This document describes, among other things, a body having at least one acoustically detectable property that changes in response to a change in a physiological condition, such as ischemia. The body is positioned with respect to a desired tissue region. At least one acoustic transducer is used to acoustically detect a change in physical property. In one example, the body is pH sensitive and/or ion selective. A shape or dimension of the body changes in response to pH and/or ionic concentration changes resulting from a change in an ischemia state. An indication of the physiological condition is provided to a user.