Techniques and systems for monitoring cardiac arrhythmias and delivering electrical stimulation therapy using a subcutaneous device (e.g. subcutaneous implantable (SD)) is described. In one or more other embodiments, SD is implanted into a patient's heart. Electrical signals are then sensed which includes moderately lengthened QRS duration data from the patient's heart. A determination is made as to whether cardiac resynchronization pacing therapy (CRT pacing) is appropriate based upon the moderately lengthened QRS duration in the sensed electrical signals. The CRT pacing pulses are delivered to the heart using electrodes. In one or more embodiments, the SD can switch between fusion pacing and biventricular pacing based upon data (e.g. moderately lengthened QRS, etc.) sensed from the heart.
