Background: Many ECG devices apply an analog or an equivalent digital first order high-pass filter as part of the ECG acquisition chain. This type of filter is known to not only reduce baseline wandering but also change the ECG signal itself. Particularly, the ST-segment of ECGs with unipolar QRS complexes can be changed considerably. To a certain degree, it is possible to restore the original ECG and therefore the correct ST-segment by inverse filtering. However, this process requires the availability of a digital representation of the filtered ECG signal which is not always the case. We present an alternative approach that can estimate the true ST-values based on only three standard ECG parameters and the high-pass filter's time constant.

Methods: Based on the high-pass filter's time constant T [s], the QRS integral A [Vs], the QRS width W [s] and the RR-interval RR [s], we derived the following formula which estimates the high-pass filter induced change in the ST-amplitude right after the QRS complex:
Δ̂AWRRT=A=e−W2TT1−e−RRTe−RR−WT−1

A given ST measurement that is based on a high-pass filtered input signal can therefore be corrected by simply subtracting the estimated difference Δ̂. This formula was derived based on a simplified and mathematically tractable ECG model. Its suitability for the use on real ECGs was evaluated by theoretical considerations and with an experimental setup including 1339 different clinical resting ECGs where the effects of a 0.05 Hz high-pass filter on the ST segment were measured and compared to the values estimated by the derived formula.

Results: The ECGs used in the experimental setup showed a mean RR interval of 842 ms and a mean QRS width of 95 ms. Furthermore, the effects of the 0.05 Hz high-pass filter showed a linear dependency on the QRS area A. On average, the observed difference in the ST measurement was –0.227 * A which is very close to the difference predicted by the derived formula:
Δ̂AW=95msRR=842msT=1/2π0.05Hz=−0.279*A.

Conclusions: The derived formula can be used to correct ST measurements by the use of just a few parameters that are easily obtained by means of automatic measurement. This feature therefore opens up the possibility of reevaluating studies that are based on AC coupled ECGs. Another potential use of this formula is in the diagnoses of patient groups that are known to have large QRS integrals such as patients with a left bundle branch block (LBBB).