Now that semi-automatic home blood pressure machines are so affordable, many people keep a close eye on their blood pressure (BP) readings at home.
Do we really need to take home BP readings?
There are some advantages:
How reliable are they?
Even in the healthcare setting it is thought that automated pressures are less reliable and generally underestimate compared to manual blood pressure readings taken with a stethoscope and sphygmomanometer. If you are worried about the accuracy of a machine, you can take your machine with you on your next GP visit and test it against a manual BP reading.
What do those figures really mean?
The word systole derives from the Greek ‘to contract’ and the word diastole from the Greek ‘to dilate’. The systolic BP is a measure of the pressure of blood ejected against the artery wall when the heart contracts and is always the higher figure. The lower figure is the diastolic BP which is the resting pressure while the heart is filling.
These pressures are measured in millimetres of mercury (mmHg) and are written as a fraction e.g.120/80 with the systolic blood pressure on the top and the diastolic pressure on the bottom.
Why do my blood pressure readings fluctuate?
Our BP varies throughout the day depending on what we are doing. At rest or when we are asleep our BP can drop 10-20mmHg. Your heart does not need to work as hard so it should be within the normal range. People who suffer from insomnia or snore heavily and suffer from obstructive sleep apnoea are the exceptions with studies showing that their BP levels do not dip at night, so they may have raised blood pressures over 24 hours which puts a greater strain on the heart.
When you are physically active your heart needs to pump harder and faster to meet the oxygen demands of your muscles and your BP rises. While BP rises during exercise, blood vessels dilate during the recovery phase and blood pressure is lower if taken after exercise. Readings taken after drinking coffee and smoking will be dramatically elevated and eating a meal that is high in salt, saturated fat or carbohydrates will raise BP. BP can also rise transiently due to anxiety, stress and excitement.
The action of your antihypertensive (blood pressure lowering) tablets will also contribute to fluctuations in your BP readings. The medications begin to work about 1 hour after taking the tablet and peaks anywhere from 4 to 15 hours later. After that, the effect of the medication slowly decreases until it is time for the next dose.
How many readings should I take?
This should be determined by why you are taking your blood pressure in the first place. If your GP has asked you to monitor, they may have specific times or situations they want you to check. If you are getting a snap shot of your blood pressure over a week, a good pattern is to do at least 12 to 14 BP measurements, with both morning and evening readings over one week. After that the frequency would depend on how stable your BP is and whether there has been any change in your medications.
Blood pressure readings can change over the day and under different circumstances. If you get dramatically fluctuating results it is a good idea to take notes about what you were doing before the test to make it easier for a doctor to see if the changes are appropriate or not.
For more information about taking your BP at home see the fact sheet on Blood Pressure on our website http://www.healthylivingnt.org.au/content/?id=128
High blood pressure treatment usually involves making lifestyle changes and if necessary, taking antihypertensive medications. Lifestyle changes for high blood pressure include:
Achieving and maintaining good blood pressure control can minimise the risk of heart disease. While it is not necessary to own a BP machine, for some it contributes to greater self management of their condition. Take a reasonable amount of readings at morning and night over a week to arrive at an average figure that can be trusted as your true blood pressure reading.
If you have any questions regarding if you should monitor your BP at home, or worried about readings you have done, speak to your GP or cardiac educator.