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Published ahead of print on September 25, 2003, doi:10.1164/rccm.200304-462OC
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American Journal of Respiratory and Critical Care Medicine Vol 168. pp. 1520-1527, (2003)
© 2003 American Thoracic Society

Differences in Sleep-induced Hypoxia between A/J and DBA/2J Mouse Strains

Arnon E. Rubin, Vsevolod Y. Polotsky, Alexander Balbir, Jerry A. Krishnan, Alan R. Schwartz, Philip L. Smith, Robert S. Fitzgerald, Clarke G. Tankersley, Machiko Shirahata and Christopher P. O'Donnell

Division of Pulmonary and Critical Care Medicine, Department of Medicine, and Department of Environmental Health Sciences, The Johns Hopkins Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland

Correspondence and requests for reprints should be addressed to Christopher P. O'Donnell, Ph.D., Room 4B61, Johns Hopkins Asthma and Allergy Center, 5501 Hopkins Bayview Circle, Baltimore, MD 21224. E-mail: codonnell{at}jhmi.edu


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In obstructive sleep apnea, hypoxic ventilatory sensitivity may affect the degree of hypoxic stress and sleep disruption that occurs in response to upper airway obstruction. We induced (1) sleep-induced hypoxia (SIH) or (2) sleep fragmentation (SF) without hypoxia for 5 days (12-hour light/dark cycle) in two inbred mouse strains with low (A/J) and high (DBA/2J) hypoxic ventilatory sensitivities. During SIH, the time to arousal (26.4 ± 1.1 vs. 21.3 ± 1.5 seconds, p < 0.025) and the severity of hypoxic exposure (nadir FIO2: 11.5 ± 0.4 vs. 13.6 ± 0.1%, p < 0.002) was greater in A/J than DBA/2J mice. Furthermore, A/J mice had a greater frequency of hypoxic events (640 ± 29 vs. 368 ± 33 events per 24 hours, p < 0.001) and total sleep time (47.5 ± 2.8% vs. 26.5 ± 2.4% per 24 hours, p < 0.0001) during SIH than DBA/2J mice. In contrast, the event characteristics and total sleep time during SF were the same in both strains. Furthermore, in the light phase, both strains showed a longer (p < 0.01) time to arousal during SIH and SF compared with the dark phase. We conclude that genetic background can influence respiratory events and sleep architecture during SIH and that the arousal threshold is subject to circadian variation. Our data imply that individuals with low hypoxic sensitivity may be at a greater risk for hypoxia-related complications of obstructive sleep apnea.

Key Words: carotid body • genetics • hypoxic ventilatory response • obstructive sleep apnea • sleep fragmentation

Sleep-disordered breathing is a prevalent condition affecting 2–4% of the adult population in the United States (1). Obstructive sleep apnea (OSA) is the most common form of sleep-disordered breathing and is characterized by repetitive periods of intermittent hypoxia and arousal from sleep (2). The intermittent hypoxia and sleep fragmentation (SF) of OSA have been implicated in numerous pathophysiologic sequelae, including neurologic, cardiovascular, and metabolic disturbances, resulting in significant morbidity and mortality (3–7). Although the magnitude of the hypoxic stimulus has been linked to the degree of pathophysiologic dysfunction in OSA (7), the factors that determine the rate and severity of intermittent hypoxia and SF are unclear.

The severity of the intermittent hypoxia and the degree of SF can vary considerably between OSA patients. Even in normal individuals, in whom airway obstruction is induced experimentally during sleep, the duration of hypoxic periods and the frequency of events exhibit marked variability (8). One potential source of the variability in apnea characteristics is due to differences in the hypoxic ventilatory response during sleep (9, 10). More specifically, the body's primary hypoxic sensing organ, the carotid body, can play a crucial role in eliciting an arousal response and terminating an apnea (11–13). Indeed, Bowes and colleagues (12) demonstrated in a dog model of OSA that carotid body denervation resulted in an escalating hypoxic stimulus that was unable to elicit an arousal response. Thus, the carotid body and hypoxic ventilatory responsiveness can play a key role in respiratory responses to airway obstruction during sleep.

