MEASUREMENT OF BLOOD PRESSURE. Department of Clinical Physiology, The Hospital for Sick Children, Great Ormond Street, London, W.C.

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1 Brit. J. Anaesth. (1962), 34, 646 MEASUREMENT OF BLOOD PRESSURE BY GERALD R. GRAHAM Department of Clinical Physiology, The Hospital for Sick Children, Great Ormond Street, London, W.C.I The arterial blood pressure is normally the resultant of the interplay of systemic flow (cardiac output) and peripheral "run-off", the amount of blood passing into and through the capillary bed, which is regulated by the amount of resistance offered to blood flow by the arteriolar "stopcocks". Being largely determined by the two variables, cardiac output and total peripheral vascular resistance, blood pressure can give only very limited information about blood flow to the organism and its parts. Yet it is the blood pressure which alone of the several variables which have been mentioned is ordinarily accessible to measurement. Fortunately, a detailed analysis of blood pressure can indirectly reveal much about the state of the circulation, and can be most usefully employed, as long as the limitations of the information so supplied are taken into account (cf. Graham, 1960). The intermittent contractile action of the heart, during which blood is propelled into the elastic aorta where some of it is momentarily "stored", causes the division into systolic and diastolic pressures, each of which depends on several factors. The systolic pressure depends largely on (a) the contractile power of the left ventricle and the amount of blood within it, and (b) the elasticity of the aorta and its major branches. The diastolic pressure depends on the interplay between the amount of blood taken up by the aorta during systolic ejection, the pressure maintained upon the blood by the elastic recoil of the aorta, and the amount of blood which passes out of the aorta and the major arterial vessels into the tissues. The various factors which determine the level and "shape" of the systolic and diastolic pressures can often be separated if the blood pressure can be analyzed in its phasic details from moment to moment and, in addition, related to such other factors as heart rate. However, the common method of indirect blood pressure measurement determines only "peak" systolic and "trough" diastolic levels (and their difference, pulse pressure); the details of the arterial pressure curve must thus be deduced by interpolation, which is often far removed from the true picture, an example of which is shown in figure 1. VALUE OF BLOOD-PRESSURE ESTIMATION As the only accessible and easily measurable parameter of cardiovascular function, the arterial blood pressure is of great importance clinically. To the anaesthetist it is, together with the heart rate, the most immediate evidence of the state of the circulation. The "volume" or amplitude of the pulse is chiefly determined by the pulse pressure (systolic minus diastolic), but also affected by the degree of counter-pressure exerted by the palpating finger. While the palpation of the artery is thus inferior to the auscultatory method in determining pulse pressure, it gives considerable information about the contour of the pulse wave. A slowly rising pulse is one of the signs of aortic stenosis (in which case the pulse pressure will probably be narrow), or of a low stroke volume. A rapidly rising pulse may be due to a large systolic discharge, as in aortic regurgitation; greater rigidity of aorta or large arteries, as in arteriosclerosis; or the summation of reflected waves, as in intense vasodilatation. The width of the pulse pressure is a function not only of stroke volume, aortic elasticity and peripheral resistance but also of heart rate. A wide pulse pressure will be a normal rinding if there is bradycardia. Obversely, a narrow pulse pressure may be expected in tachycardia. Taken in conjunction with the heart rate, the pulse pressure will however be an important indication of cardiac output and peripheral resistance, especially if it has 646

2 MEASUREMENT OF BLOOD PRESSURE 647 mm Hg FIG. 1 Blood-pressure record obtained in a child via an indwelling Riley needle in the brachial artery. Note the variation in peak systolic pressure (A) with respiration. B points to the diastolic pressure as obtained by the cuff method. The shaded area, if planimetered, gives the mean pressure. The relatively high frequency waves during the diastolic phase are vibration artefacts. Note the P-wave inversion and short P-R interval in the simultaneously recorded electrocardiogram. These abnormalities indicate a nodal rhythm. been observed to have changed. Thus, in haemorrhage the mean pressure may remain unchanged but the systolic level may have fallen while the diastolic level has risen. Narrowing of the pulse pressure will be particularly significant if the heart rate has not changed. Obviously, the blood pressure (systolic and diastolic) may be within normal limits even after severe haemorrhage if tachycardia and peripheral vasoconstriction provides sufficient compensation for the fall in cardiac output. This response may mask a completely inadequate blood volume and cardiac output. Marked, more or less rhythmic, changes in blood pressure (usually parallel changes in systolic and diastolic pressure) may be observed in various arrhythmias, such as normal respiratory sinus arrhythmia, atrial fibrillation, or different types of extrasystoles. ARTERIAL BLOOD PRESSURE DIRECT METHOD This, historically the first approach, consists of the insertion of a needle or catheter into an artery and the transmission of the pressure along a column of fluid to a suitable manometer. Choice of site. Under ordinary circumstances, a peripheral artery is chosen at its most superficial course which makes it accessible to percutaneous puncture after infiltration with a local anaesthetic. The brachial artery just above the antecubital fossa is generally the preferred site. But halfway up the arm, where the artery courses medial to the biceps muscle alongside the median nerve, is another useful site. The femoral artery just below the inguinal ligament at the fossa ovalis presents a good alternative which is often chosen, particularly in children. The radial artery is rather more difficult

