Measurement of Arterial Blood Pressure and Pulse

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Journal of Tissue Viability 1994 Vol 4 No 2

Measurement of Arterial Blood Pressure and Pulse Shyam V S Rithalia

Scientific Officer, Orthopaedic Mechanics Research Institute , University of Salford

The principles of the measurement of blood pressure are set out, together with a description of the mechanisms employed in automatic pulse monitors. The technical requirements of the measurement of pulse pressure in a variety of situations, such as in Prediatrics and the home environment are discussed. Introduction photoelectric transducers. They pick up the peripheral pressure Since the earliest days ofmodem medicine blood pressure pulse from a finger or an ear. and pulse measurements have been part of the basic clinical examination. Their origin is the pumping action Most patients have their pressure measured with a conventional of the heart. Blood pressure (BP) is the pressure exerted mercury sphygmomanometer using a stethoscope and an on the arterial walls when blood flows through the occlusive cuff placed over the brachia! artery. Accuracy of arteries. The highest value of pressure, which occurs when the heart contracts and sends outblood through the measurement depends on the experience as well as an arteries, is called the systolic blood pressure. The lowest individual's interpretation ofKorotkoff sounds4.S. Unacceptable value, which occurs when blood flows back into the errors can also occur, especially in neonates and patients with dilated heart, is called the diastolic blood pressure. The rapid heart rates or low blood pressure. In some cases direct pulse rate (P) is the number of times the heart beats or arterial blood pressure measurement is necessary, for example contracts in one minute. It is well known that the values during surgical procedures and in intensive care, particularly in of blood pressure and pulse vary significantly during the course of 24 hours according to an individual's activity 1 Three factors, Oscilloscope namely, the cardiac output, the diameter of the arteries ~,.----------. and the state or quantity of Chart Recorder Data Data ~~IAlarml blood are mainly responsible Acquisition Processing Acoustic for blood pressure level2. and/or Meter When the arteries have Visual Measure narrowed the pressure is higher than normal. KnowledgeofBP is important for assessing the condition of the systemic blood circulation Figure 1. Block Diagram Showing the Physiological Monitoring Chain and tissue perfusion. Recent studies have indicated that measurement of blood pressure is of importance when hypotensive patients where low cardiac output and peripheral classifying patients at risk of pressure sores3 • vasoconstriction make it difficultto detect pressure indirectly6·7 •

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Measurement Techniques The basis of any physiological measurement system is the biological signal. This has to be sensed, transduced or converted from one form of energy to another, displayed and recorded (Figure 1). Transducers are used to detect mechanical signals, such as blood pressure waves, and to convert them into electrical signals for transmission. A pulse rate meterusually forms an integral part of the blood pressure monitor. The rate is derived from the number of pressure waves and given as beats per minute (beats/min). On the other hand, discrete pulse sensors may be either crystal microphones or

Direct MethOcl An artery is cannulated for direct arterial blood pressure measurement Typically, a 20 gauge teflon-coated cannula is inserted percutaneously into the radial artery. The key item in the pressure monitoring system is a pressure transducer (Figure 2), attached to the monitor. The transducer may take one of several forms, from a variable resistance diaphragm to a silicon microchip. A basic system consists of an intravascular cannula connected to a rigid fluid filled catheter which communicates the pressure to an elastic diaphragm, the deflection of which is detected electrically. There is a direct relationship between the deflection of the diaphragm and the voltage. The higher the

Journal of Tissue Viability 1994 Vol 4 No 2

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Figure 2. Blood Pressure Transducers. From left to right: SE Laboratories SEM 4-88, Bentley Trantec and Gould PSO. Arrow indicates Transducer Diaphragm.

voltage, the greater the pressure. To maintain patency of the vascular system, the catheter is flushed continuously or intermittently with heparinised saline or dextrose solution. Extreme care should be exercised to eliminate all air bubbles from the fluid and the transducer should be positioned at the level of the heart 8 • There are many factors, such as changes in natural frequency and the damping coefficient of the monitoring system which determine the accuracy of the blood pressure recording9 • It is alsoimportanttorememberthattheapparatusfordirectpressure measurement is complex. This normally requires the skills of specialist technicians to calibrate, maintain and operate it. However, as long as the system remains properly maintained and correctly calibrated, the direct measurement provides true systemic blood pressure10 • The recognised contraindications and complications include, butare not limited to, poor collateral flow, severe hremorrhagic diathesis, occlusive arterial disease, arterio-spasm, hrematoma formation, and the risk of local infection or thrombosis11 •12• Indirect Method

