QRS Interval Abnormalities

QRS Complex Morphology

The QRS describes ventricular depolarisation (ventricular contraction). The Morphology explains the origin of impulse. The Origin of impulse can be from:

• SA node
• Atria
• AV node 
• Ventricle

If the QRS complex is >120 msec, it is known as a wide QRS complex. A wide QRS complex suggests that the velocity of contraction within the ventricle is slow. Thus, the ventricle takes more time for depolarization. In QRS complex

• Q wave - negative wave
• R wave - positive wave 
• S wave - negative wave

If an electrode is placed in the ventricle, the septum depolarizes first. The direction of septal depolarization is from left to right. The direction of depolarization moving away from the electrode Results in a negative Q wave. Next, the ventricular wall gets depolarized. The depolarization direction is towards the electrode and results in a positive R wave. Lastly, the base of the ventricle gets depolarized. This direction of depolarization is away from the electrode, resulting in a negative S wave.

Also read: Cardiac Murmurs: Types, Characteristics & Classification

Abnormalities of QRS complexes

The main abnormality of QRS complexes is their width. Narrow versus broad QRS complexes are caused by the Voltage (height) of the complexes.

The second abnormality is in the conducting system. Normally, the impulse originates from the SA node and travels through the conducting system to the ventricle. 

Abnormalities of the width of the QRS complex

QRS complexes are abnormally wide in the presence of bundle branch block. The Velocity of conduction within the ventricle is slow. The Conditions with a wide QRS complex are as follows:

• If the origin of the impulse is in from the ventricle (ventricle acts as a pacemaker)
  • Then, the velocity of conduction within the ventricle is slow
  • So, depolarization takes more time
  • Leads to a wide QRS complex. 
• Ventricular escape beat 
• WPW syndrome: preexcitation syndrome
  • Short PR interval and wide QRS complex (slurring of R wave - delta wave) 
• Bundle branch block
• Ventricular tachycardia
• Ventricular fibrillation 

Width of complexes

1. Narrow QRS complex 

• Duration of QRS < 100 msec 
• If the origin of the impulse is from the supraventricular area (atria/SA node/AV node) 

2. Broad QRS complex

When the duration of the QRS is> 120 msec, this happens in case of electrolyte abnormality, like Hyperkalemia—tall T wave, wide QRS. Tricyclic antidepressants like Amitriptyline, Imipramine, and Phenothiazines also cause broad QRS complexes. 

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Causes of wide QRS complexes 

• Intrinsic intraventricular conduction delay
  • Left bundle branch block or Right bundle branch block 
• Extrinsic intraventricular conduction delay
  • Hyperkalemia or Drug-induced.
• Class I antiarrhythmic drugs (sodium channel blockers)
• Tricyclic antidepressants 
• Ventricular beats
  • Premature ventricular contractions
  • Paced beats. 
  • Escaped beats.
• WPW syndrome
• Factitious ECG
  • The speed of ECG paper within the ECG machine is around 50 mm/sec.
  • Normal speed is 25 mm/sec 
• Wide QRS complex tachyarrhythmias
  • VT
  • VF
  • SVT with aberrancy 

Abnormalities of the Conduction System

• If the velocity of conduction is highest through bundle branches and Purkinje fibers, the rate of depolarization is quick, and the QRS complex duration will be 70-110 msec. 
• If the ventricle is depolarized by impulse generated within ventricle/ accessory pathway – Wide QRS complex.
• Treatment of VT depends upon the hemodynamic condition of patients.
  • For hemodynamically unstable VT, the first line of treatment is DC shock.
  • For hemodynamically stable VT, the patient is given antiarrhythmic drugs.
    • Lignocaine (depending on the cause of VT) 
    • Amiodarone

Left Bundle Branch Block (LBBB)

• In patients with LBBB, both ventricles cannot contract/depolarize simultaneously.
• The first right ventricle depolarizes, followed by the left ventricle.
• This is represented by a wide QRS complex.

Causes of LBBB 

• A2 D2 H2
  • A2: Aortic stenosis and anterior wall myocardial infarction (AWMI) 
  • D2: Digoxin toxicity and DCMP (dilated cardiomyopathy) 
  • H2: Hyperkalemia and hypertension
• Left bundle branch.
  • Has 2 fascicles
    • Anterior fascicle
    • Posterior fascicle 
• In fascicular blocks, the QRS interval need not be >120 msec, it will be >110 or < 120 msec. 

To read more about the subtopics of this topic, like Left anterior fascicular block (LAFB), Left posterior fascicular block (LPFB), ECG of incomplete RBBB, and Factitious ECG, sign up to the prepladder app and learn with the best faculties of India.

Also read: P-Wave Abnormalities: Measurement

Narrow QRS Complexes

The Narrow supraventricular complexes arise from three main places:

• SA node
• AV node
• Atria

SA Node- ECG with Normal QRS Complex

• Normal P wave
• Normal QRS complex: <100 msec.

Atria

• Narrow QRS complex or normal QRS complex
• Atrial abnormalities are seen in P wave.
  • Atrial flutter
    • Saw toothed shaped P waves.
  • Atrial fibrillation

AV node/Junctional

• Narrow QRS complex
• Absence of P wave
  • Impulse originates from AV node. 
  • Simultaneously enters atria and ventricle.
  • Wave generated by Atrial depolarization is masked with Ventricular depolarization wave i.e., QRS complex.
  • Ventricular mass is more than Atrial mass.

