Cardiac arrhythmias can be detected and treated by implantable cardioverter defibrillators (ICDs).  ICDs typically monitor an intracardiac electrogram (EGM) signal to determine a patient's heart rhythm.  When tachycardia or fibrillation are detected, electrical stimulation therapies are delivered, which may include pacing therapies and/or cardioversion/defibrillation shock therapies.  The delivery of a shock therapy can be painful to the patient and uses considerable battery charge.  As such, it is desirable to avoid delivering shock therapy when unnecessary, for example when the arrhythmia is not life-threatening and the patient is hemodynamically stable. 
An uncalibrated, oxygen saturation index can be determined using an implantable optical sensor detecting two or three light wavelengths for monitoring patient hemodynamics.  The uncalibrated oxygen saturation index can be used in detecting hemodynamically unstable arrhythmias.  The influence of motion, optical path length, sensor location, confounding physiological events or conditions, and the relationship of an uncalibrated oxygen saturation index to the physiological status of the tissue, e.g. to actual tissue oxygenation, can result in a broad statistical distribution of the responses of the oxygen saturation index to both hemodynamically unstable arrhythmias and to normal sinus rhythm resulting in reduced specificity in differentiating between the two.  A need remains for improved sensors and methods for discriminating between hemodynamically stable and unstable arrhythmias.  Such discrimination may be used in controlling the delivery of shock therapies.