In recent years, a new nonpharmacologic therapy, ventricular resynchronization therapy (VRT), has been developed to treat advanced drug-resistant heart failure with electrocardiographic (ECG) evidence of delayed interintra-ventricular conduction. The ECG is fundamental in two regards: initially, in that the indication for left ventricular or biventricular pacing requires a long QRS ± left bundle branch block (LBBB), and in follow-up, in that the post-VRT trace may show changes from baseline termed electrocardiographic remodeling. We attempt to explain the significance of these changes below.
Early electrocardiographic remodeling
In the acute phase, in the first week after biventricular pacemaker implantation, the differences between the baseline and pacing ECGs are unclear. Alonso et al reported that the only difference after a period of VRT between responders and nonresponders in 26 patients with biventricularly paced heart failure matched for age, sex, and New York Heart Association (NYHA) class was in ECG duration and axis. Responders had a significantly shorter QRS duration, correlating with improved functional and clinical status. An earlier study, by Leclercq et al, found no correlation between biventricular pacing and ECG remodeling in the acute phase.
Data from the 190 patients in the InSync Italian Registry implanted with a biventricular pacemaker showed QRS shortening after implantation. ECG remodeling tended to be greater in patients with a longer baseline PR interval (interventricular conduction time >80 ms). Three patients in a 20-patient subgroup also showed a change in QRS axis.
How are these ECG changes to be explained? Alonso et al ascribed them to the pacing site. The greater the decrease in QRS duration, the better positioned the left ventricular stimulating lead in the coronary sinus. To shorten the QRS duration, pacing must be applied as close as possible to the ventricular site of greatest conduction delay. The best stimulation site is reported to be the lateral free wall of the left ventricle. Optimal positioning increases long-term benefit. However, optimized atrioventricular delay is also very important for optimal pacing, and this tends to increase QRS duration. Review of the overall literature suggests that the QRS duration tends to decrease in the acute phase of biventricular pacing. However, this conclusion is very approximate and of little use to the cardiologist in overall clinical and instrumental patient assessment.
Electrocardiographic remodeling in chronic biventricular resynchronization therapy
Significant shortening of the paced QRS tends to be associated with improved functional and clinical status during chronic pacing. According to the InSync Italian Registry, early QRS shortening tends to be maintained during follow-up and correlates significantly with improvement in NYHA class, left ventricular systolic function, and exercise tolerance (6-minute walk test, quality of life score sheet, etc). The fact that QRS duration does not undergo further shortening in the chronic phase suggests that mechanical effects take over (mechanical escape mechanism) once electrical resynchronization is achieved.
In this regard, however, an important point must be made. The benefits of effective chronic pacing depend on optimizing atrioventricular delay. An optimal atrioventricular interval often implies prolonging the QRS with respect to baseline. This may be why clinical and hemodynamic benefit has been observed during chronic pacing despite prolongation of the QRS complex. By extension, some studies have reported only moderate correlation between QRS shortening and peak VO,.
Intrinsic QRS after chronic pacing
It is unsure whether the intrinsic QRS remodels after long periods of biventricular pacing. Comparing the intrinsic ECG with baseline after 12 months of biventricular pacing for heart failure, Vogt et al found no regression of intraventricular conduction or first degree atrioventricular block. This finding in 42 patients requires further investigation, as it may have important implications and utility in the long-term follow-up of patients paced for heart failure.
Long-term pacing for heart failure causes remodeling of the surface ECG. However, it is unclear how to interpret such changes. The surface ECG may be an insufficiently sophisticated instrument, since it only examines the electrical changes brought by pacing. In heart failure patients treated with VRT, the hemodynamic benefits observed are mostly due to mechanical improvement. Echocardiography may prove more appropriate for guiding, predicting, and monitoring the mechanical effects of VRT in heart failure patients.
Alonso C, leclercq C, Victor F, et al. Electrocardiographic predictive factors of long-term clinical improvement with multisite biventricular pacing in advanced heart failure. Am J Cardiol. 1999;84:14] 7-1421.
Hochleitner M, Hortnagl H, Ng CK, Hortnagl H, Gschnitzer F, Zechmann W. Usefulness of physiologic dual-chamber pacing in drug-resist-ant idiopathic dilated cardiomyopathy. Am J Cardiol. 1990,66:198-202. Leclercq C, Cazeau 5, Le Breton H, et al. Acute hemodynamic effects of biventricular DDD pacing in patients witfi end-stage heart failure. J Am Coll Cardiol. 1998;32:1825-1831.
Rtcci R, Ansalone G, Toscano S, et al. Cardiac resynchronization: materials, technique and results. The InSync Italian Registry. Eur Heart J Suppl. 2000;2(suppl J):J6-J15.
Vogt J, Krahnefeld O, Lamp B, et al. Electrocardiographic remodeling in patients paced for heart failure. Am J Cardiol. 2000;86(suppl 1 J:K152-K156. Zardini M, Tritto M, Bargiggia G, e» al. The InSync Italian Registry: analysis of clinical outcome and considerations on the selection of candidates to left ventricular resynchronization. Eur Heort J Suppl. 2000;2(suppl J):J 16-J22.
management; pacing; ECG remodeling; biventricular resynchronization therapy; multisite biventricular pacing