What is it?

Hypertension is persistently high pressure in the arteries that can, over time, cause damage to organs such as the kidneys, brain, eyes and heart. It is therefore a major risk factor for cardiovascular disease (myocardial infarction, ischaemic and haemorrhagic stroke) and chronic kidney disease and it is important to detect and treat hypertension early.

Arterial blood pressure, the amount of force blood exerts on the walls of the arteries, depends on the force and rate that the heart contracts as it pumps oxygenated blood from the left ventricle (compartment) of the heart into the arteries and the resistance to that flow. The amount of resistance depends on the elasticity and diameter of the smaller blood vessels and how much blood is flowing through them.

Blood pressure is dynamic; it rises and falls depending on a person’s level of activity, the time of day, and physical and emotional stresses. In healthy people it is largely controlled by the nervous system, regulated by hormones produced by the kidneys and the adrenal gland that affect the amount of sodium, potassium and fluids removed or retained by the kidneys (which therefore affects the volume of blood) and is altered by decreasing and increasing the rate the heart beats and the diameter of the blood vessels. When one or more of the regulating processes is not able to respond appropriately, the pressure of the blood may become persistently high.

Two pressures are recorded when blood pressure is measured. They are systolic pressure – the peak force on the blood vessel walls when the heart is contracting, and diastolic pressure – the pressure present when the heart is relaxing between beats. Both pressures are measured in millimetres of mercury (mm Hg) and are expressed as systolic over diastolic pressure. For instance, a blood pressure of 120/80 mm Hg corresponds to a systolic pressure of 120 and a diastolic pressure of 80 and would be reported as 120 over 80. Blood pressures are generally classified in adults as follows:

• Optimal blood pressure - a systolic of less than 120 and diastolic of less than 80 mm Hg
• Normal blood pressure - a systolic pressure between 120-129 and/or a diastolic between 80-85 mm Hg
• High normal (a state of blood pressure that is elevated above normal and that may eventually become hypertension) - a systolic pressure between 130-139 and/or a diastolic between 85-89 mm Hg
• Hypertension, Grade 1 - a systolic of 140-159 and/or a diastolic of 90-99 mm Hg
• Hypertension, Grade 2 - a systolic of 160-179 and/or a diastolic of 100-109 mm Hg
• Severe hypertension, Grade 3 – a systolic greater than or equal to 180 and/or a diastolic greater than or equal to 110 mm Hg

Usually diastolic pressure mirrors systolic pressure, but as people get older the diastolic changes less than systolic pressure and hypertension due to high systolic pressure (called isolated systolic hypertension) becomes more common. In general, the higher the blood pressure and the longer the period of high pressure, the greater the likelihood of damage.

Blood pressure in children is assessed differently to adults and is compared with the 95th percentile of children of the same age, height and sex.

Types of hypertension

Essential hypertension
In most cases the cause of hypertension is not known. This form of high blood pressure is called essential or primary hypertension. It can affect anyone but is found more frequently in men, especially those of African descent, and it becomes more common in everyone with increasing age. In most cases, high blood pressure does not cause symptoms until it begins to damage body organs. For this reason hypertension is sometimes referred to as the ‘silent killer,’ quietly increasing the risk of developing stroke, heart disease, heart attack, kidney damage, and blindness. Because hypertension is both a quiet and a common condition, blood pressure is often measured each time a patient sees their doctor.

Although it may not be possible to identify the cause, there are several things that are known to increase the risk of developing hypertension and that are known to make it worse it when it is present. These include:

• Overweight and obesity
• A lifestyle with little exercise
• Smoking
• Excessive use of alcohol
• Excessive dietary salt (sodium)
• Use of oral contraceptives (rarely)
• Use of drugs such as steroids, cocaine and amphetamines

Secondary hypertension
Hypertension may also be due to an identifiable disease. This form of high blood pressure is called secondary hypertension. It is important to identify these conditions as they may be able to be treated, allowing the blood pressure to return to normal or near normal levels. These conditions include:

