Chapter 14: Observations
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Source: NICE ([132]), O'Brien et al. ([145]).
Evidence‐based approaches
Rationale
Indications
Blood pressure measurements should be taken as follows:
- on admission or during initial assessment (NICE [126])
- when a patient is transferred to a ward setting from intensive or high‐dependency care (NICE [142])
- regularly for inpatients, as per local policy (RCP [172])
- in patients at risk of, or with known, infections (NICE [140])
- to assess response to interventions intended to correct a patient's blood pressure (Peate and Wild [157])
- pre‐operatively to establish a baseline, and post‐operatively to assess cardiovascular stability (Tait et al. [194])
- in critically or acutely ill patients, or those who are at risk of rapid deterioration and who might require close and potentially continuous monitoring (Goulden [73])
- in patients who are being transfused blood or blood products, to establish a baseline and also during and after the transfusion (NICE [129])
- in any patient who is receiving medications that could alter their blood pressure, such as epidurals or anaesthetics, antiarrhythmics, anti‐hypertensives, nitrates or vasopressors (Lin [104]).
Contraindications
There are times when certain methods of blood pressure measurement should be used with caution:
- Oscillometric blood pressure devices may not be accurate in patients with weak or thready pulse or those with pre‐eclampsia (Chen et al. [40]).
- The brachial artery should not be used to measure blood pressure in patients with arteriovenous fistulas (Adam et al. [2]).
- Patients with atrial fibrillation should have auscultatory blood pressure measurements taken, rather than oscillometric, and may require multiple readings (Marini and Dries [112]).
- Korotkoff sounds (see the section on indirect methods of measuring blood pressure below) are not dependably audible in children under the age of 1 year and many children under 5 years; therefore, ultrasound, doppler or oscillometric devices are recommended in these patients (Duncombe et al. [57]).
- Patients who have had trauma to the upper arm, a mastectomy or a forearm amputation should not have their blood pressure measured on the affected side at the brachial artery (Lin [104]).
- Oscillometric devices should be used with caution in patients with atherosclerosis and/or high or low blood pressure, as they may not measure accurately (Chen et al. [40]).
- Blood pressure should not be measured on an arm that has had brachial artery surgery or is at risk of lymphoedema (Bickley [18]).
In all cases, the manufacturer's guidance should be sought for contraindications specific to the device used (NICE [132]).
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 (Adam et al. [2]). To do this, a cannula is inserted into an artery, most commonly the radial artery, as it is easy to access and monitor (Lin [104]). The cannula has a transducer attached to it that is connected to a cardiac monitor, where the blood pressure is shown as a waveform; the cannula is also attached to a pressurized flush of solution to prevent blood backflow (Figure 14.23) (Tait et al. [194]). This method has risks of severe haemorrhage, thrombosis and air embolism; therefore, it must only be used where patients can be continuously observed (Adam et al. [2]).
Figure 14.23 Arterial line and transducer set.
Indirect
For indirect blood pressure measurement, either manual auscultatory sphygmomanometers or automated oscillometric devices are used (Duncombe et al. [57]). 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 (Babbs [9]). 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 the cuff pressure, blood re‐enters the arteries, producing vibrations in the artery during systole, enabling a pulse to be auscultated (Benmira et al. [17]). As the cuff pressure descends, the sounds cease as the artery remains open throughout the pulse wave (Chen et al. [39]). These sounds are called the Korotkoff sounds (Figure 14.24). Box 14.3 outlines which sounds relate to which phases of blood pressure.
Figure 14.24 Korotkoff sounds.
Systolic blood pressure is usually defined as being phase 1 of the Korotkoff sounds, and diastolic is usually defined as being phase 5 (Marieb and Hoehn [110], Patton [155]). However, in some patients the Korotkoff sounds may continue until the cuff is completely deflated; in such cases, phase 4 will represent the diastolic blood pressure (Pan et al. [151]). Some patients may present with an ‘auscultatory gap’; this is evident as a silence between the Korotkoff sounds, in that phase 1 may be audible but then a gap follows before the sounds of phase 2 are audible again. It is vital to establish whether the patient has an auscultatory gap as the blood pressure may be misread (missing phase 1 all together). Having an auscultatory gap is often associated with arterial stiffness.
Box 14.3
The five phases of the Korotkoff sounds
The sounds heard are called the Korotkoff sounds and have five phases:
- The first phase is clear tapping, repetitive sounds, which increase in intensity and indicate the systolic pressure.
- 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.
- The third phase is sharper and crisper sounds.
- The fourth phase is the distinct muffling of sounds, which may be heard as soft and blowing noises.
- 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.
Blood pressure measurement sites
The brachial artery is usually the favoured site for blood pressure measurement; however, in some patients this is inappropriate and so alternative sites have to be considered (Adam et al. [2]), such as the thigh, calf or wrist (Bickley [18]). However, local guidelines should be followed, as specific cuffs and different sizes of cuff can be advised, according to the blood pressure measurement site, in order to give an equal pressure to that in the brachial artery (Bickley [18]).
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 (Brown and Cadogan [31]). The patient should rest on a bed in the supine position for at least 5 minutes prior to the initial blood pressure measurement, and the measurement should be taken in this position. The patient should then stand upright and have their blood pressure taken again in the first minute (RCP [172]). A third blood pressure reading should be taken after the patient has been standing for 3 minutes (RCP [172]). While the patient is in the standing position, the practitioner should support the patient's arm at the elbow to maintain it parallel to the hips and ensure accuracy (Chen et al. [40]). Orthostatic (postural) hypotension is defined by a drop in arterial blood pressure of at least 20 mmHg for systolic and 10 mmHg for diastolic blood pressure, with symptoms (RCP [172]).
