Blood pressure measurement occurs either non-invasively or invasively, and usually refers to systemic arterial pressure measurement, but can also refer to systemic venous or pulmonary arterial pressure measurement. In 1733 the Reverend Stephen Hales was the first person to measure the blood pressure in vivo in unanaesthetised horses by direct cannulation of the carotid and femoral arteries. In doing so he observed the pulsatile nature of flow in the circulation. In 1828 Poiseuille developed the mercury manometer, and used it to measure blood pressure in a dog. The mercury manometer has, of course, become the standard technique against which other techniques are compared. The earliest numerical information on blood pressure measurement came from direct rather than indirect measurement in 1856 by Faivre, using Poiseuille’s device. However, in the last part of the nineteenth century, non-invasive measurement techniques were developed. In 1903, Codman and Cushing introduced the concept of routine intraoperative blood pressure measurement, which at the time was a revolutionary concept. Nowadays it is a fundamental part of minimal monitoring criteria. There are several techniques of non-invasive BP (NIBP) measurement, all of which function by occluding the pulse in a limb with a proximal cuff, then detecting its onset again distally, on lowering the cuff pressure. Detection methods include palpation, auscultation, plethysmography, oscillotonometry and oscillometry. Accuracy of all non-invasive techniques depends on cuff size in relation to the limb concerned, and over which artery the cuff is placed. Such techniques of NIBP measurement are necessarily intermittent. Much discussion has taken place on the accuracy of these devices, and the accuracy of diastolic pressure measurements needs improving, and there are ideas proposed for new non-invasive devices [Tooley and Magee 2009]. In the absence of a stethoscope, this technique is simple and reliable. After inflating the cuff on the upper arm to a pressure of above that of systolic, the cuff is then deflated while palpating the brachial artery and the systolic pressure is measured with a mercury column at first detection of the pulse. A study by van Bergen [1954] showed that BP can be underestimated by this method by up to 25% at 120 mmHg.
Recent Advances in Wearable Devices for Non-Invasive Sensing
