Table 1.

Summary of the principles of operation for non-invasive estimation of blood pressure (BP) using a manual auscultatory sphygmomanometer compared with automated BP measurement devices (BPMDs).

Operational step	Requirements and rationale for when using a manual auscultatory sphygmomanometer	Comparison with automated BPMDsa
Cuff placement	Select an appropriately sized inflatable compression cuff that encircles the upper arm: • Dimensions of the cuff bladder relative to the individual’s mid-arm circumference influence whether proper occlusion of the upper arm and brachial artery occurs • A cuff that is too small (undercuffing) will overestimate BP and a cuff that is too large (overcuffing) will underestimate BP • Cuffs must have an inflatable bladder length covering 75–100% of the mid upper arm circumference and a bladder width covering 37–50% of the mid-arm circumference [59] • The shape of the cuff is also important in large arm circumference where the arm tends to be conical [68]	The 75–100% and 37–50% rule for inflatable bladder dimensions do not apply here. Individualised cuff selection should be based on the mid-arm circumference range indicated on the device cuffs and each cuff available for use with the device must be included in that device’s validation testing
Arm positioned	Arm supported with the middle of the cuff positioned at mid-heart level: • Due to effects of hydrostatic pressure, if the upper arm is above or below the mid-heart level, accuracy of BP readings will be influenced • If the upper arm is too high, BP will be underestimated and if the upper arm is too low, BP will be overestimated (by approximately 0.8 mmHg per cm above or below the heart)	Same requirements and rationale
Cuff inflation	Inflate manually with bulb to at least 30 mmHg above the point where the radial pulse disappears, indicating that the brachial artery is occluded	Same requirement to occlude the brachial artery; however, this is automated in non-hybrid BPMDs. Inflation is controlled electronically by a microcomputer and pumps to a level above systolic BP (e.g., 20–40 mmHg). The level of inflation is determined by proprietary algorithms, with some using stepped cuff pressure changes. Cuff pressure is sensed by pressure transducer. Some devices measure BP during inflation
Cuff deflation	Deflation rate should be 2–3 mmHg/s or per heart rate when heart rate is very slow. A deflation rate that is too fast can significantly underestimate systolic BP and overestimate diastolic BP	Deflation rate is electronically controlled via a deflate valve using a continuous or stepped decrease approach. In most devices the rate of deflation is faster than that recommended for manual measurement for BPMDs that do not use an auscultatory method [69]
Signal	Auscultation by stethoscope placed over the brachial artery in the antecubital fossa below the lower border of the cuff to minimise noise artefact	• For the oscillometric method, the compression cuff and its entrained air volume is used to sense the volumetric changes in the brachial artery created by cardiac contraction and relaxation, resulting in volumetric and therefore pressure changes within the cuff (so-called oscillations). Cuff pressure is sensed by a solid-state pressure transducer within the internal housing of the device • A small number of BPMDs employ an automated auscultator method using a microphone embedded in the cuff with which to detect Korotkoff sounds
Signal association with systolic and diastolic BP	Korotkoff sounds denote systolic BP (phase I) and diastolic BP (phase V, or phase IV in absence of V): • Korotkoff phase I sound is the first appearance of two consecutive clear tapping sounds denoting the introduction of blood flow under the cuff • Korotkoff phase II–IV sounds change in quality as the cuff is deflated • Korotkoff phase V sound is the point at which all sounds disappear, denoting the restoration of blood flow under the cuff	• For the oscillometric method, systolic and diastolic BPs are estimated typically by characteristic ratios of an envelope fitted to the ‘oscillations’ with systolic BP at about 50% (range 45–73%) of maximal amplitude on the rising phase of the waveform envelope and with diastolic BP at about 70% (range 69–83%) of maximal amplitude on the falling phase of the waveform envelope [45, 50] • Mean arterial pressure is estimated on the oscillometric waveform envelope at the point of maximal amplitude • Digital readouts are provided for systolic and diastolic BPs and occasionally for mean arterial pressure • Algorithms for device functionality and those used to estimate mean arterial pressure, systolic and diastolic BP are closely guarded trade secrets that are not shared publicly nor independently scrutinised [45]

^aDescriptions provided are generally applicable to automated BPMDs but differences exist between manufacturers and devices.