Assessment of maternal and neonatal vital signs
Blood pressure measurement
Learning Outcomes
Having read this chapter, the reader should be able to:
Death from hypertensive disease, particularly pre-eclampsia and eclampsia, is the fourth highest direct cause of maternal deaths in the UK and is at its lowest rate since the 1985–87 triennial report (Knight et al 2014). Across the world, it is consistently in the top three causes of maternal mortality. The World Health Organization (WHO) (2011) advise that within Africa and Asia almost 10% of maternal deaths are associated with hypertensive disorders of pregnancy, which is similar to the US rates (CDC 2014), New Zealand (PMMRC 2013), and other developed countries; however, this increases to 25% in Latin America.
The midwife is ideally placed to measure blood pressure in the childbearing woman, confirming normality, detecting deviations from normal and referring the woman for further assessment. It is crucial the midwife carries out this procedure accurately as changes in blood pressure can have serious consequences for both the woman and the fetus/baby. This chapter considers the issues surrounding the accurate measurement of arterial blood pressure, physiology, influencing factors, and changes that occur during childbirth. The equipment used, technique of blood pressure measurement and factors influencing the accuracy of the recording are discussed. The chapter concludes with a discussion of venous blood pressure measurement.
Definition
Blood pressure is the force exerted by the blood on the blood vessel walls. It varies within the different blood vessels, being highest in the large arteries closest to the heart and decreasing gradually within the smaller arteries, arterioles and capillaries. Blood pressure continues to reduce as blood returns to the heart via the venules and veins. Blood pressure measurement usually reflects the arterial blood pressure, although venous pressure may also be measured. The brachial artery is generally used to measure blood pressure in the adult.
Arterial blood pressure
This is the pressure exerted on the arterial walls to facilitate blood flow around the body to ensure adequate oxygenation of the tissues and vital organs. It is not constant, increasing during ventricular contraction (systole) and decreasing when the ventricles relax (diastole). When recording blood pressure, it is important to assess both the highest and lowest levels of pressure as these reflect differing physiological responses of the cardiac cycle. The arterial blood pressure measurement is documented numerically in millimetres of mercury – e.g. systole 130 mmHg and diastole 80 mmHg, pronounced 130 over 80.
Systolic blood pressure (SBP)
This is the pressure exerted on the blood vessel walls following ventricular systole when the arteries contain the most blood and is the time of maximal pressure. SBP is determined by the:
An increase in the first two factors or a decrease in the third factor raises SBP, and vice versa.
Diastolic blood pressure (DBP)
This is the pressure exerted on the blood vessel walls during ventricular diastole when the arteries contain the least amount of blood, resulting in the least pressure being exerted on the blood vessel walls. DBP is influenced by the:
DBP is lower when these are reduced, particularly when the heart rate is slower as there is less blood remaining in the arteries.
Mean arterial pressure (MAP)
The mean arterial pressure is the average pressure required to push the blood through the circulatory system. It provides a sensitive indicator of acute changes in perfusion pressure making it a valuable tool in caring for the critically ill woman (Roberts 2006) and is a better indicator of perfusion than SBP. Perrin & MacLeod (2013) suggest a MAP >60 indicates adequate perfusion. The MAP assists with interpreting changes occurring in blood pressure measurements when the systolic and diastolic pressures alter at different rates. It can be estimated electronically or mathematically using the formula:
Alternatively, it can be calculated quickly by doubling the diastolic pressure reading, adding it to the systolic reading and dividing the total by three. Thus a blood pressure of 110/65 mmHg has a MAP of 80 mmHg while a blood pressure of 150/90 mmHg has a MAP of 110 mmHg.
Pulse pressure
This is the difference between SBP and DBP, a normal SBP is around 40 mmHg higher than the DBP (Lippincott et al 2013). A rise in pulse pressure results from an increased SBP and/or a decrease in DBP due to increased stroke volume, decreased peripheral resistance or both and is associated with infection, pyrexia, exercise and bradycardia. A decrease in pulse pressure is due to a decrease in SBP and/or an increase in DBP resulting from decreased stroke volume, increased peripheral resistance or both and can result from hypovolaemia, e.g. shock and haemorrhage. A narrowing pulse pressure can indicate increasing blood loss (Perrin & MacLeod 2013).