We have previously demonstrated that different inbred mouse strains exhibit a wide variation in hypoxic ventilatory responsiveness (14). Within the strains studied, the DBA/2J strain exhibited the largest ventilatory response to hypoxia, and the A/J strain exhibited the smallest ventilatory response to hypoxia. Furthermore, we have recently shown that the carotid body of the DBA/2J strain is four times larger, contains many more typical glomus cells, and is markedly more responsive to the neurotransmitter acetylcholine than the A/J strain (15). The purpose of this study was to determine whether genetic differences in hypoxic sensitivity and carotid body structure and function between the DBA/2J and A/J strains affect the phenotypic expression of sleep-disordered breathing. To examine genetic influences on sleep-disordered breathing severity, we have developed a murine model of sleep-induced hypoxia (SIH), in which the rate and severity of hypoxic events occur independent of upper airway properties (16). In the model, polysomnography is measured continuously, and the sleep/wake state is scored in real time, enabling the delivery of an increasing hypoxic stimulus at sleep onset, followed by reoxygenation at arousal. Responses were compared between the two inbred mouse strains, and we hypothesized that the severity of SIH would be greater in the low hypoxic-responding A/J strain than the high hypoxic-responding DBA/2J strain. In contrast, we further hypothesized that that the severity of nonhypoxic SF would be comparable between the two strains. Some of the results of these studies have been previously reported in the form of an abstract (17).


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Adult male A/J (24.3 ± 0.5 g, 109 ± 7 days of age) and DBA/2J (25.6 ± 0.9 g, 96 ± 6 days of age) mice from Jackson Laboratory (Bar Harbor, ME) were used in the study, which was approved by the Johns Hopkins University Animal Use and Care Committee and complied with the American Physiological Society Guidelines. Animals were euthanized with pentobarbital (60 mg intraperitoneally).

Procedures
In isoflurane (1–2%)-anesthetized animals, three electroencephalographic electrodes and two nuchal electromyographic electrodes were implanted as previously described (16). At 5–7 days after surgery, a connector cable from the animal was fixed above to a low friction mercury swivel, allowing 360-degree unrestricted movement of the tethered mouse. The mouse was housed in a customized chamber with a 12-hour light/dark cycle in which gas entered through inlet ports in the base and exhausted through an open hole at the apex (see online supplement for further details).

The techniques and procedures related to our online sleep/wake detection system have been previously described (16). In brief, the program used a series of investigator-determined thresholds for frequency distribution of the electroencephalogram and amplitude of the nuchal electromyogram to determine sleep/wake state over 5-second epochs. In this study using DBA/2J and A/J strains, the computer algorithm detected wakefulness and NREM sleep with an accuracy consistent with our previously published values in C57BL/6J mice (16), but during REM sleep fell below our previously published level of 85.6 ± 5.0% accuracy. However, misclassified REM periods were always scored as NREM sleep and did not affect the triggering of SIH or SF (discussed later here). Overall sleep architecture data are reported as wakefulness and sleep (NREM and REM sleep combined).

SIH and SF
SIH.
During continuous periods of wakefulness, room air was delivered through the cage at a rate of 4 L/minute. When sleep was detected for a continuous 15-second period, the output signal from the computer caused the airflow to cease and 100% N2 to be delivered at 4 L/minute until arousal occurred, at which time the delivery of room air was restored to produce reoxygenation.

SF without hypoxia.
A system was developed to cause arousal using a tactile and auditory stimulus after a comparable period of time from sleep onset as that produced with hypoxia. The sampling ports for measurement of FIO2 during SIH were adapted and used to deliver a sequentially increasing airflow stimulus until arousal occurred (10 L/minute from 0–5 seconds, 20 L/minute from 5–10 seconds, and 50 L/minute from 10–15 seconds).

Protocol
At the completion of a 3- to 5-day acclimation period in the housing chamber, 5 consecutive days of either SIH or SF were bracketed by 24-hour baseline and recovery periods during which sleep was recorded.

