How to read your QT Interval

How to read your QT Interval all by yourself, using ordinary household items

(if your household happens to own an Apple Watch, iPhone and Computer)

v1.11 April 17th-21st 2020.

There’s a chance that there’s some people out there right now, who might be a little scared of the chance they might catch the Covid-19 virus — and be one of the unlucky 20% that get ‘the bad version’ of symptoms that might require hospitalisation.

This might in turn cause some folks to consider self-medicating with the drugs du jour Hydroxychloroquine + Azithromycin + Zinc, since they seem to be readily available and in common use (for other diseases like malaria, lupus and arthritis) and everyone is talking about the anecdotal success they’re having on Covid-19 (whilst still no gold standard clinical trials have reported their outcomes) at this time. This note will probably date quickly because soon after publication, we will probably get some results from clinical trials and know much better which treatments are more effective for this disease, but at the time of writing, hcq/azm/zinc is a popular, but unproven and not evidence-based treatment option.

You may have already read that there’s a few worrying potential side effects from the short term high dose use of hcq & azm, and the main worry is QT Prolongation, which can cause heart problems or even death.

Unfortunately hcq and azm each have a small chance of causing QT Prolongation, and taking them together sounds like squaring the risk — a potential recipe for disaster. In some reports it could be around 10% of people had noticeable QT Prolongation.

Of course, here’s some disclaimers :-

  1. You shouldn’t be self-medicating. You know that don’t you? It’s dangerous!
  2. There’s no hard evidence that HCQ/AZM/Zinc even helps. All data so far is anecdotal or word of mouth and lacking in gold-standard random clinical trials. These will come and soon we’ll know one way or another whether this cocktail of drugs is helpful and we might be better able to balance the risk/reward.
  3. In the meantime, what if you’re still considering the use of HZQ/AZM/Zinc, but want to reduce the chance of toxic side effects?

So I was researching this, and a doctor friend (shout out to Dr Jack) suggested that there might be a way to use an Apple Watch to monitor your own QT because it is an ecg machine, even if only a basic single-channel one. I looked into it, and sure enough, there is a way of doing this, simply using a common or garden Apple Watch or any other home ECG device… that people can measure their own QT Interval and attempt to diagnose for themselves if they have QT Prolongation. (huge disclaimer: In theory..!)

Oh yea, while we’re on the subject of disclaimers.

4. You shouldn’t be self diagnosing nor self medicating yourself. Go talk to your doctor and do this properly! Don’t be ‘hacking your own health’ unless you have to, and make sure you understand the risks.

5. If you really aren’t gonna see a doctor… and want to diagnose your QT Interval by yourself ..

Then.. here’s how it could be done. If you choose to try it, you’re on your own! This is not ‘On Me!’ And I take no liability for your health or your ability to correctly diagnose yourself. Nor do I take any responsibility that any of this information is legit. Got that?

Ingredients :-

A recent Apple Watch (that can measure ECG) or any similar device.

An iPhone (kindof a pre-requisite if you have an Apple Watch, shirley?)

A patient (presumably, yourself)

Recipe :-

Step 1.

Wait til you’re calm so that your heart rate measurements might be considered your ‘resting heart rate’, cos you’re going to be doing this test more than once and you want a little consistency. Probably several times over the next few days, to get a ‘baseline’. Needless to say you want to do this first measurement in advance of you getting ill and taking any drugs that might risk messing with your heart rhythm.

Step 2.

Take an ECG measurement using your Apple Watch (series 4 onwards) or any similar device.

(you will find this app in your list of watch apps.). The test takes about 30 seconds.

Here’s apple’s notes on how to do this:

Your iPhone will alert you that its just received a new ECG reading.

Step 3.

Go into the Health app on your iPhone.

Click on the newly measured Electrocardiogram (ECG) report. With any luck its headed: Sinus Rhythm (which is ‘normal’). If it doesn’t say that, you might not want to go any further, and should consult your doctor over potentially irregular heart rhythms.

Step 4.

Click on your most recent ECG reading.

You will see a pretty red line on the graph of the single-channel ECG reading.

Halfway down the page, you will see a blue option that you can click to Export a PDF for your doctor.

So click that, and you’ll see your ECG report, red lines on a piece of white paper.

Click the Share button. And go ahead and Email or Airdrop it to yourself.

Send it to your doctor while you’re at it.

Step 5.

Go to your Computer. Presumably its a Mac, since you’re already an Apple fanboy/girl and you already own a Watch and a Phone so presumably you’ve also got the MacBook and the t-shirt as well.

