Validation of Constant Work Rate Cycling Endurance Time for Use in Chronic Obstructive Pulmonary Disease Clinical Trials

Rationale: A COPD Foundation working group sought to identify measures of exercise endurance, a meaningful aspect of physical functioning in everyday life among patients with chronic obstructive pulmonary disease (COPD) that is not fully accepted in regulatory decision making, hampering drug development. Objectives: To demonstrate, as we previously asserted (Casaburi COPD 2022;9:252), that constant work rate cycling endurance time is an appropriate exercise endurance measure in patients with COPD. Methods: To validate this assertion, we assembled an integrated database of endurance time responses, including 8 bronchodilator (2,166 subjects) and 15 exercise training (3,488 subjects) studies (Casaburi COPD 2022;9:520). Results: Construct validity was demonstrated: 1) peak physiologic and perceptual responses were similar for constant work rate and incremental cycling; 2) after bronchodilator therapy, there were greater increases in endurance time in patients with more severe airflow limitation; 3) after exercise training, endurance time increases were similar across airflow limitation severities; and 4) there were correlations between changes in endurance time and changes in mechanistically related physiologic and perceptual variables. Test-retest reliability was demonstrated, with consistency of changes in endurance time at two time points after the intervention. Responsiveness was confirmed, with significant increases in endurance time after active (but not placebo) bronchodilator therapy, with greater increases seen with more severe airflow limitation and after exercise training. On the basis of regression analysis using multiple anchor variables, the minimum important difference for endurance time increase is estimated to be approximately 1 minute. Conclusions: Constant work rate cycling endurance time is a valid exercise endurance measure in COPD, suitable for contributing to the evaluation of treatment benefit supporting regulatory decision making and evidence-based therapeutic recommendations.

Differentially co-expressed myofibre transcripts associated with abnormal myofibre proportion in chronic obstructive pulmonary disease

BACKGROUND

Skeletal muscle dysfunction is a common extrapulmonary manifestation of chronic obstructive pulmonary disease (COPD). Alterations in skeletal muscle myosin heavy chain expression, with reduced type I and increased type II myosin heavy chain expression, are associated with COPD severity when studied in largely male cohorts. The objectives of this study were (1) to define an abnormal myofibre proportion phenotype in both males and females with COPD and (2) to identify transcripts and transcriptional networks associated with abnormal myofibre proportion in COPD.

METHODS

Forty-six participants with COPD were assessed for body composition, strength, endurance and pulmonary function. Skeletal muscle biopsies from the vastus lateralis were assayed for fibre-type distribution and cross-sectional area via immunofluorescence microscopy and RNA-sequenced to generate transcriptome-wide gene expression data. Sex-stratified k-means clustering of type I and IIx/IIax fibre proportions was used to define abnormal myofibre proportion in participants with COPD and contrasted with previously defined criteria. Single transcripts and weighted co-expression network analysis modules were tested for correlation with the abnormal myofibre proportion phenotype.

RESULTS

Abnormal myofibre proportion was defined in males with COPD (n = 29) as <18% type I and/or >22% type IIx/IIax fibres and in females with COPD (n = 17) as <36% type I and/or >12% type IIx/IIax fibres. Half of the participants with COPD were classified as having an abnormal myofibre proportion. Participants with COPD and an abnormal myofibre proportion had lower median handgrip strength (26.1 vs. 34.0 kg, P = 0.022), 6-min walk distance (300 vs. 353 m, P = 0.039) and forced expiratory volume in 1 s-to-forced vital capacity ratio (0.42 vs. 0.48, P = 0.041) compared with participants with COPD and normal myofibre proportions. Twenty-nine transcripts were associated with abnormal myofibre proportions in participants with COPD, with the upregulated NEB, TPM1 and TPM2 genes having the largest fold differences. Co-expression network analysis revealed that two transcript modules were significantly positively associated with the presence of abnormal myofibre proportions. One of these co-expression modules contained genes classically associated with muscle atrophy, as well as transcripts associated with both type I and type II myofibres, and was enriched for genetic loci associated with bone mineral density.

CONCLUSIONS

Our findings indicate that there are significant transcriptional alterations associated with abnormal myofibre proportions in participants with COPD. Transcripts canonically associated with both type I and type IIa fibres were enriched in a co-expression network associated with abnormal myofibre proportion, suggesting altered transcriptional regulation across multiple fibre types.

The Dynamics of Locomotor Neuromuscular Fatigue during Ramp-Incremental Cycling to Intolerance

INTRODUCTION

Traditional neuromuscular fatigue assessments are not task-specific and are unable to characterize neuromuscular performance decline during dynamic whole-body exercise. This study used interleaved maximal isokinetic cycling efforts to characterize the dynamics of the decline in neuromuscular performance during ramp-incremental (RI) cycle ergometry exercise to intolerance.

METHODS

Eleven young healthy participants (10 male/1 female) performed two RI cycle ergometry exercise tests to intolerance: [1] RI-exercise with peak isokinetic power (Piso) at 80 rev·min-1 measured at baseline and immediately at intolerance from a maximal ~6 s effort; [2] RI-exercise where additional Piso measurements were interleaved every 90 s to characterize the decline in neuromuscular performance during the RI-test. Muscle excitation was measured using EMG during all Piso assessments, and pulmonary gas exchange was measured throughout.

RESULTS

Baseline Piso was 832 ± 140 W and RI-exercise reduced Piso to 349 ± 96 W at intolerance (p = 0.001), which was not different from flywheel power at intolerance (303 ± 96 W; p = 0.292). There was no reduction in Piso between baseline cycling and gas exchange threshold (GET; baseline Piso vs. mean Piso below GET: 828 ± 146 vs. 815 ± 149 W; p = 1.00). Piso fell progressively above GET until intolerance (Piso every 90 s above GET: 759 ± 139; 684 ± 141; 535 ± 144; 374 ± 117 W; each p < 0.05 vs. baseline and mean Piso below GET). Peak muscle excitation (EMG) was also reduced only above GET (73 ± 14 % of baseline, at intolerance; p < 0.05). However, the reduction in peak Piso preceded the reduction in peak muscle excitation.

CONCLUSIONS

The dynamics of the decline in neuromuscular performance (reduction in Piso and EMG) during RI-exercise are consistent with known intensity-dependent metabolic and traditional pre-post neuromuscular fatigue responses to discrete bouts of constant-power exercise.

Lack of effect of an in-line filter on cardiopulmonary exercise testing variables in healthy subjects

PURPOSE

Pathogen transmission during cardio-pulmonary exercise testing (CPET) is caused by carrier aerosols generated during respiration.

METHODS

Ten healthy volunteers (age range: 34 ± 15; 4 females) were recruited to see if the physiological reactions to ramp-incremental CPET on a cycle ergometer were affected using an in-line filter placed between the mouthpiece and the flow sensor. The tests were in random order with or without an in-line bacterial/viral spirometer filter. The work rate aligned, time interpolated 10 s bin data were compared throughout the exercise period.

