Mechanisms of Dyspnea

COVID-19 and silent hypoxemia in a minimal closed-loop model of the respiratory rhythm generator

Silent hypoxemia, or "happy hypoxia," is a puzzling phenomenon in which patients who have contracted COVID-19 exhibit very low oxygen saturation ( SaO 2 < 80%) but do not experience discomfort in breathing. The mechanism by which this blunted response to hypoxia occurs is unknown. We have previously shown that a computational model of the respiratory neural network (Diekman et al. in J Neurophysiol 118(4):2194-2215, 2017) can be used to test hypotheses focused on changes in chemosensory inputs to the central pattern generator (CPG). We hypothesize that altered chemosensory function at the level of the carotid bodies and/or the nucleus tractus solitarii are responsible for the blunted response to hypoxia. Here, we use our model to explore this hypothesis by altering the properties of the gain function representing oxygen sensing inputs to the CPG. We then vary other parameters in the model and show that oxygen carrying capacity is the most salient factor for producing silent hypoxemia. We call for clinicians to measure hematocrit as a clinical index of altered physiology in response to COVID-19 infection.

COVID-19 and silent hypoxemia in a minimal closed-loop model of the respiratory rhythm generator

Silent hypoxemia, or 'happy hypoxia', is a puzzling phenomenon in which patients who have contracted COVID-19 exhibit very low oxygen saturation (SaO2 < 80%) but do not experience discomfort in breathing. The mechanism by which this blunted response to hypoxia occurs is unknown. We have previously shown that a computational model (Diekman et al., 2017, J. Neurophysiol) of the respiratory neural network can be used to test hypotheses focused on changes in chemosensory inputs to the central pattern generator (CPG). We hypothesize that altered chemosensory function at the level of the carotid bodies and/or the nucleus tractus solitarii are responsible for the blunted response to hypoxia. Here, we use our model to explore this hypothesis by altering the properties of the gain function representing oxygen sensing inputs to the CPG. We then vary other parameters in the model and show that oxygen carrying capacity is the most salient factor for producing silent hypoxemia. We call for clinicians to measure hematocrit as a clinical index of altered physiology in response to COVID-19 infection.

Submissive hypercapnia: Why COPD patients are more prone to CO2 retention than heart failure patients

Patients with late-stage chronic obstructive pulmonary disease (COPD) are prone to CO2 retention, a condition which has been often attributed to increased ventilation-perfusion mismatch particularly during oxygen therapy. However, patients with mild-to-moderate COPD or chronic heart failure (CHF) also suffer similar ventilatory inefficiency but they remain near-normocapnic at rest and during exercise with an augmented respiratory effort to compensate for the wasted dead space ventilation. In severe COPD, the augmented exercise ventilation progressively reverses as the disease advances, resulting in hypercapnia at peak exercise as ventilatory limitation due to increasing expiratory flow limitation and dynamic lung hyperinflation sets in. Submissive hypercapnia is an emerging paradigm for understanding optimal ventilatory control and cost/benefit decision-making under prohibitive respiratory chemical-mechanical constraints, where the need to maintain normocapnia gives way to the mounting need to conserve the work of breathing. In severe/very severe COPD, submissive hypercapnia epitomizes the respiratory controller's 'can't breathe, so won't breathe' say-uncle policy when faced with insurmountable ventilatory limitation. Even in health, submissive hypercapnia ensues during CO2 breathing/rebreathing when the inhaled CO2 renders normocapnia difficult to restore even with maximal respiratory effort, hence the respiratory controller's 'ain't fresh, so won't breathe' modus operandi. This 'wisdom of the body' with a principled decision to tolerate hypercapnia when faced with prohibitive ventilatory or gas exchange limitations rather than striving for untenable normocapnia at all costs is analogous to the notion of permissive hypercapnia in critical care, a clinical strategy to minimize the risks of ventilator-induced lung injury in patients receiving mechanical ventilation.