This has normally taken the form of pressure estimation in the brachia! artery. An occlusive cuff placed over the upper arm is inflated to a pressure greater than the systolic blood pressure. (Figure 3) The cuff is then gradually deflated, while a detector system simultaneously employed determines the point at which

the blood flow is restored to the limb. The detector system need not be a sophisticated electronic device. It may be as simple as manual palpation of the radial pulse. Most commonly a mercury sphygmomanometer in conjunction with a stethoscope for avscultation, over the blood vessel for the Korotkoff sounds, is employed to define both systolic and diastolic pressures13• The potential sources of error are the mercury leakage from the manometer causing underestimation of pressure and blockage ofthe air vent which leads to underestimation ofblood pressure. Observers may also differ greatly in their interpretation of the Korotkoff sounds. Therefore other more elaborate methods of detection, suchas plethysmographic, ultrasonic and microphone detectors have been used in automatic BP and pulse monitors14• In recent years, electronic pressure and pulse monitors based on arm oscillometry have become very popular. The principle of blood pressure measurement using oscillometric technique is dependent on the transmission of intra-arterial pulsation to the occluding cuff surrounding the limb15 • The cuff is placed around the upper arm and rapidly inflated to typically about 30 mmHg above the systolic blood pressure. Thus the brachia! artery is occluded and the pressure in the cuff is measured by a pressure transducer. The pressure is then gradually decreased in steps of a few mmHg. Arterial pressure oscillations are superimposed on the cuff pressure when the blood vessel is no longer fully occluded (Figure 3). The sampling of pressure is carried out at a rate determined by the pulse or heart rate. The oscillation amplitudes are measured and used, together with an

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Journal of Tissue Viability 1994 Vol 4 No 2

empirical algorithm, to estimate the systolic and diastolic blood pressures. Typical examples of oscillometric blood pressure and pulse monitors are shown in Figure4.

Cuff Size Both the length and width ofthe occluding cuff are important for reliable measurements of blood pressure by noninvasive indirect methods 15 • The cuff should fit around the arm firmly and comfortably. A too short or too narrow cuff results in erroneously high blood pressure readings. In adults the recommended widths are 12 cm for lean or normal and 15 cm for obese or heavily

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Time---+ Figure 3. Schematic Diagram of the Oscillometric Method Arterial Pressure Oscillations are Superimposed on the Step Deflection of the Cuff Pressure

Figure 4. Omron oscillometric blood pressure and pulse monitors: a. HEM - 403C (Manual). b. HEM - 703C (automatic). c. HEM - 703CP (automatic with printer) d. HEM 815F (finger model)

muscled arms. For infants the width should be at least 40% of the arm circumference.Some recommendations have proposed the use of one cuff 35 cm x 12.5 cm in children and adults with arm circumference ofup to 42 cm. There are however difficulties in providing an appropriate cuff size for very small babies16•17 •

renal failure. Many researchers have demonstrated the importance of establishing a 'true' base line blood pressure before the initiation of hypertensive therapy. Furthermore, several studies have shown that a vast majority of patients exhibit higher BP readings in the clinic when their pressure is measured by a doctor or nurse18 •19 •

New Developments Hypertension is considered as one of the most important health risk factors in industrialised countries. It causes serious damage to the heart and arteries leading to cardiac infarct, stroke or

Therefore BP measurement in the clinic may be a less accurate predictor of ultimate risk than measurements carried out at home. Ambulatory monitoring as a research tool has also

Journal of Tissue Viability 1994 Vol 4 No 2 contributed extensive scientific knowledge of situational and periodic fluctuations in blood pressure. Although electronic monitors are relatively inexpensive and easy to operate it is important that they be properly evaluated before use in the home environment20 • Increasingly more clinicians are now advising their patients to record their BP and pulse at home over a period of up to 3 months before starting antihypertensive medication21 -23 • Self­ monitoring of blood pressure has become simpler with the development of microelectronics and oscillometric monitors. The patients no longer have to learn how to listen for Korotkoff sounds. This has also removed the bias and observer errors allowing more accurate measurement than by the conventionai technique using a stethoscope and a mercury sphygmomanometer. Recently, several studies have indicated that most patients welcome the greater involvement in their own care 24 •25 •

Acknowledgements My thanks are due to the staff of the Departmentment of Medical Illustration at the Hope Hospital, Salford, for preparing the figures. I also wish to thank Healthprime Products, Bolton, for the generous loan of oscillometric blood pressure and pulse monitors.

Address for correspondence Dr S VS Rithalia, Scientific Officer, Brian Bletchford Building, Statham Street, Salford M6 6 PU.

References 1.

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