Blood pressure measurement in pregnancy - Wiley Online Library

DOI: 10.1111/tog.12173

2015;17:91–8

Review

The Obstetrician & Gynaecologist http://onlinetog.org

Blood pressure measurement in pregnancy Hannah L Nathan MBBS BSc DFSRH,a Kate Duhig MBBS BSc MRCP,b Natasha L Hezelgrave Lucy C Chappell MBBS PhD,c Andrew H Shennan MBBS MD FRCOGd,*

MBBS BSc,

a

a

Clinical Research Fellow, Women’s Health Academic Centre, King’s College London, 10th floor, North Wing, St Thomas’ Hospital, London SE1 7EH, UK b Academic Clinical Fellow, Women’s Health Academic Centre, King’s College London, 10th floor, North Wing, St Thomas’ Hospital, London SE1 7EH, UK c Clinical Senior Lecturer, Women’s Health Academic Centre, King’s College London, 10th floor, North Wing, St Thomas’ Hospital, London SE1 7EH, UK d Professor of Obstetrics, Women’s Health Academic Centre, King’s College London, 10th floor, North Wing, St Thomas’ Hospital, London SE1 7EH, UK *Correspondence: Andrew H Shennan. Email: [email protected]

Accepted on 8 January 2015

Key content

Learning objectives

Accurate blood pressure (BP) measurement is fundamental to early diagnosis of hypertensive disorders in pregnancy. Poor auscultatory technique and lack of training leads to inaccuracies in BP measurement using sphygmomanometry with mercury and aneroid devices. Automated devices limit user error but require validation of accuracy because they tend to underestimate BP in pre-eclampsia. Systolic hypertension may better predict risk of adverse outcome (such as haemorrhagic stroke) than diastolic hypertension. Ambulatory/self-monitoring increases the number of representative readings available on which to base management, limiting unnecessary intervention. Detection of hypotension in pregnancy is crucial to the diagnosis of shock secondary to haemorrhage and sepsis.

To learn how to obtain accurate BP measurements in pregnancy. To understand the significance of hypertension in pregnancy.

Ethical issues

A large proportion of maternal deaths are associated with substandard care, often related to poor recognition of severity of hypertension or shock and need for treatment. Lack of cheap, accurate, easy-to-use BP devices in low- and middle-income countries, where risk of maternal and perinatal mortality and morbidity secondary to pre-eclampsia and shock is highest, continues to be a challenge. Keywords: blood pressure measurement / diagnosis /

hypertension / pre-eclampsia

Please cite this paper as: Nathan HL, Duhig K, Hezelgrave NL, Chappell LC, Shennan AH. Blood pressure measurement in pregnancy. The Obstetrician & Gynaecologist 2015;17:91–8.

Introduction Hypertensive disorders in pregnancy, which include preeclampsia, gestational hypertension and chronic hypertension, complicate 2–8% of pregnancies and confer risk to the health of mother and fetus.1 Pre-eclampsia is one of the three leading causes of maternal death in the UK and can result in substantial maternal morbidity, including intracranial haemorrhage, HELLP (haemolysis, elevated liver enzymes and low platelet count) syndrome and disseminated intravascular coagulation.2 In 2010 an estimated 287 000 maternal deaths occurred globally, 99% of which occurred in low- and middle-income countries (LMICs). Approximately 14% of these deaths were thought to be related to hypertensive disorders in pregnancy,3

ª 2015 Royal College of Obstetricians and Gynaecologists

although this figure may be higher when the contribution of hypertensive disorders in pregnancy to other causes of mortality (such as postpartum haemorrhage) is considered. Pre-eclampsia also contributes to one-fifth of all preterm births globally (and is the leading cause of iatrogenic preterm birth) and one-quarter of stillbirths and neonatal deaths in LMICs.1 Accurate measurement of blood pressure (BP) is crucial to the diagnosis and management of hypertensive disorders in pregnancy. BP monitoring is the most important, and frequent, screening test in the antenatal period and is undertaken by healthcare assistants, midwives, general practitioners and obstetricians on a daily basis. Accuracy of BP measurement impacts on maternal and perinatal clinical outcomes, highlighted in the 2006–2008 UK Confidential

