Table 4.

Definitions

Term	Definition
Left bundle branch block (LBBB)	For the purposes of this guideline, LBBB is defined by the 2009 AHA/ACCF/HRS Scientific Statement on recommendations for the standardization and interpretation of the electrocardiogram11 as QRS duration ≥120 ms and a broad notched or slurred R-wave in leads I, aVL, V5, and V6.
Cardiac physiologic pacing (CPP)	CPP is defined as any form of cardiac pacing intended to restore or preserve ventricular synchrony. CPP can be achieved by engaging the intrinsic conduction system via CSP (eg, HBP or LBBAP) or CRT.
Conduction system pacing (CSP)	CSP involves recruitment of the intrinsic conduction system by either HBP or LBBAP.
His bundle pacing (HBP)	HBP involves the direct stimulation of the His bundle to engage the native conduction system. Based on location and pacing outputs, HBP may be selective (isolated recruitment of the His bundle) or nonselective (recruitment of both the local septal myocardium and the His bundle).12
Left bundle branch area pacing (LBBAP)	LBBAP is ventricular pacing that is intended to engage all or any part of the left bundle branch (LBB) fascicular system. Similar to HBP, various responses can be seen based on location and pacing outputs. These include selective LBBP (direct stimulation and isolated recruitment of the LBB fibers), nonselective LBBAP (direct stimulation and recruitment of both the local myocardium and the LBB fibers), or deep septal pacing (no direct recruitment of the LBB fibers).
Cardiac resynchronization therapy (CRT)	CRT aims to restore or preserve ventricular synchrony using left ventricular (LV) stimulation at appropriately timed right ventricular (RV) sensing or stimulation. CRT most commonly refers to BiV pacing, in which a pacing lead is implanted in the RV and another on the epicardial surface of the LV via an epicardial vein. Alternatively, the LV lead may be implanted endocardially or surgically on the epicardium. LV pacing alone in some situations may also deliver CRT. CSP for patients with dyssynchrony may also be considered a form of CRT, but for the purposes of this guideline, CRT refers to use of BiV or LV pacing. These pacing locations refer to standard anatomy but may differ in certain forms of congenital heart disease.
Biventricular (BiV) pacing	BiV pacing is the most common method used to achieve CRT. It most commonly involves the use of 2 ventricular leads, 1 in the RV (apex or septum) and 1 to pace the LV via the coronary sinus or sometimes via direct placement on the epi cardium or endocardium. The LV lead is usually implanted epicardially in the coronary veins, ideally targeting an area of latest activation, which is most often the lateral or posterolateral wall. Alternatively, the LV lead may be implanted endocardially or surgically on the LV epicardium.
Substantial right ventricular pacing (RVP)	Chronic RVP may result in pacing-induced cardiomyopathy in a subset of patients. Substantial RVP may be defined as RVP that is documented to oris anticipated to exceed 40%. However, some observationalstudies have indicated that RVP exceeding 20% can also have detrimental consequences.13–15 It is acknowledged that the burden of RVP may not be accurately predictable prior to implantation and that these data are based on percentages that have been reported in patients with implanted devices. For the purposes of this document, substantial RVP refers to anticipated or actual pacing ≥ 20%−40% and less than substantial refers to anticipated or actual pacing < 20%−40%. Substantial RVP may occur due to second- or third-degree atrioventricular block or to first-degree atrioventricular block with very prolonged PR intervals.
Response to CRT/CPP	CRT “response” has been variously defined in differentstudies, without an actualconsensus on what constitutes response. Response to CRT may be defined using multiple criteria (see Table 5) in terms of improvement of clinical conditions. The terms CRT “stabilizer” or “nonprogressor” have evolved to include patients who may not derive significant reverse remodeling from CRT but seem to realize a blunting of the natural downhill progression of HF. The terms “favorable responder,” which includes the CRT stabilizer or nonprogressor, and “unfavorable responder” have been proposed to account for this. No specific response criteria have yet been postulated for other types of CPP. However, it is reasonable to apply the criteria above for all forms of CPP.