3 648 BRITISH JOURNAL OF ANAESTHESIA to puncture, because it is smaller, lies deeper and is more prone to go into spasm. Obviously, any of these sites may be chosen if it is decided to expose the artery for cannulation under direct vision. The latter approach may be necessary in small children or where continuous pressure recording makes the insertion of a tube advisable (but see below). The position of the patient and his limbs during an operation and thus the accessibility will be an important factor in choosing a site. Needles. Either a Cournand or a Riley needle gives satisfactory results. The former consists of a double needle, the internal, smaller, one being used for "finding" and piercing the artery. Once the arterial lumen is entered, the whole needle is slightly advanced, the inner one removed and the needle connected to the recording system. The Riley needle has no inner needle and is thus particularly suitable for children where small-bore needles are necessary. The insertion of needles is satisfactory for short-term arterial pressure measurements. For prolonged recordings, however, the insertion of a small polythene or nylon tube, either through a Cournand-type needle after it has been placed well into the lumen or by cannulation under direct vision, is essential. Dangers. The presence of peripheral vascular disease or haemorrhagic disorder is a contra-indication to the recording of arterial blood pressure by direct needle puncture, but the risks of thrombosis in the former and of severe bleeding in the latter must be balanced against the need for obtaining intra-arterial pressure readings. Meticulous technique and careful haemostasis, if necessary under direct vision, will reduce the dangers. Percutaneous puncture on the whole is less likely to cause bleeding in the absence of demonstrable vascular disease or haemorrhagic diathesis than puncture under direct vision in which the surrounding tissues have been disserted away. On the other hand, the latter technique allows immediate inspection and, if necessary, a small suture with an atraumatic needle may be inserted in the artery if the needle hole does not close up spontaneously. There is no doubt that even in the absence of disease arterial puncture may cause extensive bleeding, but this is a rare complication. It should be borne in mind that a large amount of blood may extxavasate into the surrounding tissues before it is recognized. Arterial spasm occurs not infrequently after either percutaneous or open puncture, but is hardly ever troublesome. The danger of thrombosis or ischaemia resulting from prolonged spasm must, however, be admitted. The generous infiltration of local anaesthetic is said to reduce the incidence of arterial spasm. Pethidine (50 mg/ml) sprinkled on to the exposed artery has been used occasionally to relieve spasm. In case of severe and prolonged arterial spasm papaverine may successfully relieve it (it is injected into the artery proximal to the site of spasm). Continuous recording. Because of the likelihood of displacement of the needle during long-term recording of the blood pressure, the insertion of a small polythene tube is preferable. It can be threaded into the artery either after this has been opened by scissors or, if at all possible, directly through a large needle which has first been placed into the arterial lumen. The difficulties of continuous recording are largely connected with the need for preventing clotting in the connections to the manometer. This requires frequent flushing from a pressurized bottle containing saline or glucose, possibly with the addition of small amounts of heparin. The likehood of clotting is reduced by making quite sure that there are no leaks in taps and other parts which would allow blood to run back. Frequency response. Questions of accuracy apart, the main reason for preferring the direct to the indirect method of recording blood pressure lies in the possibility of analyzing the detailed contour of the pressure curve. But this requires the accurate reproduction of moment-to-moment pressure changes, which is dependent on the frequency response of the entire recording system from needle via all connections and the pressure transducer to the recording apparatus itself. The frequency response of the entire system is as good as its weakest (i.e. slowest response) link. Modern transducers have an inherently high frequency response which may, however, be cancelled out either by a low frequency response of the connecting tubes or catheters (too long and of too small a calibre) or an inadequate recording system. Furthermore, frequency response is inevitably linked with the other characteristics of a recording system, i.e. its sensitivity, stability, reliability and ease of handling.

