ABC Health & Wellbeing

Fact File

Hypertension (high blood pressure)

by Peter Lavelle

Hypertension or high blood pressure affects almost three million Australians over the age of 25 and can cause serious and irreversible damage. Yet, sufferers can be completely unaware they have a problem. Fortunately, treatment is usually simple and effective.

Published 23/10/2006

[Image source: iStockphoto]

• Background
• Causes
• Effects
• Diagnosis
• Treatment
• More info

Background

Hypertension is the technical term for high blood pressure, a condition that affects almost three million Australians over the age of 25.

It's a silent condition in the sense that someone who has high blood pressure usually doesn't know about it.

Then why is it a problem? Because over time, raised blood pressure can cause gradual damage to many organs in the body. This damage may be serious and irreversible; hypertension is one of the causes of stroke, heart disease, and kidney failure. So it's important that high blood pressure is detected and kept under control. The earlier the age at which it's detected and treated, the less the damage. Fortunately, treatment is usually simple and effective.

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Causes

Blood flows through the arteries because of a pressure gradient. The heart pumps blood into the arteries and at the moment of most forceful contraction, the pressure in the arteries (in a normal person) is about 120 mm Hg. (Blood pressure is measured in millimetres of mercury or mm Hg for short. In the case of 120 mm Hg, it's the pressure required to push a column of mercury up 120 millimetres in a blood pressure measuring machine). When the heart relaxes, the pressure in the arteries falls away to a low of about 80 mm Hg, before the heart contracts again and forces it back up to 120.

So the blood pressure is given as a set of two readings; the systolic (normally around 120 mm Hg) and the diastolic (normally around 80 mm Hg).

However, blood pressure can fluctuate. During periods of physical activity the heart pumps harder and faster and the systolic blood pressure can go much higher than normal – to 180 mm Hg for example. Also, the blood pressure can be higher during times of stress and it commonly increases with age.

But in some people, blood pressure stays abnormally high, even at rest. In most cases – about 95 per cent – the exact cause isn't known, but the small arteries in the body become narrowed in calibre. The heart has to pump harder to force blood through the narrower arterial channels, raising the blood pressure. Because the cause isn't fully understood, it's sometimes called primary hypertension. (It's also sometimes called 'essential' hypertension – a rather misleading name because there's nothing essential about it. If anything it's essential to have normal blood pressure!)

In a small percentage of cases – about five per cent – hypertension is a by-product, of another disease process such as kidney disease. As well, certain rare endocrine (hormonal) diseases can cause the release of abnormal amounts of the hormones adrenaline and noradrenaline, making the heart pump harder and causing blood vessels to contract.

Some medications can cause hypertension, including non-steroidal anti-inflammatory drugs (NSAIDs), oral contraceptives, corticosteroids and some antidepressant medications.

High blood pressure often runs in families. It's more common in older people and in people who:

• are obese
• don't exercise
• eat a diet high in salt
• drink a lot of alcohol

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Effects

Hypertension is a cause of atherosclerosis; a build-up of fatty deposits in artery walls.

Hypertension is one of the chief causes of a disease of the arteries called atherosclerosis, where a build-up of fatty deposits occurs in the lining of the artery walls. These fatty deposits – called plaques – can thicken, calcify and narrow the arteries to the point where they restrict the flow of blood.

This can result in the formation of a blood clot (called a thrombus) at the site of the plaque, which may block the artery altogether. If it's severe and sudden enough, the tissues normally supplied by the artery can die from lack of oxygen. It can happen in just about any organ in the body, but it's most common in the heart, brain, the arteries of the limbs, the kidneys and the retina (the light sensitive part of eye).

Hypertension is one of the chief causes of atherosclerosis (along with smoking, being overweight, having diabetes, having higher-than-normal blood cholesterol levels, and having a family history of atherosclerosis). Hypertension may double or even quadruple the risk of cardiovascular disease. It also puts a greater strain on the heart by forcing it to pump harder against a high pressure. Heart muscle that has to work harder is more likely to fail, especially if the arteries supplying the heart muscle have also been damaged by atherosclerosis.

Atherosclerosis is a silent disease; its consequences aren't felt until damage to the organs is quite advanced. That's why its important for people – especially older people – to have their blood pressure measured, and if it's found to be high, treated.

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Diagnosis

A nurse tests the blood pressure of a patient using a sphygmomanometer. Image: Getty Images.

Hypertension is usually diagnosed in the course of a general physical checkup, or while a person is seeing a doctor for some other unrelated condition. Sometimes a person may be diagnosed as having had a stroke or a heart attack and are then found to have high blood pressure.

The standard way blood pressure is measured is by using an instrument called a sphygmomanometer, which first appeared in doctors' surgeries in the 1880s. Invented by a German, Samuel Siegfried Karl Ritter von Basch, and subsequently modified to its present design in 1896 by Italian paediatrician Scipione Riva-Rocci, it has strengthened the forearm muscles of many a nurse and GP.

The modern sphygmomanometer has an inflatable cuff that is wrapped around the upper arm and inflated with air via a rubber bulb that is repeatedly squeezed. When the pressure in the cuff gets high enough, it cuts off the blood flow in the main artery of the upper arm. The air is then slowly released from the cuff via a valve. As the pressure in the cuff falls (as measured by a mercury barometer) sounds of the blood rushing through the artery are heard through a stethoscope placed over the artery.

