Blood pressure (BP) is one of the most important vital signs of the cardiovascular system and the general health. It is recommended that BP be measured regularly for health maintenance, and this is especially important for people of advanced age and who have, or are at a high risk of develop, heart disease. BP varies continuously due to physical activity, medication, emotion and stress. Different oscillometric methods, such as stethoscopes and phonocardiograms are conventional non-invasive techniques to measure blood pressure. However, these cuff-based methods have some disadvantages, which limit their use in certain clinical or home settings, especially during certain a certain physical activity in which the cardiac output increases. A continuous BP cannot be measured using cuff-based methods because a pause of at least 1–2 minutes between two BP measurements is necessary to reduce errors in the measurement. Further, the patient may be disturbed by the inflation of the cuff and this disturbance may cause a sudden elevation of the BP. A continuous measurement of blood pressure is desirable in a multitude of clinical and home settings. For example, in perioperative care, the American Society of Anesthesiologists requires continuous perioperative BP monitoring for patients with severe systemic disease. For other patients, conventional intermittent BP monitoring (NBP) may miss important clinical information because BP is not monitored at all times. An intra-arterial catheter has been used to continuously measure BP, but this costly and invasive method increases risks of arterial injury and skin infection. Noninvasive, continuous and cuff-less BP measurement methods are thus highly desirable. Currently, the relationship between the BP and pulse transit time (PTT) has been studied extensively and the PTT has been demonstrated to be a valid and well accepted measure of BP. Since PTT can be estimated from the measured ECG and photoplethysmogram (PPG), this method has been used to develop wearable devices to measure BP continuously and conveniently
