CRT is also known as biventricular pacing. Three leads are used for biventricular pacing: Right atrial, right ventricular and left ventricular in posterolateral region. The LV lead does not have a tine or screw. The curve of the lead gives the anchorage in the coronary vein. Atrial lead is not used in those with atrial fibrillation.
Selection of patients
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. QRS duration > 120 ms is an important selection criteria for CRT implantation.
M- Mode: Septal posterior wall motion delay at papillary muscle level in parasternal short axis view > 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; Beginning of QRS to beginning of RV ejection in short axis view; difference > 40 ms is significant
Tissue Doppler Imaging: Septal to lateral wall delay in time to peak velocity > 60 sec is suggestive of dyssynchrony.
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 occur since it has neither active fixation nor passive fixation mechanisms. V to V timing may not be optimal every case.