What Does the QRS Complex Represent in the ECG Wave Tracing - Basics of ECG

The QRS complex in the ECG is visible as spikes and dips. It indicates the heart’s health and shows how the electrical activity controls the heart’s contractions. The complex is made up of three waves: the Q wave, R wave, and S wave, which show different phases of the heart’s contraction in a cycle.

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The QRS complex seems more complex than the other waves. This is because ventricular depolarization occurs in steps. In this blog, we will read what each wave in a QRS complex means. How to recognise the normal and abnormal QRS patterns? And why are these differences important?

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QRS Complex

The QRS complex is made up of the Q wave, R wave, and S wave, which occur in quick succession. The electrical signals that form the QRS complex spread through the ventricles, indicating the ventricle is ready to pump blood (ventricular depolarization). The QRS complex starts with ventricular depolarization. Not every QRS complex contains Q, R, and S waves.

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The convention is:

• The Q Wave is always negative.

• The first formed positive wave of the complex is an R Wave.

• If the QRS complex only includes an upward (positive) deflection, it is an R Wave without a Q wave.

• The S wave is the first negative deflection that comes after an R Wave.

• Under normal circumstances, the duration of the QRS complex in an adult is between 0.06 and 0.10 seconds, showing rapid conduction.

• The QRS complex is positive in leads I, aVL, V5, V6, II, III, and aVF and negative in leads aVR, V1, and V2.

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The J Point is the point where the QRS complex and ST segment meetItt is considered to be the starting point of the ST segment. The J Point is also known as the junction. It is important because it helps diagnose an ST-elevation myocardial infarction when the J Point is elevated at least two millimeters above the baseline.

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Naming of the Waves in the QRS Complex

A wave is a deflection that crosses the baseline in the ECG. The following rules apply while naming the wave.

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1. Q wave: The first negative wave before a positive wave; if it is not present, then the QRS complex has no Q wave.

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2. All positive waves are called R waves:

            The first formed positive wave is named the R wave 

            R^/  (R prime) is the name given to the second positive wave after a dip. 

            R^// is a third positive R wave (though rare) is an R-bis wave. 

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3. S wave: A negative wave after a positive R wave. 

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4. Large waves are denoted by the capital letters Q, R, S, and small waves by q, r, s.

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5. Any wave measuring beyond the three small squares is a large wave; less than three squares is a small wave.

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The net Direction of the QRS Complex

The QRS complex can be positive or negative. If the sum of positive areas above the baseline is positive compared to the negative areas below the baseline, it is a net positive wave.

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A net positive direction shows a healthy heart and normal conduction pathways, indicating electrical stability of the heart’s rhythm.

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A net negative direction of the QRS complex is suggestive of heart issues. It occurs in right ventricular hypertrophy or left bundle branch block.

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The positive and negative waves in no net direction in the QRS complex are balanced. No net direction suggests an abnormal electrical conduction system in the heart. Conditions that cause no net direction are ventricular arrhythmias or severe conduction blocks.

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Electrical Vectors in the QRS Complex

When the muscles in the heart contract, they spread as impulses in different directions and strengths. The vectors in the heart represent the direction and strength of electrical signals that make the heartbeat when the heart contracts. It creates a wave of electricity that travels through the muscles.

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It works as follows:

• The sinoatrial node in the atrium starts and sends the first electric signal.

• The signal travels through the atria (causing them to contract) and reaches the ventricles. 

• As the signal spreads, it creates vectors that show the flow of electricity in different directions.

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The QRS complex forms due to the electrical depolarisation of the ventricles. The process is a series of electrical vectors showing the direction and magnitude of electric current as it travels through the heart muscle.

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During ventricular depolarization, electrical impulses move in a systematic pattern.

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They are:

1. First vector (Q Wave): The impulse begins from the interventricular septum and travels from left to right. It creates a small negative deflection on some leads and is seen as the Q Wave, the first sign of ventricular activation.

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2. Second vector (R Wave): As the electrical activity spreads through the ventricles, the left ventricle, which has more muscle mass, the vector moves downwards and to the left. It produces a large positive deflection known as the R Wave and is prominent in leads I, II, and V4 to V6.

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3. Third vector (S Wave): The last part of depolarisation moves towards the upper right ventricle, creating a small negative deflection, which is visible as the S Wave completes the ventricular activation. Each lead on the ECG sees these vectors from different angles; this is the reason why the QRS complex can look different across the 12 leads.

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When the electrical vector is aligned towards a lead, there is a positive deflection. If it moves opposite, the deflection is negative. These vectors are helpful in interpreting the cardiac axis, identifying bundle branch blocks, and recognizing patterns in MI or ventricular hypertrophy.

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The vectors formed due to the activation of the right ventricle are not visible because they are invisible by the larger vectors generated by the left ventricle. Activation of the ventricle wall proceeds from the endocardium to the epicardium. It is due to the fact that Pukinje’s fibers run through the endocardium.

Implications of a Broad QRS Complex

A wide QRS complex has a duration greater than 0.12 seconds. It suggests the electrical impulses take longer than usual to reach the ventricles, indicating an underlying heart condition. The causes of a wide QRS complex are:

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1. Bundle branch block

The right and left bundle branch consists of Purkinje’s fibres that help to spread into the ventricular myocardium. The Purkinje fibers enable the electrical impulse conduction to the ventricles so that they can contract at the same time. A bundle branch block is when a bundle branch fails to transmit the impulse.

