Intercostal and vastus lateralis microcirculatory response to a sympathoexcitatory manoeuvre in patients with chronic obstructive pulmonary disease

Descending motor drive does not interact with muscle metaboreflex for ventilation regulation during rhythmic exercise in healthy humans

The muscle metaboreflex effect on pulmonary ventilation (V̇E) regulation is more apparent during rhythmic exercise than rest, possibly because this reflex interacts with other mechanisms regulating V̇E during voluntary contractions, such as central command. Therefore, we tested whether one part of central command, the descending component of motor execution (i.e., descending motor drive), and the muscle metaboreflex interact synergistically to regulate V̇E. Thirteen healthy adults (9 men) completed four experiments in random order under isocapnia. The muscle metaboreflex was activated by rhythmic handgrip exercise at 60% maximal voluntary contraction (MVC) force with the dominant hand. Then, the muscle metaboreflex remained active during a 4-min recovery period via postexercise circulatory occlusion (PECO), or it was inactivated, maintaining free blood flow to the dominant upper limb. During the last 2 min of the handgrip exercise recovery, participants either performed rhythmic voluntary plantar flexion with the dominant leg at 30% MVC torque to generate descending motor drive or the dominant leg's calf muscles were involuntarily activated by electrical stimulation at a similar torque level (i.e., without descending motor drive). V̇E increased to a similar level during handgrip exercise in all conditions (≈22 L/min, P = 0.364). PECO maintained V̇E elevated above recovery with free blood flow (≈17 L/min vs. ≈13 L/min, P = 0.009). However, voluntary and involuntary plantar flexion with or without PECO evoked similar V̇E responses (Δ ≈ 4 L/min, P = 0.311). Therefore, an interaction between descending motor drive and muscle metaboreflex is not ubiquitous for V̇E regulation during rhythmic exercise.NEW & NOTEWORTHY Voluntary (i.e., with descending motor drive) and involuntary (i.e., no descending motor drive) plantar flexion elicited similar ventilatory responses when postexercise circulatory occlusion was or was not used in an upper limb. These results indicate that the descending motor drive component of the central command and the muscle metaboreflex do not interact to regulate pulmonary ventilation during rhythmic exercise, which suggests that a supposed interaction between central command-muscle metaboreflex is more complex than previously thought.

Exercise-induced potentiation of the acute hypoxic ventilatory response: Neural mechanisms and implications for cerebral blood flow

A given dose of hypoxia causes a greater increase in pulmonary ventilation during physical exercise than during rest, representing an exercise-induced potentiation of the acute hypoxic ventilatory response (HVR). This phenomenon occurs independently from hypoxic blood entering the contracting skeletal muscle circulation or metabolic byproducts leaving skeletal muscles, supporting the contention that neural mechanisms per se can mediate the HVR when humoral mechanisms are not at play. However, multiple neural mechanisms might be interacting intricately. First, we discuss the neural mechanisms involved in the ventilatory response to hypoxic exercise and their potential interactions. Current evidence does not support an interaction between the carotid chemoreflex and central command. In contrast, findings from some studies support synergistic interactions between the carotid chemoreflex and the muscle mechano- and metaboreflexes. Second, we propose hypotheses about potential mechanisms underlying neural interactions, including spatial and temporal summation of afferent signals into the medulla, short-term potentiation and sympathetically induced activation of the carotid chemoreceptors. Lastly, we ponder how exercise-induced potentiation of the HVR results in hyperventilation-induced hypocapnia, which influences cerebral blood flow regulation, with multifaceted potential consequences, including deleterious (increased central fatigue and impaired cognitive performance), inert (unchanged exercise) and beneficial effects (protection against excessive cerebral perfusion).

Evaluation of the relationship between intercostal muscle oxygenation measured by near-infrared spectroscopy and exercise capacity in group E COPD patients

