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 compared to the interventricular septum leading to dyssynchrony of contraction.
The timing of the stimuli given by the pacemaker is adjusted to produce synchrony of contractions of the left ventricle. The right atrium is paced first and the ventricles after a short AV delay. Pacing of the left and right ventricles are timed in such a way that the left ventricle contracts in a synchronized fashion to improve the pumping function.
As the name suggests, CRT is implanted in those heart failure patients in whom the synchrony of the left ventricular contraction has been lost. Left ventricular pacing is done through a lead in the posterolateral tributary of the coronary sinus.
CRT alone is now re-designated as CRT-P or CRT with pacing alone. This is because there is another device known as CRT-D in which a CRT is combined with an ICD. ICD is short form for implantable cardioverter defibrillator, used to automatically shock back the heart into action in case of life threatening ventricular arrhythmias.
This X-ray shows an ICD with high voltage shocking coils in superior vena cava and right ventricle. In CRT-D, shocking leads which are thicker high voltage leads are included along with the pacing leads of CRT-P.
Battery capacity has to be more for CRT-D because a large amount of electrical energy is needed for the shocks. Naturally, the longevity of CRT-D battery is lesser than that of CRT-P. Moreover, the internal shocks given by CRT-D can be painful, though essential to save life in case of fast ventricular tachycardia or fibrillation.
CRT-D is needed in some heart failure patients who have a high risk of life threatening ventricular arrhythmias. CRT-D senses the arrhythmia using its leads and can deliver either overdrive pacing or a direct current shock for terminating the arrhythmia. The direct current shock is delivered between the electrically active can of the device and the intracardiac high voltage shocking coil.