Blood pressure is a baseline observation and a component of the National Early Warning Scoring system, as well as most of the currently employed major trauma triage tools. There is a reasonable argument that blood pressure should be routinely taken, as it can identify concerns that warrant further investigation – whether or not hospitalisation is required at that time.
Blood pressure is defined as the hydrostatic pressure exerted by blood on the walls of a blood vessel and is a result of cardiac output, blood volume, and vascular resistance (Tortora and Derrickson, 2023). Blood pressure fluctuates between the contraction (systolic) and relaxation (diastolic) phases of the cardiac cycle, with higher pressures evident during the systolic phase. From a paramedic's perspective, blood pressure generally refers to the pressure in arteries generated by the left ventricle during systole and the pressure remaining in the arteries when the ventricle is in diastole. The systolic figure refers to the highest arterial pressure attained during systole, and the diastolic reading reflects the lowest arterial pressure during diastole (Tortora and Derrickson, 2023). The blood pressure is usually expressed in terms of millimetres of mercury (mmHg) and is recorded as a systolic value over a diastolic value, for example 120/80 mmHg.
Blood pressure can be monitored for a variety of reasons including:
- Recording a baseline for future measurements
- Recording changes in response to treatments or condition
- Monitoring haemodynamic status.
Blood pressure is not a static measurement. Rather, it is a highly dynamic parameter that is subject to continuous fluctuations that include both short- and long-term variability (Parati et al, 2013). Fluctuations may be short-term (heartbeat to heartbeat, minute to minute, hour to hour) or they may be long-term (days, weeks, months or even years). The fluctuations are caused by complex interactions between extrinsic factors (behavioural and environmental) and intrinsic factors (cardiovascular regulatory mechanisms). In paramedic practice, short-term fluctuations are most likely to be of greatest significance and a good understanding of the physiology of normal blood-pressure regulation is recommended to be better able to interpret fluctuations in the ill or injured patient. The full neurohormonal control of blood pressure is beyond the remit of this article, but knowledge can be refreshed in any good anatomy and physiology textbook should it be required.
Blood pressure readings need to be interpreted within the context of the overall patient presentation and more than one reading is normally required. Blood pressure can be affected by weight, age, diet, ethnic group, time of day, pain, pregnancy, stress, exercise, alcohol and further wide-ranging medical conditions (Beevers et al, 2014). The circumstances of the measurement itself may also be a factor in blood pressure readings where rises in systolic blood pressure may attributed to the ‘white coat effect’. White coat hypertension is a condition in which a normotensive subject becomes hypertensive during blood pressure measurement but then becomes normotensive again outside of the medical environment (Beevers et al, 2014).
Measuring blood pressure accurately
In paramedic practice, blood pressure can either be monitored manually using a sphygmomanometer and stethoscope, or by use of automated oscillometric devices. There are advantages and disadvantages to both methods and paramedics need to be alert to the limitations of each.
The National Institute for Health and Care Excellence (NICE) (2023) guidelines for hypertension provide some useful general guidance in terms of measuring blood pressure and it is recommended that all clinicians who take blood pressures follow these guidelines:
- All healthcare professionals taking blood pressure measurements have adequate initial training and periodic review of their performance
- Where there is pulse irregularity (for example, owing to atrial fibrillation), palpate the radial or brachial pulse before measuring blood pressure. If pulse irregularity is present, measure blood pressure manually using direct auscultation over the brachial artery
- Healthcare providers must ensure that devices for measuring blood pressure are properly validated, maintained and regularly recalibrated according to manufacturers' instructions
- When measuring blood pressure in the clinic or in the home, standardise the environment and provide a relaxed, temperate setting, with the person quiet and seated, and their arm outstretched and supported. Use an appropriate cuff size for the person's arm.
It is worth considering whether you deviate from any of the guidelines above and, if so, do you need to make changes to your practice?
Assessing blood pressure in the out-of-hospital environment
In the out-of-hospital environment, blood pressure monitoring is undertaken through either manual sphygmomanometers (normally aneroid sphygmomanometers) or automated oscillometric devices. Both devices use a cuff with inflatable bladder to occlude blood flow through the artery and then gradually release pressure to allow flow to return. These techniques measure blood pressure indirectly so may not correlate exactly with direct blood pressure measurement.
Aneroid sphygmomanometers
The traditional measurement of blood pressure using a sphygmomanometer relies upon an inflatable cuff encircling a limb combined with a stethoscope to detect sounds made by the flow of blood as it comes under the cuff (Lewis, 2019). The practitioner inflates the cuff to a point where it exerts sufficient pressure on the underlying arm to stop blood flowing underneath, meaning that no blood flow sounds can be heard. As the valve is opened and the cuff pressure falls, blood flow resumes and sounds begin to be heard. These sounds vary in intensity and usually stop at the point of the lowest pressure within the arteries before the next pulse arrives. The initial sound approximates to the systolic pressure and the final sound to the diastolic pressure in the artery (Lewis, 2019).
