Abstract: A method of monitoring cardiac dysfunction, such as pericardial effusion, is disclosed. The method uses an indwelling probe inserted within a coronary sinus or a chamber or vessel of the heart, the probe having motion sensing means configured to sense motion of the probe based on movement of the wall of the coronary sinus or other chamber or vessel. Data is obtained from the motion sensing means and processed to monitor for cardiac dysfunction. The monitoring can be in real-time and can utilise one or more three-axis accelerometers. In some embodiments, two or more three-axis accelerometers are spaced longitudinally along an elongate body of the probe, which can increase accuracy and reliability of monitoring.