Venous blood pressure
This is the pressure exerted on the walls of the veins, reflecting venous flow to the heart (particularly circulating blood volume) and cardiac function. Central venous pressure (CVP) measures the pressure within the right atrium and is determined by:
Normal maternal arterial values
Edmunds et al (2011) suggest the normal range for a healthy adult at rest is 110–140/70–80 mmHg, while Dearsley (2013) suggests it is 100–120/60–80 mmHg. Blood pressure varies according to age and other variables. Both the National Institute for Health and Care Excellence (NICE) (2010) and the WHO (2013) define hypertension (raised blood pressure) as an SBP ≥ 140 mmHg and a DBP ≥ 90 mmHg. Lowe et al (2014) agree and specifically apply it to hypertension in pregnancy. NICE (2010) classifies hypertension as ‘mild’ – SBP 140–149, DBP 90–99; ‘moderate’ – SBP 150–159, DBP 100–109; and ‘severe’ – SBP ≥ 160, DBP ≥ 110. NICE (2008) advise that where there is a single DBP ≥110 mmHg or two consecutive readings of 90 mmHg (at least 4 hours apart) and/or significant proteinuria (1+) increased surveillance is required and if the SBP is >160 mmHg on two consecutive readings (at least 4 hours apart), treatment should be considered. Edmunds et al (2011) define hypotension (low blood pressure) as occurring when the SBP is below 100 mmHg. The UK Sepsis Trust (2014) advise that action should be taken if the SBP is <90 mmHg or there is a >40 mmHg fall from baseline as this is indicative of sepsis until proven otherwise.
Blood pressure may vary between the right and left arm; thus it is important to measure the blood pressure using both arms during the first assessment. If there is an inter-arm difference of >20 mmHg, the measurement should be repeated and if the difference remains >20 mmHg, the arm with the highest reading should be used for all subsequent measurements (BHS 2006, NICE 2010, Poon et al 2008). Both Clark et al (2012) and Verberk et al (2011) advise that a failure to recognize an inter-arm difference can result in an underestimation of, and thus undertreatment of, raised blood pressure which can have serious consequences for the woman and her fetus. Furthermore, large inter-arm differences in SBP can be indicative of atherosclerotic plaques or other vascular occlusive disease and are associated with increased cardiovascular risk (van der Hoeven et al 2013). If the woman has had a mastectomy, blood pressure should be recorded on the arm on the opposite side, as Lippincott et al (2013) caution that performing a blood pressure assessment on the arm next to the affected side may decrease an already compromised lymphatic circulation. It can also increase oedema and damage the arm.
Blood pressure changes related to childbirth
Pregnancy
Profound haemodynamic changes occur during pregnancy primarily as a result of hormonal and anatomical changes, resulting in an increased blood volume, increased cardiac output and heart rate. In the non-pregnant individual this would result in an increase in blood pressure; however, during pregnancy these changes are counterbalanced by the hormonal effects (e.g. progesterone, prostaglandin) on the blood vessel walls, resulting in decreased peripheral resistance. Pregnancy is not normally associated with significant changes in arterial or venous blood pressure. Blood pressure usually begins to decrease during the first trimester, as the hormonal effects are evident before blood volume has increased to its maximum point in the third trimester. Blood pressure begins to rise gradually from the middle of pregnancy, returning to prepregnancy levels by term. The early decrease in blood pressure is less for SBP than for DBP (Blackburn 2013). Murray & Hassell (2014) propose the SBP decreases by an average of 5–10 mmHg, whereas DBP decreases 10–15 mmHg below the baseline by 24 weeks’ gestation. There is a slight increase in the pulse pressure during the third trimester due to the differences between the SBP and DBP.
Venous pressures do not alter significantly during pregnancy, although venous pressure below the uterus does increase with gestation. This may impede venous return to the heart but does not usually create problems. However, up to 8% of women may experience a transient hypotensive episode (supine hypotensive syndrome) due to the weight of the gravid uterus compressing the inferior vena cava decreasing venous return, cardiac output and stroke volume (Blackburn 2013). This occurs more commonly when in a supine position and is quickly rectified by changing to a lateral or semi-recumbent position and it is advisable that women are not laid flat on their back during the second half of pregnancy. If this is required, a wedge should be placed under the woman’s right side and hip to provide a lateral tilt.