• Kidney disease or damage – decreases the removal of salts and fluids from the body and increases blood volume and pressure. Since hypertension can also cause kidney damage, the problem can get progressively worse if left untreated.
• Renovascular disease - narrowing of the arteries supplying the kidneys due to conditions such as atherosclerosis in older patients or fibromuscular dysplasia in younger females are also a cause of secondary hypertension.
• Obstructive Sleep Apnoea - there is an association between Obstructive Sleep Apnoea (OSA) and hypertension and treatment of OSA can improve hypertension  
• Cushing’s syndrome – a disease with increased production of the hormone cortisol by the adrenal gland.
• Hyperaldosteronism (Conn's syndrome) – a condition with overproduction of aldosterone, a hormone that helps regulate the removal of sodium by the kidneys; it may be due to an adrenal gland tumour which is usually benign.
• Phaeochromocytoma – a rare and usually benign tumour of the adrenal gland that produces excessive amounts of adrenaline, a hormone that the body uses to help it respond to stress; affected patients often have episodes of severe hypertension.
• Thyroid disease – both excessive and deficient thyroid hormone production can cause increases in blood pressure
• Pregnancy – hypertension may develop at any time during a woman’s pregnancy but is most common late in pregnancy (the last trimester), when it can occur as part of pre-eclampsia (toxaemia), a condition where there is new onset hypertension in pregnancy with evidence of organ damage (e.g. protein in the urine, kidney or liver impairment).

Tests

The goals of testing are to detect high blood pressure, confirm that it is persistent over time, find out whether it is being caused by a particular disease that could be treated, check the health of various body organs, get a baseline prior to starting treatment, and monitor blood pressure and organ health during the period of treatment.

Laboratory tests
Laboratory tests cannot diagnose hypertension, but tests are frequently requested to help evaluate and monitor organ function and specific tests are sometimes requested to detect diseases that may be causing the high blood pressure or making it worse.

General tests that may be requested include:

• Urinalysis - Urine dipstick test for blood, albumin and protein to help assess kidney function
• Haematocrit – as part of a full blood count (FBC) to evaluate the ratio of fluid to solids in the blood
• Urea and creatinine – to detect and monitor kidney disease or to monitor the effect of drug treatment on the kidneys
• Electrolytes – sodium and potassium – some high blood pressure treatments can cause high sodium and potassium loss
• Fasting glucose – to determine if blood glucose levels are normal
• Calcium – increased activity of the parathyroid glands produces an increase in serum calcium which is associated with high blood pressure
• Lipid profile – to check levels of total cholesterol, HDL cholesterol, LDL cholesterol and triglycerides which can be combined with blood pressure measurements to assess cardiovascular risk

Specific tests that may be requested because of the patient’s medical history, physical findings or general laboratory test results to help detect, diagnose and monitor conditions causing secondary hypertension include:

• Aldosterone and renin – to help detect the overproduction of aldosterone by the adrenal glands (which may be due to a tumour)
• Cortisol – to detect an overproduction of cortisol that may be due to Cushing’s syndrome
• Catecholamines and metanephrines – adrenaline, noradrenaline and their metabolites (metanephrine and normetanephrine) are used to help detect the presence of a phaeochromocytoma (a tumour of the adrenal gland) that can cause episodes of severe hypertension
• Parathyroid hormone (PTH) – if calcium is found to be increased
• Thyroid stimulating hormone (TSH) and free T4 – to detect and monitor thyroid dysfunction

Non-laboratory tests
Blood pressure measurement
This is the primary tool for detecting and monitoring hypertension. Although it can now be evaluated with a variety of electronic devices, blood pressure is traditionally and most accurately measured with a stethoscope and a blood pressure cuff (a sphygmomanometer – which includes a cuff, a bulb, and a pressure dial that reads the pressure in millimetres of mercury (mm Hg)). The cuff is placed on a patient’s upper arm and a bulb attached to the cuff is squeezed and released several times to inflate the cuff and increase pressure on the arm until the arterial blood flow is temporarily shut off.

The person taking the blood pressure listens through the stethoscope (which has been placed over the artery in the patient’s arm) while slowly releasing the air and reducing the pressure in the cuff. The pressure at which the heartbeat can be heard again is the systolic pressure. The pressure at which the sound again disappears is the diastolic pressure. The pressure is given as systolic over diastolic; for instance, 120 over 80 is a systolic pressure of 120 and a diastolic of 80 mm Hg.

