Introduction - ECG Acquisition and Automated Remote Processing - page 7

Table 1.2 Common ECG
signatures for clinical use

Clinical signature Typical values (unit) Nominal limits (unit)

P width 110 ms ±20 ms

T width 180 ms ±40 ms

PR interval 120 ms ±20 ms

QRS width 100 ms ±20 ms

QT c interval 400 ms ±40 ms

P amplitude 0.15 mV ±005 mV

T amplitude 0.3 mV ±0.2 mV

QRS amplitude 1.2 mV ±0.5 mV

QT interval
R R duration

ð QT Þ c ¼ p ð 1 : 1 Þ

Or Fridericia's formula:

QT interval
R R interval

ð QT Þ c ¼ p ð 1 : 2 Þ

3

For clinical diagnosis, the medical experts have identified some quantitative and
some qualitative signatures. Some of the common clinical signatures expressed in
quantitative manner are given in Table 1.2 against their nominal range for a
healthy adult. However, for complete feature extraction, as many as 18 clinical
signatures are reported in the literature. Some qualitative signatures used for
diagnosis are inverted P wave, pathological Q wave, ST segment elevation, etc.
Clinical interpretations of ECG signatures are available in [ 4 ].
Malfunctions in heart alter the clinical signatures in some way, and the medical
experts assess these deviations from their nominal range to classify the patient in
one or more classes. However, the actual clinical diagnosis procedure is much
more complex, since many other parameters like age, sex, hereditary symptoms,
food habits, and demographic factors are also taken into consideration, in addition
to the fact that different diseases appear in different lead(s).

1.5 ECG Lead System

ECG lead system describes internationally accepted standards for placing elec-
trodes on external body surface to pick up ECG signal. This provides a framework
for easy reproducibility, comparison, and nomenclatures for the medical and
research fraternity to describe, analyze, and interpret ECG records. The first work
on ECG lead standardization was performed by W. Einthoven, who received
Nobel Prize in Medicine for his work in the year 1924. Einthoven postulated that
human body is a perfect sphere with homogeneous density, with the human heart
located at the center. He considered three leads placed on the right arm, left arm