Effect of DC Offset on the T-Wave Residuum Parameter
The T-wave residuum (TWR) is a relatively new 12-lead ECG parameter that may reflect cardiac repolarization heterogeneity. TWR shows clinical promise and may become an important diagnostic tool if accurate, consistent, and convenient methods for its calculation can be developed. However, there are discrepancies between the methods that various investigators have used to calculate TWR, as well as some questions about basic methodology and assumptions that require resolution. The presence of a DC offset or very low frequency AC component to the ECG is often observed. Many researchers have attempted to compensate for these by high pass filters and by median beat techniques. These techniques may help minimize the contribution of a low frequency AC component to the TWR, but they will not eliminate a DC offset inherent within the instrumentation. The present study examined the presence of DC offsets in the ECG record, and their effect on TWR. Specifically, in healthy individuals, a DC offset was added to all 8 channels collectively or to each channel selectively. Even with offsets that were relatively small compared to T-wave amplitude, the addition of either collectively or individually applied offsets was observed to produce very significant changes in the TWR, affecting its value by as much as an order of magnitude. These DC offsets may arise from at least two possible sources: a transient artifact from EMG or electrode movement resulting in a transient baseline offset in one or more channels. Since highpass filters have a settling time of several seconds, these artifacts will contribute to a transitory baseline offset lasting 1020 cycles. The machine hardware may also introduce an offset. Regardless of the cause or source of a DC offset, this study demonstrates that offsets have a very significant impact on TWR, and that future studies must not ignore their presence, but rather more appropriately compensate for them.