"Here doc!" a well-meaning patient says as they drop a piece of paper into my hands. Looking closely, I realise it is a detailed log of their blood pressure for the past month which includes one or two blood pressure values on most days of the week. Some numbers appear quite high, while others appear normal.
I emphatically commend the patient's effort in maintaining this log which gives me a ballpark idea of their home blood pressure. However, let's see if there is a better way to monitor blood pressure.
When to Start Home Blood Pressure Monitoring?
On a previous post, we reviewed the issue of white coat hypertension and the importance of home blood pressure monitoring. While there is a place for formal studies using ambulatory monitors from the pathology provider, I feel self-measurement of blood pressure using one's own machine is perhaps the more preferable approach.
Apart from diagnosing new cases of hypertension, home blood pressure monitoring is also very useful for monitoring response to medication. Whenever a patient is started on a new blood pressure medication, or if there is a change in dose, home monitoring is an invaluable tool.
That said, there is no need to be obsessive or excessive with home blood pressure monitoring. Discuss with your doctor and seek their advice on the timing and frequency of home monitoring.
Eliminating Variables
There are many factors which influence blood pressure such that a single high reading performed randomly during the day doesn't give us a full picture. Even a series of random readings done on multiple days is hard to interpret.
Let's begin by taking the following steps to achieve a more valid reading (as outlined in this NPS article).
Preparation
Avoid smoking and caffeine before measurements
Comfortable ambient room temperature
Not being under significant stress or pain
Measurements to be taken before eating, medications or vigorous exercise
Note: If you are taking blood pressure medication, measurements before medication allow us to measure your blood pressure when medication effect is theoretically at its lowest.
Positioning
Both feet on the floor with legs uncrossed
Weight of arm and back supported
Appropriate sized cuff placed on bare arm, with cuff at heart level
After putting the cuff on your arm, sit for five minutes in a quiet environment without any distractions such as your phone or TV. Now, we are ready to take blood pressure readings!
Image from this RACGP article outlining principles for the standard method of home blood pressure monitoring.
Taking Blood Pressure Measurements
Use your automated machine to:
Take the first reading. I generally recommend patients to discard and not record this reading.
Wait 1-2 minutes before taking and recording another reading.
Likewise, wait another 1-2 minutes before taking and recording the final reading.
Therefore, two readings are recorded with every session.
To improve the reliability of our measurements, blood pressure is measured in this manner consistently over a period of at least 5 days.
We typically aim for recordings over a 7 day period, twice a day with measurements at the same time every morning and every evening on the same arm.
Note: If you are on blood pressure medication, and the blood pressure appears too high or too low, do NOT change treatment without first consulting your doctor.
Recording Home Blood Pressure Readings
All this information should be recorded either on paper or electronically. The average blood pressure of days 2-7 is then calculated; preferably in the format of a morning and evening average.
The Heart Foundation has both a print-friendly PDF and an electronic excel spreadsheet to record these measurements.
One of my friends has even extended this spreadsheet to include three rather than two trials to improve the reliability of measurements.
Interpreting Results and Next Steps
After completing your 7 days of measurements, it is time to bring these results to your doctor and have a discussion! Generally results in excess of 135mmHg systolic, and/or 85mmHg diastolic can be diagnostic of high blood pressure or inadequate control of hypertension. However, our target for blood pressure should be individualised, for instance in diabetes or pregnancy. Your doctor will have more to share about the nuances of interpretation and next steps in management.
Your efforts will allow your doctor to make a better-informed decision about your blood pressure management, while you have taken charge of your health and gained an invaluable tool in the management of blood pressure going forward!
Summary image below from the AHA
