The blood pressure and velocity rise rapidly as a result of the opening of the aortic valve in early systole. This spike in blood pressure and momentum travels the length of the aorta and is passed on to peripheral arteries such as the brachial, the carotid, and beyond. The thus formed pulse is an example of a traveling wave in a fluid medium that involves transport of mass and heat. The alteration of the electric field that moves the heart's muscle and the thermo-mechanical effects of pulse propagation in the vascular network creates opportunities for measurement across different modalities. The method that is considered to be the gold standard for pulse measurement is electrocardiography (ECG) . It produces crisp results because it focuses on the source (heart). Other commonly used methods, such as piezoelectric probing , photoplethysmography  and Doppler ultrasound , focus on the vascular periphery. One main characteristic of all these methods is that they require contact with the subject. There are clinical applications, however, where a contact-free method is desirable. Such applications usually involve sustained physiological monitoring of patients who are in delicate state or form; examples range from sleep studies to neonatal monitoring.
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