QRS complex

Schematic representation of normal ECG

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List of terms related to QRS complex

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Overview

The QRS complex represents electrical activation of the ventricle. Widening of the QRS complex may reflect delayed conduction in the His, bundle branch or purkinje conduction system.

Morphology of the QRS Complex

Q wave

If the first deflection of the QRS is downward, it’s called a Q wave. The Q wave represents activation of the ventricular septum. The electricity spreads from right to left through the septum.

Q waves may be normal. For example in lead I, a Q less than 1/4 of the R height, and less than one box wide, is considered normal. This is the early activation of the septum. This activation goes left — away from lead I — and is therefore negative on the ECG. “Septal Qs” are normal in I, F, V5 and V6. Qs are also generally innocent in lead III and lead V1 if no other abnormality is seen.

Q waves are “significant” if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave.

Q waves can be seen on the electrocardiogram following necrosis of the myocardium such as seen in acute MI or the replacement of electrically active tissue with electrically inert tissue as seen with Chagas disease. A q wave may also reflect septal hypertrophy in hypertrophic cardiomyopathy (HCM).

Causes of Q waves in the Absence of Myocardial Infarction

• A QS complex in lead V₁ is normal.
• A Q wave <0.03 s and <1/4 of the R wave amplitude in lead III is normal if the frontal QRS axis is between 30 and 0°.
• The Q wave may be normal in aVL if the frontal QRS axis is between 60 and 90°. Small septal Q waves are non pathological Q waves if <0.03 s and <1/4 of the R wave amplitude in leads I, aVL, aVF, and V₄-V₅-V₆
• The following may be associated with Q/QS complexes in the absence of myocardial infarction:

Chagas disease

Hypertrophic cardiomyopathy

Preexcitation syndromes

Obstructive or dilated cardiomyopathy

LBBB

RBBB

Left anterior fascicular block

LVH

RVH

Myocarditis

Acute cor pulmonale

Hyperkalemia

EKG Changes of Prior Myocardial Infarction

1. Any Q wave in V₂-V₃ ≥0.02 sec or presence of QS complex in V₂ and V₃.
2. Q wave ≥0.03 sec and ≥0.1 mV deep or presence of QS complexes in leads I, II, aVL, aVF or V₄-V₅-V₆ in any two leads of a contiguous lead grouping (I, aVL, V₆; V₄-V₅-V₆, II, III and aVF). The same criteria are used for supplemental leads V₇-V₈-V₉, and for the Cabrera frontal plane leads.
3. R wave ≥0.04 sec in V₁-V₂ and R/S >1 with a concordant positive T wave in the absence of a conduction defect.

R wave

The first upward deflection of the QRS is called the R wave. Most of the ventricle is activated during the R wave. The R wave may be prolonged if the ventricle is enlarged, and may be abnormally high (indicating strong voltage) if the ventricular muscle tissue is hypertrophied.

S wave

The S wave is any downward deflection following the R wave. Like the R wave, an abnormally large S wave may indicate hypertrophy of the ventricle.

If a second upward deflection is seen, it’s called an R-prime wave. R-prime waves are never normal, but indicate a problem in the ventricular conduction system.

QRS complexes may be described by naming the waves that form them. For example, a complex with an R, an S, and an R’ is called an RSR’ complex.

QRS Duration

• The normal duration is less than 0.12 seconds
  • If the duation is > 0.12 seconds then either left bundle branch block (LBBB), right bundle branch block (RBBB) or a non-specific intraventricular conduction delay is present.
• The precordial leads are approximately 0.01 or 0.02 seconds longer than the limb leads.

QRS Amplitude

Definitions of Low Voltage:

• If the total amplitude above and below the isoelectric line is < 5 mm in all 3 standard leads.
• An average voltage in the limb leads of < 5 mm with an average of < 10 mm in the chest leads (1).
• Should be at least 5 mm in V1 and V6 , 7 mm in V2 and V5 and 9 mm in V3 and V4.

Differential Diagnosis of Underlying Causes of Low QRS Voltage

• Amyloidosis
• Diffuse coronary artery disease
• Congestive heart failure
• Emphysema
• Prior myocardial infarction
• Myocarditis
• Myxedema
• Obesity
• Pericardial effusion
• Tamponade

Definition of Maximal QRS Voltage:

Can be up to 20 to 30 mm in lead 2 and can be up to 25 to 30 mm in the precordial leads.

Additional resources

• ECGpedia: Course for interpretation of ECG
• The whole ECG - A basic ECG primer
• 12-lead ECG library
• Simulation tool to demonstrate and study the relation between the electric activity of the heart and the ECG
• National Heart, Lung, and Blood Institute, Diseases and Conditions Index
• A history of electrocardiography
• EKG Interpretations in infants and children

Examples

• 
  Diagram shows different types of QRS shapes

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