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to increase, stimulating the production of aldosterone and antidiuretic hormone which leads to the reabsorption of water and sodium and stimulates the thirst receptors (Foxall 2009). This increases the circulatory volume of fluid and thereby blood pressure (Foxall 2009). Similarly, the reverse happens when fluid volume in the circulatory system is high.

Other mechanisms which influence blood pressure

Skeletal muscle contractions and respiration promote venous return of blood to the heart and therefore increase cardiac output. The skeletal muscles contract on movement, compressing the veins and pushing the blood towards the heart, and respiration causes a change in thoracic and abdominal pressure which acts to pump venous blood (Patton and Thiobodeau 2009). A factor which affects stroke volume is Starling’s Law of the heart which states that the force of the contraction of the heart is related to how much blood volume is in the heart (Patton and Thiobodeau 2009). The more stretched the muscle fibres are prior to contraction, the stronger the contraction and the greater volume it will pump (Foxall 2009).

Evidencebased approaches

Rationale

Indications

Blood pressure measurements should be taken as follows.

On admission to a ward, in A&E departments when a decision to admit has been made (NICE 2007b).

When a patient is transferred to a ward setting from intensive or highdependency care (NICE 2007b).

At least once every 12 hours while in hospital (NICE 2007b).

For patients at risk of, or with known, infections (Chalmers et al. 2008).

To assess response to interventions put in place to correct the patient’s blood pressure (Curran 2009, NICE 2006).

On patients preoperatively, to establish a baseline, and postoperatively to assess cardiovascular stability.

Critically or acutely ill patients, or those who are at risk of rapid deterioration, will require close and potentially continuous monitoring.

Patients who are receiving blood or blood products transfusions, to establish a baseline, during and after the transfusion (McClelland 2007).

Any patient showing any signs of shock should have frequent monitoring.

Any patient who is receiving medications which could alter their blood pressure, such as epidurals or anaesthetics.

Contraindications

There are contraindications or times when certain methods of blood pressure measurement should be used with caution.

Oscillometric blood pressure devices may not be accurate in those with weak or thready pulse or patients with preeclampsia (MHRA 2005).

The brachial artery should not be used to measure blood pressure in those with arteriovenous fistulas (Turner et al. 2008).

Patients with atrial fibrillation should have auscultatory blood pressure measurements taken, rather than oscillometric, and may require multiple readings (Williams et al. 2004).

Korotkoff sounds are not dependably audible in children under the age of 1 year, and many children under 5 years (Curran 2009). Therefore, ultrasound, Doppler or oscillometric devices are recommended (O’Brien et al. 2003).

Patients who have had trauma to the upper arm, previous mastectomy or a forearm amputation should not have blood pressure measured on the affected side at the brachial artery (Turner et al. 2008).

Oscillometric devices should be used with caution in those with atherosclerosis and/or high or low blood pressures, as they may not measure accurately (Bern et al. 2007).

The manufacturer’s guidance should be sought for contraindications specific to the device used.

Blood pressure should not be measured on an arm that has had brachial artery surgery or is at risk of lymphoedema (Bickley and Szilagyi 2009).

Methods of measuring blood pressure

There are two main methods of measuring blood pressure – direct and indirect.

Direct

The direct method enables continuous monitoring of the blood pressure and so is commonly used for critically ill patients, for example in intensive care units and theatres (Woodrow 2004a). To do this a cannula is inserted into an artery, most commonly the radial artery, as it is easy to