Blood Pressure Measurement in Clinical Practice

University Degree Subjects allied to Medicine

The aim of this assignment is to discuss ‘the knowledge that underpins an aspect of measurement of nursing care in nursing’, with particular reference to practice experience. For this purpose, the subject of blood pressure measurement will be examined. Blood pressure is a process vital to homeostasis, and the efficient operation of a number of fundamental functions within the body. The two main purposes blood pressure serves, is the circulation of blood flow throughout the body and vital organs such as the brain and coronary arteries, and the perfusion of tissues with blood in order to supply nutrients and remove waste products (Hinchliff, Norman and Schober, 1998).

Consequently, blood pressure measurement is an important observation that is commonly used in a clinical setting, to aid the diagnosis, treatment and prevention of a number of cardiovascular and related disorders. Measuring the blood pressure helps to assess the efficiency of the circulatory system. The importance of these measurements lies in the fact that recent figures show 14% of deaths from coronary heart disease in men, and 12% of such deaths in women are associated to raised blood pressure (National Heart Forum, 2002).

To effectively monitor blood pressure, it is first necessary to understand the basic physiological principles that regulate such a function. Blood pressure may be defined as the force applied by blood flow, on the resisting walls of blood vessels. This pressure varies within different areas of the circulatory system, with pressure being highest in the aorta, and gradually decreasing as blood is pumped on through the large arteries, small arteries, arterioles, capillaries and eventually the veins (Walsh, 1999).

Blood pressure is expressed as two numbers, e.g. 140/80. This is due to the fact that there are two phases within a single heart beat. Systolic pressure is the highest arterial blood pressure of a cardiac cycle, and occurs during the period when the left ventricle of the heart contracts, and blood is pumped out of the heart. Diastolic pressure occurs in the period between two contractions, when the chambers of the heart widen, and fill with blood. The difference between these pressures is called the pulse pressure.

Historically, blood pressure has been measured in millimetres of mercury (mmHg), using a sphygmomanometer and stethoscope. Although electronic devices are more widely available now, they are more expensive, and require frequent calibration. Therefore, sphygmomanometers are still the most common apparatus used for blood pressure measurement in more general clinical settings. The sphygmomanometer is an instrument used to measure arterial blood pressure. The most common type is that pioneered by the Italian Scipione Riva-Rocci in 1896 (O’Brien and Fitzgerald, 1994).

It consists of a rubber cuff fastened around the upper arm and attached to a manometer, which is a device used to measure the pressure of a gas or liquid. The pressure is usually measured using a mercury gauge, which measures the pressure by way of liquid mercury within a column calibrated in millimetres (Lewis and Kuhn, 1994), although aneroid gauges are also sometimes used.

The actual technique used in the measurement of blood pressure can often present several potential sources of error (Watson 1997). For example, measurement using a sphygmomanometer is a subjective, and can in itself be subject to error. The British Hypertension Society has established guidelines on the routine reading of blood pressure measurement, which will be summarized subsequently. By following these guidelines, at least some of these sources can be excluded. There are a number of additional factors that can also contribute to inaccurate measurements, and these will be discussed later.

In order to maintain accurate blood pressure measurements, ...

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In order to maintain accurate blood pressure measurements, it is important to use a device that is properly maintained and calibrated. The mercury level should read zero when not in use, and should be placed on a flat stable surface and kept vertical throughout the procedure. The control valve is one of the commonest sources of error in sphygmomanometers and when it becomes defective it should be replaced. The patient should be relaxed, and ideally in a lying position, although a sitting position is widely accepted. It is recommended however, that in the case of elderly or diabetic patients measurements should be taken in a standing position to exclude orthostatic hypotension (Ramsey et al, 1999). The arm should be supported at the level of the heart, and tightly fitting clothing should be removed from the upper arm as this can also contribute to an inaccurate reading.

The circumference of the arm should then be measured, and the appropriate sized cuff should be applied, without any folds or twists. A stethoscope, which should be in good condition with well fitting ear pieces, is placed on the brachial artery. The cuff is inflated by squeezing the bulb at a rapid steady pace, until the pressure exceeds that within the brachial artery. This is the point at which the pulse can no longer be heard through the stethoscope. Hinchcliffe et al (1998) recommend that the maximum inflating level of the cuff should be 20 – 30 mmHg higher than the level at which the pulse disappears. It is also advisable to minimise any background noise that may make it difficult to hear the pulse, for example if the patient is chewing or eating. It is also possible that sudden noises could possibly startle the patient, or cause vibration that may give a false reading on the sphygmomanometer.

With the stethoscope still placed over the brachial artery, the cuff should then be deflated by means of the valve, at a rate of 2mm/sec (Ramsey et al, 1999). A series of sounds known as Korotkoff sounds are then listened for, which undergo five phases. During phase I, as the cuff reaches systolic pressure, a faint but clear tapping sound is heard, the first two taps actually represent the systolic pressure and at the point at which these sounds are heard, a reading should be taken from the sphygmomanometer. The taps then increase in intensity during phase II, until they reach maximum intensity at phase III. These sounds then become muffled throughout phase IV until finally disappearing. The point at which the sounds disappear completely is known as phase V, and it is at this point the reading for diastolic pressure is taken. Both systolic and diastolic readings should be taken to the nearest 2mmHg (Lewis and Kuhn, 1994). It is also advisable to take two measurements on each visit, preferably from each arm, as there can occasionally be a “consistent difference between the two arms” (Hurst, 2002). In such cases, the highest reading is used.

