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Departments of Medicine (Cardiology) and Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Hershey, Pennsylvania 17033
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The
purpose of this study was to determine the effects of the semicircular
canals and otolith organs on respiration in humans. On the basis of
animal studies, we hypothesized that vestibular activation would elicit
a vestibulorespiratory reflex. To test this hypothesis, respiratory
measures, arterial blood pressure, and heart rate were measured during
engagement of semicircular canals and/or otolith organs. Dynamic
upright pitch and roll (15 cycles/min), which activate the otolith
organs and semicircular canals, increased respiratory rate (
2 ± 1 and 3 ± 1 breaths/min, respectively; P < 0.05). Dynamic yaw and lateral pitch (15 cycles/min), which activate
the semicircular canals, increased respiration similarly (3 ± 1 and 2 ± 1, respectively; P < 0.05). Dynamic chair rotation (15 cycles/min), which mimics dynamic yaw but eliminates neck muscle afferent, increased respiration (3 ± 1;
P < 0.05) comparable to dynamic yaw (15 cycles/min).
Increases in respiratory rate were graded as greater responses occurred
during upright (5 ± 2 breaths/min) and lateral pitch
(4 ± 1) and roll (5 ± 1) performed at 30 cycles/min.
Increases in breathing frequency resulted in increases in minute
ventilation during most interventions. Static head-down rotation, which
activates otolith organs, did not alter respiratory rate (1 ± 1 breaths/min). Collectively, these data indicate that semicircular
canals, but not otolith organs or neck muscle afferents, mediate
increased ventilation in humans and support the concept that vestibular
activation alters respiration in humans.
respiratory; ventilation; otolith organs; semicircular canals; neck afferents
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN HUMANS, STATIC HEAD-DOWN rotation (HDR) increases muscle sympathetic nerve activity and elicits skeletal muscle vasoconstriction (7, 21). We demonstrated that increases in muscle sympathetic nerve activity during static HDR are mediated by the otolith organs and not activation of central command, visual input, and nonspecific receptors from the head, baroreflexes, or neck muscle afferents (10, 17, 18, 21). Similarly, electrical stimulation of the vestibular nerve increases sympathetic nerve activity and alters regional blood flows in the cat (12, 13). Thus data from both humans and cats provide consistent experimental support for the existence of a vestibulosympathetic reflex.
Data from the cat suggest the presence of a vestibulorespiratory reflex, as indicated by increased phrenic, abdominal, and intercostal nerve activity during electrical vestibular nerve stimulation (2, 15, 23). Further support for this concept is provided by the finding that electrical vestibular nerve stimulation has inputs to both inspiratory and expiratory spinal neurons (9). Thus it appears that vestibular activation may produce alterations in inspiratory, expiratory, or whole breath respiratory measures. However, in these previous animal studies, respiration per se was not measured. Therefore, it is unclear whether vestibular activation elicits a functional vestibulorespiratory reflex. Limited data in humans suggest that otolith organs do not mediate a vestibulorespiratory reflex (7, 14). However, it is unknown if natural stimulation of the semicircular canals mediates alterations in respiration. If a vestibulorespiratory reflex is present in humans, it may be important in the context of mediating increases in ventilation during nonstationary exercise or in maintaining or increasing venous return to the heart during physical stressors such as orthostasis. Therefore, the purpose of the present study was to determine if natural vestibular stimulation alters respiration in humans. We hypothesized that activation of the semicircular canals, but not otolith organs, would increase respiration in humans. Several experiments were performed to isolate the independent and interactive influences of semicircular canals and otolith organs of the vestibular system on respiration. Additional experiments were conducted to minimize the confounding influences of neck muscle afferents. The results of the present study suggest an independent role of semicircular canal activation in mediating increases in respiration and support the concept that vestibular activation alters respiration in humans.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects
Nine healthy volunteers (5 men and 4 women) [age: 28 ± 1 (SE) yr; height: 170 ± 1 cm; weight: 68 ± 1 kg] who were normotensive, nonsmokers, nonobese, and unmedicated were studied. Written informed consent was obtained from all subjects after verbal explanation of the experimental protocol. The Institutional Review Board of the Pennsylvania State University College of Medicine approved the experiments.Experimental Design
Subjects performed several experiments isolating the respective influences of the semicircular canals and otolith organs on respiration in humans. Additional experiments were performed to minimize input from other potentially confounding variables (e.g., neck muscle afferents). The order of trials was randomized and counterbalanced. All protocols contained a 3-min baseline period followed by 1 min of the respective intervention and a 3-min period of recovery. All trials were performed with the subjects' eyes closed.Experimental Protocols
Dynamic upright pitch (n = 8). The purpose of this study was to determine if simultaneous activation of the vertical semicircular canals and otolith organs elicits changes in respiration. Upright pitch was performed with the subjects sitting. The subject's head was passively rotated in the vertical axis from the neck-extended to neck-flexed position. Two separate trials were performed at 15 and 30 cycles/min.
