The goal of this clinical trial is to learn if a high-flow nasal cannula (HFNC) - a device that delivers warm, humidified air through a small tube placed in the nostrils - can improve exercise tolerance in adults with heart failure (HF) who are entering a cardiac rehabilitation program. The main questions it aims to answer are: Can HFNC help people with heart failure exercise for a longer time during a treadmill test? and does HFNC lower how hard people feel they are working during exercise?

Researchers will compare exercise performance with HFNC to exercise performance without HFNC to see if HFNC helps people with HF tolerate exercise better. Participants will:

Complete two treadmill exercise tests, one with HFNC and one without, in a randomly assigned order.

Wait 48 to 72 hours between the two tests to allow full recovery. Have their heart rate, blood pressure, oxygen level, and breathing rate measured before and after each test.

Heart failure is a chronic syndrome characterized by reduced cardiac output, exercise intolerance, and dyspnea. Despite the well-established benefits of cardiac rehabilitation, many HF patients have severely limited exercise capacity, which restricts the intensity and duration of training they can safely perform.

High-flow nasal cannula (HFNC) delivers heated, humidified air at flow rates of 50-60 L/min. Its physiological effects include reduction of inspiratory work, washout of nasopharyngeal dead space, generation of positive end-expiratory pressure, and improved mucociliary transport. These mechanisms may facilitate exercise in patients with cardiorespiratory limitations.

STUDY DESIGN. Randomized crossover trial. Each participant acts as their own control, completing two incremental treadmill tests in random order, separated by a washout period of 48-72 hours: Condition A (experimental): HFNC via AIRVO 2™ device (Fisher & Paykel Healthcare) with asymmetric nasal cannula at 50-60 L/min, FiO₂ 0.21, temperature 34-37°C. Condition B (control): no ventilatory support device. Randomization will be performed using randomizer.org. Before the experimental condition, participants will undergo an acclimatization period in which flow is increased by 10 L/min every 5-10 minutes until reaching the target flow or maximum tolerated flow.

EXERCISE PROTOCOL. An incremental protocol will be used on the treadmill, which will consist of incline and a progressive increase in speed and grade. The test ends at voluntary exhaustion or when predefined safety stopping criteria are met.

SAFETY. The test will be stopped if any of the following occur: chest pain, severe dyspnea (Borg ≥8), nausea or presyncope, cyanosis, SpO₂ below 90%, or heart rate above 85% of age-predicted maximum. The testing site will be equipped with automated external defibrillator, bag-valve mask, emergency oxygen, and at least one team member certified in advanced life support. Direct transfer to the hospital emergency service is available on-site (Complejo Hospitalario San José).

OUTCOMES. Primary outcomes are maximum treadmill test duration (seconds) and perceived exertion (Modified Borg Scale, 0-10), measured immediately after each test. Secondary outcomes - blood pressure, heart rate, SpO₂, respiratory rate, and rate-pressure product - are measured at baseline and immediately post-test.

STATISTICAL ANALYSIS. Data normality will be assessed using the Shapiro-Wilk test (n<50). Paired t-test or Wilcoxon signed-rank test will be applied depending on data distribution. Sample size was calculated for a crossover design (two-tailed, α=0.05, power=80%, dz=0.8), yielding 23 participants; 28 will be recruited assuming 20% dropout.

CO-INVESTIGATORS: Monserrat Delgado Valverde, Monserrat Orellana Morales, Emily Quiñileo Riffo, and Sergio Silva Valverde (Escuala de Kinesiología, Universidad de Santiago de Chile). Claudia Román Ureta, Paulina Castro Olivares (Complejo Hospitalario San José).

SETTING. Cardiovascular rehabilitation program, Complejo Hospitalario San José, Independencia, Santiago, Chile.

Inclusion Criteria:

• Age 18 years or older
• Medical diagnosis of heart failure (documented in clinical record or referral)
• Electrocardiogram performed within the last 6 months
• Functional capacity class II or III according to the New York Heart Association (NYHA) classification
• Stable pharmacological treatment
• Stable and controlled comorbidities
• Available to complete two evaluation sessions
• Able and willing to provide written informed consent

Exclusion Criteria:

• Diagnosis of respiratory disease (chronic obstructive pulmonary disease, interstitial lung disease, or pulmonary hypertension)
• Myocardial infarction within the past 3 months
• Hospitalization due to heart failure within the past 4 weeks
• Cognitive impairment or inability to follow the study protocol
• Neuromuscular or osteoarticular conditions that limit treadmill test performance

Individual participant data will not be shared during the study period. Data will be stored on a password-protected server at Universidad de Santiago de Chile for 5 years after study completion. Only anonymized data, with no information that could identify participants, may be deposited in a public repository after the custody period ends. Any future use of the data will require a specific and separate consent process.