RESULTS

From rest to peak exercise, filter use increased only minute ventilation ([Formula: see text]E) (Δ[Formula: see text]E = 1.56 ± 0.70 L/min, P < 0.001) and tidal volume (VT) (ΔVT = 0.10 ± 0.11 L, P = 0.014). Over the entire test, the slope of the residuals for [Formula: see text]CO2 was positive (0.035 ± 0.041 (ΔL/L), P = 0.027). During a ramp-incremental CPET in healthy subjects, an in-line filter increased [Formula: see text]E and VT but not metabolic rate.

CONCLUSION

In conclusion, using an in-line filter is feasible, does not affect appreciably the physiological variables, and may mitigate risk of aerosol dispersion during CPET.

Current definitions of the breathing cycle in alveolar breath-by-breath gas exchange analysis

Identification of the breathing cycle forms the basis of any breath-by-breath gas exchange analysis. Classically, the breathing cycle is defined as the time interval between the beginning of two consecutive inspiration phases. Based on this definition, several research groups have developed algorithms designed to estimate the volume and rate of gas transferred across the alveolar membrane ("alveolar gas exchange"); however, most algorithms require measurement of lung volume at the beginning of the ith breath (VLi-1; i.e., the end-expiratory lung volume of the preceding ith breath). The main limitation of these algorithms is that direct measurement of VLi-1 is challenging and often unavailable. Two solutions avoid the requirement to measure VLi-1 by redefining the breathing cycle. One method defines the breathing cycle as the time between two equal fractional concentrations of lung expired oxygen (Fo2) (or carbon dioxide; Fco2), typically in the alveolar phase, whereas the other uses the time between equal values of the Fo2/Fn2 (or Fco2/Fn2) ratios [i.e., the ratio of fractional concentrations of lung expired O2 (or CO2) and nitrogen (N2)]. Thus, these methods identify the breathing cycle by analyzing the gas fraction traces rather than the gas flow signal. In this review, we define the traditional approach and two alternative definitions of the human breathing cycle and present the rationale for redefining this term. We also explore the strengths and limitations of the available approaches and provide implications for future studies.

Exercise Physiology and Cardiopulmonary Exercise Testing

Aerobic, or endurance, exercise is an energy requiring process supported primarily by energy from oxidative adenosine triphosphate synthesis. The consumption of oxygen and production of carbon dioxide in muscle cells are dynamically linked to oxygen uptake (V̇O2) and carbon dioxide output (V̇CO2) at the lung by integrated functions of cardiovascular, pulmonary, hematologic, and neurohumoral systems. Maximum oxygen uptake (V̇O2max) is the standard expression of aerobic capacity and a predictor of outcomes in diverse populations. While commonly limited in young fit individuals by the capacity to deliver oxygen to exercising muscle, (V̇O2max) may become limited by impairment within any of the multiple systems supporting cellular or atmospheric gas exchange. In the range of available power outputs, endurance exercise can be partitioned into different intensity domains representing distinct metabolic profiles and tolerances for sustained activity. Estimates of both V̇O2max and the lactate threshold, which marks the upper limit of moderate-intensity exercise, can be determined from measures of gas exchange from respired breath during whole-body exercise. Cardiopulmonary exercise testing (CPET) includes measurement of V̇O2 and V̇CO2 along with heart rate and other variables reflecting cardiac and pulmonary responses to exercise. Clinical CPET is conducted for persons with known medical conditions to quantify impairment, contribute to prognostic assessments, and help discriminate among proximal causes of symptoms or limitations for an individual. CPET is also conducted in persons without known disease as part of the diagnostic evaluation of unexplained symptoms. Although CPET quantifies a limited sample of the complex functions and interactions underlying exercise performance, both its specific and global findings are uniquely valuable. Some specific findings can aid in individualized diagnosis and treatment decisions. At the same time, CPET provides a holistic summary of an individual's exercise function, including effects not only of the primary diagnosis, but also of secondary and coexisting conditions.

A wireless patch for the monitoring of C-reactive protein in sweat

The quantification of protein biomarkers in blood at picomolar-level sensitivity requires labour-intensive incubation and washing steps. Sensing proteins in sweat, which would allow for point-of-care monitoring, is hindered by the typically large interpersonal and intrapersonal variations in its composition. Here we report the design and performance of a wearable and wireless patch for the real-time electrochemical detection of the inflammatory biomarker C-reactive (CRP) protein in sweat. The device integrates iontophoretic sweat extraction, microfluidic channels for sweat sampling and for reagent routing and replacement, and a graphene-based sensor array for quantifying CRP (via an electrode functionalized with anti-CRP capture antibodies-conjugated gold nanoparticles), ionic strength, pH and temperature for the real-time calibration of the CRP sensor. In patients with chronic obstructive pulmonary disease, with active or past infections or who had heart failure, the elevated concentrations of CRP measured via the patch correlated well with the protein's levels in serum. Wearable biosensors for the real-time sensitive analysis of inflammatory proteins in sweat may facilitate the management of chronic diseases.

Distinguishing science from pseudoscience in commercial respiratory interventions: an evidence-based guide for health and exercise professionals

Respiratory function has become a global health priority. Not only is chronic respiratory disease a leading cause of worldwide morbidity and mortality, but the COVID-19 pandemic has heightened attention on respiratory health and the means of enhancing it. Subsequently, and inevitably, the respiratory system has become a target of the multi-trillion-dollar health and wellness industry. Numerous commercial, respiratory-related interventions are now coupled to therapeutic and/or ergogenic claims that vary in their plausibility: from the reasonable to the absurd. Moreover, legitimate and illegitimate claims are often conflated in a wellness space that lacks regulation. The abundance of interventions, the range of potential therapeutic targets in the respiratory system, and the wealth of research that varies in quality, all confound the ability for health and exercise professionals to make informed risk-to-benefit assessments with their patients and clients. This review focuses on numerous commercial interventions that purport to improve respiratory health, including nasal dilators, nasal breathing, and systematized breathing interventions (such as pursed-lips breathing), respiratory muscle training, canned oxygen, nutritional supplements, and inhaled L-menthol. For each intervention we describe the premise, examine the plausibility, and systematically contrast commercial claims against the published literature. The overarching aim is to assist health and exercise professionals to distinguish science from pseudoscience and make pragmatic and safe risk-to-benefit decisions.

Critical Power and Respiratory Compensation Point Are Not Equivalent in Patients with COPD

INTRODUCTION

Several studies report that pulmonary oxygen uptake (V̇O 2 ) at the respiratory compensation point (RCP) is equivalent to the V̇O 2 at critical power (CP), suggesting that the variables can be used interchangeably to demarcate the threshold between heavy and severe intensity domains. However, if RCP is a valid surrogate for CP, their values should correspond even when assessed in patients with chronic obstructive pulmonary disease (COPD) in whom the "normal" mechanisms linking CP and RCP are impeded. The aim of this study was to compare V̇O 2 at CP with V̇O 2 at RCP in patients with COPD.