The threshold for sensing airflow resistance during tidal breathing rises in old age: implications for elderly patients with obstructive airways diseases

OBJECTIVE

to determine whether the ability of elderly subjects to detect a rise in airflow resistance is attenuated in old age, and to measure the magnitude and variability of such a change.

METHODS

we studied 124 healthy adults aged 20-86 years. Progressive external airflow resistance loading was used to measure the inspiratory and expiratory load detection thresholds (LDTs) during tidal breathing at rest.

RESULTS

the mean inspiratory LDT rose from 4.00 (3.06 SD) kPa.s/L in the 20-39 age group to 6.51 (6.20) in the 40-64 age group (NS) and 29.10 (13.58) in the 65 + age group (P < 0.00001). The inspiratory LDT was significantly correlated with age, mainly due to the higher thresholds in people over the age of 65 (r = 0.7860, P < 0.00001), but did not correlate with age-corrected forced vital capacity or respiratory rate. Expiratory LDT values and correlations were very similar. Day-to-day variability in LDTs tended to be higher in older subjects.

CONCLUSION

the threshold for detecting external resistive loads during tidal breathing rises in old age. This appears to be a consequence of ageing processes rather than pathology, and might be a manifestation of a fall in proprioceptive acuity in elderly people. This finding has clinical implications for the self-management of asthma in old age. There is a need to conduct a similar study in patients with airways disease.

Assessment of physiological capacities of elite athletes & respiratory limitations to exercise performance

Physiological assessment of athletes is an important process for the characterization of the athlete, monitoring progress and the trained state or 'level of preparedness' of an athlete, as well as aiding the process of training program design. Interestingly, the majority of physiological assessments performed on athletes can also be performed on children with disease, and therefore clinicians can learn a great deal about physiology and assessment of patient populations through the examination of the physiological responses of elite athletes. This review describes typical physiological responses of elite athletes to tests of aerobic and anaerobic metabolism and provides a specific focus upon respiratory limitations to exercise performance. Typical responses of elite athletes are described to provide the scientist and clinician with a perspective of the upper range of physiological capacities of elite athletes.

[Descriptors of breathlessness in Mexican Spanish]

OBJECTIVE

Breathlessness is the most common symptom of cardiovascular or pulmonary disease. The term encompasses a wide range of descriptors used by patients, however. Identifying those descriptors can be useful for analyzing symptoms and understanding how they arise. The aim of this study was to characterize the descriptors of breathlessness used in Mexican Spanish and to consider their association with various states of respiratory distress and cardiovascular or pulmonary disease.

MATERIAL AND METHODS

A questionnaire was based on 21 descriptors of breathlessness, some of which had no equivalents in English. The subjects included 15 healthy individuals during a cardiopulmonary stress test, 13 healthy subjects after a carbon dioxide rebreathing procedure, and 10 healthy women during pregnancy. We also included 16 patients with confirmed heart disease in stable condition, 15 patients during exacerbation of asthma, 20 with stable chronic obstructive pulmonary disease, and 15 with diffuse interstitial lung disease also in stable condition. Descriptors of breathlessness were then grouped based on the results of cluster analysis.

RESULTS

Seven clusters of phrasal descriptors were identified as possibly representative of types of dyspnea. These clusters of descriptors were categorized as follows: agitation, suffocation, smothering, inhalation, exhalation, panting, and rapidity. Associations between types of dyspnea and the groups of participants were identified based on how frequently they used the terms.

CONCLUSIONS

At least 7 clusters or groups of descriptors of breathlessness were identified as equivalent to 7 types of dyspnea; some items have no equivalent in English. Healthy subjects with respiratory distress or certain groups of patients with cardiovascular or pulmonary disease are associated with certain types of dyspnea.