Analyses
We determined the time and duration of each event and periods of wakefulness and sleep (we used previously described combining rules to produce contiguous epochs of sleep/wake) (16). The relative propensity of strains (DBA/2J versus A/J) to fall asleep was evaluated using the Andersen-Gill model of multiple failure-time survival analysis (18). Statistical significance between factors of SIH or SF, light cycle (light, dark), or changes over time (Days 1–5) was determined by analysis of variance. If the analysis of variance was significant for a factor, a Dunnett test was used to determine which means were significantly different. All the results are presented as mean ± SEM.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
SIH
Event characteristics.
A recording example of consecutive hypoxic events during NREM sleep is shown in Figure 1 for an A/J (upper tracing) mouse and a DBA/2J (lower tracing) mouse. The figure demonstrates that the A/J mouse has longer hypoxic events when compared with the DBA/2J mouse. Group data show that over the 5 days of SIH, A/J mice had hypoxic events that lasted on average 26.4 ± 1.1 seconds and were significantly longer (p < 0.025) than the average of 21.3 ± 1.5 seconds for DBA/2J mice (Figure 2A) . Consequently, the nadir FIO2 throughout the period of SIH was significantly lower (p < 0.002) in the A/J mice (11.5 ± 0.4%) than the DBA/2J mice (13.6 ± 0.1%; Figure 2B).



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  		Figure 1. A 13-minute recording example from a representative tracing in an A/J mouse (top four channels) and DBA/2J mouse (bottom four channels) during periods of sleep-induced hypoxia (SIH). For each animal, the top two channels show changes in the electroencephalogram (EEG) and electromyogram (EMG) as the animal cycles through periods of SIH and arousal. The third channel shows the staging of sleep/wake state in 5-second epochs by the online computer detection system (marked by downward tick), and the fourth channel shows the time period during which nitrogen was introduced (4 L/minute) into the chamber to produce SIH.

 


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  		Figure 2. The mean ± SE for (A) hypoxic event duration (seconds), (B) total number of hypoxic events per 24-hour period, (C) nadir fraction of inspired oxygen (FIO2; percentage), and (D) total hypoxic time (minutes per 24 hours) during 5 consecutive days of SIH (D1–D5) in A/J (left panel) and DBA/2J (right panel) mice. Over the entire 5-day period, the A/J mice exhibited a longer hypoxic event duration (26.4 ± 1.1 vs. 21.3 ± 1.5 seconds, p < 0.025), a lower nadir FIO2 (11.5 ± 0.4% vs. 13.6 ± 0.1%, p < 0.002), more hypoxic events (640 ± 29 vs. 368 ± 33 events per 24-hour period, p < 0.001), and an increased total hypoxic time (284.4 ± 21.9 vs. 133.1 ± 18.9 minutes of hypoxic time per 24-hour period, p < 0.001) compared with DBA/2J mice. The statistical differences were determined by an independent effect of strain in a two-way analysis of variance.

 
In addition to having longer more severe hypoxic events, the A/J mice also had significantly more events (p < 0.001) than the DBA/2J mice (Figures 1 and 2C). There was no statistical change in either strain in the number of events per 24 hours over the course of the 5 days of SIH. As a result of having more severe as well as more numerous hypoxic events, the A/J mice experienced more than double the hypoxic time recorded in the DBA/2J strain (p < 0.001; Figure 2D).

Sleep architecture.
The total sleep time during the 24-hour baseline period, the 5 days of SIH, and the 24-hour recovery period is shown in Figure 3 . The DBA/2J strain slept on average for 39.3 ± 3.8% of the 24-hour control period (Figure 3B, lower panel). There was a significant decrease (p < 0.003) in the amount of time slept during the 5 days of SIH to an average of 26.5 ± 2.4%, followed by an increase (p < 0.005) to 38.8 ± 2.3% during the 24-hour recovery period. In contrast, the A/J mice did not change their total sleep time during the 5 days of SIH when compared with the baseline and recovery days (Figure 3A).



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  		Figure 3. The mean ± SE for total sleep time (percentage per 24 hours) during 5 consecutive days of SIH compared (D1–D5) with 24-hour baseline and recovery periods in (A) A/J and (B) DBA/2J mice. The statistical differences between baseline and recovery periods and 5 days of SIH were determined by one-way analysis of variance within each strain.

 
Sleep propensity.
The relative hazard ratio comparing the propensity to fall asleep after an arousal in the DBA/2J relative to the A/J strain, independent of number and duration of previous hypoxic exposures, is shown in Figure 4 . In the initial 24-hour baseline period, both strains of mice were similarly likely to fall asleep after spontaneous arousals/awakenings (Figure 4). However, throughout the 5 days of SIH, the DBA/2J strain was only half as likely to fall asleep after either hypoxic arousals or spontaneous arousal/awakenings relative to the A/J strain (Figure 4; relative hazard ratio of 0.54 ± 0.084, p = 0.005). The decreased propensity for the DBA/2J mice to fall asleep after spontaneous arousal/awakenings persisted in the 24-hour recovery period (Figure 4).



