How can this device get an EKG from just your fingertips?

Factual Questions
1

I am seeing a cardiologist to evaluate an occasional arrhythmia that (I think) I have. His nurse took an EKG in the office, but he said the only way to diagnose these things is to have a trace when it is actually happening (for me that about once every month or so). He has devices that can do 30-minute or 24-hour traces but for me he recommended this device that connects to my (Android) phone.

How can a device that you touch with two fingertips possibly give you a reliable EKG when they had to hook up several electrodes to my chest and legs in the doctor’s office to get one? How can electrical activity in your heart possibly be detected at your fingertips, so as to resolve the different phases of the trace?

I could see that there would be a tradeoff between convenience and precision here, but I just don’t see how this thing can work at all.

2

The same way pulse monitors on exercise equipment do it - excellent common-mode rejection ratio.

Naturally, you only get one “trace” with this device, but that’s enough to show an obvious arrhythmia.

3

You can even pack the necessary circuitry into a business card.

But yeah, beowulff got it. The voltages may be small compared to environmental noise, but all you care about is the difference, and that’s relatively easy to measure.

The unit in my link uses an LMV341 op-amp (voltage difference amplifier, basically), which can easily measure differences in the microvolt range.

The extra probes at the doctor just get you more channels–useful, but overkill for basic triaging.

4

I’m a doctor, but not a cardiologist. And not know exactly what device they actually strapped to your fingertip -

The classic EKG requires leads that span your legs and heart. With this wide of a net, it captures voltage differences caused by your heart beat from different angles, and measures the timing and morphology as cells depolarize within your heart. The specific time intervals and depolarization patterns are linked to different arrhythmias.

Guess that you had some simple form of a pulse oximeter on your finger, just to capture the timing between beats of your heart. It’s a different form of a Holter monitor. That simpler device just tells when your heartbeats are not spaced evenly - it can’t capture the morphology of the actual waveforms. Sufficient to tell your cardiologist how often your arrhythmias are occurring, and how high/low your heart rates go.

5

The device linked in the OP is a single-lead EKG.

Some information on handheld ECG devices (including a review of the one linked in the OP) is available here:
https://www.ndsu.edu/pubweb/~grier/Comparison-handheld-ECG-EKG.html

6

I am still amazed that when electrical activity originates in the chest, you can detect it in the fingertips. Am I understanding you folks correctly?

I am getting an echocardiogram tomorrow; if the doc adds anything that you folks haven’t already said I’ll let you know.

7

Here’s a better link 1-lead to 12-lead and exercise ECG

The single lead EKG basically just detects when your heart beats, and the intervals in between. Like “beat, pause, beat”. If you have AFIB it can be something like “beat beat beat, irregularly long pause, beat”.

Some of those single lead models have extra cables that allow you to connect between say right arm to left arm. That gives a bit better picture of the depolarization event.
The greater the distance between the two points, the more accurate the vector.

If you look at the pink graph at the middle of the page, that’s a real 12 lead EKG reading that we doctors use. It looks at your heart from multiple angles, and records the different shapes of the waveforms across different lead patterns.

There’s a reason why med school was so long…

Best of luck tomorrow!