Ecg Interval on ECG Library – LITFL Basics

QT Interval

Tue, 08 Oct 2024 00:00:00 +0000

Definition

Time from the start of the Q wave to the end of the T wave
Represents time taken for ventricular depolarisation and repolarisation, effectively the period of ventricular systole from ventricular isovolumetric contraction to isovolumetric relaxation

The QT interval is inversely proportional to heart rate:

The QT interval shortens at faster heart rates
The QT interval lengthens at slower heart rates
An abnormally prolonged QT is associated with an increased risk of ventricular arrhythmias, especially Torsades de Pointes
Congenital short QT syndrome has been found to be associated with an increased risk of paroxysmal atrial and ventricular fibrillation and sudden cardiac death

How to measure the QT interval

The QT interval is usually measured in either lead II or V5-6, however the lead with the longest measurement should be used
Several successive beats should be measured, with the maximum interval taken
Large U waves (> 1mm) that are fused to the T wave should be included in the measurement
Smaller U waves and those that are separate from the T wave should be excluded
The maximum slope intercept method is used to define the end of the T wave (see below)

The ST Segment

Tue, 08 Oct 2024 00:00:00 +0000

S-T Segment

The ST segment is the flat, isoelectric section of the ECG between the end of the S wave (the J point) and the beginning of the T wave.

The ST Segment represents the interval between ventricular depolarization and repolarization.
The most important cause of ST segment abnormality (elevation or depression) is myocardial ischaemia or infarction.

Causes of ST Segment Elevation

QRS Interval

Mon, 14 Aug 2023 00:00:00 +0000

QRS Complex Morphology

Main features to consider:

Width of the complexes: Narrow versus broad.
Voltage (height) of the complexes.
Spot diagnoses: Specific morphology patterns that are important to recognise.

QRS Complex Naming Convention

Courtesy of ECGwaves.com

QRS Width

Normal QRS width is 70-100 ms (a duration of 110 ms is sometimes observed in healthy subjects). The QRS width is useful in determining the origin of each QRS complex (e.g. sinus, atrial, junctional or ventricular).

J point

Fri, 07 Apr 2023 00:00:00 +0000

The J point

The J point is the the junction between the termination of the QRS complex and the beginning of the ST segment.

The J (junction) point in the ECG is the point where the QRS complex joins the ST segment. It represents the approximate end of depolarization and the beginning of repolarization as determined by the surface ECG. There is an overlap of around 10ms.

The J point marks the end of the QRS complex, and is often situated above the baseline, particularly in healthy young males. The J point may deviate from the baseline in early repolarization, epicardial or endocardial ischaemia or injury, pericarditis, RBBB, LBBB, RVH, LVH or digitalis effect.

PR segment

Thu, 04 Feb 2021 00:00:00 +0000

↪ ECG Basics Homepage

The PR segment is the flat, usually isoelectric segment between the end of the P wave and the start of the QRS complex.

PR segment abnormalities

These occur in two main conditions:

Pericarditis
Atrial ischaemia

Pericarditis

The characteristic changes of acute pericarditis are:

PR segment depression.
Widespread concave (‘saddle-shaped’) ST elevation.
Reciprocal ST depression and PR elevation in aVR and V1
Absence of reciprocal ST depression elsewhere.

NB. PR segment changes are relative to the baseline formed by the T-P segment.