Cardiac Resynchronization Therapy
Assistant Editor-in-Chief: Atif Mohammad, MD

# Introduction
Cardiac resynchronization therapy is a relatively new mode of therapeutic modality currently being used particularly in patients with advanced heart failure.Bi-ventricular pacing or pacing one of the ventricles with bundle branch block is referred to as Cardiac Resynchronization Therapy.
Currently CRT has been approved in patients with advanced HF patients benefit from simultaneous pacing of both ventricles (biventricular or BiV pacing) or of one ventricle in patients with bundle branch block. This approach is referred to as cardiac resynchronization therapy (CRT) . CRT can be achieved with a device designed only for pacing or can be incorporated into a combination device with an ICD.
Cardiac Resynchronization therapy has been approved for patients with advanced heart failure NYHA class III,IIV or patients with LVEF 120 ms.It has gain FDA approval for patients with NYHA class I OR II heart failure in the setting of QRS delay.

# Mechanism of Benefit

## CRT in Heart Failure
CRT has been known to benefit patients with left ventricular dyssynchrony.The mechanism is till unclear but it is known to improve contractile performance of cardiac chambers and hence result in reverse remodeling improving functional capacity and decreasing clinical outcomes of mortality and repeat hospitalizations.Basically, CRT causes improved cardiac myocyte depolarization as a result of cardiac resynchronization which improves cardiac systolic function causing contraction of the left ventricle and thus reducing wall stress and mitral regurgitation.This also improves ventricular remodeling .
Improved contractility of LV in patients with HF associated with IVCD or LBBB is known to reduce myocardial energy demands and oxygen consumption.
CRT has shown to cause reverse ventricular remodeling.A study conducted in Hong Kong showed long term clinical improvement in patients treated with CRT. But it is still not clear whether,clinical improvement and ventricular remodeling correlate with each other or ventricular remodeling is necessary for reduction in clinical signs and symptoms.

## CRT in Afib
Cardiac resynchronization Therapy is mostly used in Chronic Afib in patients with a node ablation indication.CRT has shown to increases peak oxygen consumption and increased exercise duration as compared to standard RV pacing.It has shown to reduce atrial size and activation along with reduction in degrees of mitral regurgitation.It has also known to improve LVEF in patients with chronic Afib.
It is still not clearly known to benefit in patients in Afib with heart failure.
Observational studies have shown significant improvement when CRT is combined with AV node ablation.But there is not enough data to show benefits with other modes of therapy like pulmonary vein isolation.

# Major Society Guidelines
Therefore, common indications for CRT implantation according to the recent guidelines of the American Heart Association/American College of Cardiology/Heart Rhythm Society (AHA/ACC/HRS),HFSA,
- It is recommended for patients with LVEF 35 percent, a QRS duration 120 msec, and NYHA functional class III or ambulatory class IV symptoms with optimal medical therapy
- Itis reasonable for patients with LVEF 35 percent with NYHA functional class III or ambulatory class IV symptoms who are receiving optimal recommended medical therapy and who have frequent dependence on ventricular pacing
- It may be considered for patients with LVEF 35 percent with NYHA functional class I or II symptoms who are receiving optimal recommended medical therapy and who are undergoing implantation of a permanent pacemaker and/or ICD with anticipated frequent ventricular pacing
The American Society of Echocardiography made a consensus statement that patients who meet accepted criteria for CRT should not have their treatment kept on hold because of dyssynchrony results on Echocardiography.

# Procedure Technique
Prior to implantation of LV leads for pacing, basic clinical knowledge of techniques common to all ICDS and pacemakers is necessary.In addition,facilities with various types of vascular access and alternative sites of lead placements is important.
There are 4 approaches
- Transvenous approach
- LV pacing under direct visualization using cardiac surgical approach
- Transvenous via transseptal approach
- Direct approach to surface of left ventricle without thoracostomy

## Pocket
The first step is formation of pocket between the fascia and the muscle.The pocket is formed by anesthetizing the patient and blunt and sharp dissection.Usually the non-dominant side of patient is used.The size should be enough to adjust the device but not large enough to allow it to move.

## Access
Most common routes of venous access are :1)Subclavian cutdown  2)Cephalic stick
Other routes of access are Internal or External Jugular vein, Iliofemoral vein or Axillary vein.
- Subclavian Access:an 18 G needle is attached to a syringe containing 10 ml anesthetic which is inserted at the junction of the medial and middle third of clavicle just above the notch of the sternum.
- Cephalic cutdown:This is more of a surgical approach where the cephalic vein is located,lifted and incised.Due to invasive nature of this approach it is not preferred over Subclavian access.Though, decision of access route is totally based on clinical judgment of physician.

## Implantation of Leads
Most commonly, CRT devices require implantation at 3 sites-RV, RA and LV.The RV lead is most important as there should be pacing wire in case of asystole.In order to get the RV lead wire into the vein, an introducer wire is sometimes used.A guidewire then intorduces it into the RV.The RV lead is pretty floppy and a stylet that may be of variable thickness is used to make it rigid enough as o facilitate its passage inside the vein.Once inside, the guidewire can be removed.The RV lead is passed through the vein, over the tricuspid valve and into the ventricular septum midway in the right ventricle.
The RA lead can be introduced at the same point of venous access as RV lead.If both RV and RA leads are introduced at the same point of entry they can be retained by retained wire technique. Like RV lead, the RA lead was introduced in a similar fashion as the RV lead.Once entering the right atrium, the stylet was withdrawn causing the RA lead to spring back into its preformed J shape.It is then tested in a similar fashion as the RV lead.
The LV lead is the most intriguing.It is introduced through cannulation of Coronary sinus.For cannulation of Coronary sinus following things are needed:  A fixed curve or steerable curve EP catheter, obturator, LV sheath , Angiography catheter and the lead itself with stylet in place.These tools are mostly manufactured in a delivery system.This delivery system is then introduced into the vein and under flouroscopic guidance introduced into the rt. atrium and eventually into the os of coronary sinus.The LV lead implanted by an over the wire technique with use of stylet in such a way as there should be maximum distance between the the RV and LV leads.

# Stimulation
All stimulation devices, ICD, pacemaker do 2 basic things which are : sense and pace.Pacing is the ability of the device to cause depolarization and contraction of the heart.The output pulse is generally governed by 2 factors;pulse amplitude and pulse duration.At the start of implant, a capture test is done.It is done to assess the capture threshold of each chamber.Clinicians set pulse output settings normally above a capture threshold as a safety margin for each chamber.While programming the CRT device, great care should be taken as the device can capture the ventricular chamber.CRT device has also to sense incoming signals.Sensing is normally done in RA and RV.Sensitivity sensing defines the size signals the device can detect and respond to.Basically the purpose of the CRT is make the ventricles to contract in concert.So sensing the heart in the right ventricle provides sufficient info of the intrinsic activity of the heart and helps for the functioning of the CRT.
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CME Category::Cardiology