Blood pressure is the pressure exerted by the column of blood upon the walls of the vessels carrying it. The blood pressure is not fixed but varies during a cardiac cycle. When the heart contracts and squeezes the blood out, the pressure in the arteries is highest and this is known as the systolic blood pressure. After the contraction, the heart starts relaxing and the pressure in the vessels falls. This pressure during the relaxed phase is the diastolic blood pressure. A person’s blood pressure is mainly determined by the pumping action of the heart and the condition of the vessels carrying the blood.
Blood pressure is usually measured in the upper arm using an instrument called sphygmomanometer. 24 hours ambulatory blood pressure monitoring is more reliable but is needed only in special cases. Two different pressures are measured, the systolic blood pressure and the diastolic blood pressure. So, when the blood pressure is quoted 130/80, it means that the systolic blood pressure is 130 mm of Hg, and the diastolic blood pressure is 80 mm of Hg.
The normal range for systolic blood pressure is 90 to 119 mm of Hg and for diastolic blood pressure is 60 to 79 mm of Hg. Systolic blood pressure lower than 90 mm Hg and/or diastolic blood pressure lower than 60 mm Hg warrants evaluation for the cause of low blood pressure and the affected persons are almost always symptomatic. However, very high blood pressure may be present without any symptoms, especially in the early stages of the disease. Therefore, screening by routing measurements of the blood pressure is very important to recognize raised blood pressure in the early stages.
The table below gives the current standard classification of the blood pressure readings according to JNC 7 (The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure). Please note that this classification applies for the blood pressure readings in an adult. For children, the normal blood pressure values vary with the age. Also note that for normal blood pressure, the criteria for both systolic and diastolic blood pressure should be met, for other conditions, presence of either raised blood pressure (systolic or diastolic) is sufficient for the diagnosis. So a blood pressure of 124/96 is Stage 1 Hypertension and not Prehypertension. (Read more on blood pressure chart.)
Table 1: Blood Pressure Classification according to JNC 7
| Classification | Systolic BP* | Diastolic BP |
|---|---|---|
| Normal Blood Pressure | 90 – 119 | and 60 – 79 |
| Prehypertension | 120 – 139 | or 80 – 89 |
| Stage 1 Hypertension | 140 – 159 | or 90 – 99 |
| Stage 2 Hypertension | ≥ 160 | or ≥ 100 |
| Isolated Systolic Hypertension | ≥ 140 | or < 90 |
*BP – Blood Pressure, Values of blood pressure in mm Hg
A single raised blood pressure reading is not diagnosed as hypertension and assigned one of the above listed categories. For the diagnosis of hypertension, the blood pressure should be raised on two or more separate occasions. Prehypertension is slightly raised blood pressure but not enough to warrant therapy with medications. However, lifestyle modifications are recommended for prehypertension to bring the blood pressure down in the normal range. Stage 1 Hypertension and above require therapy with drugs to control the blood pressure.