Acute Sustained Hypoxia Suppresses the Cough Reflex in Healthy Subjects

    Adelaide Institute for Sleep Health, Repatriation General Hospital, Daw Park
    School of Molecular and Biomedical Science, Discipline of Physiology, University of Adelaide, Adelaide
    Department of Medicine, Flinders University, Bedford Park, South Australia, Australia

    ABSTRACT

    Rationale: An intact cough reflex is important to protect the lung from injurious substances and to clear excess secretions. A blunted cough reflex may be harmful or even fatal in respiratory disease. Hypoxia is common in respiratory disorders and has been shown to have depressant effects on respiratory sensation and ventilation. We hypothesized that it might also suppress the cough reflex.

    Objectives: To determine if acute hypoxia increases cough threshold and cough tachyphylaxis to inhaled capsaicin.

    Methods: On two occasions, 16 healthy subjects inhaled a saline control followed by doubling doses of capsaicin aerosol (range, 0.49–500 μM) every minute for 15 s during controlled ventilation ( 190% baseline) with isocapnic hypoxia (SpO2,  80%) or isocapnic normoxia, in random order. When a subject responded to a dose with five or more coughs, the next doubling dose of capsaicin was administered continuously for 60 s to assess acute tachyphylaxis.

    Main Results: The capsaicin concentration required to elicit five coughs was significantly higher during isocapnic hypoxia compared with normoxia (29.6 ± 16.0 vs. 23.4 ± 15.6 μM, p = 0.01). During continuous capsaicin inhalation, significantly more coughs were evoked in the first 10 s compared with the last (2.3 ± 0.3 vs. 1.3 ± 0.3, p < 0.01), indicating cough tachyphylaxis. However, the decrease was the same during hypoxia and normoxia (–1.3 ± 0.4 vs. –0.9 ± 0.6, p = 0.54).

    Conclusions: Acute isocapnic hypoxia suppresses cough reflex sensitivity to inhaled capsaicin. This finding raises the possibility that the cough reflex may be impaired during acute exacerbations of hypoxic-respiratory disorders.

    Key Words: capsaicin  cough reflex  tachyphylaxis

    The cough reflex is an important defensive respiratory reflex that protects the lungs from inhalation or aspiration of potentially injurious substances and clears excess secretions. A blunted cough reflex can be harmful or even fatal in the presence of severe respiratory disease (1, 2). Any factor that impairs the function of this vital protective response has the potential to increase disease severity.

    Acute hypoxia is a common feature of conditions such as pneumonia and exacerbations of chronic obstructive lung disease in which cough is an important protective reflex. Recent studies have demonstrated that acute hypoxia suppresses the perception of respiratory load, which may contribute to treatment delay (3, 4). Impairment of respiratory afferent-neural transmission below the level of the cortex appears to contribute to this depression (5). This finding raises the possibility that acute hypoxia may impair a range of protective respiratory reflexes that are subserved by subcortical structures—for example, brainstem nuclei involved in respiratory reflex integration. Respiratory brainstem neural networks and networks that modulate cough share common neuronal pathways, such as the nucleus tractus solitarii (NTS) (6). In the same way that subcortical structures mediate hypoxia-induced respiratory depression (7, 8) and potentially respiratory load sensation (5), cough may be similarly depressed during hypoxia. In support of this hypothesis, animal studies suggest that the cough reflex is impaired during acute hypoxia (9, 10). Should this also be the case in humans, it may contribute to a progressive clinical worsening in hypoxic-respiratory diseases.

    This study tested the hypothesis that acute sustained hypoxia in healthy subjects impairs the cough threshold and results in more rapid adaptation (tachyphylaxis) of the cough reflex. Inhaled capsaicin was chosen as the provocative agent as it reproducibly produces cough in a dose-dependent fashion, and has been used extensively to examine the efficacy of antitussive agents and to explore physiologic differences in cough-reflex sensitivity in healthy subjects and patient groups (1, 11–13). Changes in inspiratory flow may strongly influence cough-reflex characteristics via flow-related changes in aerosol deposition throughout the airway and/or increased activation of slowly adapting pulmonary stretch receptors via lung inflation at higher inspiratory flows (10, 14, 15). Consequently, to avoid confounding influences of hypoxic ventilatory stimulation on cough-reflex sensitivity, cough-reflex characteristics were compared during normoxia (baseline drive) and hypoxia (increased drive) at matched levels of mild voluntary hyperventilation.