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  		Figure 4. The mean ± 95% confidence interval for the relative hazard ratio of the DBA/2J strain falling asleep after arousal (either spontaneous or hypoxia-induced) as compared with the A/J strain. A relative hazard ratio that is not significantly different from 1.0, as seen on the control day, indicates an equal likelihood to fall asleep in both strains, whereas a relative hazard ratio significantly less than 1.0 (Days 1–5 of SIH and recovery day) indicates that the DBA/2J strain is not as likely to fall asleep after arousal as the A/J strain.

 
Light/dark phase analysis.
We further analyzed event characteristics and sleep architecture based on the 12-hour light/dark cycle (Table 1) . Both the A/J and DBA/2J mice had more severe hypoxic events and greater total sleep time during the light phase compared with the dark phase. The decrease in total sleep time in the DBA/2J mice during the 5 days of SIH occurred exclusively in the light phase, and no comparable changes in total sleep time in the light phase were evident in the A/J mice during the 5 days of SIH. During the 12-hour dark phase, there was no change in the total sleep time during the hypoxic period for either the A/J or DBA/2J mice.


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  		TABLE 1. The difference in event characteristics and total sleep time between the light and the dark phase in a/j and dba/2 mice during 5 days of sleep-induced hypoxia

 
SF
Event characteristics.
A representative recording of consecutive SF events for an A/J mouse is shown in Figure 5 . Group data presented in Figure 6A show that over the 5 days of SF, the time to arousal was not different between the A/J and DBA/2J mice, but both strains did exhibit a significant lengthening (p < 0.001) of the time to arousal over the course of the SF period.



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  		Figure 5. A 4- to 5-minute recording example from a representative tracing in an A/J mouse during periods of sleep fragmentation (SF) without hypoxia. The top two channels show changes in the EEG and EMG as the mouse cycles through periods of sleep and arousal. The third channel shows the staging of sleep/wake state in 5-second epochs by the online computer detection system (marked by downward tick), and the fourth channel shows the time period during which a high flow air burst was delivered into the chamber (see METHODS for details) to induce a nonhypoxic arousal. A/J mice exhibited similar event characteristics and frequency.

 


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  		Figure 6. The mean ± SE for (A) event duration (seconds) and (B) total number of events per 24-hour period during 5 consecutive days of SF without hypoxia (D1–D5) in A/J (left panel) and DBA/2J (right panel) mice. Statistical differences between strains were examined by two-way analysis of variance across strain and time factors.

 
In contrast to SIH, the number of SF events was comparable in A/J mice and DBA/2J mice (Figure 6B). Moreover, the number of events in both strains significantly increased (p < 0.025) over the 5 days of SF, a result that was not seen in SIH.

Sleep architecture.
The similar number of SF events per 24 hours detailed in Figure 6B is consistent with the A/J and DBA/2J mice exhibiting comparable total sleep times and is also in marked contrast to severely reduced total sleep time seen in DBA/2J mice during SIH (Figure 3). The pooled data for total sleep time during SF are shown for the DBA/2J and A/J mice in Figure 7 and are not different between the strains. There was, however, a significant increase in the total sleep time over the course of the 5 days of SF in the DBA/2J mice (p < 0.015) and a nonsignificant trend in the A/J mice (p = 0.63).



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  		Figure 7. The mean ± SE for total sleep time (percentage per 24 hours) during 5 consecutive days of SF without hypoxia compared with 24-hour baseline and recovery periods in (A) A/J and (B) DBA/2J mice. Statistical differences between baseline and recovery periods and 5 days of SIH were examined by one-way analysis of variance in each strain.

 
Light/dark analysis.
The light/dark cycle influenced the time to arousal in both the A/J and DBA/2J strains during SF (Table 2) . The time to arousal was significantly longer during the light phase than the dark phase in A/J and DBA/2J mice. There were no differences between strains with respect to the number of events or total sleep time in the light and dark phases.


