Dynamic response characteristics of CO2-induced air hunger

Air Hunger: A Primal Sensation and a Primary Element of Dyspnea

The sensation that develops as a long breath hold continues is what this article is about. We term this sensation of an urge to breathe "air hunger." Air hunger, a primal sensation, alerts us to a failure to meet an urgent homeostatic need maintaining gas exchange. Anxiety, frustration, and fear evoked by air hunger motivate behavioral actions to address the failure. The unpleasantness and emotional consequences of air hunger make it the most debilitating component of clinical dyspnea, a symptom associated with respiratory, cardiovascular, and metabolic diseases. In most clinical populations studied, air hunger is the predominant form of dyspnea (colloquially, shortness of breath). Most experimental subjects can reliably quantify air hunger using rating scales, that is, there is a consistent relationship between stimulus and rating. Stimuli that increase air hunger include hypercapnia, hypoxia, exercise, and acidosis; tidal expansion of the lungs reduces air hunger. Thus, the defining experimental paradigm to evoke air hunger is to elevate the drive to breathe while mechanically restricting ventilation. Functional brain imaging studies have shown that air hunger activates the insular cortex (an integration center for perceptions related to homeostasis, including pain, food hunger, and thirst), as well as limbic structures involved with anxiety and fear. Although much has been learned about air hunger in the past few decades, much remains to be discovered, such as an accepted method to quantify air hunger in nonhuman animals, fundamental questions about neural mechanisms, and adequate and safe methods to mitigate air hunger in clinical situations. © 2021 American Physiological Society. Compr Physiol 11:1449-1483, 2021.

Multidimensional aspects of dyspnea in obese patients referred for cardiopulmonary exercise testing

We investigated the contributions of obesity on multidimensional aspects of dyspnea on exertion (DOE) in patients referred for clinical cardiopulmonary exercise testing (CPET). Ratings of perceived breathlessness (RPB, Borg scale 0-10) were collected in obese (BMI ≥ 30; n = 47) and nonobese (BMI ≤ 25; n = 27) patients during two (one lower: ∼30 W; and one higher: ∼50 W) 4-6 min constant load cycling bouts. Multidimensional dyspnea profiles (MDP) were collected in the final 26 obese and 14 nonobese patients of the sample. RPB was greater (p = 0.05) in obese (3.3 ± 2.2 vs 2.4 ± 1.4) at lower work rates, but similar at higher work rates (4.9 ± 2.2 vs 4.4 ± 1.8). MDP sensory score including unpleasantness was 4.3 ± 2.2 in obese vs 2.5 ± 1.9 in nonobese (p < 0.001). The affective score was 1.9 ± 2.2 vs 0.7 ± 0.7, respectively (p < 0.01). Breathing sensations including 'air hunger', 'effort', and 'breathing at lot' were greater (p < 0.05) in obese, making these patients more frustrated/angry (p < 0.05). Obesity should be considered as a potential independent influencing factor that provokes DOE and unpleasantness when assessing breathlessness during CPET.

Aerosol furosemide for dyspnea: Controlled delivery does not improve effectiveness

Aerosolized furosemide has been shown to relieve dyspnea; nevertheless, all published studies have shown great variability in response. This dyspnea relief is thought to result from the stimulation of slowly adapting pulmonary stretch receptors simulating larger tidal volume. We hypothesized that better control over aerosol administration would produce more consistent dyspnea relief; we used a clinical ventilator to control inspiratory flow and tidal volume. Twelve healthy volunteers inhaled furosemide (40mg) or placebo in a double blind, randomized, crossover study. Breathing Discomfort was induced by hypercapnia during constrained ventilation before and after treatment. Both treatments reduced breathing discomfort by 20% full scale. Effectiveness of aerosol furosemide treatment was weakly correlated with larger tidal volume. Response to inhaled furosemide was inversely correlated to furosemide blood level, suggesting that variation among subjects in the fate of deposited drug may determine effectiveness. We conclude that control of aerosol delivery conditions does not improve consistency of treatment effect; we cannot, however, rule out placebo effect.

The Effect of Aerosol Saline on Laboratory-Induced Dyspnea

PURPOSE

In the 'placebo arm' of a recent study, we found that aerosol saline (sham treatment) produced substantial relief of laboratory-induced dyspnea (Breathing discomfort-BD) in nearly half the subjects. The sham intervention included a physiological change, and instructions to subjects could have produced expectation of dyspnea relief. In the present study, we attempted to discover whether the response to sham aerosol was driven by behavioral or physiological aspects of the intervention.