METHODS

Twenty-two COPD patients (14 male/8 female; forced expiratory volume in 1 s, 46% ± 17% pred) performed ramp-incremental cycle ergometry to intolerance (5-10 W·min -1 ) for the determination of gas exchange threshold (GET) and RCP. CP was calculated from the asymptote of the hyperbolic power-duration relationship from 3-5 constant-power exercise tests to intolerance. CP was validated with a 20-min constant-power ride.

RESULTS

GET was identified in 20 of 22 patients at a V̇O 2 of 0.93 ± 0.18 L·min -1 (75% ± 13% V̇O 2peak ), whereas RCP was identified in just 3 of 22 patients at a V̇O 2 of 1.40 ± 0.39 L·min -1 (85% ± 2% V̇O 2peak ). All patients completed constant-power trials with no difference in peak physiological responses relative to ramp-incremental exercise ( P > 0.05). CP was 46 ± 22 W, which elicited a V̇O 2 of 1.04 ± 0.29 L·min -1 (90% ± 9% V̇O 2peak ) during the validation ride. The difference in V̇O 2 at 15 and 20 min of the validation ride was 0.00 ± 0.04 L, which was not different from a hypothesized mean of 0 ( P = 0.856), thereby indicating a V̇O 2 steady state.

CONCLUSIONS

In COPD patients, who present with cardiopulmonary and/or respiratory-mechanical dysfunction, CP can be determined in the absence of RCP. Accordingly, CP and RCP are not equivalent in this group.

Associations Between Muscle Weakness and Clinical Outcomes in Current and Former Smokers

Introduction

Smokers with chronic obstructive pulmonary disease (COPD) are at increased risk of muscle weakness. There are limited data describing weakness in smokers with normal spirometry and preserved ratio-impaired spirometry (PRISm), 2 subgroups at risk of respiratory symptom burden and activity limitations. In this study, we evaluated the associations of 2 weakness measures, sit-to-stand (STS) and handgrip strength (HGS), with clinical outcomes in smokers with COPD, normal spirometry, and PRISm.

Methods

We evaluated 1972 current and former smokers from the COPD Genetic Epidemiology (COPDGene®) cohort with STS and HGS measurements at their 10-year study visit. Multivariable regression modeling was used to assess associations between weakness measures and the 6-minute walk distance (6MWD) test, the St George's Respiratory Questionnaire (SGRQ), the Short-Form-36 (SF-36), severe exacerbations, and prospective mortality, reported as standardized coefficients (β), odds ratios (ORs), or hazard ratios (HRs).

Results

Compared with HGS, STS was more strongly associated with the 6MWD (β=0.45, p<0.001 versus. β=0.25, p<0.001), SGRQ (β=-0.24, p<0.001 versus β=-0.18, p<0.001), SF-36 Physical Functioning (β=0.36, p<0.001 versus β=0.25, p<0.001), severe exacerbations (OR 0.95, p=0.04 versus OR 0.97, p=0.01), and prospective mortality (HR 0.83, p=0.001 versus HR 0.94, p=0.03). Correlations remained after stratification by spirometric subgroups. Compared with males, females had larger magnitude effect sizes between STS and clinical outcomes.

Conclusions

STS and HGS are easy to perform weakness measures that provide important information about functional performance, health-related quality of life, severe exacerbations, and survival in smokers, regardless of spirometric subgroup. This iterates the importance of screening current and former smokers for weakness in the outpatient setting.

Physical Activity, Muscle Oxidative Capacity, and Coronary Artery Calcium in Smokers with and without COPD

Introduction

Severe chronic obstructive pulmonary disease (COPD) is partly characterized by diminished skeletal muscle oxidative capacity and concurrent dyslipidemia. It is unknown whether such metabolic derangements increase the risk of cardiovascular disease. This study explored associations among physical activity (PA), muscle oxidative capacity, and coronary artery calcium (CAC) in COPDGene participants.

Methods

Data from current and former smokers with COPD (n = 75) and normal spirometry (n = 70) were retrospectively analyzed. Physical activity was measured for seven days using triaxial accelerometry (steps/day and vector magnitude units [VMU]) along with the aggregate of self-reported PA amount and PA difficulty using the PROactive D-PPAC instrument. Muscle oxidative capacity (k) was assessed via near-infrared spectroscopy, and CAC was assessed via chest computerized tomography.

Results

Relative to controls, COPD patients exhibited higher CAC (median [IQR], 31 [0–431] vs 264 [40–799] HU; p = 0.003), lower k (mean ± SD = 1.66 ± 0.48 vs 1.25 ± 0.37 min⁻¹; p < 0.001), and lower D-PPAC total score (65.2 ± 9.9 vs 58.8 ± 13.2; p = 0.003). Multivariate analysis—adjusting for age, sex, race, diabetes, disease severity, hyperlipidemia, smoking status, and hypertension—revealed a significant negative association between CAC and D-PPAC total score (β, −0.05; p = 0.013), driven primarily by D-PPAC difficulty score (β, −0.03; p = 0.026). A 1 unit increase in D-PPAC total score was associated with a 5% lower CAC (p = 0.013). There was no association between CAC and either k, steps/day, VMU, or D-PPAC amount.

Conclusion

Patients with COPD and concomitantly elevated CAC exhibit greater perceptions of difficulty when performing daily activities. This may have implications for exercise adherence and risk of overall physical decline.

Near-infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O2 diffusion capacity in humans

The final steps of the O2 cascade during exercise depend on the product of the microvascular-to-intramyocyteP O 2 ${P}_{{{\rm{O}}}_{\rm{2}}}$ difference and muscle O2 diffusing capacity (D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ ). Non-invasive methods to determineD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ in humans are currently unavailable. Muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) recovery rate constant (k), measured by near-infrared spectroscopy (NIRS) using intermittent arterial occlusions, is associated with muscle oxidative capacity in vivo. We reasoned that k would be limited byD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ when muscle oxygenation is low (kLOW ), and hypothesized that: (i) k in well oxygenated muscle (kHIGH ) is associated with maximal O2 flux in fibre bundles; and (ii) ∆k (kHIGH  - kLOW ) is associated with capillary density (CD). Vastus lateralis k was measured in 12 participants using NIRS after moderate exercise. The timing and duration of arterial occlusions were manipulated to maintain tissue saturation index within a 10% range either below (LOW) or above (HIGH) half-maximal desaturation, assessed during sustained arterial occlusion. Maximal O2 flux in phosphorylating state was 37.7 ± 10.6 pmol s-1  mg-1 (∼5.8 ml min-1  100 g-1 ). CD ranged 348 to 586 mm-2 . kHIGH was greater than kLOW (3.15 ± 0.45 vs. 1.56 ± 0.79 min-1 , P < 0.001). Maximal O2 flux was correlated with kHIGH (r = 0.80, P = 0.002) but not kLOW (r = -0.10, P = 0.755). Δk ranged -0.26 to -2.55 min-1 , and correlated with CD (r = -0.68, P = 0.015). mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ k reflects muscle oxidative capacity only in well oxygenated muscle. ∆k, the difference in k between well and poorly oxygenated muscle, was associated with CD, a mediator ofD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ . Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O2 diffusing capacity. KEY POINTS: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O2 availability, for comparison with biopsy variables. The mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ recovery rate constant in HIGH O2 availability was hypothesized to reflect muscle oxidative capacity (kHIGH ) and the difference in k between HIGH and LOW O2 availability (∆k) was hypothesized to reflect muscle O2 diffusing capacity. kHIGH was correlated with phosphorylating oxidative capacity of permeabilized muscle fibre bundles (r = 0.80). ∆k was negatively correlated with capillary density (r = -0.68) of biopsy samples. NIRS provides non-invasive means of assessing both muscle oxidative and oxygen diffusing capacity in vivo.