The tendency to altered perception of airflow resistance in aged subjects might be due mainly to a reduction in diaphragmatic proprioception

Elderly patients with asthma, particularly those above the age of 80 years, appear less able to detect early worsening of their airflow resistance and hence might not take 'rescue' bronchodilator medication promptly. No consistent explanation for the observation has been posited. We hypothesize that deterioration in the sensitivity and accuracy of inspiratory (mainly diaphragmatic) proprioception is a plausible mechanism. This contention is supported by observations that indicate the central role of afferent phrenic nerve fibres arising from mechanoreceptors in diaphragmatic muscle and entheses in the ability to sense changes in intrathoracic pressure and volume. Other sensory afferent sources appear less important in this context because the ability to detect intrathoracic pressure and volume changes is preserved in patients with heart-lung transplants (parenchymal and airway denervation), topically anaesthetized upper airways and spinal cord transection below C4 (intact phrenic function) but not cord transection above C2 (phrenic function absent) if the airways are simultaneously anaesthetized. Further support for the hypothesis comes from demonstration of reduced integrated proprioceptive function in older subjects, such as increased postural sway, reduced ability to judge changes in joint position and slower recovery from eye and hand perturbation. In the context of asthma, the detection of a change in airflow resistance depends mainly on the subconscious detection of a mismatch between the inspiratory effort and the volume change achieved; the resulting discrepancy between length (volume) and tension (muscular effort) is felt as a sensation of obstructed breathing, resulting in greater effort to breath and conscious actions such as self-medication. Our hypothesis proposes that a reduced ability to detect accurately the volume change during tidal breathing delays the sensing of the obstruction in older subjects.

Managing dyspnea and cough

Dyspnea, like pain, is a subjective experience that incorporates physical elements and affective components. Management of breathlessness in patients with cancer requires expertise that includes an understanding and assessment of the multidimensional components of the symptom, knowledge of the pathophysiologic mechanisms and clinical syndromes that are common in cancer, and familiarity with the indications and limitations of the available therapeutic approaches. Relief of breathlessness should be the goal of treatment at all stages of cancer. Good control of dyspnea will improve the patient's quality of life.

Proportional assist ventilation may improve exercise performance in severe chronic obstructive pulmonary disease

PURPOSE

Exercise tolerance is impaired in chronic obstructive pulmonary disease (COPD), in part because of a reduction in ventilatory capacity and excessive dyspnea experienced. The authors reasoned that proportional assist ventilation (PAV), a ventilator mode in which the level of support varies proportionately with patient effort, could be used during exercise to assist ventilation. The purpose of this study was to evaluate the efficacy of PAV to improve exercise endurance and related physiologic parameters in COPD.

METHODS

In 8 patients (age = 62.8 years mean, +/- 6.9 standard deviation) with severe COPD (forced expiratory volume in 1 second = 0.70 +/- 0.21 L) flow, volume, dyspnea, leg fatigue, arterial blood gases, and gas exchange were measured during constant workrate exercise (37 +/- 18 watts; i.e., 80% previously determined maximum oxygen consumption). Crossover exercise trials were performed in random order: while spontaneously breathing through the experimental circuit without assistance (control trial) and with PAV (using 9.8 +/- 2.1 cm H2O/L and 3.3 +/- 1.0 cm H2O/L/sec of volume assist and flow assist, respectively).

RESULTS

The application of PAV during exercise was well tolerated by each subject. Compared with the control measurement at equivalent time during exercise, PAV improved breathing pattern and arterial blood gases while dyspnea was reduced. Consequently, there was a significant increase in exercise duration with PAV (323 +/- 245 seconds during the control trial compared with 507 +/- 334 seconds with PAV, P = 0.02).

CONCLUSIONS

Proportional assist ventilation can improve performance during constant workrate exercise in severe COPD.

Hypoxic ventilatory response and breathlessness following hypocapnic and isocapnic hyperventilation

STUDY OBJECTIVES

To investigate the etiology of posthyperventilation (post-HV) hypoxemia following voluntary hyperventilation (VHV), we examined the effects of hypocapnic (hypo-CO2) and isocapnic (iso-CO2) VHV on the hypoxic ventilatory response (O2-response) and on the sensation of breathlessness during the O2-response.