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Enquiries into Maternal Deaths report,4 which found that the most common reason for substandard care in deaths secondary to pre-eclampsia/eclampsia was failure to recognise and treat hypertension. As well as detecting hypertension, BP devices are also essential for the detection of acute haemodynamic compromise and management of shock in pregnancy. Obstetric haemorrhage, pregnancy-related sepsis and unsafe termination of pregnancy are major contributors to approximately 46% of maternal deaths worldwide and all can present with signs and symptoms of shock.3 According to the Confidential Enquiries report,4 the majority of maternal deaths could have been avoided if early warning signs of impending collapse had been recognised and acted on earlier. Thus, the ability to measure BP accurately is an indispensible skill for obstetricians, midwives and other healthcare workers, regardless of setting, in order to prevent maternal and perinatal morbidity and mortality, both in the UK and worldwide. This review considers the importance of accuracy of BP measurement, including choice of device, technique and common errors. It also discusses the evidence-based hypertension BP thresholds at which there is increased risk of morbidity and mortality in pregnancy, and the importance of prompt medical intervention, as well as the role of vital sign monitoring in women at risk of shock.

Accuracy of BP measurement in pregnancy BP measurement is a key part of the assessment of hypertensive disorders in pregnancy, guiding diagnosis, admission, antihypertensive treatment and timing of delivery, as well as the assessment of haemodynamic shock in pregnancy, secondary to obstetric haemorrhage or sepsis. It is therefore important that all healthcare providers are aware of the issues surrounding accuracy of BP measurement in pregnancy.

Auscultatory technique The National Institute for Health and Care Excellence (NICE)5 Antenatal Care guidance recommends BP measurement at every antenatal visit and outlines the steps involved in BP measurement using the auscultatory technique. This includes use of the correct-sized cuff, initial inflation of the cuff 20–30 mmHg above the palpable systolic BP, deflation at a rate of 2 mmHg per second, recording BP to the nearest 2 mmHg and use of Korotkoff phase V to indicate diastolic BP. For example, deflating the cuff too fast will result in underestimation of the systolic BP and overestimation of the diastolic BP. Despite these clear recommendations, in clinical practice BP measurement is often not performed correctly, leading to inaccurate readings,

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because of inadequate training and equipment, time constraints or lack of awareness of the importance of BP monitoring as a screening and diagnostic test.

Korotkoff IV versus V Up until the late 1990s there was debate as to whether Korotkoff phase IV (muffling of sound, K4) or Korotkoff phase V (disappearance of sound, K5) should be used to classify diastolic BP in pregnancy. It was argued that K4 was more appropriate considering the unique haemodynamics of pregnancy and because it was thought that K5 could often extend to or near zero (since shown to be very rare).6 A randomised controlled trial published in The Lancet in 1998,7 comparing outcome in hypertensive disorders in pregnancy managed according to either K4 or K5, demonstrated that an episode of severe hypertension was more likely in women in the K4 group, mainly because diastolic hypertension was more likely to be recorded. However, the frequency of severe systolic hypertension, simultaneous systolic and diastolic hypertension, and maternal and fetal adverse clinical outcome did not differ between the two groups. Considering these findings and that K5 is better reflective of intra-arterial pressure and is far more reproducible,8 the use of K5 to classify diastolic BP was recommended and has since been included in the NICE5 Antenatal Care guidelines.

Sources of error associated with auscultatory technique BP measurement using the auscultatory technique relies on accurate transmission and interpretation of Korotkoff sounds. Although auscultation using mercury sphygmomanometry has been the gold standard of BP measurement in the past, clinicians are gradually moving away from such sphygmomanometers because of health and environmental concerns regarding the use of mercury in clinical settings. As of 2014, mercury sphygmomanometers are banned and can no longer be purchased in Europe because of environmental concerns. Some non-automated BP devices require regular calibration to ensure a leak rate (loss of air pressure) within 4 mmHg/minute and a pressure scale accurate to within 3 mmHg for any part of the pressure range.9 Observational studies assessing the calibration of BP devices used in clinical practice demonstrated that 20–25% of devices used in hospital and clinic settings had unacceptable calibration errors.9,10 Despite recommendations to record BP to the nearest 2 mmHg, a questionnaire-based study reported that only 10% of midwives and obstetricians recorded BP to the nearest 2 mmHg, with 23% recording BP to the nearest 10 mmHg.11 A study assessing BP values in women seen at antenatal clinic showed that 78% of readings obtained by clinicians ended in a zero.12 This user preference to round off BP values to a zero or five is referred to as


