Since the mid-1990s, many scholars in Europe have found that in some patients with heart failure combined with left ventricular conduction delay resulting in myocardial dyssynchrony, simultaneous electrical stimulation of the left and right ventricles will improve cardiac function and clinical symptoms. Within a short period of time, a new therapeutic approach called cardiac resynchronization therapy (CRT) was developed and validated in clinical studies as a treatment option for heart failure. During this development process, CRT achieved three firsts: the first non-pharmacological treatment for heart failure to be studied in a large clinical study with a placebo group; the first treatment to target a certain subset of heart failure patients rather than all heart failure patients; and the first treatment to be studied in humans first, with data from animal studies coming later. In general, long-term cardiac resynchronization therapy reduces left ventricular end-systolic and end-diastolic volumes by approximately ten percent after three to six months. Uniquely, if resynchronization therapy is stopped abruptly, the change in ventricular volume reduction does not immediately disappear and return to the pre-treatment state, but rather “holds up” for a period of time. This means that resynchronization therapy changes ventricular volume through “remodeling” rather than through structural changes in the ventricles brought about by independent changes in systolic and diastolic function with each cardiac cycle. Early data from several studies have shown that cardiac resynchronization therapy improves symptoms and reduces re-hospitalization rates. the COMPANION study, published in mid-2004, is one of the largest studies examining mortality rates and comparing whether resynchronization therapy (CRT) and resynchronization combined with implantable cardioverter defibrillator therapy (CRT + ICD) were superior to standard heart failure medications. The results were a twenty-four percent reduction in mortality for patients in the CRT group and a thirty-six percent reduction for patients in the CRT+ICD group (p=0.004). Although CRT contributed two-thirds of the effect, the mortality benefit in the CRT group failed to meet statistical criteria (p=0.06).The CARE-HF clinical study, published in 2005, enrolled more than 800 New York heart failure patients in Cardiac Function Class III or IV to compare the effects of CRT, but there was no ICD treatment group in this study. It was found that CRT reduced mortality by at least thirty percent (p<0.002), but the effect had to be seen after twelve months of treatment and was more pronounced with increasing time. In recent years, the REVERSE clinical study and the MADIT-CRT clinical study have been published in an attempt to investigate the use of cardiac resynchronization therapy in patients with mild heart failure in the New York Cardiac Function Class I-II category. The results showed that cardiac resynchronization therapy in these patients with mild heart failure can still reverse ventricular remodeling, reduce the chance of a first hospitalization for heart failure, and even improve heart failure survival. All clinical studies have used QRS width on the electrocardiogram as the basis for screening patients with asystolic heart failure. However, QRS width itself is not an indicator of mechanical dyssynchrony, and there is increasing evidence that mechanical dyssynchrony is a better predictor of the long-term outcome of CRT therapy. Therefore, it is possible that some patients may have wide QRS widths but no substantial cardiac dyssynchrony, and naturally CRT therapy will not be effective; it is also possible that a patient may have a completely normal QRS width on the electrocardiogram but have systolic dyssynchrony similar to that seen in other patients with a prolonged QRS. In fact, approximately 30% of patients with heart failure combined with normal QRS width have clinically significant mechanical asystole. Recently, there have been a number of studies looking for measures of mechanical dyssynchrony using instruments such as cardiac ultrasound or magnetic resonance imaging (MRI) scans. There are a number of metrics that have been developed using cardiac ultrasound in the hope of preoperatively screening heart failure patients for the effectiveness of cardiac resynchronization therapy. Cardiac resynchronization has been validated in many large clinical studies as an effective and emerging treatment modality for patients with heart failure. Although nearly 30% of the patients treated were ineffective, it is believed that the success rate and efficacy of resynchronization therapy will be improved in the near future when more appropriate mechanical dyssynchrony measures are developed and used.