Low Voltage QRS Complex

• Low voltage QRS complex
  • QRS complex in chest leads < 10 mm
  • QRS complex in limb leads < 5 mm.
• The Presence of fluid in pericardial space, the electrical activity generated within the myocardium is not transmitted properly to the leads.
  • Electrical activity picked up is less because of pericardial effusion.
  • Results in low voltage complex 

Also read: Prinzmetal’s Variant Angina

Systemic Conditions Causing Low Voltage QRS Complex 

• Endocrine disorders
  • Hypothyroidism - it causes pericardial effusion 
  • Adrenal insufficiency (Addison's disease) 
• Pleural disorders
  • Left sided pleural effusion. 
  • Left sided pneumothorax.
    • These pleural abnormalities overlap with the heart thus electrical activity is not transmitted to chest. 
• Pulmonary disorders
  • COPD (emphysema) - irreversibly abnormal dilation of lungs will overlap the heart.
• Obesity
  • Due to more fat overlapping the heart.
• Anasarca
  • Edema all over the body even within the chest
  • Electrical activity transmitted to surface of the chest wall reduces.
  • Leads to low voltage complexes. 
• Cardiac conditions
  • Cardiac amyloidosis
  • Extensive MI
  • Constrictive pericarditis 
  • Restrictive cardiomyopathy (RCMP) 

High Voltage Complex

High-voltage complexes are seen in cardiac muscle hypertrophy. When the voltage of QRS complexes is large, it follows Sokolow Lyon's criteria, which is suggestive of LVH.

When a strain pattern is present in LVH, there is ST segment depression and asymmetric T wave inversion.

Left Ventricular Hypertrophy

• Deep S wave in lead v1
  • Net depolarizing electrical activity is moving away from lead V1 - deep S wave. 
• Net depolarizing electrical activity is moving towards V5 and V6 - tall R wave. 
• Sokolow Lyon criteria
  • In LVH – SV1 + RV6 > 35mm.

Also read: High Yield Cardiology Image Based Question

Right Ventricular Hypertrophy

ECG changes in RVH

• Right axis deviation
• Tall R waves in lead V1 and V2 
• Direction of vector is towards v1 and v2. 
• Electrical activity is moving away from v5 and v6.
• In v5 and v6 - deep S wave 

Diagnostic criteria for RVH

• Right axis deviation of +110° or more 
• Amplitude of R wave in v1 - <7 mm tall or R/S ratio >. 
• Depth S wave in v5 or v6 - <3mm deep or R/S ratio <1.
• QRS duration < 120 msec (i.e., changes not due to RBBB) 

The right ventricular strain pattern is seen in RVH where ST segment depression is seen in right sided leads. There is also the Presence of asymmetrical T wave inversion. 

Frequently Asked Questions

Q. Duration of normal QRS complex 

1. 100-120 msec
2. 120-140 msec
3. 70-90 msec
4. 70-110 msec 

Ans. D. 70 - 110 msec

Q. QRS duration between 100 -120 msec suggests all of the following except?

1. Normal duration
2. Left anterior fascicular block 
3. Left posterior fascicular block
4. Left bundle branch block 

Ans: D. Left bundle branch block

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Q. Treatment of choice for the patient presented with giddiness and BP 80/60 mmHg?

1. Lignocaine bolus
2. Amiodarone bolus
3. DC shock 
4. Beta blockers 

Ans: C. DC shock 

Q. 46/M presents with h/o chest pain since 2 hrs. ECG is as follows? 

1. RVMI
2. LBBB
3. RBBB
4. PWMI

Ans: B. LBBB

Q. A patient with pulmonary embolism presented with a history of dyspnea and ECG is as follows. What is the diagnosis?

1. Sinus tachycardia
2. LBBB
3. RBBB
4. S1Q3T3 pattern 

Ans: C. RBBB

Q. Low QRS voltage on ECG indicates? 

1. Pulmonary embolism
2. Pericardial effusion
3. Cor pulmonale
4. Infective endocarditis

Ans: B. Pericardial effusion 

• If amplitude of QRS complex < 10 mm in chest leads and < 5 mm in limb leads - low voltage complex  

Q. Drugs causing QRS complex?

1. Imipramine
2. Adrenaline
3. Isoproterenol
4. Morphine

Ans: Imipramine

• Tricyclic antidepressant causes wide QRS complex.

Q. Wide QRS complex > 120 msec may be seen in all of the following except? 

1. Hyperkalemia
2. Wolff Parkinson white syndrome
3. Ventricular tachycardia
4. Left anterior fascicular block.

Ans: Left ventricular fascicular block.

• In LVFB, QRS complex < 120 msec (between 110 to 120 msec) 

Q: Endocrine condition causing low voltage complexes? 

1. Pheochromocytoma
2. Diabetes insipidus 
3. Hypothyroidism
4. Carcinoid tumor 

Ans. C. Hypothyroidism

• Due to pericardial effusion in hypothyroidism.

Q: Lung pathology with low voltage complex?

1. Bronchiectasis
2. COPD
3. Bronchial asthma
4. Bronchogenic carcinoma 

Ans: B. COPD

Q: 45/M presented with sudden onset dyspnea. ECG shown. Diagnosis?

1. Acute LVF
2. Cardiac tamponade 
3. Acute exacerbation of COPD
4. AWMI

Ans: B. Cardiac tamponade

Explanation

• ECG shows:
  • In lead II, QRS complex amplitude is large and then small alternatively - electrical alternans. 
  • Electrical alternans seen in patients with cardiac tamponade

Also Read: High-Yield Image Based Question on ECG

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