    METHODS

    Subject Selection

    The subjects were 18 young, healthy nonsmokers with no history of respiratory disease or current or recent (< 2 mo) upper respiratory tract infection, and with baseline FEV1 of more than 80% predicted; they gave informed, written consent to participate in the study. The study was approved by the Daw Park Repatriation General Hospital and Adelaide University Human Research and Ethics Committees.

    Preliminary Visit

    Pulmonary function testing, including spirometry and whole-body plethysmography, was performed to ensure normal lung function. After lung function testing, subjects were fitted with a full face mask (ComfortFull; Respironics, Murrysville, PA) with a pressure transducer in series (MP45; Validyne Engineering, Northridge, CA) and connected to the breathing circuit (Figure 1). The nose was taped closed to ensure mouth breathing (Sleek; Smith and Nephew, London, UK). After 10 min of room-air baseline breathing, each subject's ventilatory response to 15 min of isocapnic hypoxia (blood arterial O2 saturation [SpO2],  80%) was recorded (POET II model 602-3; Criticare Systems, Waukesha, WI) (5).

    Target ventilation.

    A target minute-ventilation (I) level was selected according to each subject's hypoxic ventilatory response ( 190% above baseline) as described previously (5). Subjects practiced the targeted I task during the preliminary visit for 10 to 15 min by breathing via a reservoir filled at the target flow rate with humidified compressed air (Figure 1). Subjects were instructed to maintain a constant breathing frequency and tidal volume according to the predetermined levels via real-time feedback of the inspiratory volume trace on a computer monitor (Figure 1).

    Assessment of cough-reflex threshold and acute tachyphylaxis.

    Cough challenge testing was performed at the preliminary visit for familiarization and to minimize learning effects in subsequent experiments (16, 17). Measurements were performed while subjects breathed at the target I. A Piezo electric sensor attached to the subject's neck (Sleepmate; Sleepmate Technologies, Midlothian, VA) and a dictaphone recorded cough vibration and sound. After an initial challenge with normal saline, doubling doses of capsaicin aerosol (range, 0.49–500 μM) were administered for the first 15 s of every min until five or more coughs were elicited before the next dose as described previously (18). After a subject coughed five or more times during a 60-s period, the next incremental doubling dose of capsaicin was introduced continuously for 1 min to assess acute tachyphylaxis (16). A single observer, blinded to the gas condition, identified a cough as a brief expulsive event in mask pressure associated with excursion in the vibration channel accompanied by sound recordings indicative of cough.

    Main Experimental Visits

    Subjects attended the laboratory twice, at the same time of day, approximately 1 wk apart. Subjects abstained from alcohol and caffeine for at least 12 h before each visit. The order of the two main visits (hypoxia or normoxia) was randomized between subjects via a coin toss, and subjects remained blinded to the experimental gas.

    On each occasion, after 5 min of room-air breathing, subjects were switched to the target I arm of the circuit (Figure 1) through which the experimental gas (compressed dry 9% O2 in N2, or medical air) was introduced. A manual inspiratory bleed of CO2 was used to ensure isocapnia. During hypoxia trials, the inspired O2 fraction was adjusted as necessary to maintain SpO2 of approximately 80% (5). After 30 min, cough threshold and acute tachyphylaxis were assessed as per the preliminary visit while subjects continued breathing at the targeted I. During each experiment subjects listened to music through earphones. All measurements were performed while subjects were seated upright in a comfortable chair.

    Data Analysis

    The concentrations of capsaicin required to elicit two coughs (C2) and five coughs (C5) were determined using linear interpolation of log concentration–response curves for each test. Trials in which C2 could not be accurately determined because the subject coughed more than twice during the first capsaicin dose were excluded from between-gas comparisons of C2. To further characterize the cough dose–response curve, we calculated the linear regression slope (cough sensitivity) and ordinate intercept of the log dose versus number of coughs from all available data points between the first and last threshold doses. Trials in which C5 occurred on the first capsaicin dose were excluded from the cough-sensitivity analysis because insufficient data were available to perform linear regression. Coughs during the final 1 min of continuous capsaicin nebulization were counted and grouped in 10-s bins to assess acute tachyphylaxis (16). Tachyphylaxis was defined as a reduction over time in the mean number of coughs evoked by capsaicin during the 60 s of continuous capsaicin inhalation.

