P and T wave analysis in ECG signals using Bayesian methods

1.3 - Electrocardiography 17
Figure 1.7: The P, Q, R, S, T, U waves resulting from one depolarization / repolarization cycle.
Image adapted from [Ala].
ECG intervals
Intervals and segments of the ECG are important parameters for assessing the normality or
abnormality of the space between two electrical events, thus they are generally clinically relevant.
Fig. 1.8 illustrates the normal clinical features of the electrocardiogram, which include
wave amplitudes and inter-wave timings (ECG intervals). Note that the inter-beat timing (RR
interval) is not marked. The illustration uses the typical graph-paper presentation format,
which stems from the early clinical years of electrocardiography, where analysis was done by
hand measurements of hard copies. Each box is 1 mm 2 and the ECG paper is usually set to
move at 25 mm/s. Therefore, each box represents 0.04 second in time. The amplitude scale is
set to be 0.1 mV per square, although there is often a larger grid overlaid at every five squares
(0.20 second/0.5 mV ) The values for the clinical features indicated on the graph in Fig. 1.8
are typical, although they can vary based upon gender, age, activity, and health.
The clinical interpretation of the ECG segments and intervals can be summarized as following:
• PR (or PQ) interval is measured from the beginning of the P wave to the beginning
of the QRS complex. The PR interval reflects the time the electrical impulse takes to
travel from the sinoatrial node through the A-V node and entering the ventricles. The
PR interval is therefore a good estimate of A-V node function.
• PR segment connects the P wave and the QRS complex. This coincides with the
electrical conduction from the A-V node to the His bundle and the bundle branches
and then to the Purkinje Fibers. This electrical activity does not produce a contraction
directly and is merely traveling down towards the ventricles and this shows up flat on the