ECG Interpretation: Waves, Intervals, Axis, and Abnormalities

Introduction to ECG/EKG
An Electrocardiogram (ECG or EKG) is a test that records the electrical activity of the heart over
a period of time using electrodes placed on the skin. It is a crucial tool for diagnosing various
heart conditions by analyzing the heart's rhythm and electrical conduction patterns.




Components of the ECG Waveform
The ECG waveform consists of waves, complexes, intervals, and segments that correspond to
specific electrical events in the cardiac cycle.

1. P Wave
○ What it Represents: Atrial depolarization, which leads to atrial contraction. ○
Details: The P wave reflects the electrical impulse traveling through the atria
from the sinoatrial (SA) node.
2. PR Interval
○ What it Represents: The time from the onset of atrial depolarization to the onset
of ventricular depolarization.
○ Details: It includes the P wave and the PR segment, indicating the conduction
time through the atria and the atrioventricular (AV) node.
3. PR Segment
○ What it Represents: The period between the end of the P wave and the
beginning of the QRS complex.
○ Details: Reflects the delay at the AV node, allowing the ventricles to fill with
blood before they contract.
4. QRS Complex

, ○ What it Represents: Ventricular depolarization, leading to ventricular
contraction.
○ Details: Consists of three deflections:
■ Q Wave: The initial negative deflection after the P wave.
■ R Wave: The first positive deflection after the Q wave.
■ S Wave: The negative deflection following the R wave.
5. ST Segment
○ What it Represents: The period between ventricular depolarization and
repolarization.
○ Details: Normally isoelectric (flat), changes can indicate myocardial ischemia or
infarction.
6. T Wave
○ What it Represents: Ventricular repolarization, the recovery phase of the
ventricles.
○ Details: Represents the ventricles returning to their resting state.
7. QT Interval
○ What it Represents: The total time for ventricular depolarization and
repolarization.
○ Details: Measured from the beginning of the QRS complex to the end of the T
wave.
8. U Wave (occasionally present)
○ What it Represents: Thought to represent repolarization of the Purkinje fibers.
○ Details: Not always visible and its exact significance is not entirely understood.




Isoelectric Lines in ECG
● Definition: The isoelectric line is the baseline of the ECG tracing where there is no net
electrical activity detected.
● Number of Isoelectric Lines: There is essentially one isoelectric line, but it can be
observed during different intervals.
● Occurrences:
○ PR Segment: Represents the isoelectric period after atrial depolarization.
○ ST Segment: Another isoelectric period following ventricular depolarization.
● Significance: Deviations from the isoelectric line during these segments can indicate
cardiac abnormalities such as ischemia or infarction.


Depolarization and Repolarization
● Depolarization: The process by which cardiac muscle cells change their electrical
charge from negative to positive, leading to contraction.
○ Atrial Depolarization: Reflected by the P wave.
○ Ventricular Depolarization: Reflected by the QRS complex.
● Repolarization: The process of returning to the resting negative charge after
depolarization, leading to relaxation.
○ Ventricular Repolarization: Reflected by the T wave.
● Note: Atrial repolarization occurs but is usually obscured by the QRS complex and not
visible on the ECG.