    Statistical Procedures

    Related-samples nonparametric tests (Mann-Whitney) were used to compare cough threshold measurements between gas treatments (SPSS version 12.1; SPSS Inc., Chicago, IL). Repeated-measures analysis of variance was used to examine acute tachyphylaxis time, gas effects, and gas-by-time interaction effects and to compare ventilatory parameters between gas conditions across study periods (baseline, target I, cough threshold, and tachyphylaxis) and gas-by-period interaction effects. Statistical significance was inferred when p < 0.05. Data are reported as means ± SEM unless otherwise stated.

    RESULTS

    Anthropometric Data

    A total of 16 subjects (9 males) successfully completed all of the study requirements. The mean age and body mass index of the 16 subjects were 24.5 ± 1.0 yr and 22.8 ± 0.6 kg/m2, respectively. Subjects had normal lung function (mean FEV1, 109.2 ± 2.9; forced vital capacity, 100.2 ± 3.2; and total lung capacity, 104.9 ± 3.4% predicted).

    Ventilatory Data

    Figure 2 displays I, SpO2z, and end-tidal (partial) carbon dioxide pressure (PETCO2) during baseline, targeted-ventilation, cough-threshold, and cough-tachyphylaxis periods in normoxia and hypoxia. I and PETCO2 were well matched between gas conditions across each study period (Figures 2A and 2C). There were no between-gas differences in any ventilatory parameters except, by design, SpO2, which was lower during hypoxia after the baseline period compared with normoxia (Figure 2B). With repeated coughing during the acute tachyphylaxis protocol, there was a marginal decrease in PETCO2 below baseline levels, but this occurred to the same extent under both gas conditions (Figure 2C). PETCO2 was slightly higher during the target I period compared with baseline but did not differ from baseline during cough-threshold testing (Figure 2C). As instructed, subjects achieved target I across the three targeted I periods by increasing peak inspiratory flow (36.5 ± 1.5 vs. 56.3 ± 2.9 L/min, p < 0.001) and tidal volume (0.72 ± 0.04 vs. 1.1 ± 0.06 L, p < 0.001) without changing breathing frequency (14.2 ± 0.5 vs. 15.3 ± 0.6 breaths/min, p = 0.066). There were no gas effects or gas-by-period interaction effects for these ventilatory parameters (p  0.315).

    Cough Threshold

    The initial saline dose did not evoke coughing in any subjects. In contrast, capsaicin challenge evoked a dose-dependent increase in the number of coughs. The capsaicin dose required to elicit five coughs was significantly greater during hypoxia compared with normoxia (Figure 3A). On the first capsaicin dose during the normoxia protocol, three subjects coughed five times, leaving 13 subjects available for paired cough-sensitivity comparisons. Similar to C5 cough threshold, there were significantly fewer coughs per log increment dose of capsaicin during hypoxia versus normoxia (Figure 3B). The ordinate intercept of the log capsaicin dose versus the number of coughs was also significantly lower during hypoxia compared with normoxia (1.86 ± 0.68 vs. 3.28 ± 0.80, p = 0.004). On the first capsaicin dose during at least one of the tests, 8 of the 16 subjects coughed more than two times (3 during normoxia, 3 during hypoxia, and 2 during both), leaving 8 subjects for paired C2 comparison. C2 in the remaining 8 subjects was not different between hypoxia and normoxia (18.76 ± 6.87 vs. 14.15 ± 7.94 μM, p = 0.263).

    Cough Tachyphylaxis

    Given the increased C5 cough threshold during hypoxia, a significantly higher dose of capsaicin was administered for the 1-min acute tachyphylaxis challenge during hypoxia compared with normoxia (68.9 ± 33.5 vs. 50.7 ± 31.7 μM, p = 0.018). However, the mean number of coughs elicited at the capsaicin dose producing five or more coughs (immediately before acute tachyphylaxis assessment) did not differ between hypoxia and normoxia (6.6 ± 0.4 vs. 6.2 ± 0.3, p = 0.535). During acute tachyphylaxis assessment, the mean number of coughs elicited was maximal during the second 10-s period of continuous capsaicin inhalation and decreased thereafter (Figure 4). Acute tachyphylaxis was evident by a significant analysis of variance time effect (p < 0.001) and a reduction in the mean number of coughs in the last, compared with the first, 10 s (1.3 ± 0.3 vs. 2.3 ± 0.3, p < 0.001). However, the mean number of coughs elicited throughout the 60 s was not different between hypoxia and normoxia (12.5 ± 2.1 vs. 12.6 ± 1.5, p = 0.938) and there were no gas-by-time interaction effects (p = 0.821).

