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. 2001 Oct 6;323(7316):805.
Blood pressure measurement
ABC shows absence of evidence in measuring blood pressure during pregnancy
Editor—In the short ABC on blood pressure measurement Beevers et al made several references to problems with measurement in pregnancy that have been discussed for many years without satisfactory resolution.1 There are four main points.
(1) Interestingly, the vascular sounds that are caused by vibration of the vessel wall are clearly modulated by consistent changes in patterns of flow in the brachial artery in pregnancy (figure).2
(2) As the authors indicate, automated oscillometric devices may be inaccurate; they systematically under-record K1 (phase I; onset of sounds) and K4 (phase IV; muffling of sounds) in severe pre-eclampsia by up to 40 mm Hg.3 Other circulatory conditions characterised by reduced compliance may be susceptible to similar discordances.
(3) K5 (phase V; disappearance of sounds) may be the “most accurate measurement of diastolic blood pressure in pregnancy,” but academic precision may be less relevant than pragmatic thresholds in preventing the consequences of severe pre-eclampsia and eclampsia. Measurements of K1 and K4 of 140/90 mm Hg and 160/110 mm Hg may be difficult to improve on. Perhaps the authors have alternative techniques and thresholds in mind?
(4) In the British eclampsia survey 294 out of 383 patients with eclampsia had the characteristic symptoms associated with severe hypertension, although only 70 had a diastolic blood pressure >120 mm Hg before the seizure.4 The authors who reported the survey suggested that these observations might be accounted for by an increase in “normotensive eclampsia.” An alternative explanation might be that in labour wards in the United Kingdom blood pressure is usually measured with automated oscillometric devices, which consistently under-record when compared with K1 and K4, so that women and their babies continue to be exposed to serious morbidity.
It is always helpful for obstetricians and gynaecologists to have their practices carefully scrutinised, since so much is not underpinned by rigorous evidence. The ABC emphasises the absence of such evidence. If European Union directives advise mercury to be withdrawn from the workplace I would be grateful to know how to measure blood pressure in severe pre-eclampsia so as to prevent the potentially serious consequences of this syndrome.
Sequence of vascular sounds in normotensive pregnancy. Vascular sounds were recorded with microphone over brachial artery (Toshiba HSM-05B), with concomitant Doppler recording of brachial artery waveform (Toshiba PLF703ST)
2.Quinn MJ. Korotkoff's sounds in pregnancy. Ultrasound in Obstetrics and Gynaecology. 1995;6:58–61. doi: 10.1046/j.1469-0705.1995.06010058.x. [DOI] [PubMed] [Google Scholar]
3.Quinn MJ. Automated blood pressure measurement devices: a potential source of morbidity in severe preeclampsia? Am J Obstet Gynecol. 1994;170:1303–1307. doi: 10.1016/s0002-9378(94)70146-6. [DOI] [PubMed] [Google Scholar]
4.Douglas KA, Redman CW. Eclampsia in the United Kingdom. BMJ. 1994;309:1395–1400. doi: 10.1136/bmj.309.6966.1395. [DOI] [PMC free article] [PubMed] [Google Scholar]
BMJ. 2001 Oct 6;323(7316):805.
Single measurements would not withstand 21st century peer review
Editor—There cannot be a doctor who does not know how to perform the very basic technique of blood pressure monitoring, so the reason for publishing a detailed review of the factors involved must be to improve the way routine measurements are obtained.1-1 The recommendations are therefore disappointing—specifically, the fact that the authors are prepared to advocate the use of a single reading unless there is a specific distraction.
This simply reiterates an unvalidated tradition; it would not withstand peer review in any reputable journal if newly invented. The Reverend William Hales, in the 18th century (long before ambulatory measurements), observed that minute to minute arterial pressure is remarkably variable. A single reading, therefore, means as much as a random depth measurement of a river at an unspecified time.
Worse, with an inflatable cuff the first reading is particularly unreliable, since it may simply serve to bed the cuff into position: it should be routinely discarded, rather than being the only reading. A reproducible value requires three to five subsequent readings.1-2,1-3 Because time of day and whether the day is a working day or a less stressful day also affect blood pressure, these factors should be held constant for an individual patient if readings taken on consecutive occasions months apart are to have any likelihood of sensitively detecting important changes. These will otherwise be masked by avoidable random variation; failure to detect upward trends until they stand out from the unnecessary noise imposes a potentially damaging delay.
