Visualizing Blood Pressure Data

At the risk of getting TMI with you, I’m being treated for high blood pressure. As a result, I track my blood pressure with an app called BP Watch. This allows me to print logs of my blood pressure to give to my doctor. He’s been finding it helpful in seeing whether my combination of medications and exercise is keeping my blood pressure within healthy limits.

Six months ago or so, I started adding my weight to my logs. I only weigh myself once a week. I’ve been up and down, but mostly up in weight. This fact and the existence of my blood pressure data made me curious — did my heavier times correspond to higher blood pressure? While I caution against drawing any significant conclusions from a sample of one, my personal data suggest no.

WHAT I DID

I imported my data into Excel 2013. I eliminated all the columns except: Date, Systolic, Diastolic and Weight. I didn’t really need the date column for my charts, but I left it in to document the date range the data was collected.

Then I added two worksheets – to one I added the Weight and Systolic columns, the other I added the Weight and Diastolic columns. Then in each work sheet I highlighted my two columns and clicked on “recommended charts” under the “insert” tab and accepted the scatter chart.

After that, I adjusted the vertical axis (Blood Pressure) so that it was near the bottom of my measured range instead of zero. I did this to better define the varying pressure at various weights. For Systolic, I started the vertical access at 80 and for Diastolic, used 50.

During my six months I gained roughly 14 pounds. My guess was that my measurements at the high-end of my weight ought to be noticeably higher than at the low-end of my weight. Here’s what I actually found:

Blood Pressure Chart

Systolic blood pressure plotted against weight.

 

Blood Pressure Chart

Diastolic pressure plotted against weight

 

WHAT I THINK I’M SEEING

Using the goals of 12o for Systolic BP and 80 for Diastolic BP, I think I see that while my blood pressure is mostly under control, there is room for improvement.

It appears that for any given weight in the range that I’ve been in for the past six months, I can have a range of blood pressure. Further, there is no obvious trend line pointing towards higher blood pressure at higher weights.

WHY I COULD BE MISTAKEN ABOUT THE LACK OF A RELATIONSHIP BETWEEN WEIGHT AND BLOOD PRESSURE

While this data suggest to me that there is no real relationship between blood pressure and weight, there are a number of reasons why I might be being fooled:

  1. I’m looking at the data the wrong way. Maybe if I was more sophisticated in data analysis, I’d find a trend that’s escaping me.
  2. Maybe I haven’t collected data long enough. Perhaps it takes longer for higher blood pressure to take hold at higher weights.
  3. Maybe I haven’t gained enough weight for the rise in blood pressure to emerge from the noise of variations. This is NOT an idea I plan to consciously test.
  4. Maybe my medications are swamping any effect I might get from weight gain.

If I had gathered data from hundreds of people over a wide range of weights, I’d have more confidence in either accepting or rejecting the lack of relationship that I found from a sample of one.

OTHER POTENTIAL WAYS TO LOOK AT THIS DATA

As I was mostly through this blog post, it occurred to me that it would be very helpful to run a histogram of my blood pressure numbers. While this would not shed any new light on the relationship between weight and blood pressure, it could give information on whether my blood pressure was as much under control as I think it is.

Excel does not do histograms out of the box, but I do know how to do histograms in Python and I might try to run some this weekend.

Getting back to the weight vs BP relationship, it might be helpful to calculate average blood pressures at each measured weight, then plot the average. There’s also a correlations tool in Python that I don’t fully understand but could learn more about.

WANT TO DO YOUR OWN ANALYSIS? 

If you’d like to run my data through your own analysis tools, download bpandweightdata. Or download and analyze your own data if you’re collecting it. Does Fitbit do exports?

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One Response to Visualizing Blood Pressure Data

  1. digilou says:

    To my knowledge, Fitbit doesn’t have an export feature. At least not in the app. However, I haven’t looked at their open API for devs, so maybe there’s some potential there.

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