- CRT -Cardiac resynchronisation is done by putting multiple wires and electrically organizing the contraction sequence and improving mechanical function.
- ICD-Implantable cardiovertor defibrillator shocks whenever VT or VF occur and suden death is prevented.
- CRT-D (Combo device which functions as both )
CRT is done for advanced heart failure to improve exercise capacity and hence the quality of life .It does not do any thing significant in prolonging life .ICD is again implanted in advanced LV dysfunction with either documented VT/VF or patients who are at propensity for VT .It has dramatic benefit in preventing sudden cardiac death.
Both CRT and ICD has some overlapping indication in cardiac failure. Attention young cardiologists, please realise among these two the value of ICD is many many fold higher than CRT.This fact is rarely discussed and disseminated.
True benefits of CRT is realised only when it is combined with ICD.
Summary
- Ideally all advanced cardiac failure patients should receive both ICD and CRT (CRT-D)
- ICD as stand alone therapy has a distinct role in patients with severe LV dysfunction (LV EF<30%) without wide QRS in ECG
- There is no role for CRT as a standalone procedure in cardiac failure .it should always be combined with ICD (ie CRT-D) *
*Except in patient with degenerative complete heart block , both ventricles are paced the term Bi-Vi pacing is used instead of CRT.Since LV function is normal here , there is no de-synchrony in the first place .The synchronised BIVI pacing is meant to prevent future heart failure
Final message
Always use a combo device in advanced symptomatic heart failure which is refractory to medical therapy.
After all , there need to be a life in the first place so that we can improve it . ICD ensures life while CRT tries to improve it.
Dr.S.Venkatesan MD 
As an Electrophysiologist, this is over simplification of the facts! I do not agree with this.