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  		TABLE 2. The difference in event characteristics and total sleep time between the light and the dark phase in a/j and dba/2 mice during five days of sleep fragmentation without hypoxia

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our study compared responses to SIH and SF without hypoxia between two inbred strains of mice with marked differences in hypoxic ventilatory sensitivity. A major strength of our model is the ability to examine the effect of various factors that can affect the severity of OSA in the absence of upper airway obstruction. Our data provide new insights into how genetic differences in hypoxic sensitivity may influence the severity of OSA. First, we showed that the duration and severity of hypoxic events were greater in the A/J strain, which possesses small carotid bodies and depressed hypoxic ventilatory responsiveness relative to the DBA/2J strain (14, 15). Surprisingly, in the A/J strain, the longer hypoxic events also occurred more frequently, resulting in an overall hypoxic stress that was more than double that present in the DBA/2J strain. Second, SIH caused more than a 30% reduction in total sleep time in DBA/2J mice, whereas even higher rates of SF without hypoxia did not reduce total sleep time. Thus, genetic background may influence the degree of hypoxic exposure, the frequency of events, and the total sleep time that will accompany repetitive upper airway collapse in OSA. Third, in both strains, the time to arousal with SIH remained constant and was inversely related to hypoxic sensitivity, whereas the time to arousal with nonhypoxic SF increased over time. Fourth, the threshold for arousal to a hypoxic stimulus as well as arousal to an auditory/tactile stimulus was highest during the light phase, which is the predominant sleep period for the mouse. These data suggest that variations in the circadian cycle can influence the arousal threshold, potentially leading to more severe apnea in OSA patients in certain phases of the night.

Apnea Severity Independent of Upper Airway Properties
The factors that determine the severity of OSA are complex and extend beyond the properties of the upper airway. In individuals with a propensity to upper airway collapse during sleep, many factors will determine the duration and frequency of apneic episodes. Such factors include the hypoxic and hypercapnic chemosensitivity and associated ventilatory drive, the level or threshold for arousal, and the propensity to fall asleep after an arousal has occurred (11–13, 19, 20). Indeed, the range of apnea severity was marked in a group of normal human subjects in which upper airway collapse was induced experimentally during sleep (8). The results demonstrated considerable variation between subjects with regard to both apnea characteristics and the degree of oscillation. Consequently, for a given degree of upper airway obstruction, a broad range of apnea severity could be anticipated between individuals.

Our approach using a mouse model allowed us to determine directly whether genetic differences in hypoxic ventilatory responsiveness were associated with altered "apnea characteristics." A strength of our model is that the responses are independent of airway properties, with the upper airway acting effectively as an infinitely collapsible tube whenever 15 seconds of consecutive sleep occurs. Our data show that in mice with smaller less functional carotid bodies and reduced hypoxic ventilatory sensitivity (A/J strain) the hypoxic events were longer and more severe than in mice with robust carotid body–hypoxic ventilatory reflexes (DBA/2J strain). Thus, genetic depression of hypoxic ventilatory responsiveness can account for more severe hypoxic events, independent of upper airway collapsibility.

Hypoxic Ventilatory Responsiveness and Arousal
A variety of inputs from chemoreceptors and mechanoreceptors can contribute to the arousal response that terminates a period of apnea in OSA patients. There are several respiratory stimuli that can induce arousal from sleep, including hypoxia, hypercarbia, airway occlusion, and airway irritation (11–13, 21–25). The mechanisms of arousal involve both direct and indirect connections of receptors with the central nervous system and parts of the reticular formation (12, 26). Mechanisms for causing an arousal include direct afferent stimuli from sensory receptors such as the carotid body (12) and ventilatory mechanoreceptors projecting onto the reticular arousal system (26). These ventilatory receptors may trigger arousals when a certain minute ventilation, respiratory rate, or effort is met. One unifying hypothesis is that the sum of these chemoreceptor and mechanoreceptor inputs determines the integrated level of inspiratory effort and provides a final common pathway to arousal (23).

The strain differences in event severity seen in our study may be related to the level of inspiratory effort during hypoxic exposure. We have previously shown that the tidal volume and minute ventilation increases during hypoxic exposure are greater in the DBA/2J strain than the A/J strain (14). Assuming the ventilatory response is an estimate of respiratory drive, it would be predicted that A/J mice would have longer more severe SIH events than DBA/2J mice. Our results detailed in Figure 2 show that A/J mice did have more severe SIH than DBA/2J mice and that this increased severity was sustained throughout the 5 days of exposure. Although a reduced inspiratory drive in A/J mice could account for the more severe SIH, it is also possible that this difference in event severity is not specific to hypoxia but rather represents an overall depressed level of arousability. Our data from SF without hypoxia, however, argue against a generalized reduction in arousability in the A/J strain. When the mice were aroused with tactile/auditory stimuli during the SF without hypoxia protocol, the arousal thresholds were comparable between the A/J and DBA/2J strains. Taken together, these data show that the severity of SIH in the A/J and DBA/2J strains was inversely related to the hypoxic ventilatory sensitivity rather than to a generalized level of arousability.

