Effects of continuous vs interval exercise training on oxygen uptake efficiency slope in patients with coronary artery disease

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The oxygen uptake efficiency slope (OUES) is a submaximal index incorporating cardiovascular, peripheral, and pulmonary factors that determine the ventilatory response to exercise. The purpose of this study was to evaluate the effects of continuous exercise training and interval exercise training on the OUES in patients with coronary artery disease. Thirty-five patients (59.3±1.8 years old; 28 men, 7 women) with coronary artery disease were randomly divided into two groups: continuous exercise training (n=18) and interval exercise training (n=17). All patients performed graded exercise tests with respiratory gas analysis before and 3 months after the exercise-training program to determine ventilatory anaerobic threshold (VAT), respiratory compensation point, and peak oxygen consumption (peak VO2). The OUES was assessed based on data from the second minute of exercise until exhaustion by calculating the slope of the linear relation between oxygen uptake and the logarithm of total ventilation. After the interventions, both groups showed increased aerobic fitness (P<0.05). In addition, both the continuous exercise and interval exercise training groups demonstrated an increase in OUES (P<0.05). Significant associations were observed in both groups: 1) continuous exercise training (OUES and peak VO2 r=0.57; OUES and VO2VAT r=0.57); 2) interval exercise training (OUES and peak VO2 r=0.80; OUES and VO2 VAT r=0.67). Continuous and interval exercise training resulted in a similar increase in OUES among patients with coronary artery disease. These findings suggest that improvements in OUES among CAD patients after aerobic exercise training may be dependent on peripheral and central mechanisms.

Cardiopulmonary exercise testing (CPX) is a highly reliable and well-validated approach to assessing aerobic performance and monitoring exercise tolerance in patients with cardiovascular disease (). In this context, Baba et al. () introduced the oxygen uptake efficiency slope (OUES), an objective and reproducible measure of cardiopulmonary function. The OUES is derived from the single regression analysis between oxygen uptake and minute ventilation during incremental exercise. Importantly, OUES evaluates the functional capacities of several organ systems, such as cardiovascular, pulmonary and skeletal muscle metabolism during exercise (,) in a single index. The OUES has been investigated as an index of cardiopulmonary functional reserve in patients with various conditions (  ). Of particular note, a study that followed patients with cardiovascular disease over 6 years showed that the OUES was a good prognostic indicator ().

Aerobic exercise training has been recommended as a non-pharmacological treatment for patients with coronary artery disease (CAD) (,). In this regard, continuous exercise training (CET) has been shown to promote increased cardiorespiratory fitness in CAD patients (). However, over the last few decades, there has been increasing interest in interval exercise training (IET) for cardiac rehabilitation (,). IET consists of periods of high-intensity exercise alternated with periods at lower intensity; this allows cardiac patients to complete short exercise bouts at a higher intensity than would have been possible during continuous exercise. Previous investigations (, ) have also shown that IET effectively improves the aerobic fitness of CAD patients.

Although both CET and IET have been shown to improve aerobic fitness, there is little information about the effects of the mode and intensity of exercise training on OUES in CAD patients. Thus, the purpose of this study was to evaluate the effects of CET and IET on OUES in patients with CAD.

The participants were patients admitted to the coronary care unit of the TotalCor Hospital for the diagnosis of CAD. After being discharged from the hospital, the patients enrolled in a cardiac rehabilitation program at the cardiorespiratory rehabilitation center of the Amil group. Thirty-five CAD patients (59.3±1.8 years old; 28 men, 7 women) were randomly divided into two groups: CET (n=17) and IET (n=18). This trial was designed to test the exercise-training modalities for noninferiority, with the key secondary objective of testing for superiority with respect to the OUES. The inclusion criterion was having stable CAD diagnosed by coronary angiography. Exclusion criteria included having unstable angina pectoris, complex ventricular arrhythmias, pulmonary congestion, and orthopedic or neurological limitations to exercise. Patients remained on their standard medications throughout the study, and no changes in medications were reported. The study was approved by the Ethics Committee of the Universidade Santa Cecilia (66/2011) and all of the study participants gave their written informed consent.

Graded exercise test

Maximal graded exercise tests were carried out on a programmable treadmill (DigiStress model pulsar, Brazil) before, and three months after commencing the IET or CET intervention. Gas exchange and ventilatory variables were measured continuously during the gas exchange tests, breath-by-breath, using an open-circuit spirometry procedure on an exercise-based system (SensorMedics, model Vmax 229 Pulmonary Function/Cardiopulmonary Exercise Testing Instrument, USA). The following variables were obtained breath-by-breath and expressed as 30-s averages: pulmonary oxygen uptake (VO2 mL·kg-1·min-1, standard temperature and pressure, dry) pulmonary ventilation (VE L/min body temperature and pressure, saturated), end-tidal carbon dioxide pressure (PetCO2 mmHg), ventilatory equivalent ratio for oxygen (VE/VO2), and ventilatory equivalent ratio for carbon dioxide (VE/VCO2). Before each test, the gas analyzers were calibrated using a gas mixture containing known concentrations of carbon dioxide and oxygen balanced with nitrogen, and the flow meter was calibrated using a 3-L syringe. Heart rate was continuously recorded at rest, during the graded exercise testing, and during the recovery period using a 12-lead ECG (HW Systems, HeartWare Ltda, USA). All tests in this study were performed in the same laboratory at a room temperature of 20-23°C.

The subjects performed a ramp-like progressive exercise test to exhaustion on the treadmill. The exercise workload (speed and/or slope) was increased every 1 minute with completion of the incremental part of the exercise test occurring between 8 and 12 min. The following criteria were used to define maximal effort: 1) participants demonstrated subjective evidence of exhaustion (unsteady gait, facial flushing, and hyperpnea), and either 2) peak heart rate (HR) ≥95% age-predicted maximum, or 3) maximal respiratory exchange ratio (RER) ≥1.10 ().

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