How does dehydration affect cardiovascular function specifically?

Dehydration of 2% bodyweight (approximately 1.4 kg in a 70 kg man) reduces plasma volume by approximately 3%, requiring the heart to increase heart rate to maintain cardiac output, producing a resting heart rate increase of 10–15 bpm, reduced stroke volume, and increased blood viscosity that collectively raises cardiac workload and may acutely increase risk of cardiac events in men with pre-existing coronary artery disease during exercise (Gonzalez-Alonso et al., J Physiol, 1997).

For healthy men, 2% dehydration during exercise significantly impairs performance but does not create acute cardiac risk. For men with existing coronary artery disease, hypertension, or atrial fibrillation, dehydration raises the cardiovascular burden enough to warrant attention, particularly during summer exercise, illness with fever, or in high-altitude environments. Men who take diuretics for hypertension are at particular risk of exercise-induced dehydration compounding the diuretic-mediated fluid loss, leading to episodes of dizziness, syncope, or arrhythmia.

Honesty Scale: Solid (1) for the dehydration-cardiac workload relationship during exercise.

What to do: If you take a diuretic (hydrochlorothiazide, furosemide, chlorthalidone) and exercise in heat, discuss electrolyte and hydration strategy with your physician. Do not assume the advice applies uniformly, fluid needs on a diuretic are different from those off one.

For the full picture, read The Hydration Deep Dive