SIGNAL PROCESSING and KNOWLEDGE ACQUISITION in OBJECTIVE BLOOD PRESSURE MEASUREMENT with CONVENTIONAL CLINICAL SPHYGMOMANOMETRY

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1 SIGNAL PROCESSING and KNOWLEDGE ACQUISITION in OBJECTIVE BLOOD PRESSURE MEASUREMENT with CONVENTIONAL CLINICAL SPHYGMOMANOMETRY Ventzas, D. 1 - TEI Larisa Angelopoulos A. Biomedical Eng. Cardiologist, MD ABSTRACT: The paper presents and compares the principles of auscultatory and oscillometric techniques. It focus on the signal processing and knowledge acquisition needs and techniques and finally concludes on their accuracy and reliability. KEYWORDS: Blood pressure, signal, knowledge, Korotkoff sounds, errors, auscultatory, oscillometric. INTRODUCTION: Non-invasive (i.e. non-surgical) methods sacrifice a degree of accuracy for patient safety, comfort, and convenience. Possible uses for Self Blood Pressure Measurement (SBPM) devices are white coat hypertension, systolic hypertension (higher in older), identification of hypotension, pregnancy, hypertensive patients with diabetes mellitus, blood pressure control in diabetes with macro vascular complications, resistant hypertension, failure to reduce BP with hypotensive agents. Blood pressure measurements that are chronically elevated may indicate hypertension and is a major contributing factor in heart disease and stroke since the level of arterial pressure is the basis for major diagnostic and therapeutic decisions in medicine, the measurement must be correct and maximally reproducible. PHYSIOLOGY of BLOOD PRESSURE MEASUREMENT: The introduction of innovative technologies in clinical sphygmomanometry allow not only accurate and reproducible non-invasive measurement of blood pressure but also an assessment of blood pressure as a dynamic phenomenon of the cardiovascular system. The difference between the systolic and diastolic pressure is called the pulse pressure. However, if we want to think in terms of the average force exerted against the artery wall by blood, it is worthwhile to consider a single average pressure. The mean arterial pressure (MAP) is a calculated average arterial pressure. The mean arterial pressure is defined as the diastolic pressure plus one third of the pulse pressure. Individuals blood pressure values have wide distribution. Fig. 1. Time series and averaged autospectra of beat-to-beat changes in mean arterial pressure (MAP) Table 1. Geometrical Definitions of Blood Pressure Systolic Blood Pressure (SBP): maximum blood pressure / arterial wall distension. Mean Arterial Pressure (MAP) Diastolic Blood Pressure (DBP): minimum artery pressure, smaller inward distension A normal objective blood pressure measurement depends on the person s age, sex, heredity, fitness, weight, physical conditioning, past illness, time of day, altitude, activity, climate and environment. In general, blood pressure is lower in the morning and increases throughout the day; it is lower in warm and higher in cold weather. Physical activity can have a significant short term impact on blood 1 D. Ventzas, Informatics & Telecommunications Dpt, TEI Larisa, Larisa

2 pressure, while work, exercise, smoking, diet, eating, drinking beverages containing caffeine or alcohol - even talking, laughing, or crying and emotional stress will all affect a person s blood pressure. Some useful evidence from epidemiological studies and clinical trials support for nurses and health visitors suggest, Table 2. Classifying stages of hypertension on the basis of average blood pressure levels, clinicians should specify presence or absence of target organ disease and additional risk factors. Table 2. Blood Pressure for Adults Systolic BP Diastolic BP Follow up recommended for Adults Category <120 mmhg <80 optimal <130 mmhg <85 Recheck in 2 years normal Recheck in 1 year High normal Confirm within 2 months mild Evaluate or refer to source of care within 1 month moderate Evaluate or refer to source of care within 1 week severe < 210 < 120 Evaluate or refer to source of case immediately In general, blood pressure is dependent on four main factors: cardiac output (the efficiency of the pumping process), the peripheral resistance of the vascular system (vasodilators and vasoconstrictors, viscosity), vessel elasticity and the blood volume (electrolytes). Blood pressure may increase to compensate for the reduced flow. Fig. 2. Typical recording of automatic cuff deflation (female, age = 20). The first/last notch wave is the systolic/diastolic pressure. Pulsus alternant, or beat variation in the intensity of the Korotkoff sounds, occurs in people with fluctuating systolic pressure. Fig. 3. Relative Intensity of Korotkoff sounds and phases 2

