Minute ventilation/carbon dioxide production in patients with dysfunctional breathing

Quantification of breathing irregularity for the diagnosis of dysfunctional breathing using proportional tidal volume variation: a cross-sectional, retrospective real-world study

OBJECTIVES

To develop a statistical approach that provides a quantitative index measuring the magnitude of the irregularity of the breathing response to exercise for the diagnosis of dysfunctional breathing.

DESIGN

Cross-sectional, retrospective, real-world study.

SETTING

Single-centre study.

PARTICIPANTS

A population of 209 patients investigated with cardiopulmonary exercise testing in our institution for unexplained or disproportionate exertional dyspnoea between January and July 2022.

PRIMARY AND SECONDARY OUTCOME MEASURES

A novel statistical approach providing a quantitative index-proportional tidal volume variation (PTVV)-was developed to measure the magnitude of the irregularity of the breathing response to exercise.

RESULTS

PTVV provided a reliable statistical readout for the objective assessment of DB with a prediction accuracy of 78% (95% CI: 72 to 83%). The prevalence of DB in the investigated population was high with more than half of the patients affected by moderate-to-severe DB.

CONCLUSIONS

PTVV can easily be implemented in the clinical routine. Our study suggests a possible further simplification for the diagnosis of DB with two objective criteria including PTVV and one single criterion for hyperventilation.

Dysfunctional Breathing in Children: A Literature Review

Dysfunctional breathing (DB) describes a respiratory condition that is mainly characterized by abnormal breathing patterns, affecting both children and adults, often leading to intermittent or chronic complaints and influencing physiological, psychological, and social aspects. Some symptoms include breathlessness; dizziness; palpitations; and anxiety, while its classification lies in breathing pattern disorders and upper airway involvement. Its prevalence among the pediatric population varies with a female overrepresentation, while the existence of comorbidities in DB, such as asthma, gastro-esophageal reflux, nasal diseases, and anxiety/depression, frequently leads to misdiagnosis or underdiagnosis and complicates therapeutic approaches. The basic diagnostic tools involve a detailed history, physical examination, and procedures such as structured light plethysmography, cardiopulmonary exercise testing, and laryngoscopy when a laryngeal obstruction is present. The management of DB presumes a multidimensional approach encompassing breathing retraining, disease-specific advice through speech and language therapy in the presence of laryngeal obstruction, psychotherapy for fostering self-efficacy, and surgical therapy in a structural abnormality. The current review was developed to provide a summary of classifications of DB and epidemiological data concerning the pediatric population, comorbidities, diagnostic tools, and therapeutic approaches to enhance the comprehension and management of DB in children.

Assessing breathing effort by barometric whole-body plethysmography and its relationship with prognosis in client-owned cats with respiratory distress

BACKGROUND

Cats in respiratory distress have limited tolerance for manipulation, hindering clinical monitoring. Minute volume (MV) can be utilized to rate dyspnea in humans, but its relationship with respiratory distress in cats remains poorly investigated.

HYPOTHESIS

Cats with respiratory distress will show higher MV per kg body weight (MV/BW) than normal cats, and the MV/BW increase will correlate with survival.

ANIMALS

Fifty-two cats with respiratory distress from lung parenchymal disease, pleural space disease, lower airway obstruction (LAO), or upper airway obstruction were recruited since 2014.

METHODS

This is a prospective observational study. Study cats were placed in a transparent chamber, allowing clinicians to easily observe their breathing status and record ventilation using barometric whole-body plethysmography (BWBP). Ventilatory variables of the 52 cats were compared with those of 14 historic control cats. Follow-up data, including disease category, clinical outcomes, and survival, were prospectively collected.

RESULTS

Cats in respiratory distress demonstrated significantly higher MV/BW (397 mL/kg; range, 158-1240) than normal cats (269 mL/kg; range, 168-389; P < .001). Among the etiologies, cats with LAO, parenchymal, and pleural space disease exhibited higher-than-normal MV/BW trends. A cutoff value of 373 mL/kg (1.4-fold increase) indicated abnormally increased breathing efforts (sensitivity, 67%; specificity, 93%). MV/BW was independently associated with increased cardiorespiratory mortality in cats with respiratory distress (adjusted hazard ratio 1.17, 95% confidence interval [CI] 1.02-1.35; P = .03).

