In some examples, a system includes a medical device comprising an elongate body configured to advance through layers of tissue of a patient, a lumen extending through the elongate body, a fluid line configured to supply fluid to the lumen, and a pressure sensor positioned within the lumen or the fluid line. The system may further include processing circuitry configured to receive, from the pressure sensor, a signal corresponding to the pressure of the fluid at each of a plurality of time points, determine, for each time point: a corresponding amplitude value of the signal, a difference between two amplitude values of the signal, an amplitude oscillation status of the signal, a position of the elongate body based on the difference and the amplitude oscillation status; and provide an indication of the position of the elongate body relative to the layers of tissue.