3 AUSCULTATORY METHOD PRINCIPLES - KOROTKOFF SOUNDS: With the palpatory method, the changes in blood flow associated with systolic and diastolic pressures are felt, rather than being heard. With the ausculatory method, a stethoscope is used to listen to the blood flow in the arteries. The auscultatory technique is based on the ability of the human ear to detect and distinguish sounds. This is a great advantage since it allows the clinician to determine the quality of each measurement. However, inherent in this is the possibility for measurement error due to differences in hearing acuity from clinician to clinician. Unqualified or inexperienced personnel may be more susceptible to outside noise, other interference, or inconsistent assessment of Korotkoff sounds. In an attempt to increase reproducibility, some automated devices have replaced the human ear with a microphone. In the auscultatory method the bladder is rapidly and steadily inflated to a pressure mmhg above the level, then partially unscrewed (opened) the valve and deflated the bladder at 2 mm/sec while listening for the appearance of Korotkoff sounds. The clinician uses the stethoscope to listen for the Korotkoff sounds as the cuff deflates. The sounds are divided into five phrases based on the loudness and quality of the sounds. Phase Sound Indicates 1 1 st appearance of clear repetitive loud, clear sound, faint tapping heard with increasing intensity 2 Start swishing sound, softer and longer, with the quality of an intermittent murmur, succession of murmurs, blows, or swishes may disappear if cuff is deflated slowly. 3 Intensity increases, crisper and louder, with thumping quality similar to phase 1, distinct tapping. 4 Sounds are abrupt muffling, less distinct and softer blowing quality, disappear completely, muffled sound abruptly replaces the This coincides approximately with the appearance of a palpable pulse. The pressure at which the sounds first appear corresponds to the systolic pressure. At pressures below diastolic, the cuff does not occlude the artery. Therefore, blood flow is no longer impeded. Forces on the arterial wall are dominated by haemodynamic factors. This region is useful for calculations of arterial compliance or blood flow. Diastolic Pressure (the beginning of Phase 5) in children < 13 years old and in adults that are exercising, pregnant or hyperthyroid (See Phase 5) Diastolic pressure in adults - occurs 5-10 mm Hg below Phase 4 in normal adults. In states of increased rate of blood flow it may be greater than 10 mm Hg below Phase 4, in these cases the Phase 4 sound should be used as Diastolic Pressure in adults. thumping sounds of Phase 3. 5 All sounds disappear. Phase IV, Phase V, or a combination of the two best represent diastolic pressure. Some patients may not have audible Phase IV, and in others Phase V is difficult to determine. Usually the pressures at which muffling and disappearance occur often are only within 10 mm of Hg. The onset of Phase V defines the diastolic pressure in adults. At rest, blood flow is laminar and silent. The sound of turbulent blood flow can be heard The vibrations in the artery walls are called Korotkoff vibrations Korotkoff sound may be audible all the way to 0 mmhg The BP cuff is completely deflated before and after measurement If BP is to be retaken in the same arm, wait 2 minutes before cuff reinflated to allow the release of blood trapped in the veins to enter back into circulation In children less than 13 years of age, pregnant women, and patients with high cardiac output or severe aortic regurgitation and peripheral vasodilatation, sounds are often heard at levels far below those at which muffling occurs, sometimes to levels approaching 0 mmhg. Korotkoff sounds may be difficult to hear and arterial pressure difficult to measure when arterial pressure rises at a slow rate (as in aortic stenosis), when the vessels are markedly constricted (as in shock), and when the stroke volume is reduced (as in severe heart failure). Very soft or inaudible Korotkoff sounds can often be accentuated by dilating the blood vessels of the upper extremities simply by opening and closing the fist repeatedly. Korotkoff sounds can usually be detected by auscultating with the stethoscope (or by palpation, or the Doppler ultrasound blood flow detecting) over the radial artery to obtain both systolic and diastolic pressures. A multi-processor scheme blood pressure measurement with built-in microcomputer that analyses the digitized wave, recognizes the systolic and diastolic blood pressure, samples data converted into digital data and stored in the memory, deflates step wise with certain speed. Initially the 3

