An implantable cardiac monitoring device as an A-mode ultrasound probe which is adapted to be positioned in the right ventricle of a heart, and which emits an ultrasound signal which is reflected from one cardiac segment of the left ventricle of the heart, and the ultrasound probe receives the resulting echo signal. The delay between the emission of the ultrasound signal and the reception of the resulting echo is measured, and from this delay a position of the cardiac segment is determined. The position of this cardiac segment, at least one reflecting the signal, is related to cardiac performance, and thus the monitoring device determines, from the detected position of the cardiac segment, the cardiac performance.