METHODS

Dyspnea (air hunger) was evoked by constraining tidal volume during graded hypercapnia. We measured [Formula: see text] versus BD relationship before and after aerosol saline. To minimize subjects' expectations of dyspnea relief, participants were clearly instructed that we would only deliver saline aerosol. In Protocol 1, we delivered aerosol saline with a ventilator (mimicking our prior study); in Protocol 2, we delivered aerosol without a ventilator.

RESULTS

Administration of aerosol saline had little effect on BD in this group of subjects with one exception: one subject experienced appreciable reduction in BD in Protocol 1. This treatment effect was less in Protocol 2. The two most likely explanations are (a) that procedures surrounding ventilator administration of aerosol produced a psychological placebo treatment effect even though the subject knew a drug was not given; (b) there were behavioral changes in breathing undetected by our measurements of respiratory flow and volume that altered the subjects comfort.

CONCLUSION

When the expectation of treatment effect is minimized, a significant reduction in dyspnea in response to saline placebo is uncommon but not impossible.

Exertional dyspnoea in obesity

The purpose of cardiopulmonary exercise testing (CPET) in the obese person, as in any cardiopulmonary exercise test, is to determine the patient's exercise tolerance, and to help identify and/or distinguish between the various physiological factors that could contribute to exercise intolerance. Unexplained dyspnoea on exertion is a common reason for CPET, but it is an extremely complex symptom to explain. Sometimes obesity is the simple answer by elimination of other possibilities. Thus, distinguishing among multiple clinical causes for exertional dyspnoea depends on the ability to eliminate possibilities while recognising response patterns that are unique to the obese patient. This includes the otherwise healthy obese patient, as well as the obese patient with potentially multiple cardiopulmonary limitations. Despite obvious limitations in lung function, metabolic disease and/or cardiovascular dysfunction, obesity may be the most likely reason for exertional dyspnoea. In this article, we will review the more common cardiopulmonary responses to exercise in the otherwise healthy obese adult with special emphasis on dyspnoea on exertion.

Interactions Between Dyspnea and the Brain Processing of Nociceptive Stimuli: Experimental Air Hunger Attenuates Laser-Evoked Brain Potentials in Humans

Dyspnea and pain share several characteristics and certain neural networks and interact with each other. Dyspnea-pain counter-irritation consists of attenuation of preexisting pain by intercurrent dyspnea and has been shown to have neurophysiological correlates in the form of inhibition of the nociceptive spinal reflex RIII and laser-evoked potentials (LEPs). Experimentally induced exertional dyspnea inhibits RIII and LEPs, while “air hunger” dyspnea does not inhibit RIII despite its documented analgesic effects. We hypothesized that air hunger may act centrally and inhibit LEPs. LEPs were obtained in 12 healthy volunteers (age: 21–29) during spontaneous breathing (FB), ventilator-controlled breathing (VC) tailored to FB, after inducing air hunger by increasing the inspired fraction of carbon dioxide -FiCO₂- (VCCO₂), and during ventilator-controlled breathing recovery (VCR). VCCO₂ induced intense dyspnea (visual analog scale = 63% ± 6% of full scale, p < 0.001 vs. VC), predominantly of the air hunger type. VC alone reduced the amplitude of the N2-P2 component of LEPs (Δ = 24.0% ± 21.1%, p < 0.05, effect-size = 0.74) predominantly through a reduction in P2, and the amplitude of this inhibition was further reduced by inducting air hunger (Δ = 22.6% ± 17.9%, p < 0.05, effect-size = 0.53), predominantly through a reduction in N2. Somatosensory-evoked potentials (SEPs) were not affected by VC or VCCO₂, suggesting that the observed effects are specific to pain transmission. We conclude that air hunger interferes with the cortical mechanisms responsible for the cortical response to painful laser skin stimulation, which provides a neurophysiological substrate to the central nature of its otherwise documented analgesic effects.

Using laboratory models to test treatment: morphine reduces dyspnea and hypercapnic ventilatory response

RATIONALE

Opioids are commonly used to relieve dyspnea, but clinical data are mixed and practice varies widely.

OBJECTIVES

Evaluate the effect of morphine on dyspnea and ventilatory drive under well-controlled laboratory conditions.

METHODS

Six healthy volunteers received morphine (0.07 mg/kg) and placebo intravenously on separate days (randomized, blinded). We measured two responses to a CO(2) stimulus: (1) perceptual response (breathing discomfort; described by subjects as "air hunger") induced by increasing partial pressure of end-tidal carbon dioxide (Pet(CO2)) during restricted ventilation, measured with a visual analog scale (range, "neutral" to "intolerable"); and (2) ventilatory response, measured in separate trials during unrestricted breathing.

