About Hypertension

What is hypertension?

Hypertension is a condition in which the systemic arterial blood pressure is raised. Two pressure measurements are taken when looking at blood pressure:

1. Systolic pressure – this is the pressure in arteries when the heart is contracting. It is the highest pressure during the cardiac cycle
2. Diastolic pressure – this is the pressure in the arteries between heart beats, when the heart is resting. It is the lowest pressure in the cardiac cycle.

Blood pressure is measured in millimetres of mercury (mmHg) and written as systolic over diastolic. Normally, someone is said to have hypertension when their blood pressure is 140/90mmHg. The average blood pressure range that is considered healthy is 90/60 – 139/89mmHg.

What causes hypertension?

The causes of hypertension can fall into two broad categories: essential hypertension or secondary hypertension.

Essential hypertension is diagnosed when there is no identifiable cause and is what affects the vast majority of hypertensive patients.

Secondary hypertension is hypertension with a specific known cause, such as a dysfunction in the systems that regulate the blood volume, or a heart defect. It is best treated by controlling the underlying disease or disorder, rather than by using anti-hypertensive drugs.

Physiology of blood pressure

Blood pressure in the body is determined by this equation:

Blood pressure = cardiac output x total peripheral resistance

Or: BP = CO x TPR

Cardiac output is the volume of blood that the heart pumps out each minute. This is determined by both how much blood is pumped with each contraction of the heart and how many contractions are made each minute. When the volume of blood in the body increases, the venous return to the heart is higher, causing an increase in cardiac output. Increasing the rate of contraction will also increase cardiac output, for example during exercise. Increasing cardiac output increases blood pressure.

Total peripheral resistance is the sum of the resistance that the blood encounters on its path through the body. When blood travels through a vessel with a wide diameter, it can pass through easily and the resistance is low. However, when vessels become smaller (such as when they turn into capillary beds), the blood encounters higher resistance. Increasing total peripheral resistance increases blood pressure.

Many factors can increase total peripheral resistance. These include:

• High blood adrenaline levels. Adrenaline causes the smooth muscle walls of blood vessels to constrict, so the blood encounters higher levels of resistance.
• Being underwater
• Being cold. Blood vessels in the skin constrict to help reduce heat loss.
• Atheromas. Build ups of debris in the artery wall that reduce the diameter of the artery.

Why is hypertension a problem?

Having chronic hypertension is a major risk factor for developing several other diseases or health problems. If you have high blood pressure, the heart has to increase cardiac output to reach the demand for oxygenated blood throughout the body. Over time, this can cause the heart to grow abnormally large (hypertrophic) and also weakens it.

Strokes

A stroke is the interruption of normal blood flow to the brain, either by haemorrhage or blood clot. According to a study by Whisnant et al (1996), hypertension is the main cause of 35-50% of all strokes.

Myocardial infarction

Myocardial infarction (commonly known as a heart attack) occurs when there is a blockage of a coronary artery and the heart cannot provide itself with an adequate blood supply. This leads to cell death and can cause further complications such as heart failure or aneurysm of the myocardium.

Thrombosis

Thrombosis is a blood clot in a vessel. Blood clots can be caused by turbulent blood flow, which is common in hypertensive patients due to increased cardiac output. Complications can occur when a clot breaks free and travels around the body, a condition known as embolism. This is especially dangerous as the embolus can lodge into a blood vessel and cause necrosis of the tissue that the vessel usually supplies. If this happens in a coronary artery, this can lead to myocardial infarction.

Aneurysm

Aneurysms are bulging regions of an artery caused by a weakened vessel wall. The pressure in the artery leads to the vessel swelling out like a balloon, which can eventually rupture and lead to a haemorrhage.

Chronic kidney failure

This is caused by damaging the blood vessels of the kidney over a long period of time, leading to azotaemia, the build up of waste products in the blood. If the kidney failure is chronic, the damage is not reversible and may require regular dialysis or even a kidney transplant.

Who is affected by hypertension?

• In England, hypertension affects about 1 in 3 adults
• Half of all people over the age of 75 have high blood pressure
• For people under 45 years of age, the condition affects more men than women
• People of African or Afro-Caribbean descent have a higher incidence of hypertension and develop it at an earlier age than the white population

References

“Understanding blood pressure readings”, American Heart Association. 11 January 2011.
Haslam DW, James WP (2005). "Obesity". Lancet 366(9492): 1197–209
Iemitsu M, Miyauchi T, Maeda S, Sakai S, Fujii N, Miyazaki H, Kakinuma Y, Matsuda M, Yamaguchi I (2003). “Cardiac hypertrophy by hypertension and exercise training exhibits different gene expression of enzymes in energy metabolism." Hypertens Res. Oct;26(10):829-37

Klabunde E, Cardiovascular Physiology - www.cvphysiology.com "Turbulent Flow"

Dudley C www.netdoctor.co.uk "Kidney failure (renal failure with uraemia, or azotaemia) "

Centers for Disease Control and Prevention (2011) "High Blood Pressure Facts"

Whisnant JP (1996). "Effectiveness versus efficacy of treatment of hypertension for stroke prevention". Neurology 46 (2): 301–7.

Magiakou MA, Smyrnaki P, Chrousos GP (2006). "Hypertension in Cushing's syndrome." Best Pract Res Clin Endocrinol Metab. Vol. 20(3):467-82