Chronic Obstructive Pulmonary Disease (COPD) is characterized by exercise intolerance, which is a prognostic factor of disease severity and mortality and is usually attributed to ventilatory limitations. Emerging evidence indicates that cerebral oxygenation may also be an important contributor, being impaired both at rest and during exercise in COPD patients. Furthermore, abnormal systemic hemodynamic responses and autonomic nervous system (ANS) dysfunction have been observed, which may further impair cerebral oxygenation. The type of exercise training applied in pulmonary rehabilitation (continuous versus interval) appears to influence these mechanisms differently, yet evidence on their impact on cerebral oxygenation in COPD remains scarce. The aim of this study is to compare, for the first time, the acute changes in cerebral oxygenation, CPET parameters, systemic hemodynamic responses, and autonomic responses in patients with COPD who undergo a high-intensity interval aerobic exercise (HIIE) protocol versus a moderate-intensity continuous aerobic exercise (MICE) protocol. These findings will have important clinical implications, as they may guide the choice of the most appropriate exercise modality in pulmonary rehabilitation.

This is a single-centre, prospective, randomized, cross-over clinical trial that will be conducted at "G. Papanikolaou Hospital, Thessaloniki, Greece. For the purposes of this study, COPD patients that fulfil the inclusion/exclusion criteria will be recruited from Pulmonary Outpatient Clinics of "G. Papanikolaou" General Hospital of Thessaloniki and included after signing a written informed consent form. The study protocol was approved by the Ethics Committee of the School of Medicine, Aristotle University of Thessaloniki. All procedures and evaluations will be performed according to the Declaration of Helsinki 2013 Amendment.

The study will be conducted in a total of 4 visits including baseline assessment. At baseline assessment (visit 1), all subjects will provide a detailed medical history including smoking history, comorbidities, history of acute COPD exacerbations and severity of dyspnea (based on the modified Medical Research Council dyspnea scale). Then, they will undergo a post-bronchodilation spirometry, assessment of diffusion capacity (DLCO), measurement of lung volumes and blood pressure measurements after 10min of rest. Arterial blood gases sampling (breathing room air) will be collected, to confirm lack of resting hypoxemia. Moreover, data regarding physical activity will be collected, using the short form of International Physical Activity Questionnaire (IPAQ). The eligible patients that will be recruited will be advised to refrain from food for 4h and from caffeine, alcohol, or tobacco for 12 h and receive any standard medication before their morning appointment in the research laboratory to perform the assessments described below.

Visit 2 will be conducted with a week of baseline visit. and visits 3 and 4 will take place at least 72h after visit 2. During visit 2, all recruited patients will undergo a cardiopulmonary exercise test (CPET) on a cycle ergometer using an incremental exercise RAMP protocol to exhaustion to establish peak work rate (WRpeak). For visits 3 and 4, all patients will perform in random order on different visits (separated by one week) two exercise protocols on the same cycle ergometer, a high-intensity interval aerobic exercise (HIIE) protocol and a moderate-intensity continuous aerobic exercise (MICE) protocol. The HIIE protocol will consist of 60 seconds work at 85% WRpeak followed by 60 seconds of rest and the MICE protocol will include 20 minutes of cycling at 50% WRpeak. Exercise protocols will be matched for total workload. All parameters will be recorded at rest (3 minutes), during unloaded warm-up (3 minutes), during the exercise and during 3-min recovery following the completion of exercise bouts. The rate of perceived exertion (1-10 Borg scale), legs' perceived exertion (1-10 Borg scale), dyspnea (0-10 scale), and will be assessed every 3 min and at exercise termination.

During the CPET and the protocols, respiratory gas exchange will be recorded using a metabolic cart (Ultima CPX™, MGC Diagnostics). Breath-by-breath values for minute ventilation (VE), respiratory rate (RR), tidal volume (Vt), oxygen uptake (VO2), and end-expiratory pressure of carbon dioxide (PETCO2) will be recorded, along with derived variables. Maximum Minute Ventilation (MVV) will be calculated using the formula "Forced Expiratory Volume in 1 s (FEV1) x 40". A 12-lead electrocardiogram and oxygen saturation (SpO2, finger pulse oximetry) will be continuously monitored.

