Abstract: Apparatus and methods are described for ultrasound guided optoacoustic monitoring to provide diagnostic information for many clinical applications of blood oxygenation in blood vessels and in tissues including for early diagnosis and management of circulatory shock (including that induced by hemorrhage). In certain embodiments provided herein, methods and apparatus for optoacoustics for measurement of blood oxygenation in the innominate vein are provided. In certain embodiments provided herein, are methods and apparatus for articulating an angle between an ultrasound probe and subject body to identify an optimum position for an optoacoustic probe to measure blood oxygenation in a target blood vessel or tissue.