Relationship between QRS duration and resynchronization window for CRT optimization: Implications for CRT in narrow QRS patients.

AIMS

Cardiac resynchronization therapy (CRT) response is proportional to QRS duration (QRS). We hypothesize that this is, in part, due to slower conduction velocity and hence wider range of programmed device settings that produce adequate electrical wavefront fusion and resynchronization in wider QRS patients.

METHODS

CRT patients (n = 122) with left ventricular (LV) conduction delay, sinus rhythm and intact atrioventricular node conduction were studied. Patients were categorized by QRS: narrow (<120 ms; n = 20); moderate (120-150 ms, n = 37); and prolonged (≥150 ms; n = 65). Electrocardiographic data was acquired during native rhythm and LV-only pacing at varying atrioventricular delays (AVDs). Electrical synchrony was quantified as cardiac resynchronization index (CRI) using multi‑lead electrocardiographic systems and a proprietary algorithm that quantified wavefront fusion. A Gaussian distribution equation was fitted to CRI response.

RESULTS

Peak CRI was high (87.6 ± 6.3%) and similar (p = 0.716) across QRS groups. The standard deviation of the Gaussian distribution significantly correlated with QRS (R = 0.614, p < 0.001), and progressively and significantly (p < 0.001) increased as QRS increased from narrow (34.8 ± 10.0 ms), to moderate (50.6 ± 8.4 ms), to prolonged (67.6 ± 18.3 ms). At AVDs 20 and 40 ms from optimal, CRI differed significantly (p < 0.001) between groups, with progressively higher CRI values as native QRS increased.

CONCLUSION

Electrical resynchronization with optimally programmed LV-only pacing was similar between patients with varying QRS, including patients with narrow QRS. The resynchronization window that corresponded with optimal electrical resynchronization decreased as native QRS decreased. This finding provides one potential explanation for the lack of significant benefit of CRT in narrow QRS patients in previous studies.