One of our contributors recently asked why, when we spend all our day up and about, do we measure it when we’re sat down, with our feet flat on the floor, after waiting five minutes?
Perhaps you’ve wondered the same thing? Surely if your blood pressure spikes, it’s important to measure those spikes?
This is a really good question. The first part of the answer is to say, of course, when we do ambulatory blood pressure monitoring (ABPM), we’re not sat down. This is part of the reason that ABPM is the gold standard for diagnosing high blood pressure initially.
Getting an accurate measurement
However, for home monitoring, we do recommend that people do two things. The first is to follow the instructions given here. The second is to keep a diary so that we can get an average reading.
The reason that we ask people to follow instructions and always, in the same way, is to try and get a reproducible reading.
Imagine that your blood pressure is like the height of the sea. Just like the sea, it goes up and down. This depends on the size of the waves, the position of the sun and the moon and the time of the day. Just like with measuring the sea level, your blood pressure has lots of variables that can cause it to be up and down.
Measuring something that varies
If you were trying to measure the height of the sea, you’d want to do it in the same conditions every time. By measuring it at the same point in the tide (generally highest and lowest) and taking an average over many readings, you’d get a pretty good idea about what the average sea level was, and which direction it is moving in.
Similarly, with blood pressure, we’re trying to get an average level. We know that people’s blood pressure goes up for all sorts of things, so we try and compensate for that. We do that by trying to do it in reproducible conditions. Which means being sat and relaxed, feet flat on the floor, and waiting for five minutes before taking the first reading.
By measuring your average reading, over a number of readings, over a number of days, we get a good impression of what your overall level of risk is. We have lots of data about average readings and we can use that to guide your treatment.
What about blood pressure spikes?
“But”, you may say, “what about the spikes in my blood pressure? Surely they must be important too?”
Here’s where it gets more complex. The answer is that we don’t know for sure. We think that in people with high blood pressure, the more their blood pressure varies, the greater the risk. However, what we don’t know is how much blood pressure varies in the general population. It’s only recently that home blood pressure monitors have become popular, so most of our data is based on readings from clinics. We know that those readings tend to be higher anyway. This is because of the white coat effect.
So if your blood pressure varies a lot, we’re not sure how important that is. We also don’t know how best to treat it. As side effects from over-treatment are very common with blood pressure medications, until we know more, we don’t base our treatment plans on blood pressure spikes.
As we don’t know what to do about spikes, for now, we ignore them, and treat high blood pressure on the best evidence that we currently have.
I hope this is helpful and clarifies things. As ever, let us know in the comments if it doesn’t or you have any other questions.