Import and view that file, and enlarge it really big. Zoom in so that you can see only a few peaks. You want the chart pretty big cos you’re about to annotate it.

Step 6

Either print it out, and draw the lines on pencil… or, if you’re handy, use the annotation tools of the PDF reader, or take a screenshot, load up the picture and get annotating in the photo previewer tool.

Step 7

Count the squares.

Apple’s PDF has Big squares that have longer vertical lines that are each a second apart, and tiny little squares in between the seconds- 25 of them, each one is 40ms apart. The total of 25 squares of 40 ms equals 1,000 ms (one second)

Just so you know, the distance from one peak on the graph to the next peak is your heart rate.

If you count the number of squares from peak to peak — multiply that by 40 ms, and divide into 60,000 ms (thats 60 seconds), you will get your heart rate. In my example, my peak to peak was 830 ms, and divided into 60,000 equals 72 beats per minute — which is a perfectly sensible resting heart rate. But you already knew your heart rate cos your Apple Watch app just told it to you. But its good for you to try it for yourself to get slightly confident in your graph paper back-to-school skills.

Step 8

To measure the QT Interval, you only need to draw 3 straight lines.

One of them is Horizontal and starts at the bottom of the big peak and goes to the bottom of the next peak (or at least more than half way there).

The other two lines will be vertical, and where you place them is of critical importance to measure your QT Interval.

Just before the big peak (directly to the left of it) is a tiny little wobble. Thats the start, and mark the centre of it with a Vertical line through it.

After the peak, there’s a big swooping curve, and where it falls and hits your horizontal line, mark that with another Vertical line.

The distance between those two vertical lines is your QT Interval (at rest).

You can count up the little squares between the two vertical lines, and approximate any fractional square… and multiple by 40 ms, and you will get to a number around 400 ms. That’s your QT Interval at rest.

You can stop here, as long as you always measure your QT Interval at rest, you should see vaguely consistent results.

My QT reading

That one above is what mine looks like.

I’m sure those of you that know how to read ecg’s will tell me i’m unfit and that I should do some exercise.

Step 9 is an optional ‘stretch goal’.

So if you want to account for the heart rate, then you need to use Bazett’s method… which is a simple calculation involving the Heart Rate and the QT Interval (google it)

But the easiest is to use an App to calc it for you — so download an Apple iPhone app that can calculate the QTc — Which is the QT thats been Corrected for heart Rate.

or, use this handy calculator:

So now, what do I do when I know my QT Interval?

Well, just note it down along with the date and time. And do it a few times, on a few days… so you have a baseline to compare against it another time.

If you do decide to self medicate with any drug that has a possible side effect of QT Prolongation, then you can compare back to your baseline/historic QT Interval and see if it gets any longer when medicated.

If your QT interval does get longer, consult your doctor, and, most likely, stop taking whatever you took that had an adverse effect on your QT Interval.

NB 1. Since this is so simple to do, Apple should just — in a future version of the WatchOS — just report your QTc Interval or ideally, track it over time and tell you if you ever have QT Prolongation, since it would be easy for them to do that. Much easier than you or I measuring it by hand on graph paper like we’re doing here.

NB 2. As you probably know, Hydroxychloroquine has a very long half life (of 22 days), which means it stays in your body a long time. Google says it stays in your body for 3 months, after a single dose. If you’re planning to take those tablets twice or three times a day for 5 days (like some of the doctors have reported doing), the drug will be accumulating in your body to become a high dose after several days. Thats the risk because high doses increase the risk of unwanted (and dangerous!) side effects. that, together with the azithromycin’s own side effects. This is why we have to be super careful, and ideally, NOT self-medicate. If you have a history of heart problems or have ever been told you have an irregular ecg measurement, then best not to try it!

NB 3. If you want to consider taking something with lower risk of side effects… Its been said that some supplements might be beneficial (though unproven). Try googling: Quercetin with Zinc & vitamins C & D.

NB 4. Check this page out for more technical details on this QT Interval subject matter.

NB 5. Don’t be tempted to self medicate! The best place for you if you’re seriously ill, is in a Hospital!

NB 6. If you don’t own an Apple Watch then you might consider buying an ECG reader like AliveCor’s excellent Kardia 6L device, which has 6 channels, so its way better (and cheaper) than Apple’s Watch for this purpose. And they tell me, it can report your QTc directly, so if you have one, you wont need to read the rest of this note.

Jez, London.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store