4 CPU checks the signal level, and if it is sufficient for the measurement, the wave-forms of the vascular sounds are analyzed to measure the blood pressure; otherwise another attempt may be tried automatically. The cuff pressure, the amplifiers gain, etc are readjusted by the CPU, through RS-232C interface. Fig. 4. Indirect blood pressure measurements: oscillometric and auscultatory measurement. AUSCULTATORY METHOD ERRORS: Errors in the auscultatory blood pressure method is due to equipment, observer, subject, interface-operation, interpretation measurement and technique due to poor sound transmission, distorted sounds, sounds not well heard, extraneous noise, danger of missing auscultatory gap, inaccurate reading, with results such as blood pressure level too high / low / variable, under / over estimated of systolic / diastolic pressure. Common errors are: ERRORS, ACTION and CAUSE do not highlight the 1 st sec when the heart rate calculated artifacts or poor averaged information pressure take in both arms, but differ by 5-10 mmhg we prefer the highest pressure indicating arm 20 mmhg differences orthostatic hypotension standing drops systolic pressure and increases diastolic pressure excessive patient motion leaky cuff or hose oscillometric cycling ischemia, neuropathy. Inaccuracies may be caused by arm engorged with blood daily and seasonal basis lowest during sleep and rises in the morning and throughout the day. Do measure your pressure the same time each day. Record the date and time Do rest 5 to 10 minutes before measuring your blood pressure. allow 5 to 10 minutes rest BETWEEN subsequent measurements. blood pressure history Home measurements control of the deflation rate pressure is released too quickly vacuum is formed above the mercury and the meniscus is distorted. Low sound levels indicates low systolic, high diastolic pressure Differences at 5 mmhg rejects measurements Normal pressures might indicate hypotension, if previous pressures re high Erroneous placement of stehoscope Failure to make full skin contact with its sensing bell Possibility of shock Venous engorgement of the patients arm from repeated inflations of the cuff Patient s effort to support arm raise pressures Brachial artery to heart anatomy changes pressures sound-dependent algorithms 4

5 Fig. 5. Geometry problems a. Placing the cuff on the arm b. arm and cuff level with heart and patient orthostatic positions Sphygmomanometer arrangement Fig. 6. Occasionally the Korotkoff sounds become inaudible during Phase II or III, only to reappear as the pressure in the cuff is reduced further. This period of silence is called the auscultatory gap and is common in older and hypertensive patients. Raising the arm before inflating the cuff at max systolic+30 mmhg eliminates it. Unrecognized ausculatory gap may lead to underestimation of systolic pressure or overestimation of diastolic pressure PRINCIPLES of OSCILLOMETRIC TECHNIQUE: Most automated devices make use of oscillometry. Pressure changes in the artery are recorded with every heartbeat and displayed as an oscillogram. The oscillogram consists of two parts: 1. pressure curve as the cuff inflates and deflates 2. oscillations/vibrations of the arterial wall Unlike auscultatory techniques, which measure systolic and diastolic but estimate mean arterial pressure (MAP), oscillometric devices measure the mean but estimate systolic and diastolic. The point of maximum amplitude is considered mean arterial pressure. Therefore, an error in MAP may produce inaccurate values for systolic and diastolic. The Pulse Dynamic waveform is directly affected by variations in blood flow and blood pressure. The waveform reflects, to a certain extent, the state of the cardiovascular and peripheral system. Oscillometric devices for blood pressure (BP) measurement at the wrist are becoming more widely used in clinical practice. However, systematic comparisons with standard upper-arm auscultatory BP mercury sphygmomanometry measurement at the brachial artery are scarce. A motorized pump inflates the cuff to approximately 180 mmhg (120 mmhg for neonatal patients) initially. Then we deflate cuff pressure gradually. A pressure transducer detects air pressure and sends a signal to the measurement circuits. As pressure in the cuff is reduced, blood flows in the previously occluded artery and the pressure is read by the transducer. The point at which oscillation increases sharply is defined as the 5

