Congestive heart failure (CHF) is one of the largest and most costly health care problems in the US. Many new and exciting medical therapies have been highly successful in improving symptoms and prolonging survival in patients with this disease. Nonetheless, these therapies require a high degree of compliance, are often poorly tolerated, can be extremely expensive and ultimately fail. Nonmedical (surgical) therapies, such as left ventricular assist devices, cardiomyoplasty and transplant, are reserved as therapies of last resort due to their expensive and invasive nature. Additional therapeutic options would be helpful in treating these patients. Several reports have suggested that various forms of cardiac pacing may offer some important options in the treatment of CHF patients, especially those with markedly abnormal AV conduction and intraventricular conduction delay. This issue of the Arrhythmia News will review the potential role of cardiac pacing in the treament of these patients. 

Shortened AV Delay 

One of the simplest pacing therapies used for CHF has been conventional dual-chamber pacing with AV intervals programmed to 100 ms, significantly shorter than used in standard pacing. This therapy utilized commercially-available pacing systems and positioned leads in standard atrial and ventricular locations. Several reports in the literature offered anecdotal evidence suggesting striking lessening of symptoms and up to 10% improvement in mean left ventricular ejection fraction. The initial enthusiasm accompanying these early reports was, however, somewhat tempered by the results of subsequent controlled and prospective studies. Echocardiographic studies from the Mayo Clinic demostrated significant hemodynamic improvements (increased cardiac output, increased duration of diastolic filling, and abolition of mitral regurgitation) in patients with a pre-pacing PR interval >200 ms. The study also noted no such improvement in patients with normal PR intervals. In a prospective, randomized, double-blind, crossover trial of short AV delay pacing in 12 subjects followed 12 weeks, no shortterm hemodynamic improvements were noted, no longterm improvement in symptoms were noted, and no change in ejection fraction occurred. The authors of that study harshly suggested that factors such as "placebo" effect, unblinded investigator bias in classification and beneficial effect of pacing for unsuspected bradycardia or heart block may have resulted in previous optimistic reports. 

Biventricular Pacing 

While this criticism calmed some of the initial enthusiasm for pacing in CHF, the dramatic effect noted in some patients could not be ignored. As a result, subsequent work has focused on patients with significant conduction abnormalities, in particular those patients with profound QRS widening on the surface electrocardiogram. Of note, reports from the heart failure and electrophysiology literature suggest that these patients represent a high-risk subgroup within the CHF population with significantly increased mortality. Abnormalities in intraventricular conduction result in abnormal activation of ventricular myocardium and perhaps more importantly asynchronous activation of the atrial and ventricular chambers. Conventional pacing of the ventricle, delivered in the right ventricular apex, also results in its own intraventricular conduction abnormality. As such, several investigators have suggested that a pacing therapy which activated the right and left venticle near simultaneously would be hemodynamically better, especially in patients with CHF. While activation of the right ventricle can be obtained through conventional leads, left ventricular activation must be obtained through specialized pacing leads positioned in the coronary sinus.

Several small trials have confirmed the hemodynamic benefits of "ventricular resynchronization". Larger, prospective studies investigating this hypothesis and its clinical implications are currently underway. The InSync Trial, sponsored by Medtronic and performed in Europe and Canada, is one such trial; recently presented results from this trial have demonstrated significant improvements in NYHA functional class, functional status as assessed by 6-minute walk test and quality-of-life. Preliminary data from this trial also suggest a trend toward improved left ventricular ejection fraction. The US expansion of this trial, called MIRACLE (MultiCenter InSync Randomized Clinical Evaluation), is currently underway. The Arrhythmia Service at St. Luke's-Roosevelt Hospital Center, in conjunction with the newly established Heart Failure Service, have begun full participation in the trial. 

Potential Mechanisms for Biventricular Pacing to Improve Clinical Status in Patients with Chronic Congestive Heart Failure 

• Improved ventricular contraction sequence (within the ventricle and between the right and left ventricles)
• Increased effective ventricular mass of contraction
• Improved diastolic ventricular filling
• Reduced mitral regurgitation
• Antagonized neurohormonal activation

Reproduced with permission. Published by the Arrhythmia Service of St. Luke's-Roosevelt Hospital Center, New York, New York.