According to one aspect, an implantable medical device may include an anchor assembly configured to anchor the medical device to a body lumen. The implantable medical device may also include a capsule. The capsule may include a pH sensor. The pH sensor may be configured to measure a pH of contents within the body lumen. The capsule may also include a power source, a controller, and an impedance sensor. The impedance sensor may be configured to measure an impedance within the body lumen.