Near-infrared spectroscopy (NIRS) can be used to demonstrate muscle metabolism and oxygenation. NIRS-based oximeters enable the noninvasive measurement of static and dynamic muscle oxygenation. This study aimed to evaluate the relationship between NIRS readings and exercise capacity in group E COPD patients. The prospective study included 40 patients with group E COPD who presented to our outpatient clinic between May 2021 and June 2022. The patients were evaluated with pulmonary function testing, 6-Minute Walk Test (6MWT), echocardiography, and dyspnea and quality of life assessments. NIRS muscle oxygen saturation (SmO2) levels at the start and end of the 6MWT were obtained. 6MWT distance was positively correlated with intercostal SmO2 and fingertip SO2 at the start (R = 0.679, p ≤ 0.001 and R = 0.321, p = 0.04, respectively) and end of the 6MWT (R = 0.693, p ≤ 0.001 and R = 0.635, p ≤ 0.001, respectively) and negatively correlated with the number of hospitalizations due to exacerbations in the last year and mean pulmonary arterial pressure (R =  - 0.648, p ≤ 0.001 and R =  - 0.676, p ≤ 0.001, respectively). SF-36 score was positively correlated with intercostal SmO2 at the beginning of the 6MWT (R = 0.336, p = 0.03). Intercostal SmO2 levels at the start of the 6MWT positively correlated with diffusing capacity of the lung for carbon dioxide (DLCO) (R = 0.388, p = 0.01) and ratio of DLCO to alveolar volume (DLCO/VA) levels (R = 0.379, p = 0.02), and these correlations persisted more strongly after the 6MWT (R = 0.524, p = 0.01; R = 0.500, p = 0.01, respectively). NIRS is a practical and noninvasive method for measuring muscle oxygenation and can be used as an alternative to 6MWT in the evaluation of exercise capacity in patients with group E COPD.

Altered locomotor muscle metaboreflex control of ventilation in patients with COPD

We investigated the locomotor muscle metaboreflex control of ventilation, circulation, and dyspnea in patients with chronic obstructive pulmonary disease (COPD). Ten patients [forced expiratory volume in 1 second (FEV1; means ± SD) = 43 ± 17% predicted] and nine age- and sex-matched controls underwent 1) cycling exercise followed by postexercise circulatory occlusion (PECO) to activate the metaboreflex or free circulatory flow to inactivate it, 2) cold pressor test to interpret whether any altered reflex response was specific to the metaboreflex arc, and 3) muscle biopsy to explore the metaboreflex arc afferent side. We measured airflow, dyspnea, heart rate, arterial pressure, muscle blood flow, and vascular conductance during reflexes activation. In addition, we measured fiber types, glutathione redox balance, and metaboreceptor-related mRNAs in the vastus lateralis. Metaboreflex activation increased ventilation versus free flow in patients (∼15%, P < 0.020) but not in controls (P > 0.450). In contrast, metaboreflex activation did not change dyspnea in patients (P = 1.000) but increased it in controls (∼100%, P < 0.001). Other metaboreflex-induced responses were similar between groups. Cold receptor activation increased ventilation similarly in both groups (P = 0.46). Patients had greater type II skeletal myocyte percentage (14%, P = 0.010), lower glutathione ratio (-34%, P = 0.015), and lower nerve growth factor (NGF) mRNA expression (-60%, P = 0.031) than controls. Therefore, COPD altered the locomotor muscle metaboreflex control of ventilation. It increased type II myocyte percentage and elicited redox imbalance, potentially producing more muscle metaboreceptor stimuli. Moreover, it decreased NGF expression, suggesting a downregulation of metabolically sensitive muscle afferents.NEW & NOTEWORTHY This study's integrative physiology approach provides evidence for a specific alteration in locomotor muscle metaboreflex control of ventilation in patients with COPD. Furthermore, molecular analyses of a skeletal muscle biopsy suggest that the amount of muscle metaboreceptor stimuli derived from type II skeletal myocytes and redox imbalance overcame a downregulation of metabolically sensitive muscle afferents.

Respiratory and locomotor muscle blood flow measurements using near-infrared spectroscopy and indocyanine green dye in health and disease

Measuring respiratory and locomotor muscle blood flow during exercise is pivotal for understanding the factors limiting exercise tolerance in health and disease. Traditional methods to measure muscle blood flow present limitations for exercise testing. This article reviews a method utilising near-infrared spectroscopy (NIRS) in combination with the light-absorbing tracer indocyanine green dye (ICG) to simultaneously assess respiratory and locomotor muscle blood flow during exercise in health and disease. NIRS provides high spatiotemporal resolution and can detect chromophore concentrations. Intravenously administered ICG binds to albumin and undergoes rapid metabolism, making it suitable for repeated measurements. NIRS-ICG allows calculation of local muscle blood flow based on the rate of ICG accumulation in the muscle over time. Studies presented in this review provide evidence of the technical and clinical validity of the NIRS-ICG method in quantifying respiratory and locomotor muscle blood flow. Over the past decade, use of this method during exercise has provided insights into respiratory and locomotor muscle blood flow competition theory and the effect of ergogenic aids and pharmacological agents on local muscle blood flow distribution in COPD. Originally, arterial blood sampling was required via a photodensitometer, though the method has subsequently been adapted to provide a local muscle blood flow index using venous cannulation. In summary, the significance of the NIRS-ICG method is that it provides a minimally invasive tool to simultaneously assess respiratory and locomotor muscle blood flow at rest and during exercise in health and disease to better appreciate the impact of ergogenic aids or pharmacological treatments.