For cerebral oxygenation assessment, near-infrared spectroscopy (NIRS, OxyMon, Artinis Medical Systems, Elst, the Netherlands) will continuously monitor, non-invasively, changes in cerebral oxygenation, by measuring the micromolar (μΜ) relative changes from (resting conditions for oxygenated hemoglobin (O2Hb), deoxygenated hemoglobin (HHb), total hemoglobin (tHb), and Hb-difference. The NIRS probes will be placed over the patients' left prefrontal lobe between Fp1 and F3, according to the modified international EEG 10-20 system, with the receiving sensor fixed laterally to the emitter at a distance of 4.5 cm.

Hemodynamic parameters will be continuously and non-invasively assed using a photoplethysmography apparatus (Finometer, Finapres Medical Systems, Amsterdam, The Netherlands) connected to the participants' finger. This device allows beat-to-beat tracing of systolic and diastolic blood pressure (SBP and DBP relatively, mmHg), mean arterial pressure (MAO, mmHg) and heart rate (HR). Beat-by-beat HR and stroke volume (SV, ml) will be computed using the Modelflow method, an arterial pressure wave analysis which has been shown to be a reliable alternative to invasive thermodilution techniques at rest and during exercise. Cardiac output (CO, L/min), systemic vascular resistance (SVR, mmHg.s/mL) and ejection time (EJT, s) will be estimated using the BeatScope software (Beatscope version 1a, Finometer, Finapres Medical Systems). Autonomic responses will be assessed using the non-linear components of Heart Rate Variability (HRV) with Kubios software (Kubios, version 3.3.1, Kubios Oy, 2019). The root mean square of successive differences (RMSSD), an index of parasympathetic system activity, will be calculated. The standard deviation perpendicular to the line-of-identity SD1 (an index of parasympathetic system activity), the standard deviation along the line-of-identity (SD2) and their ratio (SD1/SD2) (an index of vagal/sympathetic function) will be analysed, since these measures are not affected by changes in respiration patterns. Baroreflex sensitivity (BRS, ms/mmHg) will be assessed by examining the spontaneous fluctuations in the blood pressure as assessed by a cross-correlation method, using the same software, as described previously.

Inclusion Criteria:

• Clinical diagnosis of COPD based on smoking history (pack/years) or exposure to other risk factors and post bronchodilation forced expiratory volume (FEV1) to forced vital capacity (FVC) ratio <0.7
• Resting arterial oxygen partial pressure at room air >60 mmHg, according to recent (within a week) arterial blood gases assessment
• Disease being stable and treated optimal, based on current GOLD guidelines.
• Provision of informed written signed consent prior to study entry

Exclusion Criteria:

• Recent respiratory infection or exacerbation (as indicated by deterioration of respiratory symptoms, hospital admission and/or change in medication within the previous month), within the past 3 months
• Changes in COPD medication with the past month
• History of asthma and/or reversible airway obstruction (post bronchodilation FEV1/ FVC >0.7)
• Pulmonary hypertension
• Congestive heart failure (Left Ventricle Ejection Fraction <50% in recent echocardiography)
• Severe peripheral artery disease
• Chronic Kidney disease with eGFR < 30 ml/min (KDIGO stages 3b, 4 ,5)
• Active acute or chronic infectious diseases
• Active malignant disease or hematologic disorder under treatment
• History of neuromuscular disorder
• History of severe intellectual disability or mental disorder
• Any contradiction to moderate-high intensity exercise or cardiopulmonary exercise testing according to ATS/ACCP guidelines, such as: myocardial infarction within the past 6 months, unstable angina, syncope, symptomatic severe aortic stenosis, uncontrolled arrhythmias causing symptoms or hemodynamic compromise, acute pulmonary embolus or pulmonary infarction, thrombosis of lower extremities, suspected dissecting aneurysm, pulmonary edema, severe untreated arterial hypertension at rest (> 200 mm Hg systolic, > 120 mm Hg diastolic blood pressure), orthopedic impairment that compromises exercise performance.