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terminal digit preference and is a source of error associated with auscultation. Observer bias refers to the user adjusting the BP reading to what is preferred or what it was preconceived to be. This concept can extend to threshold avoidance, where the observer adjusts the BP reading to avoid thresholds that entail making a diagnosis or requiring intervention. Again, this is a source of error more commonly associated with auscultation.

Aneroid devices Aneroid devices remove the need to use mercury in clinical settings, but the inherent errors associated with sphygmomanometry and the use of Korotkoff sounds with auscultation remain. Furthermore, aneroid devices require more frequent maintenance and calibration than mercury sphygmomanometers. A survey of BP devices used by UK general practitioners showed that only 50% of devices had been serviced within 1 year and 24% had never been serviced,13 increasing the chance of error. Likewise, an observational study of devices used in UK general practices demonstrated that 53% of aneroid devices were reading in error by more than 3 mmHg, far more than the mercury and automated devices.14 Although a seemingly small difference from the true reading, a systematic underestimation of BP by 3 mmHg would lead to one-quarter of patients with hypertension being falsely classified as normotensive.15 As long as aneroid devices are regularly maintained and calibrated, their accuracy can be assumed to be similar to mercury devices, as shown in an observational study evaluating the accuracy of aneroid devices used clinically, compared with a calibrated mercury sphygmomanometer.16

Oscillometry: an alternative to the auscultatory technique The auscultatory technique for measuring BP requires skill and training, and is therefore prone to observer error. In recent years, there has been a shift towards the use of automated BP devices, which rely on detecting changes in the amplitude of the intra-arterial oscillometric waveforms produced during cuff deflation to determine BP. The Royal College of Obstetricians and Gynaecologists (RCOG), the British Hypertension Society and a number of international organisations have recommended that automated devices are independently validated according to a recognised protocol to ensure accuracy (for example, the Association for the Advancement of Medical Instrumentation criteria, British Hypertension Society and International protocols).17,18 Despite this recommendation, of the hundreds of commercially available automated devices, only a small number have been evaluated and even fewer have passed validation. The issue of accuracy is even more important for those devices used in pregnancy. The majority of devices validated specifically in pregnant populations fail the protocol


requirements, likely because of the haemodynamic changes of pregnancy. A particular concern is that automated devices tend to underestimate BP in women with pre-eclampsia, resulting in false classification of normotension in these high-risk women.19 This is thought to be due to specific pathological changes of pre-eclampsia, including decreased arterial vascular compliance and increased interstitial oedema, which may affect the amplitude and detection of the oscillometric waveform.20 Separate validation in pregnancy (including pre-eclampsia) is therefore recommended but only a small number of devices have passed validation for use in pregnancy, which includes pre-eclampsia (Box 1). For those automated devices that are validated for use in pregnancy (including pre-eclampsia), the obvious benefit over auscultation (mercury sphygmomanometry and

Box 1. Automated devices validated for use in pregnancy (including pre-eclampsia)

OMRON MIT Elite OMRON MIT OMRON Hem 705CP OMRON M7 Microlife WatchBP Home Microlife BP 3BTO-A Microlife BP 3AS1-2 Welch Allyn Spot Vital Signs Dinamap ProCare 400

http://www.dableducational.org/sphygmomanometers/ devices_1_clinical.html#ClinTable

aneroid) is that inaccuracies secondary to observer error are limited and BP measurement is simpler. In LMICs, community healthcare providers caring for women antenatally may have had limited training in BP measurement. In these settings, automated devices may be a more practical alternative. However, there are issues regarding cost, powering and maintenance of these devices.