4 MEASUREMENT OF BLOOD PRESSURE 649 It is usually stated that the accurate reproduction of a pressure-wave requires a frequency response of tie entire system which is the tenth harmonic of the fundamental wave frequency. At a heart rate of, say, 180 per minute the fundamental wave frequency will be at least 3 per second, requiring a frequency response of about 30 per second (the tenth harmonic). As has been pointed out above, even an adequate frequency response will be of little value, if the sensitivity of the apparatus is inadequate, if it is unstable during prolonged recording and most important of all if the amplification necessary to convert changes of pressure to a recordable trace is not linear along the entire range of pressures obtaining at the time. Severe distortions can, for example, occur if the amplification at the "peak" systolic pressure range were much greater than at the diastolic range. The fluid-filled tubing which makes the connection from artery to transducer must be as short and rigid as possible in order to reduce the frequency response of the system as little as possible. While the inertia of the fluid column is the greater the larger its mass (which argues against tubing of a large calibre), this must be balanced against the fact that frictional resistance to the movement of fluid is increased as the calibre of tubing decreases. In view of the dimensions usually obtaining within the cardiovascular system, it is best to introduce as large a tube (or needle) as possible. However, recently transducers have become available which respond accurately and sensitively to the minutest displacement caused by pressure waves: good pressure recordings may then be obtained even through relatively small-bore tubing. APPARATUS Mercury manometers. This is the oldest type, still extensively used in animal work. Its main deficiency is that the great inertia of the mercury column makes it unsuitable for anything but meanpressure determinations. Secondly, it is not easily modified for sterile and non-toxic use. Thirdly, the pressures obtained require rather cumbersome recording apparatus in which a writing arm, floating on top of the mercury column, inscribes a trace upon a kymograph. Capsule manometers. This apparatus consists of a capsule covered by a rubber or metal membrane upon which a mirror is fixed. The capsule and its connections to the artery are filled with saline. Bulging and flattening of the membrane as the pressure transmitted from the artery rises and falls causes movement of the mirror which can be recorded and quantified by reflecting light upon a suitable camera. A highly accurate pressure pulse can be obtained in this manner and an adaptation of this system is available commercially. Electronic pressure transducers (cf. Rushmer, 1961). There are now four basic types: (a) resistance-wire strain gauge, (b) capacitance manometer, (c) variable inductance pressure gauge, and (d) semi-conductor gauge. In each the principle is the conversion of displacement or distorsion of a membrane or similar device, caused by the pressure changes transmitted to it along a column of fluid, into an electrical current, which is amplified into a visual record upon a cathode-ray oscilloscope or direct-writer. All but the semi-conductor transducer are available commercially for biological work. Their main disadvantage (apart from price) is the difficulty of cleaning and sterilizing them and their relative heat sensitivity. The latter disadvantage does not apply to the semi-conductor type, which is now being adapted to biological work and may well present a considerable practical advance. In general, electrical manometers have many advantages over mechanical ones, apart from sensitivity and fidelity of response. They do not require a rigid position, in relation to either the patient or the recording apparatus, and are capable of a wide range of pressure scales. RECORDING APPARATUS Apart from the kymograph, the use of which is essentially restricted to mercury manometers, there are three ways of converting the electrical signal from the transducer after suitable amplification into a visible trace. One is to couple the transducer and amplifier to an optical galvanometer in which a light beam is reflected on to a camera. This method gives a good frequency response, but requires that the film be developed; the trace is thus not immediately available for inspection. An attempt has been made to overcome this disadvantage by using an ultraviolet light beam upon sensitive paper which, on exposure to light, produces a visible trace within a matter of seconds. However, at present the definition of the resultant trace is not very good, and the system is not suitable where an instantan-

