Measuring arterial pressure (taking blood pressure)

The pressure of the blood in the arterial system varies rhythmically, attaining its maximum during systole and lowering during diastole. This is explained as follows; when blood is ejected during systole it meets the resistance of the arterial walls and the blood contained in the arterial system; the pressure in the arteries thus increases to cause distention of the arterial walls. During diastole the arterial pressure falls and remains at a certain level due to the elastic contraction of the arterial walls and resistance of the arterioles, owing to which the blood flow into the arterioles, capillaries, and veins continue. It follows therefore that arterial pressure is proportional to the amount of blood ejected by the heart into the aorta (i. e. the stroke volume) and the peripheral resistance. Arterial pressure is expressed in millimeters of mercury column.

The normal systolic (maximal) pressure varies from 100 to 140 mm Hg and diastolic (minimal) from 60 to 90 mm Hg, The difference between systolic and diastolic pressure is called the pulse pressure (normally it is 40-50 mm Hg). Arterial pressure can be measured by a direct or indirect method. In the direct method, the needle or a cannula is introduced directly into the artery and connected to a pressure gauge. This method is mostly used in heart surgery.

Three techniques exist to take blood pressure indirectly. These are auscultatory, palpatory, and oscillographic.

The auscultatory method is commonly used in medical practice. The method was proposed by N. Korotkoff in 1905 and is used to measure systolic and diastolic blood pressure. A sphygmomanometer is used to read pressure. It consists of mercury or a spring manometer which is connected to a cuff and rubber bulb 63 used to inflate the cuff through a connecting tube. A valve on the bulb is used to admit air into the cuff and the manometer, and to hold pressure at the needed level. A more accurate instrument is a Riva-Rocci mercury manometer. This is a mercury container communicated with a thin vertical glass tube attached to a scale graduated in millimeters from 0 to 300. The pressure in the brachial artery is usually measured. To that end the arm of the patient is freed from tight clothes and a cuff is attached snugly and evenly onto the arm (a finger-breadth between the cuff and the skin). The inlet socket of the cuff should be directed downward, 2-3 cm above the antecubital fossa. The arm should be placed comfortably on a level surface, the palm up; the muscles of the arm should be relaxed (Pict. 5).

Picture 5. General recommendations about preparation to blood pressure measurement

 The phonendoscope bell is placed over the brachial artery in the antecubital space, the valve on the bulb is closed, and the air is pumped into the cuff and the manometer. The pressure of the air in the cuff that compresses the artery corresponds to the mercury level as read off the instrument scale. Air is pumped into the cuff until the pressure inside it is 30 mm above the level at which the brachial or radial artery stops pulsating. The valve is then opened slowly to release air from the cuff. Using the phonendoscope, the brachial artery is auscultated and the readings of the manometer followed. When the pressure in the cuff drops slightly below the systolic, tones synchronous with the heartbeats are heard. The manometer readings at the moment when the tones are first heard are taken as the systolic pressure. This value is usually recorded to an accuracy of 5 mmHg (e.g. 135, 130, 125mmHg, etc.). When the pressure inside the cuff equals diastolic pressure, and the blood flow is no longer obstructed, the pulsation of the vessel suddenly decreases. This moment is characterized by a marked weakening and disappearance of the tones.

Picture 6. General scheme of blood pressure measurement

Picture 7. General scheme of blood pressure measurement-2

In normal conditions an arterial pressure changes in rather appreciable limits -from 100/70 - up to 140/90 mm Hg. In a healthy female, the inferior border of normal arterial pressure (BP) compounds 90/60 mm Hg.

According to the classification of levels the BP (WHO/ISH, 1999) the following categories of the normal BP:

Optimal BP - less than 120/80 mmHg,

Normal BP - less than 130/85 mmHg,

High normal BP - 130/85 -139/89 mm Hg.

Arterial pressure of healthy subjects varies physiologically within a certain limit depending on physical exertion or emotional strain, the posture, time of meals, and other factors. The lowest pressure is normally observed at rest, before breakfast, in the morning, i.e. in conditions under which basal metabolism occurs. This pressure is therefore called basal. When pressure is taken for the first time, it may appear slightly higher than actual which is explained by the patient's response to the procedure. It is therefore recommended that pressure be taken several times at a run without taking off the cuff but only deflating it completely. The last and the least value should be considered the closest to the true pressure. A transient increase in the arterial pressure may occur during heavy exercise (especially in persons who are unaccustomed to it), in excitation after taking alcohol, strong tea, or coffee, in heavy smoking, or during attacks of intense pain. Many diseases are attended by changes in arterial pressure.

Elevation of the systolic pressure over 140 mm and of diastolic over 90 mm Hg is called arterial hypertension. A drop in the systolic pressure below 100 mm and of diastolic below 60 mm Hg is known as arterial hypotension. Longstanding elevation of arterial pressure occurs in essential hypertension, many renal diseases (glomerulonephritis, vascular nephrosclerosis), certain endocrinological diseases, and heart diseases, etc. Systolic pressure alone is sometimes elevated, whereas diastolic pressure remains normal or decreased. This causes a marked increase in the pulse pressure. This condition occurs in aortic incompetence, thyrotoxicosis, less markedly in anemia of any etiology and atherosclerotic affections of the vessels.