Appropriate BP cuff size To ensure accuracy it is important to consider the size of BP cuff used, particularly as obesity is a risk factor for pre-eclampsia and therefore those with a large arm circumference (33 cm or above) are at higher risk of developing pre-eclampsia. In a clinical setting large cuffs are often less readily available than standard cuffs. If the appropriately sized cuff is not available, that is, if a standard cuff is used on a woman with an arm circumference of more than 32 cm, BP can be overestimated.21 Conversely, if a large cuff is used on a woman with a normal arm circumference, BP can be underestimated, although this error is much less.22 It is therefore important that a variety of cuffs are available in

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the clinical setting and that arm circumference is correctly estimated or measured. If in doubt, overcuffing (using a cuff that is too large for the arm circumference) is better than undercuffing (using a cuff that is too small for the arm circumference).

Evidence-based hypertension thresholds Obstetricians use BP values to guide management in women with hypertensive disorders in pregnancy. If BP is underestimated or overestimated through inaccurate measurement, avoidable maternal and perinatal mortality and morbidity can result. Furthermore, obstetricians use BP thresholds recommended by national guidelines to aid in management decision making. If these thresholds are not supported by adequate evidence, maternal and perinatal mortality and morbidity can again result. The NICE23 guidelines on hypertension in pregnancy define mild hypertension as diastolic BP of 90–99 mmHg and/or systolic BP 140–149 mmHg, moderate hypertension as diastolic BP 100–109 mmHg and/or systolic BP 150–159 mmHg, and severe hypertension as diastolic BP of 110 mmHg or above and/or systolic BP of 160 mmHg or above. Although these definitions are concise and widely adopted, the BP thresholds that indicate the need for treatment are less clearly defined.

BP thresholds for treating severe hypertension For women with severe hypertension, there is consensus that antihypertensive treatment should be given to reduce the risk of maternal central nervous system complications, but the specific thresholds for initiating treatment are based on limited evidence. A landmark (yet small) retrospective cohort study of 28 women who sustained strokes in association with severe pre-eclampsia or eclampsia showed that all women had a systolic BP of more than 155 mmHg immediately before the stroke, whereas only 13% had a diastolic BP of at least 110 mmHg.24 A retrospective cohort study of women with eclampsia showed that posterior reversible encephalopathy syndrome, defined as the presence of neurological symptoms and signs, together with radiological findings of vasogenic cerebral oedema, occurred at lower systolic BP levels in pregnancy (mean peak systolic BP of 173 mmHg), compared with non-pregnant patients with hypertensive encephalopathy (mean peak systolic BP of 191 mmHg).25 These studies highlight the importance of prioritising the control of systolic BP over diastolic BP, and support the 2010 NICE guidelines recommending immediate treatment if systolic BP is 150 mmHg or above.23 The 2010 NICE guidelines also recommend immediate treatment if diastolic BP is at least 100 mmHg, although this is largely based on extrapolation of risk, rather than direct evidence.23 However, a 2012 nested

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case–control study investigating potential factors associated with antenatal stroke demonstrated that after adjustment for age, stroke increased by 3% (adjusted OR 1.03, 95% CI 1.00– 1.05) for every mmHg increase in highest recorded systolic BP, compared with 8% (adjusted OR 1.08, 95% CI 1.03–1.13) for every mmHg increase in diastolic BP.26

BP thresholds for treating mild-to-moderate hypertension There is a lack of consensus among the obstetric community regarding the threshold of treatment for mild-to-moderate hypertension in pregnancy. A 2014 update of the Cochrane review assessing the effects of antihypertensive treatment of mild-to-moderate hypertension during pregnancy (including those with a diagnosis of pre-eclampsia)27 demonstrated a reduction in the number of women developing severe hypertension or requiring a second treatment agent. However, well-powered studies demonstrating reductions in adverse clinical outcome (maternal stroke, progressive renal and other end-organ disease, and heart failure) following antihypertensive treatment in these cases are limited. Similarly, the review failed to demonstrate a significant effect on preterm births or caesarean sections in those given antihypertensives. The review concluded that it remains unclear whether treatment of mild-to-moderate hypertension is worthwhile. Current NICE/RCOG guidelines therefore do not recommend treating hypertension of systolic BP of 140–149 mmHg or diastolic BP of 90–99 mmHg, except in those women with target-organ damage secondary to chronic hypertension.23

BP thresholds for treating postpartum hypertension A 2005 Cochrane review on the management of postpartum hypertension28 demonstrated insufficient evidence to form robust recommendations on the thresholds for treatment. The NICE guidance23 recommends initiating treatment at the same thresholds as for during the antenatal and intrapartum period; this is largely based on expert opinion. A 2013 clinical review29 summarises the evidence for treatment options for postpartum hypertension and provides a flow diagram with management pathways.