There are multiple problems with the traditional blood pressure measurement including:
- Interpretation of sounds requires training and practice
- Hearing impairment can impact recordings
- Auscultation is difficult in a noisy environment or on a moving vehicle
- Reactions to auditory and visual cues may be slow
- Cuff may be placed incorrectly
- Poorly calibrated equipment may read incorrectly.
Non-invasive automated sphygmomanometers
The use of automated blood pressure devices has become common place in ambulance services in developed countries and they are likely to be used more frequently than manual sphygmomanometers. The majority of these devices use a technique known as oscillometry and, though it is not important to understand exactly how these machines work, the basic principles are useful. Each arterial pulse wave creates a small rise and fall in the volume of the limb, which generates an increase and then a decrease in the pressure within the encircling cuff; the device can detect these changes by the use of a solid-state transducer (Lewis, 2019). The device inflates a cuff to occlude the artery in the same way as a manual cuff does but it detects pressure waves rather than sounds and it uses algorithms to estimate the blood pressure. The cuff is not only used to occlude the artery, but also as the sensor, so the correct cuff is essential for accurate readings.
Caution
The British and Irish Hypertension Society have identified some concerns that bear repeating here. The following is a quote from their 2019 comment piece (Lewis, 2019: p350):
‘Automated oscillometric machines differ with respect to their algorithms, transducers, inflation and deflation rates, cuff sizes and materials, all of which may affect the estimation of BP [blood pressure]. These may result in significant differences in estimations of systolic and diastolic BP compared with auscultatory readings in the same patient.’
This means that caution must be applied when comparing manual with automated blood pressure readings and when comparing blood pressure readings between automated devices.
Considerations when measuring blood pressure
This next section is not going to describe the process of taking a blood pressure. Rather, it will provide evidence to support or refute current practice and clear up some of the common errors or misconceptions that abound in clinical practice (and not just in paramedic practice).
Location of the cuff
The standard location for blood pressure measurement is the upper arm, with a stethoscope positioned over the brachial artery at the crease of the elbow. The typical recommendation is that the centre of the cuff bladder should be placed directly over the brachial artery and this is normally indicated by an arrow on the cuff. This recommendation is based on tradition and, though it seems reasonable, there is little evidence to support it. Indeed, one study has found that even wide variations in the placement of the cuff relative to the brachial artery made no clinically significant difference to the reading providing the correct-sized cuff was used (Bilo et al, 2017).
The cuff does need to be high enough up the arm to allow for unhindered access to the brachial artery for auscultation and a recommendation that the lower edge of the cuff should be 2–3 cm above the antecubital fossa (Munter et al, 2019) is reasonable.
Posture of the subject
Posture affects blood pressure. Generally, blood pressure will increase from the lying to the sitting or standing position. In the clinical setting, the position of the patient should be based upon how they are most comfortable; however, the position should be noted when documenting findings and making clinical decisions. If sitting, the patient should be in a comfortable position with their back supported, feet flat on the floor and legs uncrossed.
Arm position
The position of the arm can influence blood pressure measurement. Irrespective of whether blood pressure is measured in the seated or supine position, the cuff should be at the level of the patient's right atrium (Munter et al, 2019) (mid-sternal level). If the arm is too high, readings can be falsely low or if the arm is too low, readings can be too high. If the arm in which measurement is being taken is unsupported, this results in isometric exercise which can increase blood pressure and heart rate. A recent study showed that where an arm was supported on the patient's lap or unsupported at the patient's side, the blood pressure was significantly higher than the reference reading where the patient's arm was supported on a desk (Liu et al, 2024). The cuffed arm should be supported by the observer or resting on a table at heart level.
Which arm?
There is often a small amount of variation in blood pressure between the arms of an individual but studies have generally shown that those differences are not clinically significant at a study population level; however, absolute differences at an individual level were often clinically significant (Gould et al, 1985; Lane et al, 2002; Fonseca-Reyes et al, 2012; Sharma and Ramawat, 2016). The conclusion is that failure to take blood pressures on both arms could lead to erroneous clinical decisions and it would seem logical to advocate that paramedics take the blood pressure on both arms when they first encounter a patient. This may not always be practical as it will add time on-scene and blood pressure differences may occur as a result of the process of taking a second blood pressure rather than as a pathological finding. What has been established is that higher age, higher systolic blood pressure, diabetes mellitus, and known peripheral artery disease are more likely to be related to large inter-arm differences (Kranenburg et al, 2017). It is reasonable to suggest that a second blood pressure be undertaken on the opposite arm but ensuring that the patient is relaxed and prepared prior to undertaking the second blood pressure. A second blood pressure should not be taken where there is a clinical need for rapid transportation. A systematic review and meta-analysis had identified that a difference in systolic blood pressure of 10 mmHg or more between arms might help to identify patients who need further vascular assessment. A difference of 15 mmHg or more could be a useful indicator of risk of vascular disease and death (Clark et al, 2012).
