Introduction - ECG Acquisition and Automated Remote Processing - page 6

For QRS complex, the starting is indicated by Q-onset and termination by
S-offset. Most of the practical ECG records contain some baseline modulation,
which appears as slow undulations of the equipotential segments PR, ST, and TP.
Some researchers consider the baseline voltage in the PQ segment [ 3 ], i.e.,
between the P-offset and Q-onset points. However, for some abnormal ECGs, a
distinct PQ segment is either not available or inclined, which poses difficulty for
baseline detection. In such cases, the average amplitude of the TP segment, which
is often the longest equipotential segment in an ECG wave, is taken as the baseline
voltage. However, for accurate measurement on beat-to-beat basis, wave ampli-
tudes or heights are measured with respect to a local baseline, which is obtained by
averaging the amplitudes of onset and offset points of the corresponding wave.
Often the clinicians define a J-point at the knee region of the ST segment as the
electrical neutral voltage, which represents the electrical activity exactly between
the ventricular depolarization and repolarization. However, wide variations in ST
segments in diseased heart may create problem in finding the exact location of
J-point in some records. Figure 1.4 shows the useful ECG features and wave
durations commonly used for clinical diagnosis.
Among the common clinical signatures, distance between two consecutive
R peaks, named RR-duration, signify heart rhythms and is used for heart rate
computation. The ventricular activity (depolarization and repolarization) is
recorded in the QT segment, which is the region of interest to the cardiologists for
diagnosis of major cardiac diseases. Since the heart rate of a normal patient may
change during continuous recording, a corrected QT interval named (QT) c is
sometimes used to compensate the heart rate variation, given by Bazett's formula:

Fig. 1.4 Useful clinical signatures of ECG