The extra care entailed by obtaining proper measurements is trivial beside the cost of hypertensive damage and diabetic nephropathy. The clinical consequences of hypertension are so important that the fundamental measurement deserves to be taken with proper regard to precision, and with the same care that is now taught to veterinary surgeons.1-4
1-2.Norm R, Myers M, McKay D. Is usual measurement of blood pressure meaningful? Blood Pressure Monitoring. 1999;4:71–76. [PubMed] [Google Scholar]
1-3.Michell A. Routine blood pressure measurement: application of the standard canine technique to a human. Blood Pressure Monitoring. 1996;1:385–387. [PubMed] [Google Scholar]
1-4.Bodey AR, Michell AR, Baree KC, Buranakurl C. Comparison of direct and indirect (oscillometric) measurements of arterial blood pressure in dogs. Res Vet Sci. 1996;61:17–21. doi: 10.1016/s0034-5288(96)90104-6. [DOI] [PubMed] [Google Scholar]
Editor—We agree with Quinn that several issues in relation to blood pressure measurement in pregnancy “have been discussed for many years without satisfactory resolution” and that as a consequence our review may reflect the absence of such evidence. We attempted, nevertheless, to make the most of what evidence is available and, by doing so, to help clinical practice.2-1
The general consensus from obstetricians is that K5 (disappearance of sounds) is the best measure of diastolic pressure in pregnancy, although we acknowledge that much of the evidence for outcome in pregnancy derives from K4 (muffling).2-2 We would urge our obstetric colleagues to provide the evidence on which to base future management decisions.
With regard to automated alternatives to the mercury sphygmomanometer, we would agree with Quinn's contention that these devices are not always proved to be accurate in pregnancy.2-3 As the British Hypertension Society's protocol recommends, all blood pressure measuring devices should be validated in special populations, such as pregnant women.2-4
Had Michell read our articles he would have noted our constant emphasis on the need to obtain multiple (preferably ambulatory) measurements before making any clinical decisions. The two words that he selectively quotes were used in the context of advocating one careful measurement rather than numerous careless hurried measurements. If he had read the next sentence he would not need to have repeated what we had already stated unequivocally: “As a result of the variability of measurements of casual blood pressure decisions based on single measurements will result in erroneous diagnosis and inappropriate management. Reliability of measurements is improved if repeated measurements are made.”
References
2-1.Beevers G, Lip GYH, O'Brien E. ABC of hypertension. Sphygmomanometry: factors common to all techniques. BMJ. 2001;322:981–985. doi: 10.1136/bmj.322.7292.981. . (21 April.) [DOI] [PMC free article] [PubMed] [Google Scholar]
2-2.De Swiet M. K5 rather than K4 for diastolic blood pressure measurement in pregnancy. Hypertension in Pregnancy. 1999;18:3–5. doi: 10.3109/10641959909016191. [DOI] [PubMed] [Google Scholar]
2-3.O'Brien E, Waeber B, Parati G, Staessen J, Myers MG.on behalf of the European Society of Hypertension Working Group on Blood Pressure Monitoring. Blood pressure measuring devices: recommendations of the European Society of Hypertension BMJ 2001322531–536.. (3 March.) [DOI] [PMC free article] [PubMed] [Google Scholar]
2-4.O'Brien E, Petrie J, Littler WA, de Swiet M, Padfield PL, Altman D, et al. The British Hypertension Society protocol for the evaluation of blood pressure measuring devices. J Hypertens. 1993;11(suppl 2):S43–S63. doi: 10.1097/00004872-199306000-00013. [DOI] [PubMed] [Google Scholar]
BMJ. 2001 Oct 6;323(7316):805.
Doctors who cannot calibrate sphygmomanometers should stop taking blood pressures
Editor—Professionals often use specialist equipment to diagnose or remedy a problem. If they use equipment of unknown reliability they are acting unprofessionally and failing in their duty of care3-1; their action is also reckless and unethical. Furthermore, it could result in a claim for negligence if a patient thinks that harm resulted from the use of unreliable equipment.
To ensure that they are acting professionally, safely, and ethically, professionals must immediately stop using equipment of uncertain reliability and arrange for the regular calibration and maintenance of all equipment. This is generally best done by formally delegating responsibility to a suitably qualified person or organisation.
In the ABC of hypertension the authors emphasise the professional duty to ensure that when blood pressure is measured the readings obtained are accurate.3-2 They also state that the reliability of mercury sphygmomanometers cannot be taken for granted3-3 and that some sphygmomanometers are notoriously inaccurate.3-2
The authors do not, however, give clear advice to stop using sphygmomanometers of unknown reliability. We find this surprising. The principle is well established in health care. If we are unsure about the integrity of the cold chain we throw the vaccine away; if an autoclave has not undergone routine servicing we do not use it. What is so different about sphygmomanometers? Is their use exempt from this important quality assurance principle? We think not.
In these litigious times things are about to change. It can only be a matter of time before a patient seeks damages on the basis that he or she suffered harm because the doctor used an unreliable sphygmomanometer. Without convincing evidence that the doctor's sphygmomanometer was reliable such a claim will be hard to defend. The fact that it is common for doctors to use uncalibrated equipment can no longer be relied on to provide a successful “Bolam” defence.3-4 Judges have long ruled that medical actions and treatments must be logical.3-1 Can readers justify to their patients or argue before a judge that use of an uncalibrated sphygmomanometer is logical?