Labour
Increased anxiety and pain levels can result in a rise in blood pressure, especially in the primiparous woman (Blackburn 2013). Additionally, both systolic and diastolic blood pressures increase during uterine contractions by up to 35 mmHg and 25 mmHg, respectively, as the circulation increases by 300–500 mL during a contraction, increasing cardiac output. Blackburn (2013) advises cardiac output can increase by 10–15% during the first stage and up to 50% during the second stage. Both these increases return to the baseline level once the contraction is over. It is therefore important to measure the blood pressure between contractions.
Postnatal period: maternal
Increased venous return following delivery results in higher venous pressures and cardiac output is 60–80% higher immediately after delivery. A sharp decline follows after 10–15 minutes and cardiac output stabilizes around 1-hour post-delivery (Blackburn 2013). The increased venous return to the heart also results in an increase in the size of the left atria for the first 3 days, increasing CVP (Blackburn 2013). As the blood volume and physiological effects of pregnancy decrease, blood pressure will return to its pre-pregnant level. Heiskanen et al (2011) advise the haemodynamic changes are relatively stable at 12 weeks post-delivery.
Postnatal period: baby
Blood pressure increases with gestational age; blood pressure is lower in the preterm baby than the term baby. A rise in arterial blood pressure occurs during the first few hours to days after birth, with blood pressure, particularly SBP, increasing gradually with age after 4 weeks (Blackburn 2013). Gardner & Hernandez (2011) suggest a SBP of 65–95 mmHg and DBP of 30–60 mmHg is normal during the first 6 hours of life for term babies. Blood pressure increases by 1–2 mmHg/day for the first 3–8 days and then increases by 1 mmHg/week for the next 5–7 weeks, stabilising around 2 months of age (Frost et al 2011).
Factors influencing arterial blood pressure
A variety of factors influence arterial blood pressure, including:
• Age: blood pressure increases with age due to loss of elasticity of the arterial walls.
• Position: blood pressure is about 10 mmHg higher in standing or sitting positions compared to left lateral, recumbent or supine (Blackburn 2013).
• Talking: if the woman talks while having her blood pressure measured, the DBP can increase by 5.3 mmHg and SBP by 6.2 mmHg (Zheng et al 2012). Dearsley (2013) advises the woman should not speak for at least 1 minute before commencing the procedure as this may increase blood pressure by 10–40%.
• Deep breathing can reduce blood pressure by almost 5 mmHg (Zheng et al 2012).
• Season: Blood pressure can vary by 40% depending on the time of day and month of year, with higher measurements occurring during winter months (Thomas et al 2008).
• Weight: overweight people tend to have higher blood pressure.
• Eating: blood pressure increases for 30–60 minutes following ingestion of food.
• Stress, fear, anxiety and pain can all raise blood pressure by up to 30 mmHg or more by stimulating the sympathetic nervous system; ‘white coat’ syndrome refers to anxiety-related hypertension resulting from attending a healthcare setting. O’Brien et al (2003) suggest this may cause an increase in SBP of 50–60 mmHg.
• Exercise increases blood pressure, with effects lasting 30–60 minutes.
• Distended bladder can increase blood pressure, with effects lasting 30–60 minutes.
• Hereditary factors: some people have an inherited predisposition to raised blood pressure; where one parent is hypertensive there is a 10–20% risk of the offspring developing hypertension with the risk increasing to 25–45% where both parents are hypertensive (Dungan et al 2008).
Indications
Blood pressure is often recorded as a matter of routine throughout pregnancy and labour, less so during the postnatal period. While the circumstances in which the estimation of blood pressure is undertaken can vary, they include:
• the initial booking history to establish a baseline, ideally by 10 weeks’ gestation (NICE 2008)
• at each antenatal visit (NICE 2008)
• during labour, initially and then 4-hourly (NICE 2014)
• following each epidural bolus (see Chapter 25)
• as the clinical condition dictates, e.g. shock and haemorrhage, symptoms such as headaches, visual disturbances, proteinuria or when the Modified Early Obstetric Warning Score (MEOWS) triggers further investigation and repeat of vital signs (p. 66)
• before, during and following a blood transfusion (see Chapter 49)
• following recovery from anaesthesia: with other vital sign observations, it should be taken every 30 minutes for 2 hours and then hourly until stable (NICE 2011a), although Meakin (2011) recommends recording blood pressure every 15 minutes for the first 2 hours, then every 30 minutes for 2 hours followed by hourly measurements for 2 hours, 2-hourly measurements for 2 days and 4-hourly measurements for the next 3 days.
