Editing Pacemaker (section)

== Biventricular pacing ==
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[[File:Cardiac resynchronisation therapy.png|thumb|Three leads can be seen in this example of a cardiac resynchronization device: a right atrial lead (solid black arrow), a right ventricular lead (dashed black arrow), and a coronary sinus lead (red arrow). The coronary sinus lead wraps around the outside of the left ventricle, enabling pacing of the left ventricle. Note that the right ventricular lead in this case has two thickened aspects that represent conduction coils and that the generator is larger than typical pacemaker generators, demonstrating that this device is both a pacemaker and a cardioverter-defibrillator, capable of delivering electrical shocks for dangerously fast abnormal ventricular rhythms.]]
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{{Main|Cardiac resynchronization therapy}}

Cardiac resynchronization therapy (CRT) is used for people with [[heart failure]] in whom the left and right ventricles do not contract simultaneously ([[ventricular dyssynchrony]]), which occurs in approximately 25–50% of heart failure patients. To achieve CRT, a biventricular pacemaker (BVP) is used, which can pace both the [[Interventricular septum|septal]] and lateral walls of the [[left ventricle]]. By pacing both sides of the left ventricle, the pacemaker can resynchronize the ventricular contractions.

CRT devices have at least two leads, one passing through the ''vena cava'' and the [[right atrium]] into the [[right ventricle]] to stimulate the [[Interventricular septum|septum]], and another passing through the ''vena cava'' and the right atrium and inserted through the [[coronary sinus]] to pace the epicardial wall of the left ventricle. Often, for patients in normal sinus rhythm, there is also a lead in the right atrium to facilitate synchrony with the atrial contraction. Thus, the timing between the atrial and ventricular contractions, as well as between the septal and lateral walls of the left ventricle can be adjusted to achieve optimal cardiac function.

CRT devices have been shown to reduce mortality and improve quality of life in patients with heart failure symptoms; a LV ejection fraction less than or equal to 35% and QRS duration on EKG of 120&nbsp;ms or greater.<ref>{{cite journal |vauthors=Cleland JG, Daubert JC, Erdmann E, Freemantle N, Gras D, Kappenberger L, Tavazzi L | title = The effect of cardiac resynchronization on morbidity and mortality in heart failure | journal = N. Engl. J. Med. | volume = 352 | issue = 15 | pages = 1539–49 | year = 2005 | pmid = 15753115 | doi = 10.1056/NEJMoa050496 | s2cid = 13938919 | url = http://eprints.gla.ac.uk/128401/1/128401.pdf }}</ref><ref>{{cite journal |vauthors=Bardy GH, Lee KL, Mark DB, Poole JE, Packer DL, Boineau R, Domanski M, Troutman C, Anderson J, Johnson G, McNulty SE, Clapp-Channing N, Davidson-Ray LD, Fraulo ES, Fishbein DP, Luceri RM, Ip JH | s2cid = 19118406 | title = Amiodarone or an implantable cardioverter-defibrillator for congestive heart failure | journal = N. Engl. J. Med. | volume = 352 | issue = 3 | pages = 225–37 | year = 2005 | pmid = 15659722 | doi = 10.1056/NEJMoa043399 | doi-access = free }}</ref>

Biventricular pacing alone is referred to as CRT-P (for pacing). For selected patients at risk of arrhythmias, CRT can be combined with an [[implantable cardioverter-defibrillator]] (ICD): such devices, known as CRT-D (for defibrillation), also provide effective protection against life-threatening arrhythmias.<ref name=Texas2011>{{cite journal |vauthors=Ganjehei L, Razavi M, Massumi A | title = Cardiac resynchronization therapy: a decade of experience and the dilemma of nonresponders | journal = Texas Heart Institute Journal | volume = 38 | issue = 4 | pages = 358–60 | year = 2011 | pmid = 21841860 | pmc = 3147217 }}</ref>