). Respiratory impedance was estimated by calculating the slope defining the relationship between P₁₀₀ and inspiratory flow (VT/TI) during progressive hypercapnia both without and with resistive loading (Trial 1 and Trial 4, respectively).

Group differences were compared through the use of unpaired t tests and contingency table analyses. A value of p < 0.05 was considered statistically significant.

	RESULTS

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Twenty-two subjects from 13 multiplex families were identified as meeting study eligibility criteria, living within driving distance from the laboratory, and being available for testing. Ten subjects (from eight families) had SDB and twelve subjects (from nine families) did not have SDB. These subjects were either probands or the offspring or siblings of probands. Nine control subjects from nine families were identified. A number of the controls were matched (by gender and age) with subjects from more than one experimental family. The characteristics of the study subjects are shown in Table 1. Both controls and members of SDB families (with and without SDB) were of comparable age and had similar levels of lung function and height. BMI and RDI were similar for the controls and unaffected members of SDB families, but were higher for the subjects with SDB than for members of either of the other two groups (p < 0.05).

Respiratory responses to progressive eucapnic hypoxia are shown in Table 2. The slope of the relationship between VE and oxygen saturation, which was linear over the range of values studied in all three groups of subjects, was significantly lower in members of SDB families (0.76 ± 0.46, mean ± SD) than in controls (1.32 ± 0.92) (p = 0.03). The lowest responses were observed among subjects with SDB (0.72 ± 0.32). These latter responses were significantly lower than the hypoxic responses in the control families (p < 0.05), but not statistically different from the responses of unaffected members of SDB families (0.79 ± 0.57). Differences between controls and unaffected family members did not reach conventional levels of statistical significance.

The relationship between VE and oxygen saturation is further depicted graphically in Figure 1. As can be seen, there was no difference in VE at higher oxygen saturations (i.e., at 94% saturation, VE = 16.4 ± 5.4 L/min in controls and 14.8 ± 2.9 L/min in SDB family members, respectively, p = 0.30). However, at a saturation of 82%, the differences in VE between controls and SDB family members were greater (VE = 32.3 ± 15.5 L/min in the controls and 23.9 ± 7.8 L/min in SDB family members, respectively, p = 0.06). The intercepts of the lines describing these relationships (i.e., the projected VE at 0% saturation) were significantly different for the two groups (140.6 ± 85.3 L/min for the controls and 86.0 ± 44.9 L/min for SDB family members, respectively, p = 0.03).

View larger version (9K): [in this window] [in a new window]   Figure 1.   Slopes of the relationships between VE ( y axis) and SaO2 (x axis) during progressive eucapnic hypoxic rebreathing for controls (solid line), unaffected SDB family members (longer hashed lines), and affected SDB family members (shorter hashed lines). The slope was significantly steeper for the controls than for subjects with SDB (p < 0.05), but was no different for unaffected and affected SDB family members. (See Table 2 for SDs for regression coefficients.)

Respiratory responses to progressive hyperoxic hypercapnia during unloaded breathing are summarized in Table 2. Neither VE nor P₁₀₀ varied with PCO₂ in any different manner among the three groups of subjects.

Changes in the VE and P₁₀₀ responses to hypercapnia with the addition of an inspiratory resistive load during hypercapnia are shown in Table 3. There were no significant differences in the effects of ventilatory loading on hypercapnic P₁₀₀ responses among the three groups. Subjects with SDB experienced a significantly greater decrease in the hypercapnic ventilatory response (VE) with loading than did unaffected subjects from multiplex families (p = 0.04).

The ratio of P₁₀₀ to mean inspiratory VT/TI was calculated as a measure of inspiratory impedance. The slope of the relationship between P₁₀₀ and VT/TI during progressive hypercapnia with no added respiratory resistance was similar in SDB family members and control subjects (Figure 2). With the addition of an inspiratory resistive load during rebreathing, there was a significantly greater increase in impedance (as measured by the steeper slope) in SDB family members (both affected and unaffected than in control subjects (0.17 ± 0.05 versus 0.12 ± 0.05, p = 0.04). Differences between affected (0.18 ± 0.06) and unaffected (0.16 ± 0.05) members of SDB families were not significant.