5 650 BRITISH JOURNAL OF ANAESTHESIA eous pressure record is required. The second method is to amplify the transducer signal enough to activate a writing stylus or pen or ink jet. This type of recorder is at present the most popular one. The third method is to link the transducer to a cathode-ray oscilloscope. This has the great advantage of requiring relatively little additional amplification and having a practically unlimited frequency response. Obviously, the oscilloscope can be used for monitoring pressures in connection with the other two methods of recording. This combines the advantage of an immediate highfrequency trace with a permanent record. INDIRECT METHOD This derives from the work of Riva-Rocci who in 1896 introduced the method of compressing the arm with an inflatable pneumatic cuff and observing the pressure in the cuff (measured by a mercury manometer connected with it) when the radial pulse first became palpable on slow decompression. This means of obtaining the systolic pressure has remained essentially unaltered to the present day. But it has been complemented, if not replaced, by the ausculatory method which is based on the work of Korotkow, published in He discovered that on decompressing the cuff a definite and practically unvarying sequence of sounds could be heard if a stethoscope was applied just distal to that point of compression. The level at which sounds first appear is for practical purposes the same as the systolic pressure measured by the direct method. The diastolic pressure obtained by the auscultatory method has been the subject of much controversy. Korotkow himself, and many since his day, have taken the disappearance of all sounds as the diastolic pressure (Bordley et al., 1951). But the more usual procedure is to take as the diastolic pressure the beginning of muffling or abrupt fading of the sounds (phase 4) before they disappear completely. This is the method recommended by a committee of the American and British Cardiac Societies (1939). The loudness of the various Korotkow sounds depends, in part at least, on stroke volume and velocity of blood flow. Another method consists of the oscillometric detection of the pulsation distal to the pneumatic cuff: the pressures in the cuff at the moment of full and absent deflections are taken to be systolic and diastolic pressures, respectively. A modification of this procedure records the distal pulsations by the oscillometer: systolic and diastolic pressures are determined from the shape and size of the deflections. Cuff size. The accuracy of the values obtained with the sphygmomanometer depends on the correct width of the cuff in relation to the size of the arm (or leg) which it surrounds. The pressure in the cuff is transmitted to the greatest depth at its centre. If the cuff is of the proper size and placed correctly, the pressure in the cuff (as indicated by the mercury or aneroid manometer) is that obtaining in the tissues surrounding the brachial artery and thus accurately reflects the pressures within the artery on complete occlusion and release, respectively. If, however, the cuff is too narrow, the pressure within the artery will be significantly less than that recorded from the cuff (i.e. pressure required to occlude the artery must be higher than that existing in the artery); the diastolic and systolic pressures will thus be read too high. Falsely high readings will also be obtained if the cuff is applied too loosely. The width of the cuff is of special importance when measuring blood pressure in infants and children (cf. Moss and Adams, 1962). In neonates a width of 2.5 cm is usually optimal while in older infants one of 5 cm is best. Other standard cuff widths are 7 cm, 9.5 cm, 12 cm and 18 cm. The latter is generally employed in adults. As a general rough rule, the width of the cuff in children should be at least one-third the circumference of the arm (or leg), or cover no more than two-thirds the length of the upper arm (or thigh). In any case, for practical purposes the largest size cuff is least likely to cause a false reading. If the cuff is too broad, the pulse wave may, however, be damped and the resultant recording be too low, especially if the pulse pressure is wide. Inflation of the cuff. The arm should be held or placed in such a way that the cuff is approximately at the level of the heart. The cuff is rapidly inflated, while the radial pulse is palpated to about 30 mm Hg above the point at which the peripheral pulsation is no longer detected. The diaphragm part of the stethoscope having been placed lightly over the brachial artery just below the distal edge of the cuff, the cuff pressure is quickly deflated at a rate of about 3-5 mm Hg/sec. The pressure at which sounds first appear gives the systolic pres-

