When should I worry about low HRV?

Rarely because of the number alone. Worry about symptoms: chest pain, fainting, shortness of breath, or a racing, pounding or irregular pulse. A low week with none of those is a reason to rest and look for a cause.

Low HRV sounds frightening because large studies link it to heart disease. In a 2013 pooled analysis of 21,988 people, those with the lowest SDNN had a 35 percent higher risk of a first cardiovascular event. That compares groups of people over years. It doesn’t make your number a diagnosis. Many healthy people sit around 20 ms, which HRV researcher Marco Altini calls normal in the fifties and common in younger adults.

Call emergency services

With a pounding, fluttering or skipping heartbeat, the US National Library of Medicine says to call 911 for:

• Fainting or passing out
• Chest pain
• Shortness of breath
• Unusual sweating
• Dizziness or lightheadedness

Apple notes its watch can’t detect heart attacks. If you have chest pain, call, whatever your watch shows.

See a doctor soon

• You often feel extra beats, more than 6 a minute or in groups of 3 or more.
• Your pulse is over 100 at rest, without exercise, anxiety or fever.
• You have new or different palpitations.
• You have heart disease or risk factors such as high blood pressure, diabetes or high cholesterol, and palpitations.
• Your HRV has been well below your usual for weeks, you feel run down, and nothing in your habits explains it.

Very high or jumpy numbers matter too

A sudden jump in HRV can come from an irregular rhythm, such as atrial fibrillation, which a wearable can count as variability. In a study of 290 older adults, abnormal heartbeat patterns that pushed HRV values up were common and went with a higher risk of death. Apple says its irregular rhythm alerts check only now and then and can’t catch every case. If your numbers jump around and you feel your heart flutter or skip, see a doctor.

What we’d do with a low week and no symptoms

Sleep more, skip drinks, keep training easy and check again in a week. Look back at the last week’s drinks, sleep, training and health first. That’s where the cause usually turns up.