Cardiac Resynchronization Therapy CRT

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Cardiac resynchronization therapy, known in short as CRT, is also known as heart failure device therapy.
All patients with heart failure need optimal pharmacological therapy and lifestyle modifications. But in a small subset, there is a definite role for devices. Ventricular tachycardia in a scar of old myocardial infarction may necessitate the implantation of an implantable cardioverter defibrillator.
Hypotensive ventricular tachycardia in heart failure is an important cause for sudden cardiac death as it can degenerate into ventricular fibrillation in a short time.
Those who have survived a SCD are those at a higher risk of recurrence and benefit maximum with an ICD implantation. ICD improves the life expectancy by 6 years in these high-risk individuals.
Intraventricular dyssynchrony in the presence of severe left ventricular dysfunction is an important indication for cardiac resynchronization therapy. Delay between the contractions of the septum and the lateral left ventricular wall causes reduced left ventricular stroke volume.
The important surrogate of ventricular dyssynchrony is an increased QRS duration. In CRT, septum and lateral left ventricular wall contracts simultaneously producing improvement in the left ventricular stroke volume.
This is achieved by pacing the lateral wall of the left ventricle through a coronary vein along with right ventricular endocardial pacing. CRT improves the symptomatic status and survival of heart failure patients with left ventricular dyssynchrony. But still there is a 30% non-responder rate of patients who do not respond to CRT.
QRS duration of 150 ms or more with LBBB pattern in a person with refractory heart failure will be a strong indication for cardiac resynchronization therapy. It is often associated with mechanical dyssynchrony and wasted systolic effort of the left ventricle.
CRT produces a narrowing of the QRS complexes as the right ventricle and posterolateral left ventricle are paced in synchrony, to produce a better left ventricular output.
Selection criteria for CRT: Severe heart failure NYHA class III or IV. Depressed left ventricular ejection fraction less than 35%. QRS duration 150 ms or more. Most widely used marker of dyssynchrony is surface ECG.
But it is not an absolute marker as it may not have complete correlation with mechanical dyssynchrony. Left bundle branch block is associated with dyssynchrony of lateral wall compared to the septum.
Should be in sinus rhythm for better synchronization and should be on optimal medical therapy.
Those with recent myocardial infarction or have undergone coronary revascularisation within 3 months as well as those scheduled for coronary revascularisation are excluded. This is in view of the potential for improvement in left ventricular function in the short term.
Echocardiographic parameters: M- Mode: Septal posterior wall motion delay at papillary muscle level in parasternal short axis view more than 130 ms has a sensitivity of 24% specificity of 66%.
Interventricular mechanical delay: difference between LV and RV pre-ejection period. Beginning of QRS to beginning of LV ejection in apical 4 chamber view and beginning of QRS to beginning of RV ejection in short axis view; difference more than 40 ms is significant.
Tissue Doppler Imaging: Septal to lateral wall delay in time to peak velocity more than 60 ms is suggestive of dyssynchrony.
Non responders to cardiac resynchronization therapy: About 30 % of patients do not respond to CRT. The reasons could be any one of the following: Not every patient with wide QRS has dyssynchrony and vice versa. Leads may be too close to each other to produce synchronous contraction of septum and lateral wall.
Scarred region of left ventricle can cause poor capture and synchronization. Consistent ventricular capture by spontaneous impulses can also prevent resynchronization.
This is more likely to occur in atrial fibrillation with fast ventricular rate. Attempts at AV nodal ablation to counter this problem have been tried. In sinus rhythm, this problem can be reduced by programming a lower AV delay.
Dislodgement of LV lead can also be a cause of poor synchronization. V to V timing may not be optimal in every case.
Levophase of left coronary angiogram to see tributaries of coronary sinus: Levophase of the angiogram is obtained when you continue the cine recording till the contrast passes from the arterial tree through the capillaries to the venous system.
Levophase angiogram gives an outline of the coronary sinus and its major tributaries. But it will not be enough for an excellent visualization of the venous anatomy for left ventricular lead placement for CRT.

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