The cuff and bladder
The sizing of the cuff and bladder in blood pressure measurement is paramount to accurate findings. The estimation of intra-arterial pressure by indirect means such as sphygmomanometry is predicated on a proper relationship between cuff size and the extremity. However sophisticated the measuring device used, if it is dependent upon cuff occlusion of the arm, it will be prone to inaccuracy because of miscuffing. Several dated studies noted that oversized cuffs produce falsely low readings (Sprafka et al, 1991; O'Brien, 1996) while the evidence that either too narrow or too short a bladder (undercuffing) will cause overestimation of blood pressure is unequivocal (Kallioinen et al, 2017). Care needs to be taken to ensure that the cuff is neither too large nor too small, as erroneous blood pressure readings can lead to clinical errors. The recommendation is that blood pressure cuff bladder length should be 75–100% of the patient's measured arm circumference (Munter et al, 2019) but should not overlap the patient's arm. In paramedic practice, cuff size may need to be estimated rather than measured but care should be taken to select the correctly-sized cuff.
The stethoscope (auscultation)
Bell v diaphragm
Ordinarily, the diaphragm is used to auscultate the Korotkoff sounds associated with a blood pressure. The research behind this is old and inconclusive. Some studies showed small changes (not known if they were clinically significant), while most showed no significant differences (Kallioinen et al, 2017).
Stethoscope beneath the cuff
Two old studies showed that placing the stethoscope diaphragm beneath the cuff had a significant impact on both systolic and diastolic readings (Ljungvall and Thulin, 1991; Weber et al, 1999). Ljungvall and Thulin believed that it improved the indirect measurement of blood pressure, hence the increase in systolic and reduction in diastolic blood pressure. While this may (or may not) be the case, it is important to note that there is a significant difference between the two stethoscope placements and it would be helpful to document where the stethoscope was placed.
Palpatory estimation of blood pressure
In some situations, such as a noisy road traffic collision site, it can be difficult to achieve a valid blood pressure based upon auscultatory techniques. This is also the case with those patients in whom it is difficult to determine the Korotkoff sounds. In these situations, it may be possible to estimate systolic blood pressure by palpation of the brachial or radial artery when deflating the cuff. The accuracy of the reading is open to question as not all studies have found high-levels of accuracy (for example, Sabharwal et al, 2011); however, others have found a high-level of concordance (Sahu and Bhaskaran, 2010) so it may be useful as an estimation but only where conventional methods cannot be used. The cuff should firstly be inflated to approximately 30 mmHg above where the brachial or radial pulse can no longer be felt. Then, as the cuff is deflated, the return of the pulse should be noted. This is suggested to be the approximate value of the systolic blood pressure.
Using pulse sites as an estimation of blood pressure
It has been postulated by numerous trauma courses that there is a link between blood pressure and the presence or absence of pulses at various sites. So, it has been argued that if the patient has a carotid pulse then the systolic blood pressure is 60 mmHg; if femoral pulses are present, the systolic blood pressure is 70 mmHg and if the radial pulse is present, the systolic blood pressure is greater than 80 mmHg. However, there is little support provided for these broad estimations. Deakin and Low (2000) found these figures to be an overestimate of actual intra-arterial blood pressure in a small-scale study. A more recent larger study concluded that though mean systolic blood pressure was associated with radial pulse quality, when using a binary measurement of hypotension (systolic <80 mmHg), characterisation of the radial pulse was not a reliable indicator of hypotension (Naylor et al, 2020). Patient management should not be based solely upon information gained by this technique until further evidence has been provided to either support or dispel the proposal.
The strength of the radial pulse in a trauma patient may be an acceptable method for initial rapid evaluation of trauma patients as weaker radial pulses tended to be linked to patients with lower blood pressures and worse outcomes (McManus et al, 2005; Naylor et al, 2020). However, the strength or weakness of the pulse should not be attributed a specific blood pressure.
Cuff to skin or over clothing?
While applying the cuff directly to skin remains the gold standard in most guidelines, it is important to consider whether readings can be taken over clothing where necessary. Non-invasive blood pressure (NIBP) measurements taken over a single layer of light clothing (for example, cotton shirt) have been studied on a number of occasions and the authors concluded that there were no clinically or statistically significant differences between NIBP measurements made on the sleeved and bare arm (for example, Kallioinen et al, 2017). Accordingly, where it is not possible to access skin directly, an automated blood pressure may be taken over a single layer of clothing.
Whether blood pressures are reliable when taken over multiple layers of clothing is still under discussion; however, one study does suggest that this may be feasible. The authors took measurements on the exposed arm; on the arm covered by a standardised cotton and polar fabric test sleeve; and with the arm covered by a cotton polar fabric and down jacket test sleeve. Healthy volunteers and patients were included in the study and the authors concluded that it was possible to perform reliable NIBP measurements over two and three layers of autumn or winter clothing. They stated that measuring NIBP with a clothed arm does not show clinical or statistically significant differences in comparison with measurements performed on the bare arm (Wołoszyn et al, 2019). Although it is not safe to recommend that blood pressures be routinely taken over clothing, this study provides some reassurance that the reading is likely to be accurate where it is necessary. Taking a blood pressure always seems so easy in the comfort of a classroom with fit and healthy volunteers but the reality of clinical practice is very different. Following correct principles is always important and hopefully this article will have provided some useful evidence to support your current practice or engender changes.