6 MEASUREMENT OF BLOOD PRESSURE 651 sure. The deflation should continue at about the same rate until the sounds become muffled (the diastolic pressure) and finally disappear. The reason why palpation should always be practised as well is the "auscultatory gap". For a variety of causes, such as that cuff inflation has lasted too long, there may be a silent period below the true systolic pressure although the pulse is palpable at the wrist; if a cuff has been inflated to a level within this gap, the appearance of sounds on deflation may be mistaken for the systolic pressure. The auscultatory gap is not rare in arterial hypertension and conditions associated with an increased venous pressure. Normal values. Variations in the same person at different times and discrepancies between pressure values for the two sides are such that too much reliance must not be placed on single values. In measuring the blood pressure by the indirect method one should avoid the pretence of accuracy and record values only to the nearest 5 or 10. One should always measure the pressure in each arm, and femoral pulses should always at least be palpated. It has been estimated that in some 10 per cent of normal persons the right-arm pressure is higher by at least mm Hg than the left. At birth the femoral systolic pressure is significantly higher than the brachial pressure. The two values become equal at about 9-12 months, after which the femoral pressure progressively increases above that in the arm. In children 2-15 years old, the normal pressure range is systolic and diastolic. In older children and adults up to 40 years, values of systolic and diastolic represent average figures, but deviations of ± 10 mm Hg from such "normal" levels should not be given much weight if unsupported by other evidence of disease. One other point of interest to anaesthetists is that in patients who are lying on their side, the pressure reading in the lower arm may be significantly higher (by 20 mm or so Hg) than that of the upper arm. FLUSH METHOD In infants the poor quality of the Korotkow sounds especially in the arm makes it often impossible to use the conventional method of measuring blood pressure. The flush method is a way of obtaining mean pressures. The hand or foot is tightly wrapped up to the cuff which has been placed just above wrist or ankle. The pressure in the cuff is then rapidly raised to about 200 rnm Hg and the wrapping removed. Following this the cuff pressure is slowly released (at a rate of 3-5 rnm Hg/sec) until a distinct flush occurs in the previously blanched hand or foot: this is the mean arterial pressure. Care must be taken that the child is lying quietly without crying. Sedation may be necessary. CONTINUOUS RECORDING BY INDIRECT METHOD Although contrivances are available for recording arterial pressures by automatic sphygmomanometry using the Korotkow sounds as recorded by a small microphone as indication of systolic and diastolic pressure, the equipment is very expensive and cumbersome, and not always reliable. If one is satisfied with a continuous recording of systolic pressure, the "Blood Pressure Follower"* is a simple and convenient means. A small sleeve-like cuff is put over a suitable finger, a small crystal having been placed over the digital artery. The cuff is blown up automatically until the crystal no longer picks up arterial pulsations, when the cuff deflates. The machine automatically in this way "hunts" about the systolic pressure and the cuff pressure at this level is shown on a manometer and recorded on a pen-writer. A selection of cuffs of different sizes is provided, but the method is not always easy to use in small children. It is quite suitable during anaesthesia and in other situations where a continuous record of systolic pressure is wanted over several hours. The Sonopulsef is a similar apparatus, also with a cuff encircling the base of a finger. It employs a small carbon microphone applied to the pulp of the finger to detect pulsation distal to the cuff. The source of cuff-inflating pressure is a manually inflated reservoir and not as in the "Blood Pressure Follower" a pump or a cylinder of compressed gas. Further, if the reservoir pressure is substantially above the systolic blood pressure the readings may be inaccurate. The instrument, which also contains an automatic pulse rate counter, does not record blood pressure continuously. It is used to give individual readings on a dial. It suffers from the Winston Electronic Ltd. fmedical Industrial Equipment Ltd.

7 652 BRITISH JOURNAL OF ANAESTHESIA drawback that in states of intense vasocontriction the amplitude of the capillary pulsation may be insufficient to produce a signal strong enough to give a recognizable end point. REFERENCES Bordley, J., Connor, C. A. R., Hamilton, W. F., Kerr, W. J., and Wiggens, C. J. (1951). Recommendations for human blood pressure determinations by sphygmomanometers. Circulation, 4, 503. Graham, G. R. (I960). Physiology of the cardiovascular system, in Evans and Gray (eds.), General Anaesthesia, Vol. 1. London: Butterworth. Joint Report of the Committees Appointed by the Cardiac Society of Great Britain and Ireland, and the American Heart Association (1939). Standardisation of methods of measuring the arterial blood pressure. Brit. Heart J.. 1, 261. Moss, A. J., and Adams, F. H. (1962). Problems of Blood Pressure in Childhood. Springfield, Illinois: Thomas. Rushmer, R. F. (1961). Cardiac Diagnosis. 2nd ed. Philadelphia and London: Saunders.