Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 121

Nurses routinely measure blood pressure (BP) using a device termed a sphygmomanometer (sphyg). Because of the past history of using mercury column sphygs (rarely used today because of the toxicity of mercury), BP readings recorded using digital, mercury-free devices are still expressed in mmHg.

A typical reading in a young, healthy adult would be around 120/80 mmHg.

The two figures obtained each time a blood pressure measurement is taken represent:

 The systolic BP: This is the upper figure which corresponds to the time during the cardiac cycle when the ventricles are undergoing systole (contraction) and blood is being ejected.

 The diastolic BP: This is the lower figure and corresponds to the time during the cardiac cycle when the ventricles of the heart are undergoing diastole (relaxation) and no blood is being ejected.

Currently NICE (National Institute for Health and Care Excellence) recognises BP readings of 140/90 mmHg or higher as being indicative of hypertension (high blood pressure). It is estimated that hypertension affects at least a quarter of all adults in the UK and over half of all adults in the UK over the age of 60. Hypertension is a major preventable cause of mortality in the UK, increasing the risk of MI, stroke (CVA), heart failure, chronic kidney disease and cognitive decline (NICE, 2018).

A normal BP is essential to maintain tissue perfusion (blood supply) throughout the body from the top of the scalp to the tips of the toes. BP can be affected by many parameters, but a normal BP depends on having a healthy heart to ensure adequate cardiac output (CO) and healthy blood vessels to ensure adequate blood flow. The blood vessels provide a collective resistance to blood flow with the total resistance offered by all the blood vessels in the body known as the peripheral resistance (PR).

In simple terms BP can be thought of as a product of multiplying the CO and the PR:

BP = CO × PR

As we will explore below, BP can be altered by changing the heart rate to change CO or by altering the diameter of blood vessels to change the PR.