Abstract Body (Do not enter title and authors here): Introduction
Although ECG devices with non-conventional form factors, such as wearable patches, offer enhanced convenience and comfort for users, they can introduce variability in signal characteristics, posing challenges for clinicians and ECG analysis software. To ensure accuracy and support informed decision-making, ECG software must process even the most difficult signals effectively.

Aims
To compare the signal characteristics of a standard lead II configuration with those of a modified lead II (MLII) patch configuration and to evaluate the differential impact of these configurations on the performance of ECG processing software.

Methods
Five-minute ECGs were recorded on 43 adult participants (63% male, mean age: 34.5 ± 9.1 years) using Bittium Faros cardiac monitors in two configurations: i) standard Einthoven lead II with electrodes placed across the chest cavity, and ii) a MLII patch configuration with electrodes 10 cm apart on the upper left chest. Average QRS amplitudes for both configurations were compared, using a paired T-test to determine statistical significance. ECG signals were processed with HeartKey software, and the accuracy of QRS beat detection was calculated against manual annotations. Heart rate (HR) detection accuracy was assessed using Bland-Altman analysis.

Results
The average QRS amplitude was significantly lower in the MLII patch configuration compared to the standard lead II configuration (2.3 ± 1.0 mV vs. 1.2 ± 0.7 mV, p <0.001), with 55% of participants showing a two-fold greater mean QRS amplitude in the standard configuration. Despite the reduced amplitude, HeartKey software achieved high QRS beat detection accuracy, with a mean sensitivity of >99.1% and a mean positive predictive value of >99.9% across both configurations. HR detection accuracy was high across both configurations, with an absolute mean HR difference of 0.4 bpm (95% CI: -0.5 bpm to 1.3 bpm) for the standard lead II and 0.24 bpm (95% CI: -1.4 bpm to 1.9 bpm) for the MLII patch configuration.

Conclusion(s)
Despite the notable decrease in QRS amplitude with the MLII patch configuration compared to the standard Lead II configuration, HeartKey software maintained high accuracy in detecting QRS beats and analysing heart rate.