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Review question/objective The objective of this review is to evaluate the accuracy of alternate sites for measuring non‐invasive blood pressure when the upper arm is not feasible. More specifically, the objectives are to synthesize the available evidence on the diagnostic accuracy of measuring non‐invasive blood pressure on alternate sites, using upper arm as a reference site. Background Blood pressure generally refers to the pressure exerted by the systemic blood circulation on the blood vessel walls and is one of the principal vital signs of patient monitoring.1 The systolic blood pressure (SBP) corresponds to the peak of the arterial pressure when the heart contracts and diastolic blood pressure (DBP) the trough when the heart relaxes.2 Measuring blood pressure is a common and yet essential procedure performed by healthcare professionals in the clinical settings to assess patient's cardiovascular status.3 It is also important for monitoring patients’ response to medical treatment for blood pressure abnormalities. Inaccurate measurement can result in missed or wrong diagnosis which can lead to inappropriate management of cardiovascular problems.4‐5 This is increasingly an important issue as obesity is becoming a worrying public health problem.4 Using appropriate technique to measure blood pressure is therefore vital in our care delivery and the American Heart Association has issued recommendations on proper clinical blood pressure measurement.6 Non‐invasive blood pressure readings are usually chosen, instead of invasive measurement, as the primary mode of performing routine cardiovascular assessment in the acute care settings.4,7 The readings can be obtained by either using sphygmomanometer or automated oscillometric device. The sphygmomanometer only uses one standard 12 x 23 cm cuff bladder to fit over patient's upper limb during the procedure. The assessor has to manually inflate the cuff and then places a stethoscope over the brachial artery to listen for first appearance of Korotkoff sounds. These sounds of blood flow are interpreted as the systolic pressure. The disappearance of the Korotkoff sounds is assumed diastolic pressure reading.The automated device, on the other hand, detects the initial changes in the amplitude of arterial pulsations and subsequently evaluates the entire cyclic expansion and contraction of the brachial artery. The highest peak corresponds to the mean arterial pressure. The systolic and diastolic readings are then derived from the algorithm programmed in the device. There are few factors to consider in ensuring the blood pressure reading taken is valid. Sources of errors in obtaining a valid may be related to one of the following: inappropriate cuff size, incorrect cuff placement on extremity, position of patient during the procedure, timing of consecutive measurements and patient's anatomical structures.6 Among the parameters for consideration, applying appropriate cuff size received much attention in literature. Healthcare professionals may not apply right‐sized cuff at all times because of cuff availability8, limb accessibility9 and suitability. Assessors may encounter limb sizes that are bigger or smaller than what the current range of cuff sizes can suitably measure.8 The traditional mindset of placing cuff usually at the upper arm may be related to lack of awareness or low priority to study such issue scientifically. This means the inaccurate blood pressure measurement obtained is likely left unchecked or invalidated in the medical records. Blood pressure readings would be overestimated if the cuff used is overly big on small limbs and vice versa for big‐sized limbs. This problem may be partly alleviated by exploring alternate sites of measuring blood pressure to overcome the problem when the upper arm is inaccessible. Having an alternate site for blood pressure taking would be helpful in instances when the healthcare professionals are caring for patients with intravenous devices on both arms or in trauma, surgery or with lymphedema.9 Measuring blood pressure at upper arm is also contraindicated in patients with arteriovenous fistula access on the upper limbs. One hurdle to measuring blood pressure measure on alternate sites is most non‐invasive monitoring devices are specifically designed for upper arm.2 Nonetheless, healthcare professionals face a practical problem when patient's upper arm site is not accessible. Finding suitable alternate site to measure blood pressure noninvasively becomes relevant in addressing knowledge and practice gaps. Validity of alternate sites to replace upper arm for obtaining accurate blood pressure readings had been evaluated in different populations. The study methodologies were, however, not standardized or appropriate, rendering comparisons across studies challenging. Alternate sites that were studied for the feasibility of replacing upper arm measurement included but not limited to forearm, thigh, calf and ankle. Among the sites, method studies comparing forearm blood pressure readings against that of upper arm were most commonly reported in mainstream literature. The populations studied involved the acutely ill,5 patients in emergency department2, outpatients,4 healthy3,7‐9 and obese4 volunteers in the community. Overall most studies found forearm readings to be higher than those of upper arm. However, the quality of statistical analyses was noted to be inadequate which failed to account for repeated measurements in some studies. As mentioned, the methodologies were inconsistent such the type of monitoring device used to obtain the blood pressure readings, method of measuring blood pressure and cuff sizes used in the comparison studies across locations and populations. The Bland‐Altman analyses performed in one study had found the 95% limits of agreement range +17.1 to ‐19.7 mmHg for systolic pressures; +14.5 to ‐16.5 mmHg for diastolic pressures.2 The wide range implied that the forearm and upper arm readings might not be interchangeable.5 Other alternate site studies conducted on calf and ankle also reported higher pressure readings when compared to the upper arm measurements.8 Physics principles might offer some explanation for the pressure differences between upper arm and alternate sites.3 Bernoulli's principle describes how blood vessels further away from the central aorta have slower blood flow and higher internal vessel pressure. This is, in turn, increases total pressure or resistance according to Poiseuille's law. In addition to agreement analyses, other outcomes of interests such as patient's discomfort during blood pressure monitoring at alternate site is also an important criteria. The aim of the review is to systematically examine and report the strength of evidence in studies conducted to compare the accuracy of blood pressure readings measured on all possible alternate sites against upper arm measurements. The review aims to include both published and unpublished studies. An initial search of the Cochrane Library of Systematic Reviews, the Joanna Briggs Library, the CRD database and MEDLINE did not identify existing systematic review or systematic review protocol on the same topic. The author had identified eight primary studies to provide some evidence on how the alternate site measurements might measure up to those taken at the upper arm. Appropriate methodology for performing a meta‐analysis on diagnostic studies requires further development as the available literature is limited to guide analysis. The present review referred to two available resources10‐11 as bases for designing an appraisal checklist and performing meta‐analysis.

Clinical comparison of non-invasive blood pressure measurement at different sites: a systematic review protocol