Interval QRS

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).

·       Narrow complexes (QRS < 100 ms) are supraventricular in origin.

·       Broad complexes (QRS > 100 ms) may be either ventricular in origin, or due to aberrant conduction of supraventricular complexes (e.g. due to bundle branch block, hyperkalaemia or sodium-channel blockade).

Sinus rhythm with frequent ventricular ectopic beats (VEBs) in a pattern of ventricular bigeminy. The narrow beats are sinus in origin, the broad complexes are ventricular.

Narrow QRS Complex Morphology

Narrow (supraventricular) complexes arise from three main places:

·       Sino-atrial node (= normal P wave)

·       Atria (= abnormal P wave / flutter wave / fibrillatory wave)

·       AV node / junction (= either no P wave or an abnormal P wave with a PR interval < 120 ms)

Broad QRS Complex Morphology

Broad/Wide QRS Complexes

·       A QRS duration > 100 ms is abnormal

·       A QRS duration > 120 ms is required for the diagnosis of bundle branch block or ventricular rhythm

Broad complexes may be ventricular in origin or due to aberrant conduction secondary to:

·       Bundle branch block (RBBB or LBBB)

·       Hyperkalaemia

·       Poisoning with sodium-channel blocking agents (e.g. tricyclic antidepressants)

·       Pre-excitation (i.e. Wolff-Parkinson-White syndrome)

·       Ventricular pacing

·       Hypothermia

·       Intermittent aberrancy (e.g. rate-related aberrancy)

High Voltage QRS Morphology

·       Increased QRS voltage is often taken to infer the presence of left ventricular hypertrophy.

·       However, high left ventricular voltage (HLVV) may be a normal finding in patients less than 40-45 years of age, particularly slim or athletic individuals.

·       There are multiple “voltage criteria” for left ventricular hypertrophy.

·       Probably the most commonly used are the Sokolov-Lyon criteria (S wave depth in V1 + tallest R wave height in V5-V6 > 35 mm).

·       Voltage criteria must be accompanied by non-voltage criteria to be considered diagnostic of left ventricular hypertrophy.

Low Voltage QRS Morphology

The QRS is said to be low voltage when:

·       The amplitudes of all the QRS complexes in the limb leads are < 5 mm; or

·       The amplitudes of all the QRS complexes in the precordial leads are < 10 mm

Electrical Alternans

·       This is when the QRS complexes alternate in height.

·       The most important cause is massive pericardial effusion, in which the alternating QRS voltage is due to the heart swinging back and forth within a large fluid-filled pericardium.

Spot Diagnoses

These cardiac diseases produce distinctive QRS morphologies that are important not to miss:

·       Brugada syndrome (partial RBBB with ST elevation in V1-2)

·       Wolff-Parkinson White Syndrome (delta wave)

·       Tricyclic poisoning (wide QRS with dominant R wave in aVR)

https://litfl.com/qrs-interval-ecg-library/

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