METHODS

O2-responses and visual analog scale (VAS) scores for estimating breathlessness in 10 normal subjects during the O2-response under iso-CO2 conditions and under hypo-CO2 conditions immediately following voluntary maximal HV of 3 min duration were examined.

RESULTS

Although there was no significant difference in the post-HV ventilation levels following hypo-CO2 vs iso-CO2 VHV, the VAS scores at the start of the O2-response following hypo-CO2 VHV (30.2+/-24.2 mm) were significantly higher (p<0.05) than the VAS scores at the start of the O2-response following iso-CO2 VHV (13.7+/-8.4 mm). However, VHV did not have a significant effect on the O2-response at 2 min after the VHV when the arterial O2 saturation (SaO2) was below 90%. The nonsteady-state hypo-CO2 induced by VHV greatly attenuated the O2-response below 90% SaO2 and VAS scores at 70% SaO2.

CONCLUSIONS

Elevated VAS scores immediately following the hypo-CO2 VHV, which might be independent of actual breathing levels, and the attenuation of the O2-response following the hypo-CO2 VHV were not due to input from lung and chest wall mechanoreceptors induced by the hyperpnea itself, but rather to the hypo-CO2 induced by hyperpnea.

Assessment and management of dyspnoea

The terms 'dyspnoea' or 'breathlessness' refer to an individual's subjective awareness of discomfort related to the act of breathing. Elevations in CO2 above normal levels have been shown to cause breathlessness although it is unlikely to be the sole cause of breathlessness in a clinical setting. Several studies suggest that supplemental O2 during exercise will diminish the sensation of breathlessness although not all work has confirmed this finding. Much about the role of gas exchange in dyspnogenesis remains controversial. Phrenic blockade can abolish dyspnoea in response to breath-holding, while work in quadriplegics suggests that the intercostal muscles are not involved. A separate and direct pathway from the respiratory centre to the sensory cortex has also be implicated. Threshold discrimination has established that patients with chronic airflow limitation (CAL) have a blunted response to the addition of resistive loads to breathing, while category scaling methods (e.g. the Borg scale) have added descriptive terms to these physiological measures. Questionnaires often appear limited by their subjectivity and lack of correlation with physiological changes, but remain a useful tool in the clinical setting. In regard to therapy of dyspnoea high fat diets have a theoretical advantage in the CAL group but are generally not well tolerated. Resistive training devices and exercise training in CAL have been widely researched but in general, measures of lung remain unaltered and many of the studies would suggest that they have little, if any, inpact on functional status. Beta-agonists have been widely shown to be useful in CAL patients, despite the fact that bronchodilatation is not always demonstrable. Anticholinergics have be shown to be effective bronchodilators, but whether there is an improvement in dyspnoea above that expected from improvement in lung function is unclear. Animal studies and work in normal individuals would suggest that methylxanthines have a theoretical role in CAL possibly by increasing diaphragmatic muscle strength and decaying fatigue, but toxicity and lack of clear benefit in this group suggest that they should not be used as monotherapy. There is little evidence to support the use of opioids in chronic CAL although their role in the acute dyspnoea of end-stage CAL remains defined. The use of benzodiazepines has also been disappointing. Bullectomy remains widely accepted in clinical practice. New techniques such as 'reduction surgery' for diffuse emphysema are showing promise, although still in need of further testing and validation.

A comparison of pulse oximetry and respiratory rate in patient screening

OBJECTIVE

To examine how well respiratory rate correlates with arterial oxygen saturation status as measured by pulse oximetry, and determine whether respiratory rate measurements detect oxygen desaturation reliably.