Blood pressure measurements are usually performed with the patient sitting quietly for a few minutes but may also be done in other postures, such as standing. If a patient has an elevated blood pressure, the pressure in the other arm may be measured to confirm the finding. Since blood pressure can and will vary, a diagnosis of hypertension is not made from a single measurement, but will involve multiple measurements made at different times. It is not a single high reading that the doctor is interested in, but persistent high blood pressure.

The doctor may ask the patient to wear a device that monitors and records the blood pressure at regular intervals during the day to monitor it over time. This is especially helpful during the diagnostic process and can help rule out the high measurements that only occur when the patient is in the doctor’s surgery. This is known as the ‘white coat phenomenon,’ which has been estimated to account for as much as 10-20% of suspected cases of hypertension. There are now electronic blood pressure measuring devices that can be used in the home. These can be used effectively but should be checked at intervals against the findings at the doctor’s surgery to ensure accuracy.

These forms of blood pressure measurement are considered indirect. Very rarely, a direct measurement of blood pressure may be required. This can be obtained by inserting a catheter into an artery to measure the pressure inside the blood vessel.

As part of the diagnostic process and to help evaluate the status of vital organs, the doctor may request or perform one or more of the following:

• ECG (electrocardiogram) – to evaluate the heart rate and function
• Eye examination – to look at the retina for changes in the appearance of the blood vessels (retinopathy)
• Physical examination – to help evaluate the kidneys, to look for abdominal tenderness, to listen for bruits (the sound of blood flowing through a narrowed artery), to examine the thyroid gland in the throat for any enlargement or signs of dysfunction, and to detect any other clinical signs as they present
• Imaging ultrasound of the kidneys and renal arteries

Treatment

Lifestyle changes can help lower the risk of developing hypertension. In many patients with mild high blood pressure, reaching and maintaining a healthy weight, exercising regularly, limiting dietary alcohol and salt, and stopping smoking can reduce blood pressure levels to normal and may be the only ‘treatment’ required. The risks associated with gender, race and increasing age, however, do not disappear with lifestyle changes and, in many cases, a treatment plan that includes medicines is necessary to control persistently high blood pressure.

There are several classes of drugs available to treat hypertension. Each class works differently, targeting a particular aspect of blood pressure regulation. Frequently, a patient will need to take a couple of different drugs together to achieve blood pressure control. Your doctor will work with you to select the combinations and dose that are right for you. Classes that are available include:

• Diuretics – a commonly used group of drugs that increase the removal of salt and water by the kidneys. This reduces the volume of fluid in circulation and lowers the blood pressure.
• Adrenergic blockers (alpha blockers, beta blockers, alpha-beta blockers) – work to reduce the nervous system’s rapid response to physical and emotional stress.
• ACE (angiotensin-converting enzyme) inhibitors and ARBs (Angiotensin II receptor blockers) – help prevent the constriction of arterioles (small arteries) by blocking the formation and/or action of angiotensin II, an enzyme that the body produces to constrict blood vessels and increase blood pressure.
• Calcium channel blockers – dilate arterioles by decreasing the amount of calcium that enters into the blood vessel walls and the heart muscle.
• Vasodilators – work directly on blood vessels to relax the muscles that constrict and dilate the arteries.

If a condition causing secondary hypertension can be cured (for example, by removing an adrenal tumour) or controlled (for example, by treating diabetes or thyroid disease), then blood pressure levels may fall to normal or near normal. When a cure is not possible and control of the disease consists of minimising further damage (as may occur with kidney disease), then the hypertension will be controlled with a combination of medicines, and the patient will be monitored closely to help maintain organ function and reduce the likelihood of problems arising.

An asymptomatic blood pressure of more than 180/110 mm Hg must be treated urgently. Higher blood pressures are treated as emergencies and may require admission to hospital so that intravenous drugs can be given.

Pregnant women with pre-eclampsia require rest, close monitoring, and frequent visits to their doctor’s surgery or even admission to hospital. The only real resolution for pre-eclampsia is delivery, but postponing delivery as long as possible allows the foetus more time to mature. This time delay must be balanced against the increasing danger of the development of eclampsia, with seizures and organ damage in the mother, an emergency condition that can be life-threatening for both baby and mother.

More information

Healthdirect Australia: How to lower your blood pressure

Better Health Channel: Hypertension

Heart Foundation: Blood pressure