    DISCUSSION

    The main finding of this study was that acute sustained isocapnic hypoxia increased the C5 cough threshold to inhaled capsaicin and decreased the slope of the cough-sensitivity relationship in healthy individuals. Cough tachyphylaxis to 1 min of continuous capsaicin inhalation was present but not different between gas conditions.

    Despite recognition of the physiologic and potential clinical importance of this issue (10, 19), there have been few studies conducted to investigate the effects of hypoxia on cough sensitivity. Tatar and colleagues demonstrated suppression of laryngeal and tracheobronchial cough to mechanical stimulation in cats during poikilocapnic hypoxia (9). Hypoxia-induced impairment was noted at both anatomic locations; however, tracheobronchial cough appeared particularly vulnerable to suppression. Although it is not possible to determine if tracheobronchial cough was preferentially down-regulated during hypoxia and if the effect of hypoxia on capsaicin-induced cough differs from its effect on mechanically induced cough, the findings of decreased cough-reflex sensitivity during hypoxia in the present study are in agreement with Tatar and colleagues. Two studies examined the effects of prolonged high-altitude exposure (up to 1 mo) on cough-reflex sensitivity to citric acid, the latter study under simulated altitude (20, 21). Both studies reported a small decrease in citric-acid cough threshold at extreme altitude. The authors postulate that subclinical pulmonary edema or airway-drying effects secondary to an altitude-induced increase in ventilation may have contributed to this effect (20, 21). Although these studies were not specifically designed to examine the effect of hypoxia, and caution is warranted given the small sample size, the degree of hypoxia appeared to have no effect on cough threshold when examined using a linear-regression model. However, potentially cough-provoking effects of hypobaric hypoxia may have masked hypoxic cough suppression evident in the current study in which ventilation and inspiratory flow were carefully controlled 30 min before and during normobaric hypoxia and normoxia provocation testing. However, it is also possible that capsaicin- but not citric-acid–induced cough is suppressed by hypoxia. Finally, the cough reflex exhibits plasticity (22, 23) and may be importantly influenced by the duration of hypoxia (acute vs. chronic).

    There has been some uncertainty as to the presence of cough tachyphylaxis to inhaled capsaicin. Although not specifically designed to test tachyphylaxis, several studies have reported the absence of cough adaptation to capsaicin in adults (14, 24–26). Chang and colleagues (15) performed two repeated capsaicin cough challenges separated by 10 min in children, and noted a tendency toward an increase in cough threshold during the second test, although the difference was not statistically significant (15). In a systematic examination of the long- and short-term adaptation characteristics of the cough reflex to capsaicin and citric acid, Morice and colleagues observed marked tachyphylaxis to both cough-provoking stimuli, although adaptation appeared more prominent for citric acid than capsaicin (16). Using an acute tachyphylaxis protocol similar to that of Morice and colleagues (16), we observed a similar pattern of cough adaptation during capsaicin inhalation. However, the peak number of coughs occurred slightly later in the current study. This variation is likely explained by differences in cough-provocation delivery systems and inspiratory flow rates. Together, these studies suggest that acute tachyphylaxis of the cough reflex occurs during capsaicin inhalation, an effect that is likely to be dose dependent. The current study also suggests that acute sustained hypoxia does not influence the extent and time course of acute cough tachyphylaxis.

    Possible Mechanisms Contributing to Blunted Cough-Reflex Sensitivity during Hypoxia

    The underlying physiology of the sensory and central mechanisms responsible for activating the cough reflex to various cough-provoking stimuli remains under investigation (19, 27, 28). Briefly, sensory information from stimulation of the afferent nerve endings capable of producing cough is relayed to the NTS via the vagus nerve. The neural initiation of the various respiratory-muscle contractions producing cough is believed to originate in the medulla (29). Down-regulation of the cough reflex during hypoxia could be the result of impairment at one or more levels along the cough-reflex arc.

