The college have never paid much attention to this topic. It merited a little sub-question (d) in Question 15 from the second paper of 2010. In this question, the candidates were only expected to list the benefits.
If one were to approach this issue in a "critically evaluate this therapy" kind of way, one would require a little more depth. A good review article of cardiac resynchronisation therapy is available. The benefits of CRT for heart failure are also discussed in a 2007 meta-analysis.
Rationale and Advantages of Cardiac Resynchronisation Therapy
- A nice concise explanation is offered in this 2005 NEJM article, which is an editorial in the same issue that featured the CARE-HF trial.
- In brief, if you have LBBB, the electrical activation of the lateral wall of the LV can be significantly delayed, when compared to the activation of the RV and the septum
- Dyssynchronous electrical activation results in dyssynchronous contraction, which is mechanically inefficient.
- CRT restores synchrony to ventricular contraction in patients with severe heart failure.
- The potential benefit is improved cardiac output, without much (or any) increase in myocardial oxygen consumption.
Limitations for Cardiac Resynchronisation Therapy
- Requires specialist skill to insert and adjust; hardly an emergency procedure
- To benefit, one must have LBBB, a wide QRS, and an LVEF less than 35%.
- Generally, only about 5-10% of heart failure patients will benefit
- There is a "heterogeneity of effect" in patients who do not meet the recognised criteria (read: it does them no good)
- Proximity to the left phrenic nerve may result in uncomfortable diaphragmatic stimulation during pacing
- There are too many leads; lead dislodgement occurs in 10% of patients.
Evidence in support of CRT for severe heart failure and cardiogenic shock
- The bottom line is that CRT improves NYHA grade, and it may improve mortality.
- There is strong evidence that CRT reduces mortality and hospitalisation (i.e. it is superior to AICD or medical therapy).
- CARE-HF (2005) demonstrated improved symptoms and reduced risk of death from 20% to 30% (over a 2-year period) in patients with NYHA III or IV stage of heart failure.
- Critics comment that this may be due to the built-in AICD function and the prevention of sudden cardiac death or arrhythmia-induced heart failure by these devices.