Event Frequency and Total Sleep Time
The most surprising finding of this study was that the A/J mice, which exhibited the longer and more severe periods of SIH, also had a greater number of events per 24 hours. Because the event frequency is dependent on the presence of sleep, there are two possible explanations of our data. Either the A/J is sleeping more than expected, or the DBA/2J is sleeping less than expected during periods of SIH. In a previous study in C57BL/6J mice, we observed only small decrements in total sleep time during 5 days exposure to SIH (16), similar to the A/J strain in this study, suggesting that, in fact, the DBA/2J strain is sleeping less than expected. The implication is that hypoxia reduces the propensity to fall asleep in DBA/2J strain. Our current data support this contention based on (1) the hazard ratio for the propensity to fall asleep after an hypoxic event or spontaneous arousal, which was significantly reduced in the DBA/2J strain relative to the A/J strain and (2) that the SF without hypoxia protocol did not cause a decrease in total sleep time or event frequency in the DBA/2J strain. Interestingly, the specific effect of hypoxia on the propensity to fall asleep in DBA/2J mice remained present during the recovery period at a time when the stimulus was no longer present (Figure 4). It is not possible from the results of our study to determine whether an elevated hypoxic ventilatory sensitivity was a contributing factor to the disrupted sleep homeostasis in DBA/2J mice exposed to SIH.

Caveats of Study
There are several caveats of our study that need to be acknowledged. First, we have no causal evidence that the difference in the sensitivity of the carotid body–hypoxic ventilatory reflex arc produced the strain differences in SIH. However, the finding that SF without hypoxia produced identical event characteristics and sleep architecture strongly suggests that the observed strain differences with SIH are directly related to the hypoxic nature of the stimulus. An attractive experiment would be to denervate the carotid body of DBA/2J mice and determine whether the responses to SIH become more comparable to A/J mice. Such an experiment is unlikely to be successful, however, based on the earlier work by Bowes and colleagues (12), in which experimentally induced airway obstruction during sleep in carotid body–denervated dogs failed to elicit an arousal response. Second, the respiratory reflexes causing arousal from hypoxia with an unobstructed airway may be different from the respiratory reflexes causing arousal from an asphyxic stimulus with an obstructed airway, as occurs in OSA. However, as described previously here, the inspiratory effort has been proposed as a unifying stimulus to arousal, and studies in sleeping humans have shown that resistive loading, hypoxia, and hypercapnia all produced arousal at comparable levels of peak-negative esophageal pressure (23). Thus, it would be predicted that any intrinsic strain difference in arousal threshold between mice with high and low carotid body–hypoxic ventilatory sensitivity would remain whether the stimulus to arousal is an increasing hypoxic stimulus with an unobstructed airway or an increasing asphyxic stimulus with an obstructed airway. Third, our model examined only the effects of hypoxic ventilatory sensitivity on event characteristics and sleep architecture during SIH. In OSA, airway obstruction leads to an increasing asphyxic stimulus in which both hypoxic and hypercapnic stimuli likely contribute to the expression of disease severity (19). A major strength of our model is that we can separately or additively examine the effects of hypoxia and hypercapnia on event severity and sleep architecture. Interesting future experiments would include the examination of the severity of sleep-induced hypercapnia or SIH/hypercapnia in the A/J and DBA/2J strains as well as in the leptin-deficient C57BL/6Jlepob mouse, which has a markedly reduced hypercapnic ventilatory sensitivity (27).

