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Not Too High, Not Too Low: Your Blood Pressure Matters Too

By Professor Philip Home

Blood pressure is an important force in your body; learn about how to manage your blood pressure, what it means to have high blood pressure, and how this relates to diabetes

Blood pressure is the force of your blood as it flows through your body. If your blood pressure is too high, it can damage the vessels that carry blood to many of your organs. In this article we’ll discuss high blood pressure, how it happens, how you can manage blood pressure, and how diabetes can affect your blood pressure. 

Lessons from nature: how does the giraffe get blood all the way to its head?

Giraffes have very high blood pressure compared to yours or mine. Because blood pressure drives the blood around the body and to all the body's organs, when a giraffe reaches up to eat leaves, blood must have enough force to continue to flow against gravity, all the way up to reach its brain. That build-up of pressure creates a problem though, when the giraffe puts its head down to drink – how does it stop the blood vessels and the brain (and the eyes, and so on) from being damaged when reaching down? To solve this problem, the giraffe’s body has a series of valves and other mechanisms that regulate its blood pressure according to posture.  

This tells us that, to avoid damaging organs (including the heart, the brain, and the kidneys), blood pressure must be regulated within a narrow range. This is true, too, in you and me – if you’ve ever felt faint after standing up from lying down on a hot day, that is because your blood pressure was temporarily too low to reach the height of your brain.  

Forty years ago, as a junior hospital intern, I saw people with dangerously high blood pressure that caused acute headache, brain swelling, and in extreme cases, rapid kidney failure. That does not happen much nowadays, but even moderately raised blood pressure does slowly cause damage to the kidneys, the blood vessels of the brain (leading to risk of strokes), and the heart (leading to risk of heart attacks) over time.  

Why does high blood pressure occur, and why does it cause problems?

Your kidneys filter blood to produce urine, getting rid of waste products that are otherwise toxic to the body. These filters are tiny, sensitive structures, and are driven by the pressure of the blood (but they depend on blood pressure being at the right level). The pressure is regulated by the kidney itself; however, if the pressure is wrong, the filtration structures become damaged, and they slowly scar, seize up, and lead to kidney failure. 

High blood pressure can result from many things. For example, eating lots of salt leads to excess water in the body, as does overeating (for reasons that are not understood), leading to raised blood pressure. Once any kidney damage occurs, it is harder for the body to get rid of excess salt, making the problem worse. Another way that high blood pressure can occur is through high levels of fat in the blood. Fat damages the walls of the arteries, causing them to stiffen. As a result, the heart pushes blood into the inflexible arteries and blood pressure increases. Fat can also cause the blood vessels to be more likely to break (which could lead to a stroke) or the vessel wall to form clots (which could lead to a heart attack).     

Stress can also raise blood pressure, but this is usually just for a short period of time. Sudden strong exercise also raises blood pressure in this way. There is little evidence that these short-term stressors matter if your blood vessels are healthy; the same is true for chronic mental stress (like the pressures of everyday life and work), unless the stress causes you to consume food and liquids in amounts that are not healthy.

High blood glucose and blood pressure

Living for years with high blood sugar levels can damage the filtration structures in the kidney, as well as cause damage to blood vessels. Therefore, it is not surprising that blood pressure management is even more important in people with diabetes than in the general population. Indeed, targets for blood pressure are often lower for people with diabetes because high blood glucose levels already increase risk of stroke and kidney failure. Doubling that risk due to high blood pressure is a big problem. Click here to read the American Diabetes Association’s position statement on diabetes and high blood pressure. 

Note: while blood pressure tends to increase in everyone with age, this is secondary to our lifestyle – blood pressure targets should remain the same, even if they become more difficult to maintain.   

How does blood pressure management help the body?

As noted above, the old emergency problems of very high blood pressure have largely disappeared with modern treatments. But reducing blood pressure is still important as it reduces the risk of heart failure by taking strain off the heart.

For the long-term problems of kidney failure and stroke, there is evidence from many clinical trials over the last 30 years that blood pressure management with medication can greatly reduce risk. This has been shown in studies of people with type 1 diabetes and those with type 2 diabetes. A surprising (but happy) finding was that blood pressure treatment also slowed the progression of diabetes-related eye damage. In general, all blood pressure-lowering medications reduce your risk of other complications, and taking more than one medication may be needed to achieve targets. However, one group of medications that specifically helps blood vessels of the kidney are RAS blockers (renin-angiotensin system blockers); these are particularly helpful if someone has early kidney damage. 

Attention to healthy eating is also important, including attention to salt intake and calories. Blood pressure measurements can show the effects of diet quickly (within seven days), but damage is prevented only by long-term management.

Measuring blood pressure

This used to be done by healthcare professionals, but the stress of clinical appointments often caused a person’s blood pressure to increase (called 'white-coat hypertension'). Manual measurement was replaced by automatic systems that people wear for 24 hours, providing useful information on periods of both activity and rest.  

Now a preferred approach to blood pressure is self-measurement with a battery-driven air pump and an automatic sensor. These are not expensive, but be sure to choose one that is quality-assured and worn on the upper arm. I use a blood pressure meter made by Omron. For people with bigger arms the cuff size may need to be longer than usual.  

Even using a home blood pressure meter can be stressful. In clinics, health professionals are advised to allow you to rest for five minutes before taking the measurement while you are sitting. But if you try a device at home and check your blood pressure every five minutes, you will find that sitting blood pressure levels actually settle over about 20 minutes. At first you might need to do this every day for three days, and on both arms (or according to directions from your healthcare team); for follow-up, weekly or monthly checks may be adequate. Always write your blood pressure measurements down to show to your healthcare professional.  

If you don’t have a blood pressure condition, checking once a year is fine.  

Blood pressure targets

You should aim to keep your blood pressure target constant from year to year. Blood pressure measurements have two numbers: the higher number represents the pressure immediately after each heart contraction, and the lower number is the pressure immediately before the next contraction. This is written as something like “128/82.” Targets are often set for less than 130/80, or below 120/80 for a younger person. However, your healthcare professional may suggest a different personal target if, for example, you have difficulties reaching standard targets due to medication intolerance, or you have reasons for tighter management (like evidence of early kidney damage). You may need both lifestyle changes and multiple medications to achieve your target.  

Medications affect people differently, and there is no way to predict what side effects you might experience. Medications can also interact with one another if you take several drugs to manage your diabetes. Your healthcare professional will recommend an alternative if you have a significant problem with your blood pressure medicine, and will teach you about situations (such as a gut infection, or a medical procedure) in which changes to treatment are important.  

Blood pressure is naturally lower during pregnancy. High blood pressure can damage the developing baby and mother at levels only slightly above the non-pregnant range. If you are pregnant, your healthcare team will check your blood pressure regularly, but you can still measure it yourself. Some drugs might affect the developing baby, so treatment, if needed, might differ from your usual choices.  

The take-home message

Look after your blood pressure, and it will look after you. Ensure your healthcare professional pays attention to this risk factor regularly. Be particularly careful if you already have kidney or eye damage from diabetes, or if you suffer a stroke in older age. Managing blood pressure can be easier than managing blood glucose, but both work together and are important for protecting your health.  

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