METHODS

Respiratory rate (RR) and oxygen saturation (SaO2) were measured prospectively on 12,096 consecutive adult emergency department triage patients at a university medical center. Respiratory rate was measured by counting ausculated breath sounds for 1 min. Pulse oximetry was used to measure SaO2. Measurements were analysed by age (with one group for 18-19 year olds, groups for every 10 yr from age 20 to age 60, and groups for every 5 yr for subsequent ages). Pearson correlation coefficients were calculated for each age group as well as the weighted average coefficient. Cases having oxygen saturation below 90% were examined to determine how frequently they exhibited increased RR (increased RRs were defined as any rate in the upper five percentile by age.

RESULTS

Correlation coefficients ranged from 0.379 to -0.465 with a weighted mean of -0.160. Coefficients for ages 18 through 70 years (representing 10,740 patients) all had magnitude < 0.252. Overall, only 33% of subjects with oxygen saturation below 90% exhibited increased RR.

CONCLUSIONS

Respiratory rate measurements correlate poorly with oxygen saturation measurements and do not screen reliably for desaturation. Patients with low SaO2 do not usually exhibit increased RR. Similarly, increased RR is unlikely to reflect desaturation.

Management of dyspnea and cough in patients with cancer

The understanding and treatment of dyspnea in the cancer patient are where the science of pain management was 15 or 20 years ago. Very few studies have examined the pathophysiologic mechanisms that cause dyspnea in cancer patients, and few investigators have evaluated therapeutic strategies to control dyspnea in this patient group. The optimal therapy for dyspnea is treatment of the underlying cause. When this is not possible, opioids and phenothiazines provide effective symptomatic relief in most cases, but many unanswered questions remain. Are these the optimal drugs, and what are their optimal doses? What are the effects of chronic dosing? Which is the best route of administration? How serious are the risks of respiratory depression? A clear consensus supports the aggressive treatment of pain in terminally ill cancer patients, even if death is hastened as an unintended consequence. No such position has yet been reached in the management of dyspnea in the same population. As a result, dyspnea is addressed only very late in the course of the disease, perhaps reducing the patient's quality of life and function at earlier stages and resulting in a very small "therapeutic window" in the terminal phase. Clearly, a need exists for more research to determine the most effective management of this common and very distressing symptom.

Dyspnea sensation and chemical control of breathing in adult twins

To examine possible genetic influence on the sensation of dyspnea and on load compensation, we conducted a twin study using healthy adult pairs (10 monozygotes, MZ, and 9 dizygotes, DZ). The ventilatory response to progressive hypercapnia (HCVR) was examined under three different conditions: hyperoxia (PETO2 > 150 mm Hg), hypoxia (PETO2 maintained at 50 to 55 mm Hg), and hyperoxia with an inspiratory flow-resistive load (17 mm H2O/L/s), with simultaneous assessment of the dyspnea sensation by visual analog scale (VAS). Although the VDZ/VMZ ratio (VMZ and VDZ are within-pair variances in MZ and DZ, respectively) for the slope value of the minute ventilation-PETCO2 regression line was not different from 1 in hyperoxia either with or without an inspiratory load, it was significantly larger than 1 in hypoxia (F = 5.17, p < 0.05), suggesting that a genetic influence on HCVR existed only in the presence of hypoxia. During 3% CO2 inhalation, the VDZ/VMZ ratio for the tidal volume (VT) was larger than 1 in hyperoxic HCVR with loading (F = 7.89, p < 0.01), and that for respiratory frequency (f) was larger than 1 only in hypoxic HCVR (F = 3.59, p < 0.05). At a PETCO2 of 55 mm Hg, the VT ratio was larger than 1 under all conditions (F = 5.91, p < 0.05; F = 6.99, p < 0.05; F = 3.75, p < 0.05; respectively), and the f ratio was significantly larger than 1 again only in hypoxic HCVR (F = 3.48, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Dyspnea, anxiety, and depression in chronic respiratory impairment

