Bipolar ECG leads recorded from closely spaced electrodes are challenging in any context. When they are positioned distally with respect to the source field (far-field), the recovery of clinically useful signal content represents an even greater challenge. Due to the increased interest in wellness devices, particularly wrist worn devices, there is a renewed interest in recovering ECG signals from distally located bipolar leads.
Ten bipolar leads were simultaneously recorded at various locations along the left arm. At the same time, a conventional proximally recorded, chest Lead I was also recorded and stored. This process was repeated for 11 healthy subjects and ECGs were recorded for a period of approximately six minutes for each subject and sampled at a frequency of 2048 Hz. Subsequently QRS detection was performed on the Lead I using an automated algorithm. Wavelet based filtering using Daubechies 4 wavelet decomposition and soft thresholding was applied to each of the 10 additional leads for each subject. The same QRS detection approach was applied to each of the filtered leads. QRS detection performance was assessed against that determined from Lead I for each subject.
Of all leads investigated it was found that a lead positioned transversally (using BIS gelled electrodes) on the upper arm proved the best accuracy against the benchmark QRS detection (SEN = 99.8%, PPV = 98.4%). The most distally positioned bipolar lead which was positioned using dry electrodes faired least favourable (SEN = 27.2%, PPV = 20.2%).
Our results show that a reasonable level of QRS detection accuracy can be achieved from some electrode positions in the far field. Whilst the quality of the ECG signals may not be adequate for comprehensive clinical interpretation this level of accurate QRS detection may still be useful in applications such as heart rate variability analysis.