CRT Effect of LV Size and QRS Duration in LBBB

Study Questions:

What is the impact of body and heart size on cardiac resynchronization therapy (CRT) response rate in men and women with nonischemic cardiomyopathy, and how does QRS duration (QRSd) relate to this?

Methods:

Patients with nonischemic cardiomyopathy with New York Heart Association (NYHA) class III/IV and “true” left bundle branch block (LBBB) were evaluated. Left ventricular mass (LVM) and end-diastolic volume were measured by echocardiography. Positive response was defined by left ventricular ejection fraction (LVEF) improvement after implantation of CRT.

Results:

This was a retrospective analysis of 130 patients (55% women). QRSd was 165 ± 20 ms. LVEF increased from 19 ± 7% to 32 ± 14% (p < 0.001) during 2-year follow-up. The response rate was 79% (78% when QRSd <150 ms vs. 80% when QRSd ≥150 ms; p = 0.8). Body surface area, QRSd, and LVM were lesser in women, but QRSd/LVM ratio was greater (p < 0.0001). Female CRT response was greater: 90% vs. 66% in males (p < 0.001). CRT response rate increased progressively to a QRSd plateau between 150-170 ms, then decreased. Among women, peak effect was observed between 135-150 ms, thereafter declined, with male response rate lower, but with gradual increase as QRSd lengthened. Sex-specific differences were unaltered by body surface area, but resolved with integration of LVM or end-diastolic volume.

Conclusions:

Application of strict LBBB criteria and correction with body surface area did not explain differences between men and women in the QRSd-response relationship. On the other hand, QRSd normalization for heart size using LV mass or volume did.

Perspective:

Differences in CRT effectiveness between men and women have been noted, with women responding at higher rates and lower absolute QRSd. Prior studies suggested that women already derive benefit with QRSd <150 msec, while most men only at QRSd >150 msec. It has been suggested that this difference may be related to mass effect (i.e., men who have larger LV mass experience some of the widening of the QRS due to the need to repolarize a greater LV mass). The present study appears to support this hypothesis and shows that QRSd normalized for LV mass (or LV volume) resolved sex-specific differences in CRT efficacy in men and women. It also explains the small decrease in response rates when QRSd >175, which resolved with QRSd/LVM or QRSd/LV-end-diastolic-volume correction, suggesting that mass or volume effect dominates when QRSd is excessively prolonged in heart failure patients. The authors have applied a stricter definition of LBBB promulgated by Straus and others, who propose that “true” LBBB requires QRS notching or slurring in leads I, AVL, V5, or V6. That by itself was not sufficient to resolve the sex-related differences.


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