The electronic blood pressure measuring devices previously mentioned work using the same principle as the manual sphygmomanometer. By using a pressurising and depressurising cycle systolic and diastolic pulses can be detected in the cuff, and are picked up by way of a microphone or sensor under the cuff. Such devices are commonly found in clinical areas where patients are completely immobile, for example intensive care units.

Ambulatory blood pressure monitoring, (ABPM) is an increasingly popular method of recording blood pressure. It permits the measurement of blood pressure over a prolonged period of time, usually 24 hours (Hurst, 2002). The process uses an electronic device that is continuously attached to the patient. Consequently, the patient may be removed from a clinical environment that could possibly affect a true reading. By taking a large number of readings in such a way, a true picture of the patients usual long-term blood pressure can be extrapolated. This system is however more expensive than more conventional methods.

The British Hypertension Society (2000) recommend that all adults should have their blood pressure routinely measured at least every five years up until the age of eighty. There is no single normal blood pressure for healthy individuals as blood pressure can vary due to a number of factors. However, it has been established that an optimal reading for a healthy adult should be less than 120mmHg systolic pressure, and 80mmHg diastolic pressure (Tortora and Grabowski, 2002).

Diabetes is a disease that is known to bear influence on blood pressure. Those suffering the condition will have a notably wider variance in pulse pressure than those unaffected by the condition. The target set by the British Hypertension Society for these individuals is a measurement below 140/80mmHg. What is classed as a normal to high reading for those not suffering from diabetes will have a systolic blood pressure level between 135 and 139mmHg, and a diastolic blood pressure of between 85 and 89mmHg. Individuals who have been identified within this range should be reassessed annually (British Hypertension Society, 2000). In the case of those individuals whose blood pressure exceeds 160/100mmHg diastolic pressure, the British Hypertension Societies recommendations suggest intervention by way of drug therapy.

There are a number of common factors that influence blood pressure, and measurements within a healthy individual can fluctuate within a wide range whilst still being classed as normal. For example exertion will increase pulse rate, thus raising blood pressure. It is therefore recommended that when taking such observations, it should be established that a patient has not recently undertaken any exercise. Extreme emotion can also provoke such results.

Age is a highly influential factor in the level of an individual’s blood pressure. As we age, both the systolic and diastolic pressure gradually rise (Watson, 1997). A rule of thumb is that systolic pressure is age in years plus one hundred in a healthy individual. Blood pressure also varies dependant on the time of day (Lewis and Timby, 1993), with the pressure being lowest in the morning, and rising later in the day and early evening. Gender also bears prevalence on blood pressure, with women generally having a lower reading than a man of the same age.

Raised blood pressure is a common condition that presents no “specific clinical manifestations until target organ damage develops” (McAlister and Straus, 2001:908). Consequently, many patients do not realise they have such a problem until they suffer some form of cardiovascular related failure. The clinical term for consistently high blood pressure is hypertension, and is a more specific condition than a generally raised blood pressure. In order for a patient to be diagnosed with hypertension, any elevation in blood pressure must be persistently higher than that of normal for the individual’s age. Tortora and Grabowski (2002) classify a blood pressure value for an adult suffering from hypertension, at a systolic pressure of above 140mmHg and a diastolic pressure of above 90mmHg, and this value is widely accepted throughout clinical studies.

Hypertension is a common condition that affects a large number of the population. The fact that initially it presents no symptoms, has earned it the dubious title of “the silent killer” (Tortora and Grabowski, 2002:758). Clearly, the need for early detection and intervention is of paramount importance. Hypertension can be classified into two distinct categories, essential or primary hypertension and secondary hypertension. In most people diagnosed with hypertension, no specific cause can be found, and this is known as essential hypertension. It is commoner in older people and in Afro Caribbean individuals (Long et al, 1995), though it can occur at any age and in any race.

There are many different causes for secondary hypertension in the remaining cases. Each of these are quite rare and usually contributed to underlying problems, such as renal disease or problems with the adrenal gland. Medication is also a known factor. Despite the fact there are no clear identifiable causes for primary hypertension, there are a number of factors that are thought to contribute to the condition, and it is probably attributed to a combination of these. Obesity, stress, low birth weight, high salt intake, neurovascular anomalies, the sympathetic nervous system and insulin resistance are just some of the factors which are thought to play a role (Hurst, 2002).

A phenomenon that is becoming more common is that of “White coat effect”. The white coat effect can be defined as a rise in blood pressure associated with the procedure of having blood pressure measured. It occurs in many patients suffering from hypertension, and its clinical importance is that patients with hypertension may appear more hypertensive than is actually the case. In such cases, ambulatory blood pressure measurement is recommended.

In conclusion, blood pressure can be a highly complex measurement to both undertake and further treat. Accurate measurement is of utmost importance. McAlister and Strauss (2001) indicate that if diastolic pressure is consistently underestimated by as little as 5mmHg, the inaccuracy could serve to deny almost two thirds of hypertensive patients treatment. Alternatively, overestimating by the same figure could potentially double the current figures for hypertensive patients, half of who would have been inappropriately labelled and treated.

To take an accurate reading, guidelines should be consistently adhered to, and contributory factors should always be taken into consideration. All associated equipment should be regularly maintained and checked before and after use. In order to fully appreciate any findings of a blood pressure measurement, a full history of the patient should be taken prior to making any diagnosis on the results. In addition to basic observations in such areas, the nurses’ role should further serve to promote healthy living in individuals, in order for such conditions to be prevented at cause.

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