No role for isolated incremental rise in BP The use of an isolated incremental rise in BP to define hypertension in pregnancy is now not recommended in the guidelines.23 Research has demonstrated that women with an incremental rise (for example, 30 mmHg systolic BP/ 15 mmHg) from booking whose BP remained under the threshold of 140/90 mmHg had normal pregnancy outcomes.30

Future research to improve evidence base In response to a call for more robust evidence, an international multicentre randomised controlled trial on


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the management of mild-to-moderate hypertension in pregnancy (Control of Hypertension in Pregnancy [CHIPS] study) is in progress and preliminary results are available.31 Women with non-proteinuric hypertension were randomised to either ‘tight’ (target diastolic BP 85 mmHg) or ‘less tight’ BP control (target diastolic BP 100 mmHg). There were similar rates of the primary (perinatal) outcome (a composite of pregnancy loss or high-level neonatal care for more than 48 hours in the first 28 days of life) between the two groups (‘less tight’ [31.4%] versus ‘tight’ [30.7%]; adjusted OR 1.02, 95% CI 0.77–1.35) and secondary outcomes (one or more serious maternal complications) (3.7% versus 2.0%, respectively; adjusted OR 1.74, 95% CI 0.79–3.84) between the two groups. However, women receiving ‘less tight’ control were more likely to develop severe hypertension (‘less tight’ [40.6%] versus ‘tight’ [27.5%]; adjusted OR 1.80, 95% CI 1.35–2.38). Therefore it is suggested that treatment to a target diastolic BP of 85 mmHg is optimal, considering the risk of adverse cerebrovascular outcomes at higher systolic BP.31

Recommended lower limit of diastolic BP The NICE guidelines23 recommend keeping diastolic BP above 80 mmHg during pregnancy in women receiving antihypertensive treatment, owing to the concern that overtreated hypertension could compromise uteroplacental and fetal circulation, and lead to an increase in small-forgestational-age infants. This recommendation is supported by a meta-regression analysis of the relationship between feto-placental growth and use of antihypertensives, demonstrating that a 10 mmHg fall in BP was associated with a 145g decrease in birthweight.32 However, concern has been raised regarding the meta-regression design, which is prone to all the biases of observational studies.27 In contrast, the Cochrane review on mild-to-moderate hypertension in pregnancy27 demonstrated no overall effect on the relative risk (RR) of having a small-for-gestational-age baby (RR 0.97; 95% CI 0.80–1.17) in those receiving antihypertensive treatment compared with placebo.

The use of mean arterial pressure Evidence is conflicting regarding the utility of mean arterial pressure (MAP) in pregnancy. A 2008 predictive accuracy systematic review and meta-analysis demonstrated that MAP measured in the first and second trimester of pregnancy may be a better predictor of pre-eclampsia (area under the receiver operator curve (AUROC) 0.76 (95% CI 0.70–0.82) than systolic BP (0.68; 95% CI 0.64–0.72) or diastolic BP (0.66; 95% CI 0.59–0.72).33 Despite this, the AUROC value is modest and lack of familiarity with MAP thresholds and the difficulty in obtaining MAP from many devices makes its clinical utility limited. This is reflected in the current NICE


guidelines,23 which do not refer to MAP for diagnosing or treating hypertensive disorders in pregnancy. Further studies are required to better establish the predictive value of MAP in hypertensive disorders in pregnancy and the thresholds that should trigger intervention.

Ambulatory/self-monitoring White-coat hypertension describes a group of individuals who are normotensive in their home environment, but who present with an elevated BP to a healthcare provider. Whitecoat hypertension has been shown to be an independent predictor of cardiovascular risk.34 A prospective observational study of women with chronic and white-coat hypertension in pregnancy demonstrated that 40% of those with white-coat hypertension developed hypertensive disorders in pregnancy and 8% developed pre-eclampsia, significantly fewer than the 22% with chronic hypertension who developed pre-eclampsia (P