In order to examine the relationship of dyspnea to anxiety and depression, the authors rated dyspnea using several methods in 50 patients with chronic respiratory impairment. Anxiety and depression were measured by the Symptom Checklist-90 and the Symptom Questionnaire. Results varied with the method of assessing dyspnea. Physician-rated dyspnea was significantly associated with patients' self-ratings of breathlessness as well as with pulmonary function tests, but not with any of the self-rating scales of emotions. Self-rated breathlessness was significantly associated with self-rated depression. In multiple regression analyses, depression was predictive of breathlessness. When the sample was limited to patients with chronic obstructive pulmonary disease, the results remained the same. The patients were significantly more depressed and anxious than matched family practice patients. In the study of the complex relationship of dyspnea to physical and emotional factors, it is desirable to use more than one measure of dyspnea because the results depend in part on the method of assessment.

Dyspnea-limited response in chronic obstructive pulmonary disease: reduced unsupported arm activities

Dyspnea, the sensation of uncomfortable breathing, is the primary activity-limiting symptom leading to reduced functional ability in chronic obstructive pulmonary disease (COPD). Patients with severe COPD report a marked increase in the sensation of dyspnea with routine tasks that require arm use, especially activities necessitating unsupported arm elevation. Dyspnea is associated with alterations in respiratory muscle function, such as an increase in muscle force requirement, a reduction in respiratory muscle strength and endurance, and an increase in the recruitment of the rib cage and accessory muscles. Unsupported arm exercise (UAE) further compromises respiratory muscle capacity for ventilation because it requires the muscles' concomitant recruitment in the maintenance of chest wall stabilization. This article presents respiratory muscle mechanisms leading to reduced UAE, methods of measuring unsupported arm endurance, and treatment strategies to improve unsupported arm activity endurance in patients with COPD.

The pattern of respiratory muscle recruitment during pursed-lip breathing

Data from the present study indicate a change in the pattern of chest wall muscle recruitment and improved ventilation with pursed-lip breathing (PLB) in COPD. Pursed lip breathing led to increased rib cage and accessory muscle recruitment during inspiration and expiration, increased abdominal muscle recruitment during expiration, decreased duty cycle of the inspiratory muscles and respiratory rate, and improved SaO2. In addition, PLB resulted in no change in pressure across the diaphragm and a less fatiguing breathing pattern of the diaphragm. Changes in chest wall muscle recruitment and respiratory temporal parameters concomitant with the increased SaO2 indicate a mechanism of improving ventilation with PLB while protecting the diaphragm from fatigue in COPD. Alterations in the pattern of respiratory muscle recruitment with PLB may be associated also with the amelioration of dyspnea. Further investigation is necessary to explore the relationship between the pattern of respiratory muscle recruitment during PLB and dyspnea.

Correlations between dyspnea, diaphragm and sternomastoid recruitment during inspiratory resistance breathing in normal subjects

The purpose of this study was to determine the relationship between recruitment of the DI and SM muscles measured as EMG signal amplitudes, the pattern of respiratory muscle recruitment measured with inductive plethysmography, and the intensity of the sensation of dyspnea, measured with 100 mm VAS. Eighteen normal subjects between the ages of 33 and 47 breathed under two conditions: normal controlled breathing and breathing against an inspiratory resistance at 60 percent of their maximal inspiratory pressure (MIP). The PM, RR, duty cycle (TI/TTOT, and VT were held constant. During resistance breathing, VAS dyspnea was increased when EMG-DI decreased; EMG-SM increased in association with the sensation of dyspnea. During inspiratory resistance breathing, dyspnea markedly increased and rib cage and accessory muscle recruitment was the predominant pattern of breathing. These data suggest that dyspnea may be associated with the recruitment of the accessory respiratory muscles rather than the recruitment of the diaphragm.

An approach to dyspnea in cancer patients

There are many potential causes of dyspnea in the patient with cancer. Ultimately, a sense of increased respiratory effort is common to all of these diverse situations. An organized approach to dyspnea in the cancer patient is presented based on psychophysical principles, and treatment modalities are suggested.