These sounds are called Korotkoff sounds. The pressure at which sound is first heard as the cuff is released is the systolic pressure. The pressure at which the last sound is heard – that is, the pressure at which the sound disappears as the blood returns to its silent, unimpeded flow – is the diastolic pressure.

One problem with the sphygmomanometer is the mercury it uses, which is poisonous and can be released into the environment, after an old machine is broken or discarded, for example. So increasingly, the sphygmomanometer is being replaced with automatic electronic measuring devices which do the same thing but are more accurate, easier to use, and can be used by patients for home blood pressure monitoring.

Because blood pressure fluctuates, doctors don't diagnose hypertension on the basis of one abnormal reading. Usually three consecutive abnormally high readings – taken on different occasions – are needed before a diagnosis of hypertension can be made.

The point at which a high blood pressure reading is considered abnormal depends a lot on a person's age. The National Heart Foundation of Australia recommends that people under the age of 65 years should have a blood pressure at rest of no greater than 130/85 mm Hg. Those over 65 years of age should aim for a blood pressure reading of no more than 140/90 mm Hg.

If the blood pressure is found to be consistently high, a doctor will do a physical examination to see if there is any underlying disease causing the hypertension (usually there's not). The examination can also reveal whether there are signs of damage to the body's organs such as absent pulses in the limbs, evidence of artery disease in the retina of the eye, or microscopic traces of blood in the urine (a sign of kidney disease).

Just because a person's blood pressure is found after three checks to be normal, doesn't mean it will always be normal. Blood pressure should be checked every now and then. The earlier hypertension is diagnosed and brought under control, the less damage there will be to the heart, brain, kidney and other organs.

The Heart Foundation advises that if your blood pressure is normal and you have no personal or family history of the condition, a check every two years and during routine visits to your doctor is recommended. If it is approaching the upper limit of normal (or higher) or you have a personal or family history of high blood pressure, stroke or heart attack, it is best to have it checked more frequently, as advised by your doctor.

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Treatment

If the blood pressure is only mildly elevated (160/100 mm Hg or less), it can often be successfully treated by adopting some lifestyle changes. These include:

• stopping smoking
• eating a balanced, low-fat, low-salt diet, with lots of fruit, vegetables and grains
• exercising regularly (30 minutes a day of aerobic exercise such as swimming, jogging or cycling)
• reducing weight if you're overweight

In about a third of cases this will be enough to bring it back to normal. If not, the next step is for a doctor to prescribe drugs to lower the blood pressure. There's a fairly wide range of drugs available, with differing mechanisms of action. Some relax the heart, others relax the smooth muscles in the small arteries so they widen and allow blood to pass more easily, thus lowering the blood pressure.

One drug is usually given in low dose at first and the dose gradually increased until the blood pressure is brought under control. Sometimes, another, or several more drugs will need to be added to bring it under control. Finding the right drug, or combination of drugs is a trial and error process.

A drug may produce side effects in some people or may have adverse interactions with other drugs and so may need to be discontinued and another drug substituted. For some people, after a period of months or years, the drug(s) can be stopped, and if the blood pressure stays down, they won't need to be taken again. But more usually, they have to be continued for life.

The major classes of antihypertensive drugs are:

Diuretics or "water pills" (thiazide, hydroclorathiazide, chlorathalidone and indapamide). These are older but still commonly used drugs which work by helping the kidneys to pass accumulated salt and water. This decreases the amount of fluid in the body and so lowers blood pressure. Diuretics also cause blood vessels to dilate, reducing the resistance to blood flow, and therefore its pressure. Some types of diuretics cause the kidneys to excrete potassium so potassium supplements may be needed.

Beta-blockers (such as propranolol, atenolol, nadolol, pindolol and labetolol). These relax the heart by blocking the actions of hormones such as adrenaline and noradrenaline which make the heart pump harder.

Alpha-blockers (eg prazosin). These also block the effects of adrenalin and noradrenaline on the blood vessels, relaxing and dilating them.

Vasodilators (hydralazine and minoxidil). These drugs relax the smooth muscle of the arteries, which causes them to dilate and so reduce the resistance to blood flow. (These are not used much these days.)

Calcium-channel blockers (nifedipine, nicardipine, verapamil and diltiazem). These work by blocking the flow of calcium in the muscles of the heart and blood vessels, causing the blood vessels to dilate.

Angiotensin converting enzyme (ACE) inhibitors (captopril, enalapril, perindopril, ramipril, quinapril and lisinopril) These block the action of a hormone (angiotensin II) that narrows blood vessels.

Angiotensin receptor blockers (candesartan, irbesartan, telmisartan, eprosartan). These work on the same system as ACE inhibitors.

Centrally acting drugs (methyldopa and clonidine). These also dilate blood vessels, but they do so by acting on the part of the brain that controls blood pressure (through nerve impulses to the heart and blood vessels), rather any direct effects on the heart and blood vessels themselves.

One curious fact is that, for reasons we don't understand, blood pressure has been coming down in Western counties – it's dropped about 2 mg on average since the mid 1980s. The effect is seen in people with both normal and high blood pressure, so it isn't due to more aggressive treatment of hypertension. It might be due to better diet and more exercise.

Reviewed by Professor Garry Jennings, Director, Baker IDI Heart & Diabetes Institute

Updated 19/01/2007