CONCLUSIONS AND CLINICAL IMPORTANCE

Breathing efforts in cats can be noninvasively quantified using BWBP. Measurement of MV/BW could serve as a prognostic index for monitoring cats experiencing respiratory distress.

Cardiorespiratory abnormalities in ICU survivors of COVID-19 with postacute sequelae of SARS-CoV-2 infection are unrelated to invasive mechanical ventilation

Postacute sequelae of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (PASC) often leads to exertional intolerance and reduced exercise capacity, particularly in individuals previously admitted to an intensive care unit (ICU). However, the impact of invasive mechanical ventilation (IMV) on PASC-associated cardiorespiratory abnormalities during exercise remains poorly understood. This single-center, cross-sectional study aimed to gather knowledge on this topic. Fifty-two patients with PASC recruited ∼6 mo after ICU discharge were clustered based on their need for IMV (PASC + IMV, n = 27) or noninvasive support therapy (PASC + NIS, n = 25). Patients underwent pulmonary function and cardiopulmonary exercise testing (CPX) and were compared with a reference group (CONTROL, n = 19) comprising individuals of both sexes with similar age, comorbidities, and physical activity levels but without a history of COVID-19 illness. Individuals with PASC, irrespective of support therapy, presented with higher rates of cardiorespiratory abnormalities than CONTROL, especially dysfunctional breathing patterns, dynamic hyperinflation, reduced oxygen uptake and oxygen pulse, and blunted heart rate recovery (all P < 0.05). Only the rate of abnormal oxygen pulse was greater among PASC + IMV group than PASC + NIS group (P = 0.05). Mean estimates for all CPX variables were comparable between PASC + IMV and PASC + NIS groups (all P > 0.05). These findings indicate significant involvement of both central and peripheral factors, leading to exertional intolerance in individuals with PASC previously admitted to the ICU, regardless of their need for IMV.NEW & NOTEWORTHY We found cardiorespiratory abnormalities in ICU survivors of severe-to-critical COVID-19 with PASC to be independent of IMV need. Overall, both group of patients experienced dysfunctional breathing patterns, dynamic hyperinflation, lower oxygen uptake and oxygen pulse, and blunted heart rate responses to CPX. PASC seems to impact exertional tolerance and exercise capacity due to ventilatory inefficiency, impaired aerobic metabolism, and potential systolic and autonomic dysfunction, all of these irrespective of support therapy during ICU stay.

[The role of the cardiopulmonary exercise test and pulmonary rehabilitation in long COVID-19]

INTRODUCTION

Long COVID refers to persistent symptoms, lasting more than 4 weeks after acute SARS-CoV-2 infection, even though the infection itself has been successfully controlled and remedied. Patient complaints are diverse, and the underlying physiopathological mechanisms are not well understood. Dyspnea and muscle fatigue are among the most commonly reported symptoms.

STATE OF THE ART

Cardiopulmonary exercise test (CPET) has been recognized as a useful tool in investigation of unexplained dyspnea. In patients with chronic lung disease, pulmonary rehabilitation is a program designed to counteract dyspnea, to increase exercise capacity and to improve quality of life.

PERSPECTIVES

Publications on CPET and pulmonary rehabilitation are needed in order to deepen comprehension and enhance management of long-COVID-19.

CONCLUSIONS

CPET reports have shown that symptoms persisting in the aftermath of acute SARS-CoV-2 infection may be related to deconditioning, a common occurrence after ICU stay, to cardiac dysautonomia subsequent to critical infections and, finally, to dysfunctional breathing subsequent to mild infections. These findings justify pulmonary rehabilitation, which has proven to be effective regardless of the severity of the initial infection, not only immediately after hospital discharge, but also at later points in time.

Reference Values of Cardiopulmonary Exercise Test Parameters in the Contemporary Paediatric Population

BACKGROUND

The evaluation of health status by cardiopulmonary exercise test (CPET) has shown increasing interest in the paediatric population. Our group recently established reference Z-score values for paediatric cycle ergometer VO_2max, applicable to normal and extreme weights, from a cohort of 1141 healthy children. There are currently no validated reference values for the other CPET parameters in the paediatric population. This study aimed to establish, from the same cohort, reference Z-score values for the main paediatric cycle ergometer CPET parameters, apart from VO_2max.