Implications
There are three immediate implications of the studies reported here to the clinical syndrome of OSA. First, in patients with OSA, the severity of the disease is known to be dependent on the properties of the upper airway and the myriad of reflexes that are elicited when the airway obstructs during sleep. However, it has not been possible to separate the contribution of reflexes resulting from upper airway obstruction to the overall expression of the disease. We now show that differences in the carotid body–ventilatory reflex arc can be associated with a doubling of the overall hypoxic stress, independent of any effects on upper airway function. The implication of this finding is that individuals with a relative insensitivity of the carotid body–hypoxic ventilatory reflex arc or individuals who develop a time-related desensitization with disease progression may be at increased risk for the neurologic, cardiovascular, and metabolic sequelae of OSA. Second, the presence or absence of hypoxia at arousal could affect the event duration and the sleep architecture, suggesting that event characteristics and degree of sleep disruption may differ between patients with frank obstructive events and significant hypoxemia and UARS patients with frequent arousals, but little or no hypoxemia. Finally, we observed a significant circadian variation in the arousal threshold to both SIH and SF without hypoxemia. Extrapolating these data to clinical OSA suggests that an increasing arousal threshold could account for the tendency of events to become longer and more severe as the night progresses (28, 29), independent of any alterations in upper airway properties.


   FOOTNOTES
 
Supported by National Institutes of Health Grants HL-66324, HL-63767, HL-68715, HL-61596, HL-50712, and ES-03819.

This article has an online supplement, which is accessible from this issue's table of contents online at www.atsjournals.org

Conflict of Interest Statement: A.E.R. has no declared conflict of interest; V.Y.P. has no declared conflict of interest; A.B. has no declared conflict of interest; J.A.K. has no declared conflict of interest; A.R.S. has no declared conflict of interest; P.L.S. has no declared conflict of interest; R.S.F. has no declared conflict of interest; C.G.T. has no declared conflict of interest; M.S. has no declared conflict of interest; C.P.O. has no declared conflict of interest.

Received in original form April 1, 2003; accepted in final form September 20, 2003


