The heart's rhythmic beating or contraction is determined by the flow of an excitatory electrical wave-front along a specialized cardiac conduction system. In the presence of altered conduction, such as a bundle branch block or an intra-ventricular conduction delay, abnormal cardiac contraction or dyssynchrony occurs. The delay can occur in the specialized conduction system (electrical dyssynchrony) or myocardium (mechanical or structural dyssynchrony).1 Techniques to improve both electrical and mechanical synchrony in patients with bundle branch block were initially done on adults. Research in this field began to appear in the last decade when dual chamber pacing was first used as adjunctive therapy for adults with medically refractory heart failure.2 Acute studies showed that atrioventricular (AV) synchronous pacing with a short AV delay improved cardiac output and exercise duration in patients with heart failure and a prolonged PR interval.3 The beneficial effects of AV resynchrony (optimizing AV conduction times with pacing) were shown to be due to increased diastolic filling time, and reduction in mitral or tricuspid valve regurgitation. The results of long-term studies, however, did not demonstrate consistent improvement in ejection fraction or NYHA functional class with DDD pacing.4 Since then, Cardiac Resynchronization Therapy (CRT) has established itself as a proven therapy for congestive heart failure in adults, with patients showing improvement in exercise tolerance, quality of life, and survival.1, 5, 6 More recently, the technique of utilizing CRT to stimulate the heart from novel or multiple sites has been applied to pediatric patients.7-9 Children with chronic heart failure have received CRT successfully as an adjunctive therapy.
One of the major limitations of CRT is the objective assessment of whether cardiac output and ventricular function are improved. As well, the precise location of where to pace the heart in order to optimize hemodynamic function needs to be determined. The objective assessment of successful CRT is a difficult clinical issue and should ideally be performed non-invasively. Traditional two-dimensional echocardiographic and Doppler indices have been used to assess the efficacy of CRT and include measuring cardiac output, looking at ventricular ejection times, visually assessing wall motion, and measuring the length of diastole using the mitral "E" and "A" waves. As most of the existing techniques are limited to assessing global function, more detailed methods of assessment are necessary in order to fully assess and optimize CRT.10, 11 Tissue Doppler Imaging (TDI) offers a more detailed analysis of regional cardiac function and allows quantitative measures to be obtained. TDI operates at high frame rates and can non-invasively map cardiac activation and add information related to the degree and location of cardiac dyssychrony. TDI and its derivatives allow: (1) measurement of myocardial velocities, which is based on the detection of the Doppler shift caused by the motion of myocardial tissue during the cardiac cycle; (2) visualization of tissue tracking, which color-codes tissue segments with similar displacements according to a color map; (3) measurement of regional strain rates, which describes the rate of deformation, or how quickly a segment of tissue shortens or lengthens; and (4) measurement of regional strain, which describes the deformation of an object (in this case, tissue) relative to its original state. 12, 13
Cardiac pacing in children is done most often following cardiac surgery for congenital heart disease (CHD). This pacing is usually temporary. Following open heart surgery children frequently exhibit cardiac dyssynchrony secondary to conduction abnormalities or regional wall motion abnormalities. Often, damage to the conduction system is an unavoidable result of the operation itself. Regardless of the extent of the conduction abnormality, most patients operated on for congenital heart disease undergo a period of decreased cardiac function related to several factors, including: pre-existing myocardial disease; cardiopulmonary bypass; and residual cardiac lesions.7, 14 The decrease in cardiac performance and, therefore, the risk to the patient's life, can be aggravated by the presence of cardiac dyssynchrony. The benefits of CRT are just beginning to receive attention in the setting of pediatric post-operative cardiac care. 15
We hope to demonstrate that CRT is beneficial in the care of post-operative patients undergoing open-heart surgery for repair of congenital heart defects. We will be using state-of-the-art TDI for assessing cardiac dyssynchrony, and using it as a tool for monitoring therapy. This study has tremendous potential for application to all patients undergoing open-heart surgery for repair of congenital heart defects. If it can be demonstrated that CRT can improve post-operative outcomes in this population, significant morbidity and mortality can be avoided, Intensive Care Unit (ICU) and hospital stays shortened, and the associated health care costs reduced.
