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access and monitor (Foxall 2009). The cannula has a transducer attached to it and is attached at the external end to a cardiac monitor where the blood pressure is shown as a waveform; it is also attached to a pressurized flush of solution to prevent blood backflow (Foxall 2009). This method has potential risks of severe haemorrhage, thrombosis and air embolism (Foxall 2009); therefore, it must only be used where patients can be continuously observed (Woodrow 2004a).

Indirect

For indirect blood pressure measurement, either manual auscultatory sphygmomanometers or automated oscillometric devices are used (Bern et al. 2007). Oscillometric devices electronically measure blood pressure by measuring the oscillation of air pressure in the cuff, so when the artery begins to pulse it causes a corresponding oscillation of cuff pressure (Levick 2010). Manual auscultatory blood pressure involves occluding the artery by use of a pressurized cuff and then gradually releasing the pressure; when the systolic blood pressure exceeds cuff pressure, blood reenters the arteries briefly, during systole, enabling a pulse to be palpated and producing vibrations in the artery (Levick 2010). As the cuff pressure descends, the sounds cease as the artery remains open throughout the pulse wave (Kacmerek et al. 2005).

Systolic blood pressure is usually defined as being at stage 1 of the Korotkoff sounds and diastolic at stage 5 (Curran 2009, Marieb and Hoehn 2010, NICE 2006, Patton and Thiobodeau 2009). See Box 12.1 for the Korotkoff sounds and the five phases. However, in some patients the Korotkoff sounds may continue until the cuff is completely deflated; in such cases stage 4 will represent the diastolic blood pressure (Williams et al. 2004). The auscultatory gap represents silence between the Korotkoff sounds and may sometimes be present; it is often associated with arterial stiffness, and it is vital not to mistake this for the actual blood pressure (Bickley and Szilagyi 2009). See Figure 12.11 for the Korotkoff sounds.

Figure 12.11 Korotkoff sounds.

Reproduced from Bickley and Szilagyi (2009).

Box 12.1 Korotkoff sounds

The sounds heard are called the Korotkoff sounds and have five phases.

1. The first phase is the clear tapping, repetitive sounds which increase in intensity and indicate the systolic pressure.

2. The second phase is murmuring or swishing sounds heard between systolic and diastolic pressures.

Some people may have an auscultatory gap – a disappearance of sounds between the second and third phases.

1. The third phase is sharper and crisper sounds.

2. The fourth phase is the distinct muffling of sounds which may sound soft and blowing.

3. The fifth phase is silence as the cuff pressure drops below the diastolic blood pressure. This disappearance is considered to be the diastolic blood pressure.

(NICE 2006, O’Brien et al. 2003)

Alternative sites/methods

Blood pressure measurement at the thigh

There may be some patients for whom brachial artery blood pressure measurement is inappropriate, therefore, alternative sites have to be considered. To measure the blood pressure in the thigh, the patient should be prone with the bladder centred over the posterior popliteal artery and the stethoscope placed over the artery below the cuff (Bickley and Szilagyi 2009). When the appropriate sized cuffs are used, they should give an equal pressure to that in the arm (Bickley and Szilagyi 2009).

Measurement of orthostatic blood pressure

Orthostatic blood pressure measurement may be indicated if the patient has a history of dizziness or syncope on changing position (Lahrmann et al. 2006). The patient needs to rest on a bed in the supine position for 10 minutes prior to the initial blood pressure measurement being taken, then they should stand upright and have their blood pressure taken again within 3 minutes (Bickley and Szilagyi 2009). While in the standing position, the practitioner should support the patient’s arm at the elbow, to maintain it at the correct level and ensure accuracy (O’Brien et al. 2003). Orthostatic hypotension