Gabriel Sarah's heart is a medical mystery.
Since 2012, the 37-year-old pediatric anesthesiologist has had ventricular tachycardia, causing the lower chamber of his heart to beat abnormally fast.
"It can cause syncopal episodes, which are times when I lose consciousness, I can't see, I can't hear, the room goes black and I can collapse," Sarah told CNBC.
His doctors don't know why.
"I've had a bunch of studies — genetic studies, imaging studies, including an MRI of my heart, an echocardiogram — and everything has been negative, which is good and bad," Sarah said.
He's one of millions of Americans with abnormal heart rhythm, or cardiac arrhythmia. The most common form, called atrial fibrillation, affects as many as 6 million people in the U.S., and at its most serious, it can lead to heart failure or stroke.
There are therapies that can treat abnormal heart rhythm, but first doctors need good ways to monitor it. For decades, cardiologists have used devices known as Holter monitors, which are about the size of a deck of cards. They're typically worn for two days, with multiple electrodes connected to the chest and torso.
Sarah tried that, as well as an event monitor, worn for a month.
"To wear something like that for 48 hours or a month is very cumbersome," he said. "It prevents you from really getting very active exercise, or doing anything requiring minimal exertion. It's difficult to wear under your clothing, it's difficult to work, and it's difficult to travel."
Times when, as Sarah points out, it's most important to track the heartbeat.