What causes a sudden increase in resting heart rate?

A sudden increase in resting heart rate (above 10–15 bpm above personal baseline, sustained for more than 48–72 hours) warrants prompt medical evaluation to rule out: (1) new-onset atrial fibrillation (the most important diagnosis to exclude, AF causes an irregularly elevated heart rate and significantly increases stroke risk); (2) infection or inflammatory illness (acute cortisol and cytokine elevation raises heart rate); (3) dehydration or significant blood loss; (4) anemia; (5) hyperthyroidism; or (6) pulmonary embolism (sudden elevated resting heart rate is a classic presentation) (Brady & Perron, Emerg Med Clin North Am, 2006).

A wearable that shows a sudden unexplained rise in resting heart rate from a man's established baseline, without exercise load change, alcohol, illness, or sleep disruption, is a legitimate clinical signal. I have seen patients whose Apple Watch's elevated heart rate notification was the first indication of new-onset atrial fibrillation. Wearables cannot diagnose AF (only an ECG can), but the heart rate elevation pattern (irregular rate vs. consistently elevated rate) can prompt appropriate clinical investigation.

Honesty Scale: Solid (1) for the differential diagnosis of sudden elevated resting heart rate. Promising (2) for wearable-detected elevated resting heart rate as a clinical signal.

What to do: If your resting heart rate is suddenly 15+ bpm above your personal baseline for 3+ days without obvious cause, call your physician. This is not an emergency room call in the absence of symptoms, but it is a same-week physician evaluation trigger. Do not wait for your next annual physical.

For the full picture, read The Resting Heart Rate Deep Dive