Implantable medical devices are available for preventing or treating cardiac arrhythmias by delivering anti-tachycardia pacing therapies and electrical shock therapies for cardioverting or defibrillating the heart.  Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to a number of rate zones in order to detect episodes of tachycardia or fibrillation. 
Upon detecting an abnormal rhythm, the ICD delivers an appropriate therapy.  Pathologic forms of ventricular tachycardia can often be terminated by anti-tachycardia pacing therapies.  Anti-tachycardia pacing therapies are followed by high-energy shock therapy when necessary.  Termination of a tachycardia by a shock therapy is commonly referred to as “cardioversion.”  Ventricular fibrillation (VF) is a form of tachycardia that is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy.  Termination of VF is commonly referred to as “defibrillation.”  Accurate arrhythmia detection and discrimination are important in selecting the appropriate therapy for effectively treating an arrhythmia and avoiding the delivery of unnecessary cardioversion/defibrillation (CV/DF) shocks, which are painful to the patient. 
In past practice, ICD systems have employed intra-cardiac electrodes carried by transvenous leads for sensing cardiac electrical signals and delivering electrical therapies.  Emerging ICD systems are adapted for subcutaneous or submuscular implantation and employ electrodes incorporated on the ICD housing and/or carried by subcutaneous or submuscular leads.  These systems, referred to generally herein as “subcutaneous ICD” or “SubQ ICD” systems, do not rely on electrodes implanted in direct contact with the heart.  SubQ ICD systems are less invasive and are therefore implanted more easily and quickly than ICD systems that employ intra-cardiac electrodes.  However, greater challenges exist in reliably detecting cardiac arrhythmias using a subcutaneous system.  The R-wave amplitude on a SubQ ECG signal may be on the order of one-tenth to one-one hundredth of the amplitude of intra-ventricular sensed R-waves.  Furthermore, the signal quality of subcutaneously sensed ECG signals are likely to be more affected by myopotential noise, environmental noise, patient posture and patient activity than intra-cardiac myocardial electrogram (EGM) signals. 
The ability of a subcutaneous ICD to detect tachyarrhythmias and reject noise depends on its ECG signal characteristics.  ECG vectors with higher amplitude R-wave waves, higher frequency (high slew rate) R-waves, higher R/T wave ratios, lower frequency signal (e.g., P and T waves) around R-waves, lower susceptibility to skeletal myopotentials, and greater R-wave consistency from cycle to cycle are preferred to ECG vectors without these attributes.  A subcutaneous ICD with a minimum of 2 ECG leads or vectors (using a minimum of 3 electrodes) in a plane may use these physical vectors to generate virtual ECG vectors using a linear combination of the physical vector ECGs.  However, choosing the optimal vector may sometimes be a challenge given the changing environment of a subcutaneous system.  As such, systems and methods that promote reliable and accurate sensing detection of arrhythmias using optimal available sensing vectors to sense ECG signals via subcutaneous electrodes are needed.