Chapter 4. Blood pressure measurement
Introduction
There are many ways in which midwives monitor the wellbeing of clients. The group of skills that involves measuring physical characteristics are often referred to as ‘observations’, one of which is blood pressure measurement. The measurement of blood pressure provides valuable information about a woman’s health. It can be used to monitor her body’s response to pregnancy, labour and birth. It is often recorded as a baseline measure in each of these circumstances to enable the carer to detect changes that may result from disease or obstetric intervention. Thus the midwife needs to understand what influences a woman’s blood pressure, how to measure blood pressure accurately and the significance of the observation made.
Consider the following scenario in relation to the measurement of blood pressure. Try to identify what you need to know in order to interpret this situation.
Suzie’s heart was pounding. The cuff felt tight and uncomfortable on her arm and then mercifully began to deflate. The midwife looked intently at the machine, listening. She then turned to the woman and said, ‘That’s fine’.
The following questions might have come to mind:
■ Why is the midwife taking her blood pressure?
■ How does the heart affect blood pressure, why was her heart pounding?
■ What is a cuff, how is it applied and inflated, when is it deflated?
■ Why was the cuff uncomfortable, what could be done to avoid discomfort?
■ What is the machine called, what does it measure, are there different types, which is best?
■ What is the midwife listening to, how does she listen, what does she use?
■ How did the midwife know that the blood pressure was ‘fine’, what is normal?
You will find the answers to these questions within the chapter. 
Definition
Blood pressure can be defined as: ‘the force exerted by the blood on the vessel walls’ (Johnson & Taylor 2006:55).
Background physiology
The circulatory system
Blood flow through the circulatory system is made possible due to the blood pressure gradient; blood pressure is highest near to the heart in the arteries and reduces as the blood flows away from the heart to the arterioles and the venous system.
The heart
The vascular and arterial systems
Systemic circulation
Blood rich in oxygen leaves the left ventricle of the heart via the aorta. The aorta divides into arteries and subdivides further into arterioles then capillaries. It is in the capillaries that exchange of gases, metabolic waste and nutrients takes place. The capillaries join to form venules, which further unite to form veins. Blood eventually returns to the right atrium of the heart from the lower body via the inferior vena cava and from the upper body via the superior vena cava.
Pulmonary circulation
Deoxgenated blood leaves the right ventricle of the heart via the pulmonary artery. This artery divides into two branches delivering the blood to the lungs, where the vessels divide into arterioles and capillaries as in the systemic circulation. Oxygenated blood is collected in venules and veins returning to the left side of the heart via four pulmonary veins.
Structure of blood vessels
All blood vessels comprise three layers, except capillaries, which are made of a single epithelial layer. The layers are:
Tunica intima – the inner layer made up of a single layer of endothelial cells.
Tunica media – the middle layer made up of elastic tissue and smooth muscle. In the aorta this layer is mostly elastic tissue with some muscle but as the arteries become smaller the proportion changes to become mostly muscle. In the venous system the vessel wall is thinner and more distensible, thus the veins are capable of carrying about 60% of the circulating blood volume (Stables & Rankin 2005).
Tunica adventicia – the outer layer made up of fibrous connective tissue.
Control of blood pressure
The blood pressure must be capable of responding to the demands of the body, and will vary accordingly. A range of factors influence arterial pressure including:
Cardiac output – (stroke volume × heart rate). Increased cardiac output leads to an increase in blood pressure, for example, during exercise. Cardiac output is also increased in response to sympathetic nerve stimulation following stress.
Arteriole peripheral resistance – The arteriole walls contain smooth muscle. When this smooth muscle contracts the lumen of the vessel decreases and the blood pressure increases. When smooth muscle relaxes the lumen of the vessel increases and the blood pressure is reduced. The renin-angiotensin system, which is stimulated in response to underperfusion of the glomerular apparatus in the kidney, leads to constriction of the arterioles and hence a rise in blood pressure.
Blood volume – A reduction in blood volume leads to a reduction in blood pressure, for example following haemorrhage. Although blood volume increases in pregnancy this does not lead to an increase in blood pressure because of reduced arteriole peripheral resistance.
Blood viscosity – Increased viscosity due to an increased number of red blood cells or plasma proteins leads to increased blood pressure. Severe anaemia could lead to reduced blood pressure.
The impact of pregnancy on blood pressure
During pregnancy there is an increase in cardiac output and circulating blood volume. However, due to the impact of progesterone relaxing the smooth muscle within the blood vessel walls, peripheral arteriole resistance is reduced and blood pressure remains within normal limits. There is virtually no change in systolic pressure during pregnancy but the diastolic pressure tends to reduce in the first two trimesters and return to pre-pregnancy values during the third trimester (Stables & Rankin 2005).
Most women have normal blood pressure throughout pregnancy, however, hypertensive disease in pregnancy is a significant cause of maternal death, resulting in 18 deaths in the 3-year period covered by the latest confidential enquiry (Lewis 2008). Although some women already have some degree of hypertensive disease, there is a pregnancy-specific disorder, called pre-eclampsia, that affects approximately 7% of all pregnancies (Sibai 1998). Pre-eclampsia may lead to serious complications in the mother including renal and hepatic failure, clotting disorders and haemorrhage and the baby may suffer the consequences of intrauterine growth restriction or premature birth (Enkin et al 2000).
The effects of pre-eclampsia can be minimized through prompt action. It is therefore vital that the woman’s blood pressure is carefully assessed throughout the antenatal period so that appropriate care and treatment can be initiated if hypertension is discovered. Box 4.1 provides some working definitions developed by APEC for use in the care of women with hypertension in pregnancy.
