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This paper presents a novel adaptive algorithm for calibrating non-invasive pulse transit time (PTT) measurements to arterial blood pressure (BP). This new algorithm allows complete calibration of PTT to BP without the use of an oscillometric blood pressure cuff or external pressure sensor. Further, the algorithm can be used to continually update the identified parameters in the calibration equation while the patient is wearing the device. The technique utilizes natural patient motion to generate a known change in the transmural pressure (input) acting on the arteries monitored by our device to produce a measurable change in pulse transit time (output). The natural motion includes varying the height of the sensor relative to the heart to alter hydrostatic pressure at the measurement site and adjusting proximal joint posture to vary the external arterial pressure at the measurement site. This new algorithm is applied to a unique wearable sensor architecture that combines two in-line PPG sensors, one located at the ulnar artery of the wrist and one located at the digital artery of the little finger along with a multi-axis accelerometer for height measurement. Initial human subject tests results using the new algorithm and device will be presented.
Date of Conference: 20-25 Aug. 2008