Equipment
Sphygmomanometer
The basis of measuring blood pressure is to exert a measured pressure on an artery usually with a sphygmomanometer, consisting of a manometer (pressure gauge) and an inflatable cuff. As the cuff inflates, the blood flow is occluded, as the pressure is released blood begins to pulsate and flow through the artery which can be detected via a pressure sensor in the cuff (oscillatory) or the resulting sounds can be heard through a stethoscope (auscultatory) depending on the type of sphygmomanometer used.
Different types of sphygmomanometer are available and can be divided into two categories: auscultatory and oscillatory (electronic). Aneroid and mercury column manometers are the two forms of auscultatory sphygmomanometer available. Auscultatory sphygmomanometers, particularly those that are mercury-based, are considered to be the gold standard for blood pressure measurement (Parati & Ochoa 2012) but are used less in clinical practice because of health and safety concerns should the mercury be spilled. Stergiou et al (2011) suggest that the deflation rates of electronic devices are slightly faster (3.5–4 mm/sec) than the recommended auscultatory rate (2–3 mm/sec), resulting in underestimation of SBP and overestimation of DBP. The sensor and pressure transducer have a limited lifespan and De Greef et al (2010) advise they can be prone to losing calibration over time. Electronic devices may not measure blood pressure accurately if a pulse irregularity exists, emphasizing the importance of palpating either the radial or brachial pulse prior to measuring blood pressure to ensure the most appropriate equipment is used (NICE 2011b). Parati & Ochoa (2012) suggest that oscillatory manometers operate under a completely different principle to auscultatory devices and therefore should not be considered to be a true alternative to mercury devices.
To ensure accurate blood pressure comparisons between different recordings, measurements should be undertaken on similar equipment. It is important to record the type of sphygmomanometer used.
A third type of sphygmomanometer has been developed – a hybrid of the auscultatory and oscillatory devices. It has both automated oscillometric and manual auscultatory features and the user decides which to utilize. It uses an electronic pressure gauge as a substitute for the mercury column but has a visual representation of the column and the blood pressure can be seen to lower, as with the mercury column (Stergiou et al 2012). Parati & Ochoa (2012) and Tasker et al (2010) agree it is a suitable replacement for the mercury sphygmomanometer. Although not widely used at present, it may become more popular.
De Greef et al (2010) found calibration errors with both auscultatory and oscillatory manometers; 25% of devices had an unacceptable calibration error, highlighting the importance that all sphygmomanometers should be maintained properly, with manometers being recalibrated every 6–12 months to maintain accuracy. Parati & Ochoa (2012) advise the calibration should be against mercury sphygmomanometers and Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) (2009) recommend mercury sphygmomanometers should be available as reference standards for clinical validation studies of non-mercury manometers until such a time as an internationally recognized alternative is available. Turner et al (2012) suggest that lack of sphygmomanometer calibration can result in both over- and underdetection of hypertension, proposing that systolic hypertension is undetected in 20% of adults but might be falsely detected in 15% of adults. Diastolic hypertension may be undiagnosed in 28% of adults and falsely diagnosed in 31% adults, indicating that some women will be wrongly treated for hypertension and some who should be treated will be missed. The date of the last calibration should be marked on the machine.
The tubing should also be checked regularly for signs of deterioration and replaced accordingly. The control valves should be able to hold a pressure of 200 mmHg for 10 seconds, allowing for a rate of fall of 1 mmHg per second (Jolly 1991); it is important for the control valves to be checked regularly to ensure the free passage of air without undue force.
Aneroid manometers
This type of manometer has a circular gauge encased in glass, with a needle that points to numbers. Pressure variations within the inflated cuff cause metal bellows within the gauge to expand and collapse, moving the needle up and down the gauge. Prior to use, the needle should be set at zero.
These are lightweight, compact and portable but less accurate than mercury column manometers as the metal parts are liable to expand and contract with temperature changes. Aneroid manometers should undergo biomedical calibration on a regular basis to increase accuracy. Skirton et al (2011) suggest further research is needed on the use of aneroid manometers before they can be considered as a replacement for mercury-based manometers.
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