ECG / EKG Components

Components of ECG

Each ECG cycles consists of 5 waves: P, Q, R, S, T corresponding to different phases of the heart activities. The P wave represents the normal atrium (upper heart chambers) depolarization; the QRS complex (one single heart beat) corresponds to the depolarization of the right and left ventricles (lower heart chambers); the T wave represents the re-polarization (or recovery) of the ventricles. To interpret ECG, one needs to focus on the frequency (heart rate), regularity, shape and size of each individual waves and the timing and interaction between waves. The following diagram shows the components of a ECG cycle.

P Wave

The P wave occurs when both left and right atria are full of blood and the SA node fires. The signal causes both atria to contract and pump blood to the ventricles (lower chambers). Any abnormality is generally associated with the SA node and the atria.

Lead II and V1 are the best leads to observe p-waves.

General characteristics :

• Less than 100ms in duration
• Less than 2.5mm in amplitude

Examples of abnormal P waves

Abnormal Patterns	Possible Causes
Inverted	Atrial depolarization is in a different direction Sinoatrial block: Pacemaker is no longer the SA node and is subsumed by another part of the heart. The heart is in an abnormal location or orientation within the chest (dextrocardia).
Greaten than 2.5mm in amplitude	Right atrial enlargement
Longer than 100ms with two lobes	Left atrial enlargement
Invisible or absent	Impairment of conduction at the sinoatrial node. Sinoatrial block: SA node fails to discharge and AV junction takes over as the pacemaker.

PQ Segment

When the signal from the SA node arrives the AV node, the signal is slowed and paused for a short period to allow blood from the atria to fill the ventricles.

General characteristics :

• 120 - 200ms in duration from the beginning of P wave to the beginning of QRS complex (PR interval)

Examples of abnormal PR interval

Abnormal Patterns	Possible Causes
Greater than 200ms in duration	AV block: Impairment of the conduction path between the atria and ventricles of the heart
Less than 120ms in duration	Wolff-Parkinson-White Syndrome - pre-excitation of the ventricles due to an accessory pathway

Q Wave

The Q wave is generated when the AV nodes releases the signal that travels through the inter-ventricular septum. 

General characteristics :

• Less than 2mm or 3ms in duration
• Less than 25% of the corresponding R wave amplitude
• Normal Q wave in lead III may diminish or disappear on deep inhaling because of the change in the heart relative position; however the infarction related Q wave persists.

R Wave

R wave is the first positive deflection after the P wave regardless whether a Q wave exists or not. As the signal continues from the AV node and spreads to the ventricles, the signal triggers a contraction on the left ventricle that pumps blood out of the ventricle.

There may be multiple R waves.

S wave

S wave is generated when the basal parts of the ventricles are depolarized resulting in the contraction of the right ventricle.

QRS Complex

The Q, R and S waves together are referred to as QRS complex (even if some of its components are missing). It is the electrical forces generated by ventricular depolarization and represents the pumping action of the ventricles. 

General characteristics :

• 80 - 120ms in duration

Abnormal Patterns	Possible Causes
Greater than 120ms in duration	Bundle branch blocks: Defect of the heart's electrical conduction system. Wolff-Parkinson-White Syndrome - pre-excitation of the ventricles due to an accessory pathway
Less than 120ms in duration	Wolff-Parkinson-White Syndrome - pre-excitation of the ventricles due to an accessory pathway

ST Segment

The ST segment marks the time for the ventricles to pump the blood to the lung and body. In normal situations, it serves as the base line from which to measure the amplitudes of the other waveforms. 

Examples of abnormal ST segment

Abnormal Patterns	Possible Causes
Depressions	Ischemia - decrease in blood supply caused by obstructions in blood vessels.
Elevations	Myocardial infarctions - damaged heart issues.

T Wave

After the contraction empties the blood in the ventricles, they begin to relax, which is marked by the T wave.

General characteristics :

• The normal T wave is asymmetrical; the first half has a more gradual slope than the second half.

Examples of abnormal T wave

Abnormal Patterns	Possible Causes
Inverted	Ischemia - decrease in blood supply caused by obstructions in blood vessels
Tall peaked	Hyperkalemia - abnormally high concentration of potassium ions in blood when associated with flat P waves, and wide QRS complexes
Flat	Hypokalemia - abnormally low concentration of potassium ions in blood when associated with flat U waves and U waves taller than T waves