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Cardiac Surgery
Technical Note
Interpreting an ECG Tracing
An electrocardiogram (ECG) graph of potential cardiac
difference (voltage) against time measures the waves of
electrical excitation passing through the heart. It consists of:
P Wave: Atria Interpretation
Electrical impulse conduction from the SA Node through-out the
atria is seen on the ECG as the P wave (Fig. 1).
1) The shape and duration of P waves may indicate atrial
enlargement.
2) Absence of a P wave may indicate atrial fibrillation.
3) A saw toothed P wave may indicate atrial flutter.
PR Segment: AV Node Delay
The AV node delays electrical conduction so that the atria and
ventricles don’t contract at the same time, and blood flows
effectively from the atria to the ventricles. The delay in the
AV node forms much of the PR segment on the ECG. Part of
atrial repolarization can also be represented by PR segment.
It is usually 120 to 200 ms long. A PR interval of over 200 ms
may indicate a first degree heart block. A short PR interval may
indicate a pre-excitation syndrome via an accessory pathway
Fig. 1: A typical ECG tracing of a normal heartbeat (shown
that leads to early activation of the ventricles. Variable PR
above) consists of a P wave, a QRS complex and a T
intervals may indicate other types of heart block. PR segment
wave.
depression may indicate atrial injury or pericarditis.
QRS Complex
As the branches of the Bundle of His taper out to produce numerous Purkinje fibers, individual groups
of myocardial cells are stimulated to contract. This spread of electrical activity throughout the ventricular
myocardium produces the QRS complex. The QRS complex takes 0.08 to 0.12 sec (80 to 120 ms). Not every
QRS complex contains a Q wave, a R wave, and a S wave. The duration, amplitude, and morphology of the
QRS complex is useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy,
myocardial infarction and other disease states.
ST Segment
The ST segment connects the QRS complex and the T wave and has a duration of 0.08 to 0.12 sec (80 to 120
ms). Flat, down-sloping, or depressed ST segments may indicate coronary ischemia. ST segment elevation may
indicate myocardial infarction.
T Wave: Ventricle Repolarization (Recovery)
Absolute refractory period: from the beginning of the QRS complex to the apex of the T wave.
Relative refractory (vulnerable) period: last half of the T wave.
1) T waves can signal coronary ischemia, Wellens’ syndrome, left ventricular hypertrophy, or CNS disorder.
2) Tall or “tented” symmetrical T waves may indicate hyperkalemia.
3) Flat T waves may indicate coronary ischemia or hypokalemia flutter.
Reference: Wikipedia
ECGInterpretation(CV-58-tn)RevC2013USltr
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