Ultra Low Power Photometry for Pulse Oximetry Applications

Removing patient cables from the hospital environment, through the use of wireless sensors, improves hygiene, convenience and standard-of-care [1] [2]. In the drive to eliminate cable clutter, vital signs monitoring (VSM) is "going wireless." This, in turn, is driving a trend for battery powered VSM sensors such as Saturation of Peripheral Oxygen (SpO2), Blood Pressure (BP), and Electro-cardiogram (ECG) sensors with a resultant demand for ultra-low-power circuits and algorithms. The architecture of the optical SpO2 pulse oximeter, which measures blood oxygenation and heartrate, is described with a focus on the drivers and contributors to system power. Two concepts for reduction of power in the pulse oximeter are explored. Firstly, an algorithm which modulates LED current according to the instantaneous heartbeat pulse phase is demonstrated in hardware and software. Secondly, an inductor centric LED driver, which provides the power efficiency of a switched mode current source and the system accuracy of a linear current source is introduced with feasibility demonstrated by circuit and system simulation.Clinical Relevance— The techniques discussed enable longer battery life for the SpO2 wireless VSM which, in turn, improves hygiene, convenience and, most importantly, mobility of the patient in the clinical setting.