Box 4.1
Definitions of hypertension in pregnancy
| Hypertension: | A diastolic blood pressure of 90mmHg or above |
| Pre-existing hypertension: | Hypertension that existed pre-pregnancy or at booking (before 20 weeks) |
| New hypertension: | Hypertension at or after 20 weeks of pregnancy |
| Pre-eclampsia: | New hypertension and the presence of significant proteinurea (greater than or equal to 300mg per 24 hours) at or after 20 weeks of pregnancy. |
| (PRECOG 2004:7) |
During labour, blood pressure rises due to the increased cardiac output asssociated with the physical exertion involved and exacerbated by pain and anxiety. Each contraction of the uterus adds 300–500ml to the circulating volume (Stables & Rankin 2005). Postnatally the blood pressure should return to pre-labour values.
National guidelines
NICE guidance (NICE 2008) recommends that blood pressure is measured at each antenatal visit, which for primigravida means a minimum of 10 recordings. The intervals for this observation are: ideally before 10 weeks, 16, 25, 28, 31, 34, 36, 38 and 40 weeks of pregnancy. Women who have already had a baby would have a minimum of seven antenatal checks, missing out a check at 25, 31 and 40 weeks of pregnancy.
The NICE Intrapartum Care Guidelines (NICE 2007) stipulate that the woman’s blood pressure should be recorded when labour is first suspected and 4-hourly during the first stage of labour. Once the second stage of labour has been diagnosed the recommendation is that blood pressure is recorded hourly and then again after the baby is born.
The NICE Postnatal Care Guidelines (NICE 2006) recommend that the woman’s blood pressure should be recorded within 6 hours of the birth, as a minimum. If the diastolic reading is greater than 90mmHg it should be repeated in 4 hours. If it does not fall within this time the woman should be assessed for pre-eclampsia.
The charity Action on Pre-Eclampsia (APEC) have produced a Pre-Eclampsia Community Guideline (PRECOG) for use in the community which is endorsed by the Royal Colleges of Midwives, Obstetricians and General Practitioners and the National Childbirth Trust (see resources).
Professional guidelines
To practise midwifery in the United Kingdom, midwives must work within the professional framework of the Nursing and Midwifery Council.
The activities of a midwife as described in the Midwives rules andstandards (NMC 2004a, pp 36–37) include:
…monitor normal pregnancies…to recognize the warning signs of abnormality in the mother…monitor progress of the mother in the postnatal period.
All activities undertaken must be documented in accordance with the NMC Guidelines for records and record keeping (NMC 2005).
The measurement of blood pressure
The measurement of blood pressure, using the brachial artery in the arm, reflects arterial pressure. The pressure the blood exerts on the artery walls when the ventricles of the heart contract is called systolic pressure. It is measured in millimetres (mm) of mercury (Hg) and is normally in the range of 100–140mmHg. The pressure the blood exerts on the artery walls when the ventricles relax is called diastolic pressure and this is normally in the range of 60–90mmHg. Blood pressure is recorded systolic over diastolic and therefore a ‘normal’ blood pressure would be recorded as 120/75mmHg.
Methods of measuring blood pressure
Blood pressure is measured using either the oscillatory or the auscultatory method.
Oscillatory method
The development of oscillatory or automatic devices is big business with a range of appliances available both for home and hospital use. Although there are automated devices that accurately measure blood pressure during pregnancy, a meta-analysis of validation studies of such devices showed that they under-read by clinically significant amounts in women with pre-eclampsia (Shennan & Waugh 2003). Whenever there is concern about a value given by an automated device the blood pressure should be checked again using the auscultatory method described below.
Auscultatory method
The traditional method for measuring blood pressure is by auscultation of the flow of blood through the brachial artery at the antecubital fossa. The instrument used to give a value to the sounds heard is called a sphygmomanometer and the instrument used to make the sounds audible is called a stethoscope.
Equipment
There are two methods of measuring blood pressure using the auscultatory method: the mercury and aneroid sphygmomanometers.
Mercury sphygmomanometer – This type of manometer has mercury contained in a glass column, with ascending numbers either side of the column. The column of mercury needs to be upright and the level observed at eye level. It is the most accurate method giving consistent readings and is heralded as the ‘gold standard’ for accurate blood pressure measurement (Valler-Jones & Wedgbury 2005). However, it is bulky, heavy and contains mercury which is hazardous to health. The Medicines and Healthcare products Regulatory Agency (MHRA) released a bulletin from the Medical Devices Agency (MDA 2000, p 4) advising the NHS to consider substituting mercury devices with alternative mercury-free products when the ‘opportunity arises’. Many NHS Trusts have replaced their mercury syphygmomanometers with aneroid instruments.
Aneroid sphygmomanometer – This type of manometer has a circular gauge encased in glass, with a needle that points to numbers. These are lightweight, compact and portable but less accurate than the mercury devices. They should be re-calibrated on a regular basis.
Inflation cuff – Sphygmomanometers work using the principle of a bladder within a cuff being inflated around the upper arm to occlude the flow of blood through the brachial artery. The bladder is then gradually deflated by means of a valve in a hand held pump, until the blood flows through the artery again. This blood flow makes a noise that is heard (auscultated) through a stethoscope. These sounds are called Korotkoff sounds (Guyton 1997). The midwife listens to the Korotkoff sounds (see Table 4.1) and observes the dial of the aneroid device (or column of mercury) to measure the blood pressure.
| Korotkoff phase | Sound heard through the stethoscope |
|---|---|
| 1 Systolic pressure | First faint tapping sounds that gradually become louder |
| 2 | Sounds become fainter again but more swishing |
| Auscultatory gap | Sounds may disappear for a short time in some clients |
| 3 | Return of clear sounds again, may be even louder than K1 |
| 4 | Sudden muffling of sounds that are softer in intensity |
