High blood pressure causes damage to blood vessels

Blood pressure is the force exerted by circulating blood on the inside walls of blood vessels. The pressure wave transmitted by the movement of blood along the arteries with each heartbeat is felt as the pulse. Two components of blood pressure can be distinguished: the highest pressure (systolic blood pressure, SBP) occurs each time the heart pushes blood into the arteries and the lowest pressure (diastolic blood pressure, DBP) occurs when the heart fills with blood. The mean arterial pressure is the average pressure over one complete cardiac cycle. Blood pressure is determined by the interplay of cardiac output and peripheral vascular resistance. Vasoconstriction leads to an increase in total peripheral resistance and blood pressure. The majority of patients with hypertension have a normal cardiac output but a raised peripheral resistance which is determined by the presence of small arterioles (Beevers et al., 2001). An increased blood pressure leads to damage of the arterial walls and to atherosclerosis.

Definition of high blood pressure in current clinical guidelines

High blood pressure, medically referred to as hypertension, is usually defined on the basis of thresholds for systolic and diastolic blood pressure, measured in mmHg. However, systolic and diastolic blood pressure show a continuous, consistent and independent relationship with cardiovascular diseases. Mortality from cardiovascular diseases increases linearly and progressively with increasing blood pressure levels above 115 mmHg systolic and 75 mmHg diastolic, as explained further in the Consequences section (Lewington et al., 2002). Given the continuous nature of this association, any classification of hypertension based on cutoff values is arbitrary. Despite this fact, for practical reasons and to guide clinicians, definitions and classifications of high blood pressure have been issued and continuously modified by major societies and organisations. Recent guidelines have been issued by:

• the World Health Organisation/ International Society of Hypertension;
• the American Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure; and
• the European Society of Cardiology (ESC) and European Society of Hypertension (ESH).

In addition, most countries, such as, for example, the United Kingdom, France and Germany have adopted national hypertension guidelines and recommendations.

Thresholds for hypertension should only be used under certain conditions

The 2007 ESH/ ESC Guidelines emphasise the arbitrariness of thresholds to classify hypertension. For clinical purposes and to facilitate treatment approaches in daily practice, the guidelines propose a classification scheme that may be used under the following conditions (ESH/ESC Task Force, 2007):

• When SBP and DBP fall into different categories a risk assessment and treatment decision should be made for the higher category;
• Isolated systolic hypertension (only SBP is elevated) should be graded according to the same SBP values indicated for systolic-diastolic hypertension. The association of high SBP with low DBP can be regarded as an additional risk;
• Thresholds for hypertension and the need for drug treatment should be regarded as flexible based on the total cardiovascular risk.

European guidelines recommend different hypertension thresholds than American guidelines

As shown in the tables below, the existing European guidelines recommend different hypertension thresholds than the American guidelines.

European Guidelines for the classification of blood pressure levels (mmHg) (ESH/ESC Task Force, 2007)

Isolated systolic blood pressure should be graded (1,2,3) according to systolic blood pressure values in the ranges indicated, provided that diastolic values are <90 mmHg. Grades 1, 2 and 3 correspond to classification in mild, moderate and severe hypertension, respectively. These terms have now been omitted to avoid confusion with quantification of total cardiovascular risk.

American Guidelines for the classification of blood pressure for adults (US Department of Health and Human Services, 2004)