MEASUREMENTS AND MAIN RESULTS

We determined the Pet(CO2) that produced a 60% breathing discomfort rating in each subject before morphine (median, 8.5 mm Hg above resting Pet(CO2)). At the same Pet(CO2) after morphine administration, median breathing discomfort was reduced by 65% of its pretreatment value; P < 0.001. Ventilation fell 28% at the same Pet(CO2); P < 0.01. The effect of morphine on breathing discomfort was not significantly correlated with the effect on ventilatory response. Placebo had no effect.

CONCLUSIONS

(1) A moderate morphine dose produced substantial relief of laboratory dyspnea, with a smaller reduction of ventilation. (2) In contrast to an earlier laboratory model of breathing effort, this laboratory model of air hunger established a highly significant treatment effect consistent in magnitude with clinical studies of opioids. Laboratory studies require fewer subjects and enable physiological measurements that are difficult to make in a clinical setting. Within-subject comparison of the response to carefully controlled laboratory stimuli can be an efficient means to optimize treatments before clinical trials.

Effects of sequential swallowing on drive to breathe in young, healthy adults

Sequential swallowing is the act of swallowing multiple times, without pausing. Because sequential swallowing requires breath-holding, it seems likely that it could increase the drive to breathe. This study was designed to determine if sequential swallowing is accompanied by an increased drive to breathe in young, healthy adults. We predicted that sequential swallowing would be accompanied by prolonged breath-holding in most cases, and that this would be followed by a recovery phase during which ventilation would increase for a brief period. Results showed that not only did healthy participants increase ventilation after sequential swallowing, they also experienced breathing discomfort (dyspnea) despite the fact that they usually continued to breathe during the swallowing sequence. Given that these effects are observable in young, healthy adults, it seems reasonable to assume that individuals with respiratory and/or neurological compromise would also have an increased drive to breathe during sequential swallowing.

Validation of a three-factor measurement model of dyspnea in hospitalized adults with heart failure

OBJECTIVE

The purpose of this study was to validate a 3-factor measurement model of dyspnea sensory quality (WORK-EFFORT, TIGHTNESS, SMOTHERING-AIR HUNGER) originally derived in patients with exacerbated chronic obstructive pulmonary disease.

METHODS

In this validation study, adult patients with heart failure were enrolled after hospital admission (median hospital day 1) and asked to rate the intensity of dyspnea sensory quality descriptors on the day of enrollment (study day 1; N = 119) and in a recall version for the day of admission (study day 0; n = 97).

RESULTS

Confirmatory factor analysis demonstrated good model fit for both days. Cronbach's α for each factor was greater than .87 for both study days.

CONCLUSION

This is the first study to validate a previously specified measurement model of dyspnea sensory quality in an independent sample. Results indicate that measurement of dyspnea sensory quality in exacerbated cardiopulmonary disease does not necessarily require disease-specific questionnaires.

The affective dimension of laboratory dyspnea: air hunger is more unpleasant than work/effort

RATIONALE

It is hypothesized that the affective dimension of dyspnea (unpleasantness, emotional response) is not strictly dependent on the intensity of dyspnea.

OBJECTIVES

We tested the hypothesis that the ratio of immediate unpleasantness (A(1)) to sensory intensity (SI) varies depending on the type of dyspnea.

METHODS

Twelve healthy subjects experienced three stimuli: stimulus 1: maximal eucapnic voluntary hyperpnea against inspiratory resistance, requiring 15 times the work of resting breathing; stimulus 2: Pet(CO(2)) 6.1 mm Hg above resting with ventilation restricted to less than spontaneous breathing; stimulus 3: Pet(CO(2)) 7.7 mm Hg above resting with ventilation further restricted. After each trial, subjects rated SI, A(1), and qualities of dyspnea on the Multidimensional Dyspnea Profile (MDP), a comprehensive instrument tested here for the first time.

MEASUREMENTS AND MAIN RESULTS

Stimulus 1 was always limited by subjects failing to meet a higher ventilation target; none signaled severe discomfort. This evoked work and effort sensations, with relatively low unpleasantness (mean A(1)/SI = 0.64). Stimulus 2, titrated to produce dyspnea ratings similar to those subjects gave during stimulus 1, evoked air hunger and produced significantly greater unpleasantness (mean A(1)/SI = 0.95). Stimulus 3, increased until air hunger was intolerable, evoked the highest intensity and unpleasantness ratings and high unpleasantness ratio (mean A(1)/SI = 1.09). When asked which they would prefer to repeat, all subjects chose stimulus 1.