RESULTS

In this cross-sectional study, 909 healthy children aged 5-18 years old underwent a CPET. Linear, quadratic, and polynomial mathematical regression equations were applied to identify the best CPET parameters Z-scores, according to anthropometric parameters (sex, age, height, weight, and BMI). This study provided Z-scores for maximal CPET parameters (heart rate, respiratory exchange ratio, workload, and oxygen pulse), submaximal CPET parameters (ventilatory anaerobic threshold, VE/VCO₂ slope, and oxygen uptake efficiency slope), and maximum ventilatory CPET parameters (tidal volume, respiratory rate, breathing reserve, and ventilatory equivalent for CO₂ and O₂).

CONCLUSIONS

This study defined paediatric reference Z-score values for the main cycle ergometer CPET parameters, in addition to the existing reference values for VO_2max, applicable to children of normal and extreme weights. Providing Z-scores for CPET parameters in the paediatric population should be useful in the follow-up of children with various chronic diseases. Thus, new paediatric research fields are opening up, such as prognostic studies and clinical trials using cardiopulmonary fitness outcomes. Trial registration NCT04876209-Registered 6 May 2021-Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT04876209 .

Dysfunctional breathing symptoms, functional impact and quality of life in patients with long COVID-19: a prospective case series

BACKGROUND

Dysfunctional breathing is increasingly recognised after SARS-CoV-2 infection, but the associated symptoms, functional impact and quality of life have not been systematically studied.

METHODS

This study describes a prospective case series of 48 patients with dysfunctional breathing based on compatible symptoms and an abnormal breathing pattern during cardiopulmonary exercise testing. Patients with underlying disease that could explain these symptoms were excluded. Median time from COVID-19 to evaluation was 212 (IQR 121) days. Self-administered questionnaires, including the Nijmegen questionnaire, Short-Form (36) Health Survey (SF-36), Hospital Anxiety and Depression Scale, modified Medical Research Council scale, post-COVID-19 Functional Scale, and specific long COVID symptoms, were the outcome measures.

RESULTS

On average, mean V'O₂ was preserved. Pulmonary function tests were within limits of normality. Hyperventilation, periodic deep sighs/erratic breathing and mixed types of dysfunctional breathing were diagnosed in 20.8%, 47.1% and 33.3% of patients, respectively. After dyspnoea, the five most frequent symptoms using the Nijmegen scale with a cut-off of ≥3 were faster/deeper breathing (75.6%), palpitations (63.8%), sighs (48.7%), unable to breathe deeply (46.3%) and yawning (46.2%). Median Nijmegen and Hospital Anxiety and Depression Scale scores were 28 (IQR 20) and 16.5 (IQR 11), respectively. SF-36 scores were lower than the reference value.

CONCLUSIONS

Long COVID patients with dysfunctional breathing have a high burden of symptoms, functional impact and a low quality of life, despite no or negligible organic damage.

ERS International Congress 2022: highlights from the Respiratory Clinical Care and Physiology Assembly

It is a challenge to keep abreast of all the clinical and scientific advances in the field of respiratory medicine. This article contains an overview of the laboratory-based science, clinical trials and qualitative research that were presented during the 2022 European Respiratory Society International Congress within the sessions from the five groups of Assembly 1 (Respiratory Clinical Care and Physiology). Selected presentations are summarised from a wide range of topics: clinical problems, rehabilitation and chronic care, general practice and primary care, mobile/electronic health (m-health/e-health), clinical respiratory physiology, exercise and functional imaging.

Exercise responses and mental health symptoms in COVID-19 survivors with dyspnoea

Objectives

Dyspnoea is a common persistent symptom post-coronavirus disease 2019 (COVID-19) illness. However, the mechanisms underlying dyspnoea in the post-COVID-19 syndrome remain unclear. The aim of our study was to examine dyspnoea quality and intensity, burden of mental health symptoms, and differences in exercise responses in people with and without persistent dyspnoea following COVID-19.

Methods

49 participants with mild-to-critical COVID-19 were included in this cross-sectional study 4 months after acute illness. Between-group comparisons were made in those with and without persistent dyspnoea (defined as modified Medical Research Council dyspnoea score ≥1). Participants completed standardised dyspnoea and mental health symptom questionnaires, pulmonary function tests, and incremental cardiopulmonary exercise testing.

