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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.

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).

Last Review Date: April 27, 2023

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