An implantable medical device identifies onset and/or progression of left ventricular dysfunction (LVD). The implantable medical device receives a signal that represents electrical activity within a heart, e.g., an electrogram signal, and digitally processes the signal to assess left ventricular function. In particular, the implantable medical device measures at least one characteristic of QRS complexes within the signal, and assesses left ventricular function based on measurements. The implantable medical device may, for example, measure the width of the QRS complexes and/or the amplitude of the R-waves within the QRS complexes. The implantable medical device may alert a patient or clinician of the onset or progression of LVD, and may control delivery of therapies, such as rate-responsive pacing and cardiac resynchronization pacing, based on the measurements. The implantable medical device may also control delivery of a drug by an implanted drug delivery device, e.g., drug pump, based on the measurements.