Results

Exertional dyspnoea intensity and unpleasantness were increased in the dyspnoea group. The dyspnoea group described dyspnoea qualities of suffocating and tightness at peak exercise (p<0.05). Ventilatory equivalent for carbon dioxide (V'_E/V'_CO2) nadir was higher (32±5 versus 28±3, p<0.001) and anaerobic threshold was lower (41±12 versus 49±11% predicted maximum oxygen uptake, p=0.04) in the dyspnoea group, indicating ventilatory inefficiency and deconditioning in this group. The dyspnoea group experienced greater symptoms of anxiety, depression and post-traumatic stress (all p<0.05). A subset of participants demonstrated gas-exchange and breathing pattern abnormalities suggestive of dysfunctional breathing.

Conclusions

People with persistent dyspnoea following COVID-19 experience a specific dyspnoea quality phenotype. Dyspnoea post-COVID-19 is related to abnormal pulmonary gas exchange and deconditioning and is linked to increased symptoms of anxiety, depression and post-traumatic stress.

The utility of cardiopulmonary exercise testing in athletes and physically active individuals with or without persistent symptoms after COVID-19

Introduction

Cardiopulmonary exercise testing (CPET) may capture potential impacts of COVID-19 during exercise. We described CPET data on athletes and physically active individuals with or without cardiorespiratory persistent symptoms.

Methods

Participants' assessment included medical history and physical examination, cardiac troponin T, resting electrocardiogram, spirometry and CPET. Persistent symptoms were defined as fatigue, dyspnea, chest pain, dizziness, tachycardia, and exertional intolerance persisting >2 months after COVID-19 diagnosis.

Results

A total of 46 participants were included; sixteen (34.8%) were asymptomatic and thirty participants (65.2%) reported persistent symptoms, with fatigue and dyspnea being the most reported ones (43.5 and 28.1%). There were a higher proportion of symptomatic participants with abnormal data for slope of pulmonary ventilation to carbon dioxide production (VE/VCO2 slope; p<0.001), end-tidal carbon dioxide pressure at rest (PETCO2 rest; p=0.007), PETCO2 max (p=0.009), and dysfunctional breathing (p=0.023) vs. asymptomatic ones. Rates of abnormalities in other CPET variables were comparable between asymptomatic and symptomatic participants. When assessing only elite and highly trained athletes, differences in the rate of abnormal findings between asymptomatic and symptomatic participants were no longer statistically significant, except for expiratory air flow-to-percent of tidal volume ratio (EFL/VT) (more frequent among asymptomatic participants) and dysfunctional breathing (p=0.008).

Discussion

A considerable proportion of consecutive athletes and physically active individuals presented with abnormalities on CPET after COVID-19, even those who had had no persistent cardiorespiratory symptomatology. However, the lack of control parameters (e.g., pre-infection data) or reference values for athletic populations preclude stablishing the causality between COVID-19 infection and CPET abnormalities as well as the clinical significance of these findings.

Cardiopulmonary exercise testing applied to respiratory medicine: Myths and facts

Cardiopulmonary exercise testing (CPET) remains poorly understood and, consequently, largely underused in respiratory medicine. In addition to a widespread lack of knowledge of integrative physiology, several tenets of CPET interpretation have relevant controversies and limitations which should be appropriately recognized. With the intent to provide a roadmap for the pulmonologist to realistically calibrate their expectations towards CPET, a collection of deeply entrenched beliefs is critically discussed. They include a) the actual role of CPET in uncovering the cause(s) of dyspnoea of unknown origin, b) peak O₂ uptake as the key metric of cardiorespiratory capacity, c) the value of low lactate ("anaerobic") threshold to differentiate cardiocirculatory from respiratory causes of exercise limitation, d) the challenges of interpreting heart rate-based indexes of cardiovascular performance, e) the meaning of peak breathing reserve in dyspnoeic patients, f) the merits and drawbacks of measuring operating lung volumes during exercise, g) how best interpret the metrics of gas exchange inefficiency such as the ventilation-CO₂ output relationship, h) when (and why) measurements of arterial blood gases are required, and i) the advantages of recording submaximal dyspnoea "quantity" and "quality". Based on a conceptual framework that links exertional dyspnoea to "excessive" and/or "restrained" breathing, I outline the approaches to CPET performance and interpretation that proved clinically more helpful in each of these scenarios. CPET to answer clinically relevant questions in pulmonology is a largely uncharted research field: I, therefore, finalize by highlighting some lines of inquiry to improve its diagnostic and prognostic yield.

