Cardiac resynchronization therapy
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|Cardiac resynchronization therapy|
Cardiac resynchronisation therapy (CRT) is the insertion of electrodes in the left and right ventricles of the heart, as well as on occasion the right atrium, to treat heart failure by coordinating the function of the left and right ventricles.
CRT is indicated in patients suffering from a low ejection fraction (typically <35%) indicating heart failure, 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, with access to the ventricles most commonly via the subclavian vein, although access may be conferred from the axillary or cephalic veins. Right ventricular access is direct, while left ventricular access is conferred via the coronary sinus (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.
The key indication for CRT is left bundle branch block (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 what we consider heart failure.
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 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 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.
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, is generally no larger than a pocket watch and has inserts for the electrode leads.
Key complications include:
- Dissection or perforation of coronary sinus which can in turn cause pericardial effusion
- Inability to cannulate coronary sinus (approximately 5% of patients)
- Myocardial perforation, pneumothorax and infection, all of which have an incidence of less than 1%
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.
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