A Conceptual Framework for Use of Increased Endurance Time During Constant Work Rate Cycle Ergometry as a Patient-Focused Meaningful Outcome in COPD Clinical Trials

The Chronic Lung Disease Biomarker and Clinical Outcome Assessment Qualification Consortium (CBQC) evaluates the potential of biomarkers and outcome measures as drug development tools. Exercise endurance is an objective indicator of treatment benefit, closely related to daily physical function. Therefore, it is an ideal candidate for an outcome for drug development trials. Unfortunately, no exercise endurance measure is qualified by regulatory authorities for use in trials of chronic obstructive pulmonary disease (COPD) and no approved COPD therapies have claims of improving exercise endurance. Consequently, it has been challenging for developers to consider this outcome when designing clinical trials for new therapies. Endurance time during constant work rate cycle ergometry (CWRCE), performed on an electronically braked stationary cycle ergometer, provides an exercise endurance measure under standardized conditions. Baseline individualized work rate for each participant is set using an incremental test. During CWRCE the patient is encouraged to continue exercising for as long as possible. Although not required, physiological and sensory responses (e.g., pulmonary ventilation, heart rate, dyspnea ratings) are frequently collected to support interpretation of endurance time changes. Exercise tolerance limit is reached when the individual is limited by symptoms, unable to maintain pedaling cadence or unable to continue safely. At exercise cessation, exercise duration is recorded. An CWRCE endurance time increase from the pre-treatment baseline is proposed as a key efficacy endpoint in clinical trials. In COPD, improved exercise endurance has a direct relationship to the experience of physical functioning in daily life, which is a patient-centered, meaningful benefit.

Heart Rate Variability on 10-Second Electrocardiogram and Risk of Acute Exacerbation of COPD: A Secondary Analysis of the BLOCK COPD Trial

Introduction

Autonomic dysfunction is common in chronic obstructive pulmonary disease (COPD), and worse autonomic function may be a marker of risk for acute exacerbations of COPD (AECOPD). Heart rate variability (HRV) is a measure of autonomic function. Our objective was to test whether lower (worse) HRV is a risk factor for AECOPD.

Methods

We measured standard deviation of normal RR intervals (SDNN) and root mean square of successive RR interval differences (RMSSD) on 10-second electrocardiograms (ECGs) performed at screening and day 42 in participants in the Beta Blockers for the Prevention of Acute Exacerbations of COPD trial ( BLOCK-COPD), a placebo-controlled trial of metoprolol for prevention of AECOPD. We used Cox-proportional hazards models to test if these HRV measures were associated with risk of any AECOPD, and separately, hospitalized AECOPD. We tested associations using baseline HRV measures and incorporating HRV measures from day 42 as a time-varying covariate. We also tested for interactions with metoprolol assignment.

Results

Of 532 trial participants, 529 (forced expiratory volume in 1 second [FEV1 ]41 ± 16.3 % predicted) were included in this analysis. We did not find a significant association between HRV measures and risk of AECOPD when all participants were analyzed together. There was a significant interaction between RMSSD and assignment to metoprolol on time to first hospitalized AECOPD; in the placebo group greater RMSSD was associated with a lower risk of hospitalized AECOPD (adjusted hazard ratio0.71, 95% confidence interval: 0.52 to 0.96, per 10 ms increase) but there was no association in the metoprolol group.

Conclusions

Autonomic dysfunction as measured by HRV may be a risk factor for AECOPD. Future studies should analyze longer HRV recordings and their performance in broader samples of people with COPD, including those on beta-blockers.

Chronotropic index during 6-minute walk and acute respiratory events in COPDGene

BACKGROUND

Lower heart rate (HR) increases during exercise and slower HR recovery (HRR) after exercise are markers of worse autonomic function that may be associated with risk of acute respiratory events (ARE).

METHODS

Data from 6-min walk testing (6MWT) in COPDGene were used to calculate the chronotropic index (CI) [(HR immediately post 6MWT - resting HR)/((220 - age) - resting HR)] and HRR at 1 min after 6MWT completion. We used zero-inflated negative binomial regression to test associations of CI and HRR with rates of any ARE (requiring steroids and/or antibiotics) and severe ARE (requiring emergency department visit or hospitalization), among all participants and in spirometry subgroups (normal, chronic obstructive pulmonary disease [COPD], and preserved ratio with impaired spirometry).

RESULTS

Among 4,484 participants, mean follow-up time was 4.1 years, and 1,966 had COPD. Among all participants, CI-6MWT was not associated with rate of any ARE [adjusted incidence rate ratio (aIRR) 0.98 (0.95-1.01)], but higher CI-6MWT was associated with lower rate of severe ARE [0.95 (0.92-0.99)]. Higher HRR was associated with a lower rate of both any ARE [0.97 (0.95-0.99)] and severe ARE [0.95 (0.92-0.98)]. Results were similar in the COPD spirometry subgroup.

CONCLUSION

Heart rate measures derived from 6MWT tests may have utility in predicting risk of acute respiratory events and COPD exacerbations.

A randomized, crossover, placebo controlled, double blind trial of the effects of tiotropium-olodaterol on neuromuscular performance during exercise in COPD

Exercise intolerance in COPD is associated with dyspnea, reduced inspiratory capacity (IC) and occurs with a neuromuscular "power reserve" i.e. an acute ability to increase isokinetic locomotor power. This power reserve is associated with resting FEV₁/FVC suggesting that treatments to target pulmonary function may protect neuromuscular performance and extend whole-body exercise in COPD. We, therefore, tested whether combination long-acting β-agonist and muscarinic antagonist bronchodilator therapy (LAMA+LABA; Stiolto Respimat®) would ameliorate the decline in neuromuscular performance and increase endurance time during constant power cycling at 80% peak incremental power. Fourteen COPD patients (4 female; 64[58,72] years; FEV₁ 67[56,75]% predicted; median[25^th,75^th percentile]), participated in a randomized, placebo-controlled cross-over trial (NCT02845752). Pulmonary function and cardiopulmonary exercise responses were assessed before and after 1 week of treatment, with 2 weeks washout between conditions. Performance fatigue was assessed using a ~4-second maximal isokinetic cycling effort at pre-exercise, isotime and intolerance. Isotime was the shorter exercise duration of the two treatment conditions. Significance was assessed using ANOVA with treatment as fixed factor and subject as random factor. FEV₁ was greater with LAMA+LABA vs. placebo (1.81[1.58,1.98] L vs 1.72[1.29,1.99] L; P=0.006), but IC at isotime, performance fatigue at isotime and constant power endurance time were not different between condition (each P>0.05). A modest (~95 mL) FEV₁ increase in following 1 week of combination LAMA+LABA treatment did not alleviate neuromuscular performance fatigue or enhance cycle exercise tolerance in mild to severe COPD patients with largely preserved "static" lung volumes.

