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Cardiac resynchronisation therapy (CRT or CRT-P) is the insertion of
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
s in the left and right ventricles of the heart, as well as on occasion the right
atrium Atrium may refer to: Anatomy * Atrium (heart), an anatomical structure of the heart * Atrium, the genital structure next to the genital aperture in the reproductive system of gastropods * Atrium of the ventricular system of the brain * Pulmona ...
, to treat
heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, ...
by coordinating the function of the left and right ventricles via a
pacemaker An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart ei ...
, a small device inserted into the anterior chest wall. CRT is indicated in patients with a low
ejection fraction An ejection fraction (EF) is the volumetric fraction (or portion of the total) of fluid (usually blood) ejected from a chamber (usually the heart) with each contraction (or heartbeat). It can refer to the cardiac atrium, ventricle, gall bladder, ...
(typically <35%) indicating
heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, ...
, where electrical activity has been compromised, with prolonged QRS duration to >120 ms. The insertion of electrodes into the ventricles is done under
local anesthetic A local anesthetic (LA) is a medication that causes absence of pain sensation. In the context of surgery, a local anesthetic creates an absence of pain in a specific location of the body without a loss of consciousness, as opposed to a general a ...
, with access to the ventricles most commonly via the
subclavian vein The subclavian vein is a paired large vein, one on either side of the body, that is responsible for draining blood from the upper extremities, allowing this blood to return to the heart. The left subclavian vein plays a key role in the absorptio ...
, although access may be conferred from the axillary or
cephalic vein In human anatomy, the cephalic vein is a superficial vein in the arm. It originates from the radial end of the dorsal venous network of hand, and ascends along the radial (lateral) side of the arm before emptying into the axillary vein. At the e ...
s. Right ventricular access is direct, while left ventricular access is conferred via the
coronary sinus In anatomy, the coronary sinus () is a collection of veins joined together to form a large vessel that collects blood from the heart muscle (myocardium). It delivers deoxygenated blood to the right atrium, as do the superior and inferior vena ...
(CS). CRT defibrillators (CRT-D) also incorporate the additional function of an implantable cardioverter-defibrillator (ICD), to quickly terminate an abnormally fast, life-threatening heart rhythm. CRT and CRT-D have become increasingly important therapeutic options for patients with moderate and severe heart failure. CRT with pacemaker only is often termed "CRT-P" to help distinguish it from CRT with defibrillator (CRT-D).


Indications

The key indication for CRT is
left bundle branch block Left bundle branch block (LBBB) is a conduction abnormality in the heart that can be seen on an electrocardiogram (ECG). In this condition, activation of the left ventricle of the heart is delayed, which causes the left ventricle to contract late ...
(LBBB) of the heart, a cardiac abnormality leading to delayed left ventricular contraction. LBBB causes a QRS prolongation of ≥120 ms on the electrocardiogram, contributing to poor left ventricular coordination and reduced systolic function, thereby reduced ejection fraction (<35%). This reduction in ejection fraction is considered heart failure. Heart failure patients are generally considered if in class II or III heart failure. Current
National Institute for Health and Care Excellence The National Institute for Health and Care Excellence (NICE) is an executive non-departmental public body of the Department of Health and Social Care in England that publishes guidelines in four areas: * the use of health technologies withi ...
(NICE) guidelines state that CRT-D device placement is inappropriate for class IV heart failure, but placement of CRT-P devices may be appropriate in certain circumstances.


Method

CRT requires the placement of an electrical device for biventricular pacing, along with placement of (at least) two pacing leads, to facilitate stable left ventricular and right ventricular pacing. For all elements, the first stage of the process is local anaesthetic followed by incision to allow for approach from the appropriate vein. From here, the leads and device can be inserted.


Right ventricular lead placement

A
venipuncture In medicine, venipuncture or venepuncture is the process of obtaining intravenous access for the purpose of venous blood sampling (also called ''phlebotomy'') or intravenous therapy. In healthcare, this procedure is performed by medical labor ...
is made, and a guide wire inserted into the vein, where it is guided, with use of real time X-ray imaging, through to the right ventricle. The guide wire is then used to assist in the placement of the electrode lead, which travels through the venous system into the right ventricle where the electrode is embedded.


Left ventricular lead placement

This is generally performed subsequent to RV lead placement, with the RV lead providing a backup in case of accidental damage to the electric fibers of the heart, causing an asystolic event. As with the RV lead, a guide wire is first inserted, allowing for the insertion of a multi-delivery catheter. The catheter is subsequently maneuvered to the opening of the coronary sinus in the right atrium. From here a
contrast media A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...
is injected, allowing the surgical team to obtain a coronary sinus phleobogram to direct the placement of the lead into the most suitable coronary vein. Once the phlebogram has been obtained, the multi-delivery catheter is used to guide in the lead, from the chosen vein of entry, into the right atrium, through the coronary sinus and into the relevant cardiac vein. Left ventricular lead placement is the most complicated and potentially hazardous element of the operation, due to the significant variability of coronary venous structure. Alterations in heart structure, fatty deposits, valves and natural variations all cause additional complications in the process of cannulation.


Device placement

The device is inserted in a subcutaneous pocket created by the surgeon, the choice of left or right side of the chest wall is determined mainly by the patient's preference or location of preexisting device. The device, similar to that of a traditional
pacemaker An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart ei ...
, is generally no larger than a pocket watch and has inserts for the electrode leads.


Benefits

Several studies have also shown that CRT can decrease mortality, reverse left ventricular remodeling, and improve quality of life, walking distance, and peak oxygen uptake ( VO2 max). A 2013 study showed that CRT improved the left ventricular ejection fraction (LVEF) by an average of 10.6% 12 months after placement.


Complications

Key complications include: * Dissection or perforation of coronary sinus which can in turn cause
pericardial effusion A pericardial effusion is an abnormal accumulation of fluid in the pericardial cavity. The pericardium is a two-part membrane surrounding the heart: the outer fibrous connective membrane and an inner two-layered serous membrane. The two layers of t ...
* Inability to cannulate coronary sinus (approximately 5% of patients) * Bleeding and pocket haematoma, each of which with an incidence of less than 1% * Myocardial perforation,
pneumothorax A pneumothorax is an abnormal collection of air in the pleural space between the lung and the chest wall. Symptoms typically include sudden onset of sharp, one-sided chest pain and shortness of breath. In a minority of cases, a one-way valve i ...
and
infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable di ...
, all of which have an incidence of less than 1%


Technology

Several research papers have proposed software platforms for planning and guiding the implantation of CRT devices. This research proposes using pre-operative images to characterize tissue and left ventricle activation to identify potential target regions for deploying the CRT leads.


References

{{Cardiac procedures Cardiac electrophysiology Implants (medicine)