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Young T, Palta M, Dempsey J, Skatrud J, Weber S, Badr S. The occurrence of sleep-disordered breathing among middle-aged adults. N Engl J Med 1993;328:1230–1235.[Abstract/Free Full Text]
  2. Gastaut H, Tassinari CA, Duron B. Polygraphic study of diurnal and nocturnal (hypnic and respiratory) episodal manifestations of Pickwick syndrome. Rev Neurol (Paris) 1965;112:568–579.[Medline]
  3. Shahar E, Whitney CW, Redline S, Lee ET, Newman AB, Javier NF, O'Connor GT, Boland LL, Schwartz JE, Samet JM. Sleep-disordered breathing and cardiovascular disease: cross-sectional results of the Sleep Heart Health Study. Am J Respir Crit Care Med 2001;163:19–25.[Abstract/Free Full Text]
  4. Ip MS, Lam B, Ng MM, Lam WK, Tsang KW, Lam KS. Obstructive sleep apnea is independently associated with insulin resistance. Am J Respir Crit Care Med 2002;165:670–676.[Abstract/Free Full Text]
  5. Nieto FJ, Young TB, Lind BK, Shahar E, Samet JM, Redline S, D'Agostino RB, Newman AB, Lebowitz MD, Pickering TG. Association of sleep-disordered breathing, sleep apnea, and hypertension in a large community-based study: Sleep Heart Health Study. JAMA 2000;283:1829–1836.[Abstract/Free Full Text]
  6. Peppard PE, Young T, Palta M, Skatrud J. Prospective study of the association between sleep-disordered breathing and hypertension. N Engl J Med 2000;342:1378–1384.[Abstract/Free Full Text]
  7. Punjabi NM, Sorkin JD, Katzel LI, Goldberg AP, Schwartz AR, Smith PL. Sleep-disordered breathing and insulin resistance in middle-aged and overweight men. Am J Respir Crit Care Med 2002;165:677–682.[Abstract/Free Full Text]
  8. King ED, O'Donnell CP, Smith PL, Schwartz AR. A model of obstructive sleep apnea in normal humans. Role of the upper airway. Am J Respir Crit Care Med 2001;163(4):1031.[Free Full Text]
  9. Kimura H, Niijima M, Edo H, Tatsumi K, Honda Y, Kuriyama T. The effect of hypoxic depression on genioglossal muscle-activity in healthy-subjects and obstructive sleep-apnea patients. Sleep 1993;16:S135–S136.[Medline]
  10. Sato F, Nishimura M, Shinano H, Saito H, Miyamoto K, Kawakami Y. Heart rate during obstructive sleep apnea depends on individual hypoxic chemosensitivity of the carotid body. Circulation 1997;96:274–281.
  11. Bowes G, Townsend ER, Bromley SM, Kozar LF, Phillipson EA. Role of the carotid body and of afferent vagal stimuli in the arousal response to airway occlusion in sleeping dogs. Am Rev Respir Dis 1981;123:644–647.[Medline]
  12. Bowes G, Townsend ER, Kozar LF, Bromley SM, Phillipson EA. Effect of carotid body denervation on arousal response to hypoxia in sleeping dogs. J Appl Physiol 1981;51:40–45.[Abstract/Free Full Text]
  13. Fewell JE, Taylor BJ, Kondo CS, Dascalu V, Filyk SC. Influence of carotid denervation on the arousal and cardiopulmonary responses to upper airway-obstruction in lambs. Pediatr Res 1990;28:374–378.[Medline]
  14. Tankersley CG, Fitzgerald RS, Kleeberger SR. Differential control of ventilation among inbred strains of mice. Am J Physiol Regul Integr Comp Physiol 1994;36:R1371–R1377.
  15. Yamaguchi S, Balbir A, Schonfield B, Coram J, Tankersley C, Fitzgerald RS, O'Donnell CP, Shirahata M. Structural and functional differences of the carotid body between DBA/2J and A/J strains of mice. J Appl Physiol 2003;94:1536–1542.[Abstract/Free Full Text]
  16. Tagaito Y, Polotsky VY, Campen MJ, Wilson JA, Balbir A, Smith PL, Schwartz AR, O'Donnell CP. A model of sleep-disordered breathing in the C57BL/6J mouse. J Appl Physiol 2001;91:2758–2766.[Abstract/Free Full Text]
  17. Rubin AE, Balbir A, Smith PL, Schwartz AR, Fitzgerald F, Tankersley CG, Shirahata M, O'Donnell CP. Phenotypes of sleep disordered breathing (SDB) are different in two inbred mouse strains [abstract]. Am J Repir Crit Care Med 2003;165:A419.
  18. Andersen PK, Gill RD. Cox's regression model for counting process: a large sample study. Ann Stat 1982;10:1100–1120.[CrossRef]
  19. Khoo MC, Gottschalk A, Pack AI. Sleep-induced periodic breathing and apnea: a theoretical study. J Appl Physiol 1991;70:2014–2024.[Abstract/Free Full Text]
  20. Pack AI, Gottschalk A. Mechanisms of ventilatory periodicities. Ann Biomed Eng 1993;21:537–544.[Medline]
  21. Berthon-Jones M, Sullivan CE. Ventilatory and arousal responses to hypoxia in sleeping humans. Am Rev Respir Dis 1982;125:632–639.[Medline]
  22. Gleeson K, Zwillich CW, White DP. Chemosensitivity and the ventilatory response to airflow obstruction during sleep. J Appl Physiol 1989;67:1630–1637.[Abstract/Free Full Text]
  23. Gleeson K, Zwillich CW, White DP. The influence of increasing ventilatory effort on arousal from sleep. Am Rev Respir Dis 1990;142:295–300.[Medline]
  24. Sullivan CE, Murphy E, Kozar LF, Phillipson EA. Waking and ventilatory responses to laryngeal stimulation in sleeping dogs. J Appl Physiol 1978;45:681–689.[Abstract/Free Full Text]
  25. Sullivan CE, Kozar LF, Murphy E, Phillipson EA. Arousal, ventilatory, and airway responses to bronchopulmonary stimulation in sleeping dogs. J Appl Physiol 1979;47:17–25.[Abstract/Free Full Text]
  26. Hugelin A, Cohen MI. The reticular activating system and respiratory regulation in the cat. Ann N Y Acad Sci 1963;109:586–603.
  27. O'Donnell CP, Schaub CD, Haines AS, Berkowitz DE, Tankersley C, Schwartz AR, Smith PL. Leptin prevents respiratory depression in obesity. Am J Respir Crit Care Med 1998;159:1477–1484.
  28. Montserrat JM, Kosmas EN, Cosio MG, Kimoff RJ. Mechanism of apnea lengthening across the night in obstructive sleep apnea. Am J Respir Crit Care Med 1996;154:988–993.[Abstract]
  29. Charbonneau M, Marin JM, Olha A, Kimoff RJ, Levy RD, Cosio MG. Changes in obstructive sleep-apnea characteristics through the night. Chest 1994;106:1695–1701.[Abstract/Free Full Text]



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