6 systolic pressure. As the cuff deflates further, oscillation amplitude increases to a maximum, then decreases. The point of peak oscillation amplitude is defined as the mean arterial pressure. The point at which the system detects a loss of oscillation amplitude is defined as the diastolic pressure. The oscillometric technique uses a pressure transducer and electronic data processing to non invasively determine systolic, mean, and diastolic pressures. Fig. 7. Oscillatory blood pressure measurement the oscillations begin at approximately the level of systolic pressure and reach their greatest amplitude at the level of mean arterial pressure. Diastolic pressure is a derived value. Fig. 8. Digital Representation of Pressure Waveform in Brachial Artery in the oscillometric method Section I Section II Section III Section IV Cuff Pressure > Systolic Systolic Pressure > Cuff Cuff Pressure Between Mean Cuff Pressure Less than Blood Pressure Pressure > Mean and Diastolic Pressures Diastolic Pressure artery remains fully the blood spurts through the occluded-no blood flow artery at a high velocity Pressure waves generated by heartbeat create pulsatile pressure signal; highly representative of aortic activity "super-systolic" portion, the artery remains fully occluded. Pressure waves generated by cardiovascular activity create pulsatile arterial distension proximal to the occlusion. Unlike later oscillations, the occlusion prevents blood flow from contributing to the measured pressures (i.e. the pressures are dominated by forces directly generated by the heart). creates a Bernoulli effect causing a phase shift in the pressure signal As the cuff deflates, increasing portions of the cardiac cycle generate pressures that exceed cuff pressure. Therefore, an increasing volume of blood flows through the artery. The increased flow causes an increase in the amplitude and phase shift of the pressure signal. When cuff pressure reaches mean arterial pressure, the forces produced by blood flow balance the cuff pressure. Mean arterial pressure is the average pressure in the artery, and is marked by the second triangle icon. As cuff pressure continues to decrease, it is no longer sufficient to occlude the artery. The alleviation of cuff pressure causes the magnitude of the pressure signal to decrease. This is reflected in the decreasing magnitude of the waveform. At pressures lower than diastolic, the cuff does not occlude the artery. Therefore, blood flow is no longer impeded. This final portion of the waveform is known as the sub-diastolic range of measurement In this region, the forces on the arterial wall are dominated by hemodynamic factors. Therefore, any arterial wall distention can be used for calculations of arterial compliance (change in volume/change in pressure), or blood flow based on changes in hemodynamic parameters. 6

7 source of additional cardiovascular information Pattern recognition is used to determine systolic blood pressure Systolic pressure is marked by the first triangle icon.. When the time-dependent signal reaches a characteristic DBP point, the pattern recognition algorithm identifies DBP (marked by the third triangle icon). ACCURACY OF PULSE DYNAMICS: In clinical trials, pulse dynamics correlated well with both auscultatory and invasive methods. Pulse dynamics produces measurements that are usually higher than auscultatory measurements for the same patient with expected differences of 5-15 mmhg systolic and 5-10 mmhg diastolic. Pulse dynamics compares well with invasive measurements, which are considered standard for accuracy. Clinical studies produced correlation coefficients of 0.94/0.95/ 0.91 for systolic/mean/diastolic. Movement artifact and certain arrhythmia are easily detectable by glancing at the measurement waveform. Signal noise (artifact) due