CONCLUSIONS

(1) Maximal respiratory work is less unpleasant than moderately intense air hunger in this brief test; (2) unpleasantness of dyspnea can vary independently from perceived intensity, consistent with the prevailing model of pain; (3) separate dimensions of dyspnea can be measured with the MDP.

Coughing induced by airway irritation modulates the sensation of air hunger

The aim of this study was to clarify whether coughing elicited by airway irritation can modulate the sensation of air hunger. Using a visual analog scale (VAS), we measured air hunger for 30 s following breakpoint of a breath-hold in healthy young subjects who were asked to resume their breathing according to four patterns (free breathing, citric acid inhalation, voluntary cough, and panting) before and/or after airway anaesthesia. We also measured air hunger for citric acid-induced coughing and voluntary coughing without the preceding breath-holding. The free breathing after breakpoint of breath-holding causes an immediate relief of air hunger (VAS median values at 5, 15 and 25 s after breath-hold: 39, 0 and 0), whereas voluntary coughing causes a delay in the relief of air hunger (67, P < 0.05; 17, P < 0.05; and 0, NS) and a slower relief occurred during citric acid-induced coughing (81, P < 0.01; 49, P < 0.05; and 12, P < 0.05). Conversely, the voluntary coughing and citric acid-induced coughing per se failed to induce air hunger. Inhalation of lidocaine aerosol completely abolished the cough response to citric acid inhalation causing an immediate relief of air hunger, whereas airway anaesthesia alleviated only slightly the air hunger during voluntary coughing. The changes in air hunger during the panting were similar to those during the voluntary coughing observed before airway anaesthesia and were not affected by airway anaesthesia (VAS at 15 s point before versus after anaesthesia: 18 versus 15; NS). Coughing induced by airway irritation per se does not generate the sensation of air hunger but can aggravate it, presumably by vagally mediated mechanisms and/or central mechanisms.

Speaking-related dyspnea in healthy adults

PURPOSE

To reveal the qualities and intensity of speaking-related dyspnea in healthy adults under conditions of high ventilatory drive, in which the behavioral and metabolic control of breathing must compete.

METHOD

Eleven adults read aloud while breathing different levels of inspired carbon dioxide (CO(2)). After the highest level, participants provided unguided descriptions of their experiences and then selected descriptors from a list. On a subsequent day, participants read aloud while breathing high CO(2) as before, then rated air hunger, physical exertion, and mental effort (with definitions provided). Recordings were made of ventilation (with respiratory magnetometers), end-tidal partial pressure of CO(2), transcutaneous PCO(2), oxygen saturation, noninvasive blood pressure, heart rate, and the speech signal.

RESULTS

Unguided descriptions were found to reflect the qualities of air hunger, physical exertion (work), mental effort, and speech-related observations. As CO(2) stimulus strength increased, participants experienced increased perception of air hunger, physical exertion, and mental effort. Simultaneous increases were observed in ventilation, tidal volume, end-inspiratory and end-expiratory volumes, expiratory flow during speaking, nonlinguistic junctures, and nonspeech expirations.

CONCLUSION

Two qualities of speaking-related dyspnea--air hunger and physical exertion--are the same as those reported for many other types of nonspeech dyspnea conditions and, therefore, may share the same physiological mechanisms. The mental effort quality associated with speaking-related dyspnea may reflect a conscious drive to balance speech requirements and ventilatory demands. These findings have implications for developing better ways to evaluate and manage clients with respiratory-based speech problems.

Time course of air hunger mirrors the biphasic ventilatory response to hypoxia

Determining response dynamics of hypoxic air hunger may provide information of use in clinical practice and will improve understanding of basic dyspnea mechanisms. It is hypothesized that air hunger arises from projection of reflex brain stem ventilatory drive (“corollary discharge”) to forebrain centers. If perceptual response dynamics are unmodified by events between brain stem and cortical awareness, this hypothesis predicts that air hunger will exactly track ventilatory response. Thus, during sustained hypoxia, initial increase in air hunger would be followed by a progressive decline reflecting biphasic reflex ventilatory drive. To test this prediction, we applied a sharp-onset 20-min step of normocapnic hypoxia and compared dynamic response characteristics of air hunger with that of ventilation in 10 healthy subjects. Air hunger was measured during mechanical ventilation (minute ventilation = 9 ± 1.4 l/min; end-tidal Pco2 = 37 ± 2 Torr; end-tidal Po2 = 45 ± 7 Torr); ventilatory response was measured during separate free-breathing trials in the same subjects. Discomfort caused by “urge to breathe” was rated every 30 s on a visual analog scale. Both ventilatory and air hunger responses were modeled as delayed double exponentials corresponding to a simple linear first-order response but with a separate first-order adaptation. These models provided adequate fits to both ventilatory and air hunger data ( r2 = 0.88 and 0.66). Mean time constant and time-to-peak response for the average perceptual response (0.36 min−1 and 3.3 min, respectively) closely matched corresponding values for the average ventilatory response (0.39 min−1 and 3.1 min). Air hunger response to sustained hypoxia tracked ventilatory drive with a delay of ∼30 s. Our data provide further support for the corollary discharge hypothesis for air hunger.