Longitudinal association between muscle loss and mortality in ever-smokers

BACKGROUND

Body composition measures, specifically low weight or reduced muscle mass, are associated with mortality in chronic obstructive pulmonary disease (COPD), but the effect of longitudinal body composition changes is undefined.

RESEARCH QUESTION

Is the longitudinal loss of fat-free mass (FFM) associated with increased mortality including in those with initially normal or elevated body composition metrics?

STUDY DESIGN AND METHODS

Participants with complete data for at least one visit in the COPDGene (n=9,268) and ECLIPSE studies (1,760) were included and followed for 12 and 8 years, respectively. Pectoralis muscle area (PMA) was derived from thoracic CT scans and used as a proxy for FFM. A longitudinal mixed sub-model for PMA and a Cox proportional hazards sub-model for survival were fitted on a joint distribution using a shared random intercept parameter and Markov chain Monte Carlo parameter estimation.

RESULTS

Both cohorts demonstrated a left shifted distribution of baseline FFM, not reflected in BMI, and an increase in all-cause mortality risk associated with longitudinal loss of PMA. For each one cm2 PMA loss, mortality increased 3.1% (95% CI 2.4, 3.7, p<0.001) in COPDGene, and 2.4% (95% CI 0.9, 4.0, p<0.001) in ECLIPSE. Increased mortality risk was independent of enrollment values for BMI and disease severity (BODE index quartiles) and was significant even in participants with initially greater than average PMA.

INTERPRETATION

Longitudinal loss of PMA is associated with increased all-cause mortality, regardless of BMI or initial muscle mass. Consideration of novel screening tests and further research into mechanisms contributing to muscle decline may improve risk stratification and identify novel therapeutic targets in ever-smokers.

Objectively Measured Physical Activity in Patients with COPD: Recommendations from an International Task Force on Physical Activity

Physical activity (PA) is of key importance for health among healthy persons and individuals with chronic obstructive pulmonary disease (COPD). PA has multiple dimensions that can be assessed and quantified objectively using activity monitors. Moreover, as shown in the published literature, variable methodologies have been used to date to quantify PA among individuals with COPD, precluding clear comparisons of outcomes across studies. The present paper aims to provide a summary of the available literature for the rationale behind using objectively measured PA and proposes a standardized methodology for assessment, including standard operating procedures for future research. The present paper, therefore, describes the concept of PA, reports on the importance of PA, summarizes the dimensions of PA, provides a standard operating procedure on how to monitor PA using objective assessments, and describes the psychometric properties of objectively measured PA. The present international task force recommends implementation of the standard operating procedure for PA data collection and reporting in the future. This should further clarify the relationship between PA and clinical outcomes, test the impact of treatment interventions on PA in individuals with COPD, and successfully propose a PA endpoint for regulatory qualification in the future.

Bioenergetic Mechanisms Linking V˙O2 Kinetics and Exercise Tolerance

We hypothesize that the V˙O2 time constant (τV˙O2) determines exercise tolerance by defining the power output associated with a "critical threshold" of intramuscular metabolite accumulation (e.g., inorganic phosphate), above which muscle fatigue and work inefficiency are apparent. Thereafter, the V˙O2 "slow component" and its consequences (increased pulmonary, circulatory, and neuromuscular demands) determine performance limits.

Identifying a Heart Rate Recovery Criterion After a 6-Minute Walk Test in COPD

BACKGROUND

Slow heart rate recovery (HRR) after exercise is associated with autonomic dysfunction and increased mortality. What HRR criterion at 1-minute after a 6-minute walk test (6MWT) best defines pulmonary impairment?.

STUDY DESIGN AND METHODS

A total of 5008 phase 2 COPDGene (NCT00608764) participants with smoking history were included. A total of 2127 had COPD and, of these, 385 were followed-up 5-years later. Lung surgery, transplant, bronchiectasis, atrial fibrillation, heart failure and pacemakers were exclusionary. HR was measured from pulse oximetry at end-walk and after 1-min seated recovery. A receiver operator characteristic (ROC) identified optimal HRR cut-off. Generalized linear regression determined HRR association with spirometry, chest CT, symptoms and exacerbations.

RESULTS

HRR after 6MWT (bt/min) was categorized in quintiles: ≤5 (23.0% of participants), 6-10 (20.7%), 11-15 (18.9%), 16-22 (18.5%) and ≥23 (18.9%). Compared to HRR≤5, HRR≥11 was associated with (p<0.001): lower pre-walk HR and 1-min post HR; greater end-walk HR; greater 6MWD; greater FEV1%pred; lower airway wall area and wall thickness. HRR was positively associated with FEV1%pred and negatively associated with airway wall thickness. An optimal HRR ≤10 bt/min yielded an area under the ROC curve of 0.62 (95% CI 0.58-0.66) for identifying FEV1<30%pred. HRR≥11 bt/min was the lowest HRR associated with consistently less impairment in 6MWT, spirometry and CT variables. In COPD, HRR≤10 bt/min was associated with (p<0.001): ≥2 exacerbations in the previous year (OR=1.76[1.33-2.34]); CAT≥10 (OR=1.42[1.18-1.71]); mMRC≥2 (OR=1.42[1.19-1.69]); GOLD 4 (OR=1.98[1.44-2.73]) and GOLD D (OR=1.51[1.18-1.95]). HRR≤10 bt/min was predicted COPD exacerbations at 5-year follow-up (RR=1.83[1.07-3.12], P=0.027).

CONCLUSION

HRR≤10 bt/min after 6MWT in COPD is associated with more severe expiratory flow limitation, airway wall thickening, worse dyspnoea and quality of life, and future exacerbations, suggesting that an abnormal HRR≤10 bt/min after a 6MWT may be used in a comprehensive assessment in COPD for risk of severity, symptoms and future exacerbations.

Power Reserve at Intolerance in Ramp-Incremental Exercise Is Dependent on Incrementation Rate

INTRODUCTION

The mechanism(s) of exercise intolerance at V˙O2max remain poorly understood. In health, standard ramp-incremental (RI) exercise is limited by fatigue-induced reductions in maximum voluntary cycling power. Whether neuromuscular fatigue also limits exercise when the RI rate is slow and RI peak power at intolerance is lower than standard RI exercise, is unknown.

METHODS

In twelve healthy participants, maximal voluntary cycling power was measured during a short (~6 s) isokinetic effort at 80 rpm (Piso) at baseline and, using an instantaneous switch from cadence-independent to isokinetic cycling, immediately at the limit of RI exercise with RI rates of 50, 25, and 10 W·min-1 (RI-50, RI-25, and RI-10). Breath-by-breath pulmonary gas exchange was measured throughout.