to patient movement, inadequate cuff placement, or other factors will produce waveform irregularities that may be visually detected. This allows clinicians to discard suspicious measurements, and gives them more insight into the cardiovascular condition of the patient. Fig. 9. Pressure Waveform in the oscillometric method patient movement (a) pressure arrhythmia (b) The existence of positive or negative spike which appears in the first wave of Tone-Segment, especially steep up rising spike, for the diastolic blood pressure, which appears in up-slope of the wave of Tone-Segment. Several factors influence the accuracy of an automated device such as distorted signal. Systematic and random errors in measurement can occur at each of the interactionary points of the technique (arteries, cuff, sensor, processing). accurate transmission and interpretation of a signal (Korotkoff sound or pulse wave) readings should be accurate, representative, interpretable, reproducible, standardised Fig. 10. Pulse Dynamic Oscillometrics vs. Auscultatory Method (ANSI/AAMI SP ). Studies and pattern recognition technology validated the use of Pulse Dynamics in the non invasive low cost measurement of arterial compliance and left ventricular contractility. 7

8 The mercury manometer is preferred and recommended over the aneroid manometer (prone to mechanical alterations that affect its accuracy) and is less likely to become decalibrated. The mercury reservoir should be full so that the upper curve of the meniscus is at the zero level before the cuff is inflated. The mercury should rise and fall freely in the column. The column should be kept vertical. supine to standing results in slight decrease in systolic blood pressure, slight increase or no change in diastolic blood pressure and a mild increase in heart rate postural hypotension, observed in elderly patients under antihypertensive therapy secondary hypertension. systolic pressure in the legs is up to 20 mmhg higher than in the arms the diastolic pressures in the legs is virtually identical to this in the arms In elderly patients who have sclerotic, calcified vessels, it is likely that the systolic pressure is overestimated by the indirect method of measurement. "pseudohypertension" blood pressure of pregnant women is more complicated because of the wide pulse pressure and the need to record both Phase IV and Phase V as sounds is heard to 0 mmhg. In dialysis patients or undergone a mastectomy, blood pressure measured in opposite arm In patients with cardiac dysrhythmias and atrial fibrillation or frequent premature beats, the systolic pressure varies widely from beat to beat In a patient with slow heart rate, reduce the pressure in the cuff gradually (2 mmhg per heartbeat) to avoid underestimation of systolic pressure and over-estimation of diastolic pressure Disparity in pressure between the two arms may occur in patients with congenital heart disease, peripheral vascular disease, unilateral neurological and musculoskeletal abnormalities, and aortic dissection. In hypertensive patients the arms with the higher pressure should be used The Device The Patient Interface-Operation Interpretation Measurement Appropriate cuff size, Blood pressure levels are with the palpatory lowest and most stable Auto Zero - larger cuff needed for affected by emotional, method avoid under readings Automatically those with a large arm environmentaland physical inflation of the cuff establishes zero circumference stimuli, so effort should in patients with pressure be made to standardize auscultatory gap reference before conditionsof measurement, and over inflation in each or series of keeping extraneous those with very low measurements influences to a minimum. blood pressure Bladder length of The subject must be Palpate the brachial Ambulatory Blood Inflate cuff 30 approximately comfortably seated with artery and place the Pressure Measurement mm Hg above times the bladder width the midpoint of the upper cuff so that the refers to repeated Doppler systolic and cuff length 80% x arm at the level of the midline of the measurements made pressure circumference heart (approximately the bladder is over the away from the medical Recommended cuff size fourth intercostals space, arterial pulsation, environment with a Newborn 3 x 6 cm when the individual is then wrap and portable, automatic seated). secure the cuff (self-inflating) recorder Infant 5 x 