Excess ventilation and exertional dyspnoea in heart failure and pulmonary hypertension

Increased ventilation relative to metabolic demands, indicating alveolar hyperventilation and/or increased physiological dead space (excess ventilation), is a key cause of exertional dyspnoea. Excess ventilation has assumed a prominent role in the functional assessment of patients with heart failure (HF) with reduced (HFrEF) or preserved (HFpEF) ejection fraction, pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH). We herein provide the key pieces of information to the caring physician to 1) gain unique insights into the seeds of patients' shortness of breath and 2) develop a rationale for therapeutically lessening excess ventilation to mitigate this distressing symptom. Reduced bulk oxygen transfer induced by cardiac output limitation and/or right ventricle-pulmonary arterial uncoupling increase neurochemical afferent stimulation and (largely chemo-) receptor sensitivity, leading to alveolar hyperventilation in HFrEF, PAH and small-vessel, distal CTEPH. As such, interventions geared to improve central haemodynamics and/or reduce chemosensitivity have been particularly effective in lessening their excess ventilation. In contrast, 1) high filling pressures in HFpEF and 2) impaired lung perfusion leading to ventilation/perfusion mismatch in proximal CTEPH conspire to increase physiological dead space. Accordingly, 1) decreasing pulmonary capillary pressures and 2) mechanically unclogging larger pulmonary vessels (pulmonary endarterectomy and balloon pulmonary angioplasty) have been associated with larger decrements in excess ventilation. Exercise training has a strong beneficial effect across diseases. Addressing some major unanswered questions on the link of excess ventilation with exertional dyspnoea under the modulating influence of pharmacological and nonpharmacological interventions might prove instrumental to alleviate the devastating consequences of these prevalent diseases.

Diagnostic tests and subtypes of dysfunctional breathing in children with unexplained exertional dyspnea

BACKGROUND

Inappropriate hyperventilation during exercise may be a specific subtype of dysfunctional breathing (DB).

OBJECTIVE

To assess whether Nijmegen questionnaire and hyperventilation provocation test (HVPT) are able to differentiate inappropriate hyperventilation from other DB subtypes in children with unexplained exertional dyspnea, and normal spirometry and echocardiography.

METHODS

The results were compared between a subgroup of 25 children with inappropriate hyperventilation (increased V'E/V'CO₂ slope during a cardiopulmonary exercise test (CPET)) and an age and sex matched subgroup of 25 children with DB without hyperventilation (median age, 13.5 years; 36 girls). Anxiety was evaluated using State-Trait Anxiety Inventory for Children questionnaire.

RESULTS

All children were normocapnic (at rest and peak exercise) and the children with hyperventilation had lower tidal volume/vital capacity on peak exercise (shallow breathing). The Nijmegen score correlated positively with dyspnea during the CPET and the HVPT (p = 0.001 and 0.010, respectively) and with anxiety score (p = 0.022). The proportion of children with a positive Nijmegen score (≥19) did not differ between hyperventilation (13/25) and no hyperventilation (14/25) groups (p = 0.777). Fractional end-tidal CO₂ (FETCO₂ ) at 5-min recovery of the HVPT was < 90% baseline in all children (25/25) of both subgroups. Likewise, there was no significant difference between the two subgroups for other indices of HVPT (FETCO₂ at 3-min recovery and symptoms during the test).

CONCLUSION

The validity of the Nijmegen questionnaire and the HVPT to discriminate specific subtypes of dysfunctional breathing, as well as the relevance of the inappropriate hyperventilation subtype itself may both be questioned.