Dynamic lateral pitch (n = 8). The purpose of this study was to determine if activation of the vertical semicircular canals elicited changes in respiration. Subjects were positioned in the lateral decubitus position. The subject's head was held in line with the dorsal aspect of the spine. The subject's head was passively rotated in the horizontal axis from the neck-extended to neck-flexed position. Two separate trials were performed at 15 and 30 cycles/min.
Dynamic roll (n = 9). The purpose of this study was to determine if simultaneous activation of horizontal and vertical semicircular canals and otolith organs elicits changes in respiration. Dynamic roll was performed with the subjects sitting. The subject's head was passively moved (ear down) from the head-erect position to the point of maximal roll on the left and right. Two separate trials were performed at 15 and 30 cycles/min.
Dynamic yaw (n = 9). The purpose of this study was to determine if activation of the horizontal semicircular canals, but not otolith organs, elicits alterations in respiration. Dynamic yaw was performed with the subjects sitting. The subject's head was passively moved through yaw rotation ("no" head rotation) from the nose-forward position to the point of maximal yaw to the left and right at a rate of 15 cycles/min. This experiment assisted in the determination of whether responses observed during upright pitch and roll were caused by simultaneous activation of semicircular canals and otolith organs or solely by semicircular canals.
Dynamic chair rotation (n = 5). The purpose of this study was to determine if horizontal semicircular canal engagement, without input from neck muscle afferents, elicits alteration in respiration. Because the whole body was rotated in this experiment, it is possible to compare responses noted during dynamic yaw to identify if neck muscle afferents played a significant role in mediating the observed responses. The subject's torso, legs, and arms were securely fastened in a swivel chair. An investigator moved the chair through a field of rotation similar to that produced by the head movement in the dynamic yaw experiment at a rate of 15 cycles/min.
Static HDR (n = 9). The purpose of this experiment was to determine if otolith organ activation alters respiration. Subjects lay prone with their heads extending over the end of an examination table such that the head could be maximally rotated down (i.e., pitch rotation) without interference from the table. At baseline, the subjects had their necks extended and chins supported as previously described (21). After the baseline period, the chin support was removed, and the subject's head was passively lowered to the point of maximal rotation.
Static head rotation in lateral decubitus position (n = 9). The purpose of this experiment was to determine if responses during static HDR were mediated by otolith organs or neck muscle afferents. Static lateral head rotation (LHR) mimics the movement of static HDR, but it does not activate the otolith organs. This protocol was performed with subjects in the lateral decubitus position with their heads supported in line with the dorsal aspect of their spines. After a baseline period, the head was passively rotated to the point of maximal rotation or until the chin contacted the chest.
Measurements
Respiratory measures were taken, breath by breath, by a metabolic cart (Sensormedics, Yorba Linda, CA) interfaced with a Macintosh computer through a MacLab analog-to-digital converter (8E, ADInstruments, Milford, MA). A face mask that covered both the mouth and nose was securely fastened to the subject's head before data collection. The face mask included a two-way valve that was directly connected to a pneumotachograph and allowed measurement of inspiratory, expiratory, and whole breath respiratory measures and minimized dead space. The facemask allowed subjects to breathe comfortably and spontaneously through either their noses or mouths.Continuous measurements of arterial blood pressure (Finapres, Louisville, CO), R-R interval (electrocardiogram), and respiratory measures were made through all experiments. Respiratory measures included: inspiratory time (ms), expiratory time (ms), tidal volume (ml), breathing frequency (breaths/min), and minute ventilation (l/min). The right hand, used for measuring arterial blood pressure, was maintained at heart level throughout all trials. All data were collected (MacLab 8E) and routed to a computer for continuous collection and for data-monitoring purposes.
Data Analysis
Respiratory measures were analyzed off line using the raw signals generated from the pneumotachograph and metabolic cart. Respiration was assessed as inspiratory, expiratory, and whole breath measures. Baseline data were averaged over the 3-min collection period. Responses were compared using a repeated-measures analysis of variance. Significance was set at P < 0.05. All data are presented as means ± SE.| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Responses to Dynamic Upright Pitch
Inspiratory and expiratory time decreased and breathing frequency (Table 1 and Fig. 1) increased during upright pitch at both 15 and 30 cycles/min. The increased breathing frequency was graded as greater responses occurred during upright pitch (5 ± 2 breaths/min) at 30 cycles/min than at 15 cycles/min
(2 ± 1 breaths/min). Tidal volume decreased during the
30-cycles/min trial. Minute ventilation, tidal volume, and heart rate
were unchanged by upright pitch. Mean arterial pressure decreased
slightly at the slower pitch rate.
