  Background: Accurate detection of QRS complexes during mobile,
ultra-long-term ECG monitoring is challenged by instances of high heart rate,
dramatic and persistent changes in signal amplitude, and intermittent
deformations in signal quality that arise due to subject motion, background
noise, and misplacement of the ECG electrodes. Purpose: We propose a revised
QRS detection algorithm which addresses the above-mentioned challenges. Methods
and Results: Our proposed algorithm is based on a state-of-the-art algorithm
after applying two key modifications. The first modification is implementing
local estimates for the amplitude of the signal. The second modification is a
mechanism by which the algorithm becomes adaptive to changes in heart rate. We
validated our proposed algorithm against the state-of-the-art algorithm using
short-term ECG recordings from eleven annotated databases available at
Physionet, as well as four ultra-long-term (14-day) ECG recordings which were
visually annotated at a central ECG core laboratory. On the database of
ultra-long-term ECG recordings, our proposed algorithm showed a sensitivity of
99.90% and a positive predictive value of 99.73%. Meanwhile, the
state-of-the-art QRS detection algorithm achieved a sensitivity of 99.30% and a
positive predictive value of 99.68% on the same database. The numerical
efficiency of our new algorithm was evident, as a 14-day recording sampled at
200 Hz was analyzed in approximately 157 seconds. Conclusions: We developed a
new QRS detection algorithm. The efficiency and accuracy of our algorithm makes
it a good fit for mobile health applications, ultra-long-term and pathological
ECG recordings, and the batch processing of large ECG databases.