Analysis of Influencing Factors for Exercise Ventilation Efficiency of COPD Patients

Dynamic pulmonary hyperinflation and abnormal air exchange are the primary causes of the exercise limitation of chronic obstructive pulmonary disease (COPD) patients. During exercise, COPD sufferers' lungs are dynamically hyperinflated. Increased inefficient ventilation reduces ventilation efficiency and causes a mismatch between ventilation volume and blood flow. The ventilatory equivalent for CO2 (VeqCO2) is a physiological parameter that can be measured using cardiopulmonary exercise testing. Therefore, the aim of this exploratory study was to perform cardiopulmonary exercise testing on people with COPD, investigate the impact of static pulmonary function on ventilation efficiency under the exercise state, and screen the predictive indicators of ventilation efficiency. Subject. The aim of this study was to look at the factors that influence the exercise ventilation efficiency of people with COPD. Method. A total of 76 people with COPD were recruited during the stable period. Age, gender, body height, body mass, and other basic information were recorded. The body mass index (BMI) was determined, and forced vital capacity (FVC), forced expiratory volume in one second (FEV1), residual volume/total lung capacity (RV/TLC), diffusing capacity of the lung for carbon monoxide (DLCO), and DLCO divided by the alveolar volume (DLCO/VA) were measured. The ventilatory equivalent for carbon dioxide (VE/VCO2) under the rest state (EqCO2rest), anaerobic threshold (EqCO2at), and maximum exercise state (EqCO2 max) were calculated to investigate the influencing factors for ventilation efficiency of people with COPD. Results. FEV1% was negatively correlated with EqCO2rest (r = -0.277, P value <0.05); FEV1/FVC % was negatively correlated with EqCO2rest and EqCO2at (r = -0.311, -0.287, P value <0.05); DLCO% was negatively correlated with EqCO2rest, EqCO2at, and EqCO2max (r = -0.408, -0.462, and -0.285, P value <0.05); DLCO/VA% was negatively correlated with EqCO2rest, EqCO2at, and EqCO2max (r = -0.390, -0.392, and -0.245, P value <0.05); RV/TLC was positively correlated with EqCO2rest and EqCO2at (r = 0.289, 0.258, P-value <0.05). The prediction equation from the multivariable regression analysis equation was Y = 40.04-0.075X (Y = EqCO2, X = DLCO/VA%). Conclusions. As the degree of ventilatory obstruction increased, the ventilation efficiency of the stable people with COPD under the exercise state showed a progressive decrease; the ventilation efficiency of the people with COPD decreased significantly under the maximum exercise state, and the ventilation capacity and diffusion capacity were the significant factors that affected the exercise ventilation efficiency. The diffusion function may predict the maximum ventilation efficiency and enable primary hospitals without exercise test equipment in developing countries to predict and screen patients at risk for current exercise based on limited information.

Dysfunctional breathing diagnosed by cardiopulmonary exercise testing in ‘long COVID’ patients with persistent dyspnoea

Background

‘Long COVID’-associated dyspnoea may persist for months after SARS-CoV-2 infection. Among the causes of persistent dyspnoea, dysfunctional breathing (DB), defined as an erratic or inappropriate ventilation at rest or exercise, has been observed, but little is known about its occurrence and pathophysiology among individuals with ‘long COVID’. We aimed to describe the occurrence and identify clinical predictors of DB among patients following SARS-CoV-2 infection.

Methods

Cardiopulmonary exercise testing (CPET) was performed in 51 SARS-CoV-2 patients (median age, 64 years (IQR, 15)); male, 66.7%) living with ‘long COVID’ and persistent dyspnoea. CPET was classified into three dominant patterns: respiratory limitation with gas exchange abnormalities (RL); normal CPET or O₂ delivery/utilisation impairment (D); and DB. Non-parametric and χ² tests were applied to analyse the association between CPET dominant patterns and demographics, pulmonary function tests and SARS-CoV-2 severity.

Results

Among 51 patients, DB mostly without hyperventilation was found in 29.4% (n=15), RL in 54.9% (n=28) and D in 15.7% (n=8). When compared with RL individuals, patients with DB were younger, had significantly less severe initial infection, a better transfer capacity for carbon monoxide (median 85% (IQR, 28)), higher oxygen consumption (22.9 mL/min/kg (IQR, 5.5)), a better ventilatory efficiency slope (31.6 (IQR, 12.8)), and a higher SpO₂ (95% (IQR, 3)).

Conclusions

Our findings suggest that DB without hyperventilation could be an important pathophysiological mechanism of disabling dyspnoea in younger outpatients following SARS-CoV-2 infection, which appears to be a feature of COVID-19 not described in other viral diseases.