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Responses to Dynamic Lateral Pitch
Dynamic lateral pitch significantly reduced inspiratory and expiratory time and increased breathing frequency (Table 1 and Fig. 1). The increased breathing frequency was graded as greater responses occurred during lateral pitch (4 ± 1 breaths/min) performed at
30 cycles/min than at 15 cycles/min (2 ± 1 breaths/min). Tidal volume was reduced during lateral pitch at 30 cycles/min. No changes in
minute ventilation or heart rate were observed during lateral pitch.
Mean arterial pressure decreased during lateral pitch of 15 cycles/min.
When comparing responses to upright and lateral pitch, the changes seen in inspiratory time, expiratory time, breathing frequency, and mean arterial pressure during the two movements were not significantly different from each other, suggesting that the results were similar regardless of which semicircular canals were activated (i.e., horizontal or vertical) or if otolith organs were activated.
Response to Dynamic Head Roll
Dynamic roll decreased inspiratory time, expiratory time, and mean arterial pressure (Table 2). Breathing frequency was significantly increased (Fig. 1), whereas tidal volume remained unchanged. Minute ventilation increased during roll at 30 cycles/min.
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Responses to Dynamic Yaw
Inspiratory and expiratory times were decreased by dynamic yaw, and both breathing frequency (Table 3 and Fig. 2) and minute ventilation were increased. Tidal volume, heart rate, and mean arterial pressure were unchanged by dynamic yaw.
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Response to Dynamic Chair Rotation
Consistent with dynamic yaw, inspiratory and expiratory times were decreased, whereas breathing frequency (Table 3 and Fig. 2) and minute ventilation were increased. Tidal volume, heart rate, and mean arterial pressure were unchanged during chair rotation.Changes in inspiratory time, expiratory time, minute ventilation, and breathing frequency during dynamic yaw and chair rotation were not significantly different from each other. The similarly increased breathing frequency during dynamic yaw and chair rotation (Fig. 2) suggests that the semicircular canals and not the otolith organs or neck muscle afferents mediated the responses.
Responses to Static HDR
Inspiratory time, expiratory time, breathing frequency, tidal volume, and minute ventilation did not change during static HDR (Table 4). However, static HDR did elicit a small tachycardia with no change in mean arterial pressure.
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Responses to Static LHR
Static LHR did not significantly affect any respiratory measures (Table 4). Static LHR elicited a small increase in heart rate but did not alter mean arterial pressure.| |
DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The present study determined the ability of the vestibular system to mediate changes in respiration in humans. These data indicate that semicircular canal activation, but not otolith organ activation, alters respiration in humans. Data supporting this conclusion come from the demonstration of a similar increase in breathing frequency when semicircular canals are activated independently (chair rotation) as well as when otolith organs and neck muscle afferents are simultaneously activated (upright pitch and roll). Additionally, horizontal and vertical semicircular canal activation elicits similar increases in respiration (upright and lateral pitch). Finally, when otolith organs are activated without semicircular canal engagement (static HDR), no changes in breathing frequency occur. Collectively, these data suggest an independent influence of semicircular canal activation on breathing frequency in humans.
The present data are consistent with data from cats that demonstrate a vestibulorespiratory reflex, as suggested by increased phrenic, abdominal, and intercostal nerve activity during electrical vestibular nerve stimulation (15, 23). However, because respiration was not measured in these experiments, it is unclear whether this vestibular activation produced functional alterations in respiration. Moreover, it is unclear what effect vestibular activation would have on respiration in humans. It is important to emphasize that the stimulus applied to the vestibular system in these cat studies is very unique. Electrical stimulation of the vestibular nerve applies a gross stimulation that provides nonselective activation of the vestibular system. This electrical stimulation protocol should activate afferent feedback from both the otolith organs and semicircular canals. Thus it is not possible to determine the respective influence of each. In the present study, efforts were made to selectively identify which vestibular apparatus mediated a given response. Dynamic head movements that alter input to both the otolith organs and semicircular canals (i.e., upright pitch and roll) and selectively to the semicircular canals (yaw) and otolith organs (static HDR) were used.
