CRT-P and CRT-D

CRT stands for cardiac resynchronization therapy. CRT is used in heart failure with reduced ejection fraction. According to the Universal Definition and Classification of Heart Failure, heart failure with reduced ejection fraction has left ventricular ejection fraction of 40% or less. CRT is a type of pacemaker in which three chambers of the heart are paced, right atrium, right ventricle and left ventricle.
This picture illustrates what reduced ejection fraction means. Ejection fraction is the fraction of end diastolic volume which is pumped out from the left ventricle during systole. Normal ejection fraction is around 60 to 70%.
In some persons with heart failure and reduced ejection fraction, contraction of different parts of the left ventricle loses its natural synchrony, so that part of the contractile effort is wasted. Usually ECG will show a wide QRS complex with a left bundle branch block pattern in those with left ventricular dyssynchrony.
In left bundle branch block, there is delayed activation of the left ventricular lateral wall