Hypoxic and hypercapnic drives to breathe generate equivalent levels of air hunger in humans

Anecdotal observations suggest that hypoxia does not elicit dyspnea. An opposing view is that any stimulus to medullary respiratory centers generates dyspnea via "corollary discharge" to higher centers; absence of dyspnea during low inspired Po(2) may result from increased ventilation and hypocapnia. We hypothesized that, with fixed ventilation, hypoxia and hypercapnia generate equal dyspnea when matched by ventilatory drive. Steady-state levels of hypoxic normocapnia (end-tidal Po(2) = 60-40 Torr) and hypercapnic hyperoxia (end-tidal Pco(2) = 40-50 Torr) were induced in naive subjects when they were free breathing and during fixed mechanical ventilation. In a separate experiment, normocapnic hypoxia and normoxic hypercapnia, "matched" by ventilation in free-breathing trials, were presented to experienced subjects breathing with constrained rate and tidal volume. "Air hunger" was rated every 30 s on a visual analog scale. Air hunger-Pet(O(2)) curves rose sharply at Pet(O(2)) <50 Torr. Air hunger was not different between matched stimuli (P > 0.05). Hypercapnia had unpleasant nonrespiratory effects but was otherwise perceptually indistinguishable from hypoxia. We conclude that hypoxia and hypercapnia have equal potency for air hunger when matched by ventilatory drive. Air hunger may, therefore, arise via brain stem respiratory drive.

The perception of respiratory work and effort can be independent of the perception of air hunger

Dyspnea in patients could arise from both an urge to breathe and increased effort of breathing. Two qualitatively different sensations, "air hunger" and "respiratory work and effort," arising from different afferent sources are hypothesized. In the laboratory, breathing below the spontaneous level may produce an uncomfortable sensation of air hunger, and breathing above it a sensation of work or effort. Measurement of a single sensory dimension cannot distinguish these as separate sensations; we therefore measured two sensory dimensions and attempted to vary them independently. In five normal subjects we obtained simultaneous ratings of air hunger and of work and effort while independently varying PCO(2) or the level of targeted voluntary breathing. We found a difference in response to the two stimulus dimensions: air hunger ratings changed more steeply when PCO(2) was altered and ventilation was constant; work or effort ratings changed more steeply when ventilation was altered and PCO(2) was constant. We conclude that "air hunger" is qualitatively different from "work and effort" and arises from different afferent sources.

Self-control and external control of mechanical ventilation give equal air hunger relief

Elevated end-tidal partial pressure of CO2 (PET(CO2)) causes air hunger; this sensation becomes intense with a relatively small rise in PET(CO2) if ventilation is held constant. Spontaneously breathing subjects increase ventilation in response to CO2, thereby greatly diminishing air hunger. In healthy subjects and ventilator-dependent patients, experimenter-induced increases in ventilator tidal volume (VT) relieve air hunger even if PET(CO2) is kept elevated. We addressed two questions: (1) Can paralyzed, ventilator-dependent patients use the sensation of air hunger to effectively control ventilator VT using nonrespiratory motor pathways; and (2) Do subjects obtain more relief when in control of their own ventilator? Four subjects were trained to increase ventilator VT using a mouth-operated switch. Subjects' ratings of air hunger intensity in response to elevated PET(CO2) were compared during three conditions: (1) constant VT; (2) subject-controlled VT; and (3) experimenter-controlled VT. When given control of their ventilator, all subjects increased VT in response to increased PET(CO2), thereby relieving air hunger. Air hunger relief was similar when the experimenter mimicked these VT changes. These results suggest that: (1) ventilator-dependent patients can use sensation, conscious decisions, and nonrespiratory motor pathways to achieve an appropriate respiratory response to increased PCO2 and (2) control of one's own ventilation is unimportant in these circumstances.