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 (measured in millimetres of mercury – mmHg) usually reflects the arterial blood pressure although venous pressure may also be measured.

This is the pressure exerted on the arterial walls. Arterial blood pressure facilitates 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.

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 measures the pressure within the right atrium and is determined by:

• the volume of blood entering the right atrium (venous return)

• right ventricular function

• venous tone

• intrathoracic pressure.

The normal range for a healthy adult is 100–140/60–90 mmHg (Bailey et al 2008), but varies according to age and other variables. Both the National Institute for Health and Clinical Excellence (NICE 2006) and the World Health Organization (WHO 2003) define hypertension (raised blood pressure) as a systolic blood pressure of 140 mmHg or above and a diastolic blood pressure of 90 mmHg or above. Bailey et al (2008) define hypotension (low blood pressure) as occurring when the systolic pressure is below 100 mmHg. Blood pressure may vary between the right and left arm. Consequently Beevers et al., 2001a, British Hypertension Society, 2009a, NICE (National Institute for Health and Clinical Excellence), 2006 and Poon et al., 2008 recommend that blood pressure should be measured in both arms initially to determine if significant differences are present (20 mmHg systolic pressure, 10 mmHg diastolic pressure). Thereafter, the same arm should be used to measure blood pressure to ensure consistency.

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 effect of progesterone on the blood vessel walls, resulting in decreased peripheral resistance. Pregnancy is not normally associated with significant changes in arterial blood pressure. Blood pressure usually begins to decrease during the first trimester, as the effects of progesterone 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 much less for systolic pressure than for diastolic pressure (Blackburn 2007). Murray & Hassell (2009) suggest the systolic blood pressure decreases by an average of 5–10 mmHg, whereas the diastolic blood pressure can decrease by up to 10–15 mmHg below the baseline by 24 weeks’ gestation. 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, some women may experience a transient hypotensive episode (supine hypotensive syndrome) due to the weight of the gravid uterus compressing the inferior vena cava, impeding venous return. This occurs more commonly when in a supine position and is quickly rectified by changing to a lateral or recumbent position and it is advisable that women are not laid flat on their back during the second half of pregnancy.

Increased anxiety and pain levels can result in a rise in blood pressure. Additionally, both systolic and diastolic blood pressures increase during uterine contractions; the greater increase is seen in the systolic values. Both these increases return to the baseline level once the contraction is over (Blackburn 2007). It is therefore important to estimate the blood pressure between contractions.

Increased venous return following delivery results in higher venous pressures. As the blood volume and physiological effects of pregnancy decrease, the blood pressure will return to its prepregnant level.

Blood pressure increases with gestational age; blood pressure is lower in the preterm baby than the term baby. A rapid rise in arterial blood pressure occurs during the first week of life, with blood pressure increasing gradually with age (Swinford et al 2006).

The basis of measuring blood pressure is to exert a measured pressure on an artery (commonly brachial), usually with a sphygmomanometer, which comprises a manometer (pressure gauge) and an inflatable cuff. The blood flow is occluded and as the pressure is released and blood begins to flow through the artery, different sounds can be heard through a stethoscope (auscultatory). Oscillatory sphygmomanometers detect the pulsation of blood flow via a pressure sensor in the cuff, avoiding the need to listen for sounds.

Different types of sphygmomanometer are available and can be divided into two categories: (1) auscultatory (manual) and (2) oscillatory (automated, electronic). Aneroid and mercury column manometers are the two forms of auscultatory sphygmomanometer available and are seen more commonly in the community setting. Auscultatory sphygmomanometers are considered to be the gold standard for blood pressure measurement [Medicines and Healthcare Products Regulatory Agency MHRA (Medicines and Healthcare Products Regulatory Agency), 2006 and Beevers et al., 2001a consider mercury manometers to be more accurate than aneroid devices. Oscillatory systolic values tend to be higher than auscultatory values, but oscillatory diastolic values tend to be lower than auscultatory values. 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. The use of oscillatory manometers is increasing, particularly within hospitals.

All sphygmomanometers should be maintained properly, with manometers being recalibrated every 6–12 months to maintain accuracy. Turner et al (2006) suggest that lack of sphygmomanometer calibration can result in both over- and underdetection of hypertension, proposing that systolic hypertension is not detected in 20% 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.