These data indicate that semicircular canal activation produces increases in respiration in humans. Support for this conclusion comes from the demonstration of an increased breathing frequency during upright pitch, roll, and yaw. A possible confounding factor in these experiments may be the influence of neck muscle afferents. However, the chair rotation trial demonstrates similar increases in breathing frequency and ventilation to yaw rotation, suggesting that increases in respiration were likely mediated by the semicircular canals but not by neck muscle afferents. Additionally, demonstration of similar increases in breathing frequency during upright and lateral pitch, roll, and yaw suggests that regardless of whether the otolith organs are activated (upright pitch and roll) or not (yaw and lateral pitch) similar responses were demonstrated. These data indicate that alterations in respiration were mediated by the semicircular canals. These findings are consistent with previous results demonstrating increased respiration during caloric stimulation that selectively activates the horizontal semicircular canals (11). However, unlike caloric stimulation, which can elicit nausea, the current study demonstrates that natural engagement of the semicircular canals mediates respiratory changes.
In humans, whole body vertical displacement increases breathing frequency and tidal volume (3). Similar responses are reported in animals during vertical displacement, and these responses are attenuated by vestibular nerve transection (22). These data suggest activation of a functional vestibulorespiratory reflex by the otolith organs in humans and animals. However, these findings are in contrast to the present and several previous demonstrations of a lack of influence of otolith organs on respiration in humans (7, 14). It is likely that these vertical displacement trials applied input to other homeostatic centers that may have independently modified respiration (i.e., arousal response). In this context, both pulmonary and respiratory muscle stretch receptors alter respiration (1, 6). Thus the present data provide useful information in the context of the influence of otolith organ activation on respiration per se in humans.
Summary
The present study comprehensively examined the influence of vestibular activation on respiration in humans. These data demonstrate that semicircular canal activation increases respiratory rate in humans independent of neck muscle afferents. However, altered otolithic engagement, elicited by static HDR, does not alter inspiratory, expiratory, or whole breath respiratory measures. These data provide support for a functional vestibulorespiratory reflex in humans during activation of the semicircular canals but not otolith organs.Perspectives
A functional vestibulorespiratory reflex may be important during nonstationary exercise. Exercise elicits rapid and marked increases in respiration. The mechanisms underlying these abrupt increases are not well established. A possibly overlooked contributor to exercise hyperpnea is the vestibular system. Some forms of nonstationary exercise (i.e., running) apply input to the vestibular centers due to linear acceleration and vertical displacement. Thus it is possible that vestibular activation during nonstationary exercise in which the head is accelerated and displaced in respect to gravity may mediate, in part, increases in respiration. These suggestions are speculative but appear to be an attractive hypothesis to an area that has perplexed physiologists for years. The present results suggest that engagement of semicircular canals may contribute to hyperpnea during nonstationary exercise in humans. These increases in ventilation would assist in meeting the augmented oxygen demand elicited by locomotion.A vestibulorespiratory reflex could further be important in the context of arterial blood pressure regulation and adaptation to the upright posture. Assumption of the upright posture increases respiratory muscle activity in both humans (5, 8) and cats (nose-up tilt) (19). These alterations may aid in regulating venous return to the heart (16, 20). However, it is possible that alterations in respiratory and thoracic muscle activity may not be vestibular in origin (1, 6). On the basis of these concerns, it appeared that studies employing more isolated techniques of activating the vestibular system were needed. Recent data further suggest an important role of natural vestibular activation (i.e., tilting) in increasing respiratory muscle (rectus abdominis) electromyographical activity in cats. Additionally, these increases in respiratory muscle activity are diminished by vestibular nerve transection (2). These data suggest that the vestibular system participates in adaptation to the upright posture and are consistent with the previous demonstration of the importance of the vestibular system during orthostatic stress (4).
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ACKNOWLEDGEMENTS |
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National Heart, Lung, and Blood Institute National Research Service Award Grants HL-67624 (K. D. Monahan) and HL-58503 (C. A. Ray), National Aeronautics and Space Administration Grant NAG9-1034 (C. A. Ray), National Space and Biomedical Research Institute Grant NCC-9-58-168 (C. A. Ray), and an Established Investigator Grant from American Heart Association (C. A. Ray) supported this project.
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FOOTNOTES |
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Address for reprint requests and other correspondence: C. A. Ray, Penn State College of Medicine, The Milton S. Hershey Medical Center, Division of Cardiology H047, 500 Univ. Drive, Hershey, PA 17033-2390 (E-mail: caray{at}psu.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpregu.00568.2001
Received 17 September 2001; accepted in final form 15 November 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Significantly different from 15 cycles/min, P < 0.05.