RESULTS

Baseline Piso was not different among RI rates (analysis of variance; P > 0.05). Tolerable duration increased with decreasing RI rate (RI-50, 411 ± 58 s vs RI-25, 732 ± 93 s vs RI-10, 1531 ± 288 s; P < 0.05). At intolerance, V˙O2peak was not different among RI rates (analysis of variance; P > 0.05), but RI peak power decreased with RI rate (RI-50, 361 ± 48 W vs RI-25, 323 ± 39 W vs RI-10, 275 ± 38 W; P < 0.05). Piso at intolerance was 346 ± 43 W, 353 ± 45 W, and 392 ± 69 W for RI-50, RI-25, and RI-10, respectively (P < 0.05 for RI-10 vs RI-50 and RI-25). At intolerance, in RI-50 and RI-25, Piso was not different from RI peak power (P > 0.05), thus there was no "power reserve." In RI-10, Piso was greater than RI peak power at intolerance (P < 0.001), that is, there was a "power reserve."

CONCLUSIONS

In RI-50 and RI-25, the absence of a power reserve suggests the neuromuscular fatigue-induced reduction in Piso coincided with V˙O2max and limited the exercise. In RI-10, the power reserve suggests neuromuscular fatigue was insufficient to limit the exercise, and additional mechanisms contributed to intolerance at V˙O2max.

Dynamic airway function during exercise in COPD assessed via impulse oscillometry before and after inhaled bronchodilators

Assessing airway function during exercise provides useful information regarding mechanical properties of the airways and the extent of ventilatory limitation in COPD. The primary aim of this study was to use impulse oscillometry (IOS) to assess dynamic changes in airway impedance across a range of exercise intensities in patients with GOLD 1-4, before and after albuterol administration. A secondary aim was to assess the reproducibility of IOS measures during exercise. Fifteen patients with COPD (8 males/7 females; age = 66 ± 8 yr; prebronchodilator FEV₁ = 54.3 ± 23.6%Pred) performed incremental cycle ergometry before and 90 min after inhaled albuterol. Pulmonary ventilation and gas exchange were measured continuously, and IOS-derived indices of airway impedance were measured every 2 min immediately preceding inspiratory capacity maneuvers. Test-retest reproducibility of exercise IOS was assessed as mean difference between replicate tests in five healthy subjects (3 males/2 females). At rest and during incremental exercise, albuterol significantly increased airway reactance (X₅) and decreased airway resistance (R₅, R₅-R₂₀), impedance (Z₅), and end-expiratory lung volume (60% ± 12% vs. 58% ± 12% TLC, main effect P = 0.003). At peak exercise, there were moderate-to-strong associations between IOS variables and IC, and between IOS variables and concavity in the expiratory limb of the spontaneous flow-volume curve. Exercise IOS exhibited moderate reproducibility in healthy subjects which was strongest with R₅ (mean diff. = -0.01 ± 0.05 kPa/L/s; ICC = 0.68), R₅-R₂₀ (mean diff. = -0.004 ± 0.028 kPa/L/s; ICC = 0.65), and Z₅ (mean diff. = -0.006 ± 0.021 kPa/L/s; ICC = 0.69). In patients with COPD, exercise evoked increases in airway resistance and decreases in reactance that were ameliorated by inhaled bronchodilators. The technique of exercise IOS may aid in the clinical assessment of dynamic airway function during exercise.NEW & NOTEWORTHY This study provides a novel, mechanistic insight into dynamic airway function during exercise in COPD, before and after inhaled bronchodilators. The use of impulse oscillometry (IOS) to evaluate airway function is unique among exercise studies. We show strong correlations among IOS variables, dynamic hyperinflation, and shape-changes in the spontaneous expiratory flow-volume curve. This approach may aid in the clinical assessment of airway function during exercise.

Objectively measured physical activity as a COPD clinical trial outcome

BACKGROUND

Reduced physical activity is common in COPD and is associated with poor outcomes. Physical activity is therefore a worthy target for intervention in clinical trials, however, trials evaluating physical activity have used heterogeneous methodologies.

RESEARCH QUESTION

What is the available evidence on the efficacy and/or effectiveness of various interventions to enhance objectively measured physical activity in patients with COPD, taking into account minimal preferred methodological quality of physical activity assessment?

STUDY DESIGN AND METHODS

In this narrative review, the COPD Biomarker Qualification Consortium (CBQC) task force searched three scientific databases for articles that reported the effect of an intervention on objectively-measured physical activity in COPD. Based on scientific literature and expert consensus, only studies with ≥7 measurement days and ≥4 valid days of ≥8 hours of monitoring were included in the primary analysis.

RESULTS

37 of 110 (34%) identified studies fulfilled the criteria, investigating the efficacy and/or effectiveness of physical activity behavior change programs (n=7), mobile health or eHealth interventions (n=9), rehabilitative exercise (n=9), bronchodilation (n=6), lung volume reduction procedures (n=3) and other interventions (n=3). Results are generally variable, reflecting the large variation in study characteristics and outcomes. Few studies show an increase beyond the proposed minimal important change of 600-1100 daily steps, indicating that enhancing physical activity levels is a challenge.

INTERPRETATION

Only a third of clinical trials measuring objective physical activity in people with COPD fulfilled the pre-set criteria regarding physical activity assessment. Studies showed variable effects on physical activity even when investigating similar interventions.

Distinct glycolytic pathway regulation in liver, tumour and skeletal muscle of mice with cancer cachexia

Energetically inefficient inter-organ substrate shuttles are proposed contributors to cachexia-related weight loss. Here, we examined glycolytic pathway metabolites, enzyme activity and transport proteins in skeletal muscle, liver and tumours of mice with cachexia-related weight loss induced by colon-26 cancer cells. Skeletal muscle of cachexic mice had increased [L-lactate]/[pyruvate], LDH activity and lactate transporter MCT1. Cachexic livers also showed increased MCT1. This is consistent with the proposal that the rate of muscle-derived lactate shuttling to liver for use in gluconeogenesis is increased, that is, an increased Cori cycle flux in weight-losing cachexic mice. A second shuttle between liver and tumour may also contribute to disrupted energy balance and weight loss. We found increased high-affinity glucose transporter GLUT1 in tumours, suggesting active glucose uptake, tumour MCT1 detection and decreased intratumour [L-lactate]/[pyruvate], implying increased lactate efflux and/or intratumour lactate oxidation. Last, high [L-lactate]/[pyruvate] and MCT1 in cachexic muscle provides a potential muscle-derived lactate supply for the tumour (a 'reverse Warburg effect'), supporting tumour growth and consequent cachexia. Our findings suggest several substrate shuttles among liver, skeletal muscle and tumour contribute to metabolic disruption and weight loss. Therapies that aim to normalize dysregulated substrate shuttling among energy-regulating tissues may alleviate unintended weight loss in cancer cachexia. SIGNIFICANCE OF THE STUDY: Cachexia is a serious complication of cancer characterized by severe weight loss, muscle atrophy and frailty. Cachexia occurs in roughly half of all cancer patients, and in up to 80% of patients with advanced disease. Cachexia independently worsens patient prognosis, lowers treatment efficacy, increases hospitalization cost and length of stay, and accounts for 20-30% of cancer-related deaths. There are no effective treatments. Our findings suggest several substrate shuttles among liver, skeletal muscle and tumour contribute to metabolic disruption and weight loss in cancer cachexia. Identifying therapies that normalize dysregulated substrate shuttling among energy-regulating tissues may protect against cachexia-related weight loss.