10 cm snugly around the in patients engaging in Child/lean cm subjects'bare upper their usual activities adult arm. Cuff loose Adult cm Adult large cm alarm limits operating mode cuff / hose connections isometric exercise during the measurement will transiently raise the blood pressure level subject lying or seated, heart level blood pressure should be determined with the patient standing as well as while sitting. When the patient is standing, support the raised arm at the level of the heart. When the subject is lying down, the arm should be at the side of the body, slightly raised application results in overestimation of the pressure. Place the head of the stethoscope over the brachial artery pulsation, just above and medical to the antecubital fossa but below the lower edge of the cuff, and hold firmly in place In arrhythmia, blood pressure is level variable and we take multiple measurements and average eye level with the manometer If reading gives systolic pressure below 140 / diastolic below 85, no further readings needed systolic greater than mean pulse rate out of range Deflate the cuff quickly and completely 8

9 cuff application 1. Cuff Wrapped too closely Blood pressure recording too high 2. Applied over clothing - Inaccurate reading 3. Palpatory pressure omitted - Danger of missing auscultatory gap 4. Inflation level too low - Underestimation of systolic pressure 5. Inflation rate too slow - Diastolic pressure too high 6. Deflation rate too fast - Systolic pressure too low 7. Deflation rate too slow - Diastolic pressure too high cuff orientation periodic recalibration. based on detection of Korotkoff sounds, and others use the oscillometric technique. Badly located brachial pulse; Doppler assists Automatic Calibration Sensor dependent geometry from the bed, at the level of the middle of the chest. Standing readings are needed for those in whom a significant postural drop could occur (e.g. the elderly, in hypertensives under treatment, or in the presence of other situations where postural hypotension is possible) With the subject standing for at least a minute, take at least one reading of the standing blood pressure with the arm extended and supported, plus a record of the pulse rate. Most people exhibit this circadian rhythm, which consists of a blood pressure decrease of approximately 15-25% during the night-time, with increases to daytime levels again in the morning. patient should as far as possible be relaxed and the arm well supported longer and wider cuff needed for adequate compression of the brachial artery in the obese patient with large upper arm - place the centre of the bladder over the brachial artery pulse Pulsus paradoxus is a normal fall in systolic pressure that occurs during inspiration, in patients with constrictive pericarditis and severe pulmonary disease or restrictive cardiomyopathy arrhythmia, heart valve disease Patients with arterial disease as SLE (Lupus Erythematosis), Raynaud syndrome have decreased elasticity of arterial wall. Restricted movement / vibration of the arterial wall causes a flat signal. quality of the stethoscope patient motion or physiology Inflate the cuff rapidly to 70 mmhg, and increase by 10 mmhg increments while palpating the radial pulse. Note the level of pressure at which the pulse disappears and subsequently reappears during deflation. Wait 30 seconds before a 2nd measurement if the 1st is unsatisfactory. error of overestimating the pressure when measuring with a cuff that is too small for an obese arm, can be considerable and can lead to unnecessary therapy An auscultatory gap, often detected in patients with high systolic pressure, is normal relationship between the blood pressure and pulse rate If levels are higher (hypertensive), repeated readings at 2 minute intervals required with a concurrent record of the pulse rate Since blood pressure to be more labile in elderly patients, it is particularly important to obtain several base line determinations before making diagnostic or therapeutic decisions. conventional mercury sphygmomanometer automated devices are more inaccurate at higher pressures. This could be due to various factors including a higher nonlinear resistance in the vessels. operator dependent Maximal Inflation Level (MIL) Determine the pressure to which to inflate the cuff for the measurement of the systolic blood pressure. Distorted sounds due to long stethoscope tubing