191
Electrocardiogram
9.5.1 TIME-DOMAIN ANALYSIS
As discussed earlier, the most significant time-domain feature in the ECG is the
duration of the heart cycle. The heart cycle duration is frequently derived by measuring the time span from one R wave to the next. Other features are essentially the
duration of each wave (i.e., the duration of QRS complex) and the time separation
among the waves (e.g., the time interval between T and P waves, i.e., TP interval).
Two specific time features are further described in the following.
An important time domain feature is the duration of QRS complex as described
earlier. Generally, the QRS complex is identified by the characteristic shape and relative stable time constant in the pattern. Another feature of interest is the time interval
between the T wave and the subsequent P wave. The importance of this measure
shows the separation between two important events, i.e., the pulse rate of the sinus
node, which is expressed in the P wave, and the repolarization phenomenon that is
the origin of the T wave.
9.5.2 FREQUENCY-DOMAIN ANALYSIS
In any ECG waveform, the QRS complex is well localized as the high-frequency
region. The P and T waves are mainly the low-frequency components. The ST segment in the ECG is time restricted with mostly low-frequency content.
The normal ECG and the deviating ECG often have significantly different frequency contents. Since the normal heart rate is in the range of 60–100 beats per
minute, the fibrillation can exceed 200 beats per minute. In addition to the frequency
differences, the depolarization and repolarization ramps also change under diseased
conditions, requiring a much wider frequency bandwidth to describe each different
phenomenon.
The standard ECG can be described by the first eight harmonics of the heart
rate in the Fourier domain. This provides a basic representation as shown in Figure
9.18. Figure 9.18a shows the nine-electrode recording for a normal ECG as shown
in Figure 9.5, and Figure 9.14b illustrates the superposition of the first eight harmonics reconstructing the ECG from Figure 9.3. Any minor high-frequency deviation from the normal ECG often creates variations in the ECG that requires a much
larger number of harmonics to describe the frequency-domain features of the ECG.
As a rule of thumb, a frequency analysis should span no less than the frequency
range of 0–100 Hz for an apparently normal ECG. Arrhythmias may require a
frequency analysis up to 200 Hz. However, entering even higher-frequency spectra, the spectrum will be dominated by noise and will not contribute additional
information.
An important disease that is often detected in the frequency domain is sinus
tachycardia. A sinus rhythm of higher than 100 beats per minute is called sinus
tachycardia. Similar conditions often occur as a physiological response to physical
exercise or physical stress, but, in diseased cases, the condition results from congestive heart failure. More specifically, if the sinus rhythm is irregular such that the
longest PP or RR intervals exceed the shortest interval by 0.16 s, the situation is
diagnosed as sinus arrhythmia. This condition is very common in all age groups but
Electrocardiogram
9.5.1 TIME-DOMAIN ANALYSIS
As discussed earlier, the most significant time-domain feature in the ECG is the
duration of the heart cycle. The heart cycle duration is frequently derived by measuring the time span from one R wave to the next. Other features are essentially the
duration of each wave (i.e., the duration of QRS complex) and the time separation
among the waves (e.g., the time interval between T and P waves, i.e., TP interval).
Two specific time features are further described in the following.
An important time domain feature is the duration of QRS complex as described
earlier. Generally, the QRS complex is identified by the characteristic shape and relative stable time constant in the pattern. Another feature of interest is the time interval
between the T wave and the subsequent P wave. The importance of this measure
shows the separation between two important events, i.e., the pulse rate of the sinus
node, which is expressed in the P wave, and the repolarization phenomenon that is
the origin of the T wave.
9.5.2 FREQUENCY-DOMAIN ANALYSIS
In any ECG waveform, the QRS complex is well localized as the high-frequency
region. The P and T waves are mainly the low-frequency components. The ST segment in the ECG is time restricted with mostly low-frequency content.
The normal ECG and the deviating ECG often have significantly different frequency contents. Since the normal heart rate is in the range of 60–100 beats per
minute, the fibrillation can exceed 200 beats per minute. In addition to the frequency
differences, the depolarization and repolarization ramps also change under diseased
conditions, requiring a much wider frequency bandwidth to describe each different
phenomenon.
The standard ECG can be described by the first eight harmonics of the heart
rate in the Fourier domain. This provides a basic representation as shown in Figure
9.18. Figure 9.18a shows the nine-electrode recording for a normal ECG as shown
in Figure 9.5, and Figure 9.14b illustrates the superposition of the first eight harmonics reconstructing the ECG from Figure 9.3. Any minor high-frequency deviation from the normal ECG often creates variations in the ECG that requires a much
larger number of harmonics to describe the frequency-domain features of the ECG.
As a rule of thumb, a frequency analysis should span no less than the frequency
range of 0–100 Hz for an apparently normal ECG. Arrhythmias may require a
frequency analysis up to 200 Hz. However, entering even higher-frequency spectra, the spectrum will be dominated by noise and will not contribute additional
information.
An important disease that is often detected in the frequency domain is sinus
tachycardia. A sinus rhythm of higher than 100 beats per minute is called sinus
tachycardia. Similar conditions often occur as a physiological response to physical
exercise or physical stress, but, in diseased cases, the condition results from congestive heart failure. More specifically, if the sinus rhythm is irregular such that the
longest PP or RR intervals exceed the shortest interval by 0.16 s, the situation is
diagnosed as sinus arrhythmia. This condition is very common in all age groups but