Peripheral venous pressure changes during exercise are associated with adverse Fontan outcomes

OBJECTIVE

Elevated central venous pressure (CVP) plays an important role in the development of adverse Fontan outcomes. Peripheral venous pressure (PVP) has been validated as a surrogate for CVP in Fontan patients. We hypothesised that PVP in response to exercise will be associated with a greater prevalence of Fontan morbidity.

METHODS

Adult Fontan patients had cardiopulmonary exercise testing (CPET) with PVP monitoring in the upper extremity between 2015 and 2018. PVP at rest, during unloaded cycling and at peak exercise was compared between those with and without adverse Fontan outcomes including arrhythmia, unscheduled hospital admissions, heart failure requiring diuretics, need for reintervention and a composite outcome of the above morbidities, heart transplantation and death.

RESULTS

Forty-six patients with a mean age at CPET of 26.9±9.5 years. During exercise, PVP increased from 13.6±3.5 mm Hg at rest, to 16.5±3.9 mm Hg during unloaded cycling, to 23.0±5.5 mm Hg at peak exercise. Unloaded and peak PVP were more strongly associated than resting PVP with all adverse outcomes, except reintervention (composite outcome: resting PVP: OR 2.8, p=0.023; unloaded PVP: OR 6.1, p=0.001; peak PVP: OR 4.0, p<0.001). Cut-offs determined using ROC curve analysis had high specificity for the composite outcome (88% unloaded PVP ≥18 mm Hg; 89% peak PVP ≥25 mm Hg).

CONCLUSION

Higher PVP at unloaded and peak exercise was strongly associated with a higher prevalence of adverse Fontan outcomes. Minimally invasive PVP monitoring during CPET may serve as a useful tool for risk stratifying individuals with a Fontan.

The effect of long-acting dual bronchodilator therapy on exercise tolerance, dynamic hyperinflation, and dead space during constant work rate exercise in COPD

We investigated whether dual bronchodilator therapy (glycopyrrolate/formoterol fumarate; GFF; Bevespi Aerosphere) would increase exercise tolerance during a high-intensity constant work rate exercise test (CWRET) and the relative contributions of dead space ventilation (V_D/V_T) and dynamic hyperinflation (change in inspiratory capacity) to exercise limitation in chronic obstructive pulmonary disease (COPD). In all, 48 patients with COPD (62.9 ± 7.6 yrs; 33 male; GOLD spirometry stage 1/2/3/4, n = 2/35/11/0) performed a randomized, double blind, placebo (PL) controlled, two-period crossover, single-center trial. Gas exchange and inspiratory capacity (IC) were assessed during cycle ergometry at 80% incremental exercise peak work rate. Transcutaneous [Formula: see text] (Tc[Formula: see text]) measurement was used for V_D/V_T estimation. Baseline postalbuterol forced expiratory volume in 1 s (FEV₁) was 1.86 ± 0.58 L (63.6% ± 13.9 predicted). GFF increased FEV₁ by 0.18 ± 0.21 L relative to placebo (PL; P < 0.001). CWRET endurance time was greater after GFF vs. PL (383 ± 184 s vs. 328 ± 115 s; difference 55 ± 125 s; P = 0.013; confidence interval: 20-90 s), a 17% increase. IC on GFF was above placebo IC at all time points and fell less with GFF vs. PL (P ≤ 0.0001). Isotime tidal volume (1.54 ± 0.50 vs. 1.47 ± 0.45 L; P = 0.022) and ventilation (52.9 ± 19.9 vs. 51.0 ± 18.9 L/min; P = 0.011) were greater, and respiratory rate was unchanged (34.9 ± 9.2 vs. 35.1 ± 8.0 br/min, P = 0.865). Isotime V_D/V_T did not differ between groups (GFF 0.28 ± 0.08 vs. PL 0.27 ± 0.09; P = 0.926). GFF increased exercise tolerance in patients with COPD, and the increase was accompanied by attenuated dynamic hyperinflation without altering V_D/V_T.NEW & NOTEWORTHY This study was a randomized clinical trial (NCT03081156) that collected detailed physiology data to investigate the effect of dual bronchodilator therapy on exercise tolerance in COPD, and additionally to determine the relative contributions of changes in dead space ventilation (V_D/V_T) and dynamic hyperinflation to alterations in exercise limitation. We utilized a unique noninvasive method to assess V_D/V_T (transcutaneous carbon dioxide, Tc[Formula: see text]) and found that dual bronchodilators yielded a moderate improvement in exercise tolerance. Importantly, attenuation of dynamic hyperinflation rather than change in dead space ventilation was the most important contributor to exercise tolerance improvement.

Muscle Oxidative Capacity Is Reduced in Both Upper and Lower Limbs in COPD

INTRODUCTION

Skeletal muscle atrophy, weakness, mitochondrial loss, and dysfunction are characteristics of chronic obstructive pulmonary disease (COPD). It remains unclear whether muscle dysfunction occurs in both upper and lower limbs, because findings are inconsistent in the few studies where upper and lower limb muscle performance properties were compared within an individual. This study determined whether muscle oxidative capacity is low in upper and lower limbs of COPD patients compared with controls.

METHODS

Oxidative capacity of the forearm and medial gastrocnemius was measured using near-infrared spectroscopy to determine the muscle O2 consumption recovery rate constant (k, min) in 20 COPD (Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2/3/4, n = 7/7/6) and 20 smokers with normal spirometry (CON). Muscle k is linearly proportional to oxidative capacity. Steps per day and vector magnitude units per minute (VMU·min) were assessed using triaxial accelerometry. Differences between group and limb were assessed by two-way ANOVA.

RESULTS

There was a significant main effect of group (F = 11.2, ηp = 0.13, P = 0.001): k was lower in both upper and lower limb muscles in COPD (1.01 ± 0.17 and 1.05 ± 0.24 min) compared with CON (1.29 ± 0.49 and 1.54 ± 0.60 min). There was no effect on k of limb (F = 1.8, ηp = 0.02, P = 0.18) or group-limb interaction (P = 0.35). (VMU·min) was significantly lower in COPD (-38%; P = 0.042). Steps per day did not differ between COPD (4738 ± 3194) and CON (6372 ± 2107; P = 0.286), although the difference exceeded a clinically important threshold (>600-1100 steps per day).

CONCLUSIONS

Compared with CON, muscle oxidative capacity was lower in COPD in both upper (-20%) and lower (-30%) limbs. These data suggest that mitochondrial loss in COPD is not isolated to locomotor muscles.

