Ivabradine for Heart Rate Reduction During Exercise in Healthy Adults

During exercise, working muscles require more oxygen. The heart meets this demand in two ways: by beating faster (heart rate) and by pumping more blood with each beat (stroke volume). Heart rate increases steadily with exercise intensity until exhaustion, but the amount of blood pumped with each beat tends to plateau at moderate intensities. Why the amount of blood pumped with each beat becomes limited before heart rate, however, is not well understood. Studying these mechanisms is difficult because most drugs that lower heart rate also affect how strongly the heart pumps, making it hard to separate the two effects. One medication that can help with this is ivabradine, which is used to treat people with heart disease who have high resting heart rates. It slows the heart rate without changing how strongly the heart pumps. This study will investigate whether ivabradine lowers heart rate during exercise in healthy younger and older adults, and whether a higher dose lowers heart rate more. The results will help us better understand how heart rate influences the heart's ability to supply oxygen to the body during exercise, and what limits the ability to exercise as people age. Participants will: * Attend three visits at SportsCardiologyBC in Vancouver * Complete a cycling exercise test at each visit to measure heart rate and how hard they can exercise * Take ivabradine (a heart-rate-lowering medication) before the exercise test at Visits 2 and 3 (a lower dose at Visit 2 and a higher dose at Visit 3) * Be monitored for side effects including slow heart rate, visual symptoms, and headache.

Inclusion Criteria: * Healthy males and females aged 19-39 years, and 60-79 years * Engaging in regular aerobic exercise (e.g., running, cycling, swimming, hiking, field, ice, or racquet sports) at least once per week Exclusion Criteria: * History of any cardiovascular condition, including hypertension * Use of any chronic medication * History of any respiratory condition, including asthma * Smoking, including cannabis or vaping in the past year * History of diabetes mellitus, cancer, hepatic, renal or any other chronic illness * Seated blood pressure ≥130/90 or \150 minutes per week of moderate-vigorous intensity exercise (to avoid endurance training-related bradycardia) * Any condition that prevents vigorous exercise from being performed safely * Seated resting heart rate \