considerable extraneous noise, surrounding recordings averaged Digit Inaccurate reading Record blood pressure to nearest 2 mmhg automatic and selfinflating adult-paediatricneonatal mode Adjusting Measurement Interval [min], EAN/PULSE RATE / MEAN CYCLE SUGGESTIONS FOR FURTHER WORK: The pressure signal is caused by the cuff / artery interaction. This signal can be analysed to measure more information than the mean arterial pressure or the estimated systolic and diastolic blood pressures. combined intermittent and continuous measurement techniques improve SNR more or no microphones for differential Korotkoff recognition ECG gating techniques may be used to minimize artifacts Constant pressure oscillometric measurement of pulses generated at a low, constant pressure, permit continuous beat-to-beat arterial blood pressures to be measured, whereas the traditional 9

10 oscillometric method is limited to certain oscillometric pulses at different pressures allowing only for intermittent measurement of blood pressure. Volume-oscillometric method is based on arterial volume oscillations instead of pressure oscillations. The infrasound, ultrasound, plethysmography, electrical conductivity (impedance) and arterial tonometry and combined techniques attempt to improve the auscultatory method by detecting low frequency Korotkoff vibrations below 50 Hz, including sub-audible vibrations; a waveform is produced, similar to the pressure-generated oscillometric waveform. CONCLUSION: Despite the relative lack of evidence for the widespread use of monitoring home blood pressure in hypertension it is clear that the market is growing for such devices. The greater availability and advertising has resulted in an increasing interest by the general public. As a result cheaper, lighter and easier to use devices have been produced. SBPM should ensure that their operation is optimised and reduce patients cardiovascular risk. The paper compared the principles of auscultatory and oscillometric techniques and presented their signal processing needs and knowledge acquisition for accuracy and reliability. REFERENCES 1. The British Hypertension Society. Blood pressure measurement CD ROM. London: BMJ Books, Eckert S, Gleichmann U, Zagorski O, Klapp A. Validation of the Omron R3 blood pressure self-measuring device through simultaneous comparative invasive measurements according to protocol of the German Institute for Validation. Blood Pressure Monit; 2: , Geddes LA, Voelz M, Combs C, Reiner D, Babbs CF. Characterization of the oscillometric method for measuring indirect blood pressure. Ann Biomed Eng 1982; 10: B. P. Imholz, G. J. Langenwouters, G. A. van Montrans, G. Parati, J. van Goudoever, K. H. Wesseling, W. Wieling, and G. Mancia, Feasibility of ambulatory, continous 24-hour finger arterial pressure recording, Hypertension, 21(1), 65 73, Jones DW, Frohlich ED, Grim CM, Grim CE, Taubert KA, for the Professional Education Committee, Council for High Blood Pressure Research. Mercury sphygmomanometers should not be abandoned: An advisory statement from the Council for High Blood Pressure Research, American Heart Association. Hypertens 2001; 37(2): O'Brien E, Petrie J, Littler WA, de Swiet M, Padfield PD, Dillon MJ. Blood pressure measurement: recommendations of the British Hypertension Society. 3rd ed. London: BMJ Publishing Group, O'Brien E. State of the market in 2001 for blood pressure measuring devices. Blood Pressure Monitoring 2001; 6: O'Brien E, Waeber B, Parati G, Staessen J, Myers M (2001) Blood pressure measuring devices: recommendations of the European Society of Hypertension. BMJ; 322: P. Rogers, V. Burke, P. Stroud, I.B. Puddey Comparison of oscillometric blood pressure measurements at the wrist with an upper-arm auscultatory mercury sphygmomanometer, Clinical & Experimental Pharmacology & Physiology, Vol 26 Is. 5-6 p. 477, May/June Sykes M K, Vickers M D, Hull C J 1991 Principles of measurement and monitoring in anaesthesia and intensive care. Blackwell Scientific Publications, Oxford 11. Staessen J, Thijs L and the participants of the First International Consensus Conference on SBPM. (2000) Developments of diagnostic thresholds for automated self-measurement of blood pressure in adults. Blood Press Monitor 5: Stergiou G, Skeva I, Zourbaki A, Mountokalakis D (1998) Self-monitoring of blood pressure at home: how many measurements needed?, Journal of Hypertension 16:6;