Apparatus (20) is provided including an inflatable bladder (40), adapted to be coupled to a blood vessel (22) of a subject (18) carrying oxygenated blood, such that an interior of the bladder (40) is in fluid communication with the blood. The apparatus also includes a piston (48) in mechanical communication with the bladder (40); a motor (60), adapted to synchronize contraction and expansion of the bladder (40) with a cardiac cycle of the subject (18) by applying a motor force to the piston (48); and a spring (54), adapted to apply a spring force to the piston (48).
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