View larger version (29K): [in this window] [in a new window]   Figure 2.   Respiratory impedance (expressed as the slope between P100 and VT/TI during hypercapnia without an added inspiratory resistance (left panel ) and with an added inspiratory resistance (right panel ) in controls (black bars) and members of SDB families (gray bars) (affected and unaffected). No differences were noted in impedance measured during hypercapnia in the unloaded trials (left panel ). With resistive loading, impedance levels with increasing hypercapnia were significantly greater in the members of SDB families than in controls (p < 0.05). There were no differences in these slopes for affected and unaffected SDB family members.

	DISCUSSION

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In this study, we exploited a unique data base of carefully characterized families with and without SDB to assess the role of ventilatory control mechanisms in familial SDB. Selection of subjects with little underlying comorbidity minimized confounding effects related to mechanical impairment or chronic sleep fragmentation. Selection of subjects from multiplex families also may have identified subjects who were genetically at increased risk for SDB, and who might therefore have constituted a more homogeneous group than other samples. Comparing these subjects with individuals from unaffected families also allowed us to assess the role of potentially inherited ventilatory-control abnormalities in familial SDB. The findings of blunted hypoxic responses and increased respiratory impedances during loading in individuals from families with multiply affected members suggest that a genetic susceptibility to SDB may be related to abnormalities in ventilatory-control mechanisms and the regulation of airway patency.

The blunted response to hypoxia in members of affected families in this cohort (Table 2, Figure 1) resembles our previous finding in a single family having multiple members affected by SDB that was recruited from a different cohort in Rhode Island (10). The normal levels of lung function of this single family as well as in the Cleveland cohort suggest that the observed depression of ventilatory responses was not likely to have been due to mechanical obstruction. Furthermore, the lower hypoxic responses among individuals with mild or treated SDB also suggests that ventilatory control patterns in these family members were not the result of sleep fragmentation or hypoxemia. Put another way, the blunted hypoxic response to hypoxia in the subjects studied was more likely a primary phenomenon that may precede the development of SDB.

Abnormalities in the chemoregulation of breathing have been postulated to contribute to respiratory instability (15). Such abnormalities have been associated with various hypoventilation syndromes related and unrelated to SDB (16- 19). The role of chemoregulation in the pathogenesis of SDB unassociated with daytime hypoventilation is less clear, however. Studies of ventilatory control in SDB have been difficult to interpret owing to patient heterogeneity and to the underlaying obesity and airflow obstruction of many patients studied, which by themselves may alter ventilatory responses (19- 22). Additionally, severe SDB may cause sleep fragmentation and nocturnal hypoxemia, both of which may cause reversible alterations of ventilatory responses during wakefulness, as measured in laboratory animals (11) and in humans (23).

Our findings suggest that abnormal chemoregulation plays a role of familial SDB. Blunted chemosensitivity could contribute to prolonging the duration of apneas and/or lead to the propagation of apneas during sleep. In SDB, it has been shown that the magnitude of the hypoxic ventilatory drive during wakefulness correlates inversely with the magnitude of maximal arterial desaturation that occurs during sleep (20), a phenomenon that might be related to impaired arousal responses prolonging apneas. Fundamental differences in chemoregulation have also been suggested to underlie the pathogenesis of upper-airway obstruction during infancy, as suggested by the demonstration of different breathing patterns in response to hypoxia in infants with SDB as compared with control infants (21, 22). Further indirect evidence of an association between chemical respiratory responses and apneic activity may come from studies of the effects of sex-hormone administration, which may influence the level of apneic activity, perhaps through effects on the hypoxic ventilatory response (23, 24).

