A system includes a processor and an output device. The processor is configured to: (a) receive electrical signals indicative of measured positions of (i) one or more chest position sensors attached externally to a chest of a patient, and (ii) one or more back position sensors attached externally to a back of the patient； (b) compare between (i) a first shift between the measured positions and respective predefined positions of the one or more chest position sensors, and (ii) a second shift between the measured positions and respective predefined positions of the one or more back position sensors； and (c) produce an alert in response to detecting a discrepancy between the first and second shifts. The output device is configured to output the alert to a user.