    The central nervous system is believed to have caudal to rostral sensitivity to the depressant effects of hypoxia (7, 30). This supports a centrally located origin for hypoxia-induced depression of cough—for example, the brainstem. In support of this hypothesis, the NTS appears particularly sensitive to hypoxia and has been proposed to be a key mediator of hypoxic ventilatory depression via a -aminobutyric acid (GABA)–mediated pathway (31). All vagal respiratory sensory afferent neurons mediating cough relay through the NTS. Thus, this is potentially a primary site for hypoxia-induced down-regulation of cough, perhaps via elaboration of GABA at this site. The GABA agonist baclofen has been shown to decrease cough sensitivity to capsaicin in healthy individuals (32). This action is believed to be largely centrally mediated, although a peripheral depressant action of GABA is also possible (33). Hypoxia has also been shown to increase central nervous system levels of endogenous opioids (7), and some opiate receptor agonists have been shown to have antitussive properties via inhibition of the central component of cough (34). Cough can also be voluntarily suppressed, highlighting the role of inhibitory cortical projections to the cough neural network (35). Central activation of inhibitory pathways or down-regulation of facilitatory pathways to the cough neuronal network may contribute to impaired cough-reflex sensitivity in what may be a part of a hypoxia-sensitive "central inhibitory network" (8).

    While central depressant effects may play a key role in mediating down-regulation of cough during hypoxia, a role for peripheral depression cannot be excluded. Respiratory afferent neural transmission appears to be suppressed below the level of the cortex during acute hypoxia in healthy individuals (5). This finding raises the possibility that respiratory sensory depression may occur as low down in the neurosensory axis as the primary sensory nerve ending. Indeed, primary receptor function has been shown to be impaired during hypoxia in other receptor systems (36).

    Methodologic Considerations

    Unlike C5 cough threshold, the C2 cough threshold did not differ between gas conditions. This most likely reflects a type II error given the small sample size for this comparison and that the linear-regression slope (cough sensitivity) derived across all measured capsaicin doses was significantly reduced during hypoxia. Similarly, a lack of between-gas difference in cough tachyphylaxis may reflect type II error.

    PETCO2 was not precisely controlled at eucapnic levels during targeted ventilation and tachyphylaxis periods. However, this is unlikely to affect the main conclusions given that PETCO2 was not different from baseline during cough threshold testing and modest differences during targeted ventilation and tachyphylaxis periods were not different between gas conditions. In addition, although the presence of capsaicin-induced cough was defined in a manner consistent with the literature (17), it is possible that some of the expiratory events identified as cough may have been other expiratory reflexes.

    Finally, although we hypothesize that down-regulation of the cough reflex during hypoxia occurs due to sensory or central depression or both, it remains possible that provocant deposition and/or respiratory and airway mechanics were affected by hypoxia. However, these would appear to be unlikely explanations of our findings. First, by design, inspiratory flow, breath timing, and volume were matched between gas conditions. Second, although one study reported a small dilatory effect of hypoxia on airway caliber, attributed to changes in ventilatory pattern from normoxic conditions (37), most studies have shown no effect of hypoxia on respiratory function or airway mechanics (38, 39).

    Conclusions

    This study has demonstrated that acute sustained hypoxia depresses cough-reflex sensitivity to inhaled capsaicin in healthy individuals. This finding raises the possibility that vital protective respiratory defense mechanisms may be impaired during acute exacerbations of hypoxic-respiratory disease, such as pneumonia, bronchiectasis, and chronic obstructive pulmonary disease. Several studies have emphasized how an absent or blunted cough reflex may render patients vulnerable to increased morbidity and mortality (2, 40, 41).

    Although acute cough serves as a fundamental protective mechanism, chronic cough can be a problematic symptom and is one of the most common reasons for patients to seek medical attention (42). Several studies have demonstrated that cough sensitivity heightened during periods of respiratory disease may be reversed on recovery (1, 43). To date, there have been no studies conducted in acutely hypoxic patients. Although cough is likely further influenced by disease, the results of this study in healthy individuals suggest that acute hypoxia may impair the cough reflex.

    Acknowledgments

    The authors thank Jenny Casanova, Liz Learhinan, and Paul Henshall of the Repatriation General Hospital Pharmacy Department for their assistance with preparation of capsaicin solutions. They thank David Schembri and the Respiratory Function Unit staff, Repatriation General Hospital, for valuable assistance with lung function measurements. They also thank Dr. Stuart Mazzone for his helpful comments on the manuscript.

    FOOTNOTES

    Supported by the National Health and Medical Research Council of Australia.

    Originally Published in Press as DOI: 10.1164/rccm.200509-1455OC on December 1, 2005

    Conflict of Interest Statement: None of the authors have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

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