In contrast to our finding of blunted hypoxic ventilatory responses in SDB, Wilcox and colleagues have demonstrated greater hypoxic ventilatory responses in a group of males with severe SDB and obesity (29). The reason for this discrepant finding is not clear. Perhaps the greater comorbidity in the sample reported by Wilcox and colleagues influenced their results. Ventilatory responses in SDB have been reported to vary according to the degree of underlying hypertension (30), a common comorbidity associated with both obesity and SDB. Our study subjects were generally healthy, had less severe SDB, and were less obese than the sample described by Wilcox and colleagues. Consequently, the ventilatory responses in our subjects were less likely to have been influenced by the potential effects of these comorbidities. Ventilatory responses in subjects with severe SDB may also be influenced by adaptations to chronic disease states. This, also, was likely to be less influential in the subjects with mild or treated SDB who participated in our study.

Hypercapnic ventilatory responses during unloaded conditions were no different in members of SDB families than in controls (Table 2). Abnormalities in the ventilatory responses to CO₂ contribute to respiratory destabilization and promote periodic breathing (31, 32). However, the relationship of hypercapnic responses to SDB is less clear. Although blunted responses have been demonstrated in SDB, these findings have been most notable in patients with daytime hypoventilation (19, 21). Additionally, hypercapnic ventilatory responses may be related more to acquired than to genetic factors (33). Specifically, in SDB, treatment of upper-airway obstruction may reverse the blunting of the hypercapnic ventilatory response (22, 34, 35). Moreover, the hypercapnic response is quite variable, and may be influenced by behavioral factors (e.g., anxiety) that could reduce the likelihood of detecting significant abnormalities (36).

The availability of breath-by-breath measures of mouth pressure and airflow allowed us to estimate respiratory impedance during progressive hypercapnia. The finding of a significantly greater increase in the relationship between P₁₀₀ and VT/TI with increasing hypercapnia during resistive loading in members of SDB families as compared with controls (Figure 2) suggests that subjects from SDB familiesboth those with mild SDB and those with no breathing disturbancemay be predisposed to airway collapsibility and may be more likely to develop upper-airway obstruction during sleep as the balance between upper-airway and chest-wall activation changes, or intrathoracic airway pressure during inspiration becomes more subatmospheric. This finding is consistent with the observations made by Lavie and colleagues, who described increased apneic activity occurring in family members of SDB patients as compared with controls following a nasal occlusion experiment (37). They interpreted their findings as suggesting that in certain individuals, a genetic predisposition to SDB may be related to increased upper-airway resistance and airway instability.

The possibility that a propensity for airway collapsibility in our subjects was related to structural abnormalities rather than to abnormalities in the neuromechanical control of upper-airway muscles cannot be excluded. However, cephalometric radiographs, available for 17 subjects, demonstrated only modest differences in craniofacial structure between subjects with SDB and controls (i.e., those with SDB had a longer hyoid- mandibular plane distance and shorter distance between the posterior nasal spine and basion (data not shown). No differences were noted in the cephalometric measurements of controls and unaffected SDB family members (who also demonstrated increased impedances with loading). Additionally, respiratory impedance measured during hypercapnia without resistive loading was no different in controls and SDB family members. Thus, it appears unlikely that the greater impedances observed during loading in the SDB family members were due to large differences in bony craniofacial features.

Of interest were the comparable levels of hypoxic responses in unaffected and affected SDB family members (Table 2). If a blunted hypoxic response does constitute one of the risk factors for SDB, and is present in some unaffected SDB family members, it may be that they lack other risk factors that interact to increase the predilection for SDB. For example, affected members of SDB families in the present study differed from unaffected members by their higher BMI and in their response to resistive loading (Table 3), either or both of which might be expected to add the additional factor to exceed the threshold for the development of clinical SDB. Stated in another way, if SDB is a disease with a multifactoral basis, it may be that its expression requires the operation of a number of risk factors, including inherited abnormalities in ventilatory control, obesity, and upper-airway structure.

In summary, these data suggest that the familial aggregation of SDB is in some instances based on inherited abnormalities in respiratory control, specifically the hypoxic but not the hypercapnic ventilatory response. In addition, the greater airway impedances among SDB family members indicate that the upper airway of these subjects is susceptible to excess collapsibility during conditions of mild inspiratory loading. Although the specific bases for these deficits are not clear, further genetic studies should consider the role of inherited physiologic factors that influence airway patency. Tests of ventilatory control during wakefulness may be useful for characterizing intermediate phenotypes in this syndrome.