A patient QRS duration can be received or determined, such as using one or more patient physiological sensors. A portion of the QRS duration can be determined, such as a right or left ventricular activation time. In an example, the right ventricular activation time can be determined by identifying an onset of a QRS complex and an R-wave peak in the QRS complex. In an example, when the QRS duration exceeds a threshold duration, and the RV activation time does not exceed a second threshold duration, an indication of a cardiac conduction dysfunction can be provided, such as for discriminating between left bundle branch block and right bundle branch block.