Transcutaneous PCO2 for Exercise Gas Exchange Efficiency in Chronic Obstructive Pulmonary Disease

Gas exchange inefficiency and dynamic hyperinflation contributes to exercise limitation in chronic obstructive pulmonary disease (COPD). It is also characterized by an elevated fraction of physiological dead space (V_D/V_T). Noninvasive methods for accurate V_D/V_T assessment during exercise in patients are lacking. The current study sought to compare transcutaneous PCO₂ (TcPCO₂) with the gold standard-arterial PCO₂ (PaCO₂)-and other available methods (end tidal CO₂ and the Jones equation) for estimating V_D/V_T during incremental exercise in COPD. Ten COPD patients completed a symptom limited incremental cycle exercise. TcPCO₂ was measured by a heated electrode on the ear-lobe. Radial artery blood was collected at rest, during unloaded cycling (UL) and every minute during exercise and recovery. Ventilation and gas exchange were measured breath-by-breath. Bland-Altman analysis examined agreement of PCO₂ and V_D/V_T calculated using PaCO₂, TcPCO₂, end-tidal PCO₂ (P_ETCO₂) and estimated PaCO₂ by the Jones equation (PaCO₂-Jones). Lin's Concordance Correlation Coefficient (CCC) was assessed. 114 measurements were obtained from the 10 COPD subjects. The bias between TcPCO₂ and PaCO₂ was 0.86 mmHg with upper and lower limit of agreement ranging -2.28 mmHg to 3.99 mmHg. Correlation between TcPCO₂ and PaCO₂ during rest and exercise was r²=0.907 (p < 0.001; CCC = 0.941) and V_D/V_T using TcPCO₂ vs. PaCO₂ was r²=0.958 (p < 0.0001; CCC = 0.967). Correlation between PaCO₂-Jones and P_ETCO₂ vs. PaCO₂ were r²=0.755, 0.755, (p < 0.001; CCC = 0.832, 0.718) and for V_D/V_T calculation (r²=0.793, 0.610; p < 0.0001; CCC = 0.760, 0.448), respectively. The results support the accuracy of TcPCO₂ to reflect PaCO₂ and calculate V_D/V_T during rest and exercise, but not in recovery, in COPD patients, enabling improved accuracy of noninvasive assessment of gas exchange inefficiency during incremental exercise testing.

SARS-CoV-2 RapidPlex: A Graphene-Based Multiplexed Telemedicine Platform for Rapid and Low-Cost COVID-19 Diagnosis and Monitoring

The COVID-19 pandemic is an ongoing global challenge for public health systems. Ultrasensitive and early identification of infection is critical in preventing widespread COVID-19 infection by presymptomatic and asymptomatic individuals, especially in the community and in-home settings. We demonstrate a multiplexed, portable, wireless electrochemical platform for ultra-rapid detection of COVID-19: the SARS-CoV-2 RapidPlex. It detects viral antigen nucleocapsid protein, IgM and IgG antibodies, as well as the inflammatory biomarker C-reactive protein, based on our mass-producible laser-engraved graphene electrodes. We demonstrate ultrasensitive, highly selective, and rapid electrochemical detection in the physiologically relevant ranges. We successfully evaluated the applicability of our SARS-CoV-2 RapidPlex platform with COVID-19-positive and COVID-19-negative blood and saliva samples. Based on this pilot study, our multiplexed immunosensor platform may allow for high-frequency at-home testing for COVID-19 telemedicine diagnosis and monitoring.

The anaerobic threshold: 50+ years of controversy

The anaerobic threshold (AT) remains a widely recognized, and contentious, concept in exercise physiology and medicine. As conceived by Karlman Wasserman, the AT coalesced the increase of blood lactate concentration ([La^- ]), during a progressive exercise test, with an excess pulmonary carbon dioxide output ( V ̇ C O 2 ). Its principal tenets were: limiting oxygen (O₂ ) delivery to exercising muscle→increased glycolysis, La^- and H⁺ production→decreased muscle and blood pH→with increased H⁺ buffered by blood [HCO₃ ^- ]→increased CO₂ release from blood→increased V ̇ C O 2 and pulmonary ventilation. This schema stimulated scientific scrutiny which challenged the fundamental premise that muscle anoxia was requisite for increased muscle and blood [La^- ]. It is now recognized that insufficient O₂ is not the primary basis for lactataemia. Increased production and utilization of La^- represent the response to increased glycolytic flux elicited by increasing work rate, and determine the oxygen uptake ( V ̇ O 2 ) at which La^- accumulates in the arterial blood (the lactate threshold; LT). However, the threshold for a sustained non-oxidative contribution to exercise energetics is the critical power, which occurs at a metabolic rate often far above the LT and separates heavy from very heavy/severe-intensity exercise. Lactate is now appreciated as a crucial energy source, major gluconeogenic precursor and signalling molecule but there is no ipso facto evidence for muscle dysoxia or anoxia. Non-invasive estimation of LT using the gas exchange threshold (non-linear increase of V ̇ C O 2 versus V ̇ O 2 ) remains important in exercise training and in the clinic, but its conceptual basis should now be understood in light of lactate shuttle biology.

Factors determining training-induced changes in V̇O2max, critical power, and V̇O2 on-kinetics in skeletal muscle

Computer simulations, using the "P_i double-threshold" mechanism of muscle fatigue postulated previously (the first threshold initiating progressive reduction in work efficiency and the second threshold resulting in exercise intolerance), demonstrated that several parameters of the skeletal muscle bioenergetic system can affect maximum oxygen consumption (V̇O_2max), critical power (CP), and oxygen consumption (V̇O₂) on-kinetics in skeletal muscle. Simulations and experimental observations together demonstrate that endurance exercise training increases oxidative phosphorylation (OXPHOS) activity and/or each-step activation (ESA) intensity, the latter, especially in the early stages of training. Here, new computer simulations demonstrate that an endurance training-induced increase in OXPHOS activity and decrease in peak P_i (Pi_peak), at which exercise is terminated because of exercise intolerance, result in increased V̇O_2max and CP, speeding of the primary phase II of V̇O₂ on-kinetics, and decreases V̇O₂ slow component magnitude, consistent with their observed behavior in vivo. It is possible, but remains unknown, whether there is a contribution to this behavior of an increase in the critical P_i (Pi_crit), above which the additional ATP usage underlying the slow component begins, and a decrease in the activity of the additional ATP usage (k_add). Thus, we offer a mechanism, involving P_i accumulation, Pi_crit and Pi_peak, of the training-induced adaptations in V̇O_2max, CP, and the primary and slow component phases of V̇O₂ on-kinetics that was absent in the literature.NEW & NOTEWORTHY A mechanism of the training-induced changes in V̇O_2max, critical power, and V̇O₂ on-kinetics in skeletal muscle reported in the literature is postulated. It involves the self-driving "P_i double-threshold" mechanism of muscle fatigue underlying exercise inefficiency, the slow component of the V̇O₂ on-kinetics, and termination of exercise. It is proposed that an increase in OXPHOS activity and decrease in peak P_i at which exercise terminates are responsible for the training-induced changes in the muscle bioenergetic system.