Techniques are provided for evaluating mechanical dyssynchrony within the heart of patient in which a pacemaker, implantable cardioverter-defibrillator (ICD) or other medical device is implanted. In one example, a set of cardiogenic impedance signals are detected along different sensing vectors passing through the heart of the patient, particularly vectors passing through the ventricular myocardium. A measure of mechanical dyssynchrony is detected based on differences, if any, among the cardiogenic impedance signals detected along the different vectors. In particular, differences in peak magnitude delay times, peak velocity delay times, peak magnitudes, and waveform integrals of the cardiogenic impedance signals are quantified and compared to detect abnormally contracting segments, if any, within the heart of the patient. Warnings are generated upon detection of any significant increase in mechanical dyssynchrony. Diagnostic information is recorded for clinical review. Pacing therapies such as cardiac resynchronization therapy (CRT) can be activated or controlled in response to mechanical dyssynchrony to improve the hemodynamic output of the heart.