Background: Cardiac resynchronisation therapy (CRT) is an important advance in the treatment of chronic heart failure. The aim of CRT is biventricular capture in all beats. However, inadequate delivery of biventricular pacing is still seen in about 30% of patients with an implanted CRT device. Device interrogation is a routine approach to assess CRT delivery. However, some reports indicate that analysis of 24-h electrocardiogram (ECG) may provide additional and important information regarding CRT function.

Aim: Assessment of the adequacy of CRT delivery based on device interrogation and analysis of QRS morphology during 24-h ECG recording in patients with preserved sinus rhythm (SR).

Methods: We analysed 24-h Holter ECG recordings and data from device interrogation devices in 43 patients with preserved SR (age 56 ± 23 years, 9 women and 34 men). The obtained results were compared in an independent manner. Assessment of adequacy of CRT delivery by 24-h ECG was based on the occurrence of QRS variability, defined as a change in R wave amplitude in lead V1 by > 3 mm and/or change in QRS duration by > 40 ms and/or change in the R/S ratio. Adequate CRT delivery, i.e. complete resynchronisation, was defined as more than 95% of pacing without the defined QRS variability.

Results: Both methods allowed independent assessment of CRT delivery (p < 0.05 by the Fisher’s exact test). In multivariate analysis, factors that were independently associated with incomplete resynchronisation included ventricular arrhythmias (each 100 ventricular beats per day increased the risk of incomplete resynchronisation 1.14-fold; confidence interval [CI] 1.036–1.25, p = 0.007), maximum heart rate (HR) (each increase by 10 bpm increased the risk 3.3-fold; CI 1.36–7.9, p = 0.008), QRS duration at the minimum HR (each increase by 10 ms increased the risk 1.74-fold; CI 1.075–2.8, p = 0.024), and the programmed atrioventricular delay (each increase by 10 ms increased the risk 2.15-fold, CI 1.18–3.9, p = 0.013).

Conclusions: In patients with preserved SR, device interrogation and evaluation of 24-h ECG are complementary methods to evaluate adequate CRT delivery. Therefore, both methods should be taken into account when assessing CRT function.