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The ventricle whose bundle branch is blocked will have to wait for electrical impulses to spread from the other ventricle. This spread of the impulse from the other ventricle occurs outside the conduction system and takes longer. 

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2. Hyperkalaemia

Hyperkalaemia or elevated potassium levels in the blood cause a wide QRS complex. Higher potassium levels affect the heart’s electrical activity and slow depolarization of the ventricles. It forms wide QRS complexes, which may cause life-threatening arrhythmias.

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3. Medications

Certain medications, such as anti-arrhythmia drugs and tricyclic antidepressants, cause wide QRS complexes as a side effect.

4. Ventricular rhythm

When the heart’s electrical impulses originate from the ventricles rather than the atria, they cause the wide QRS complex, indicating ventricular rhythm. It causes the heart rate to slow down and symptoms such as fatigue, weakness, and even fainting to appear.

5. Pre-excitation (Wolf Parkinson White syndrome)

An extra electrical pathway in the heart, which causes the preexcitation of ventricles, leads to a wide QRS complex during tachycardia episodes.

6. Aberrant ventricular conduction 

Aberrant ventricular conduction, or aberrancy, occurs when electrical impulses unusually travel through the ventricles. It causes a wide QRS complex to form. It happens during episodes of rapid atrial rates or in the presence of certain conduction disturbances.

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Amplitude of the QRS Complex

The amplitude of the QRS complex refers to the height or depth of the waves (the R wave and S wave)in the complex.

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It tells us about the

• electrical activity in the heart. 

• Abnormalities of structure.

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They indicate conditions such as left ventricular hypertrophy or myocardial infarction, and changes in amplitude suggest a range of cardiac issues that require further evaluation.

1. R wave 

The amplitude of the R wave is the height of the first positive deflection in the QRS complex.

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• A tall R wave indicates strong electrical activity and healthy ventricular function.

• A small R wave suggests ischemia, obesity, COPD, or pericardial effusion.

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The amplitude of R-waves provides information about the heart's structure and helps in diagnosing issues in the heart’s performance.

R wave peak time: It is the moment when the R wave reaches its highest point during the QRS complex. It is important as it reflects the speed of ventricular depolarization. A delayed R wave peak time can lead to conduction abnormalities as bundle branch block or other intraventricular conduction delays.

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R wave progression: Normal R wave progression in the chest leads implies that the R wave gradually increases in amplitude from V1 to V5, then diminishes in amplitude from V5 to V6. The S wave undergoes the opposite.

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Abnormal R-wave progression occurs in:

• Myocardial infarction: The necrotic myocardium does not generate an electric potential, and therefore, there is a loss of R wave amplitude in the ECG.

• Cardiomyopathy: It causes an increase or decrease in R wave amplitude depending on the type and severity of the condition. Amplitude increases in hypertrophic cardiomyopathy.

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2. Q wave 

A Q Wave is considered pathological based on the amplitude and its duration. 

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• Duration > 0.03 seconds.

• Amplitude > 25% of the R wave amplitude.

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The pathological Q waves are significant if they exist in two contiguous leads, such as aVF and III or V4 and V5.

3. Normal Q wave variants

1. Septal Q waves: The septal Q wave is a small Q wave that appears in the left precordial lead V5 and V6 during ventricular depolarization. It results from the depolarization of the ventricular septum. A prominent septal Q wave indicates a healthy electrical pathway through the septum, ensuring effective heart function.

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2. Respiratory Q Wave: The respiratory Q Wave is a normal variant that can appear on an ECG. It is influenced by respiratory cycles, varies in amplitude, and changes with changes in intrathoracic pressure during inhalation and exhalation. It leads to small deflections that appear as Q Waves in certain leads.

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3. Small Q Waves: They are considered benign and found in various leads of the ECG. They are present due to individual and anatomical variations of the heart's orientation in the chest. The small deflections do not suggest any pathology; they are narrow and shallow.

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4. Abnormal pathological Q waves

Abnormal Q waves signify underlying health conditions. Wider and deeper than normal Q waves indicate an old myocardial infarction.
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Pathological Q Waves are seen in

• Left-sided pneumothorax.

• Dextrocardia is a condition in which the heart is placed on the right side of the chest.

• Perimyocarditis.

• cardiomyopathy.

• Amyloidosis.

• Bundle branch blocks.

• Pre-excitation syndromes (WPW syndrome).

• Ventricular hypertrophy acute cor pulmonale.

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Tips to Maintain the QRS Complex

A lifestyle that promotes heart health helps to maintain a normal QRS complex and reduces the risk of cardiac complications.

1. Do regular exercise

Walking, running, cycling, or swimming improves blood circulation and keeps the heart healthy. One hundred fifty minutes of exercise a week strengthens the heart muscles and reduces weight and stress.

2. Eat a healthy diet

Fruits, vegetables, whole grains, and lean proteins should be eaten regularly. Blood pressure and strain on the heart are reduced by taking less salt in the diet.

3. Avoid smoking and alcohol

Quit smoking and alcohol; it improves heart health as smoking damages blood vessels, and excessive alcohol disrupts heart rhythms.

4. Reduce stress levels

Stress,  heart rate, and blood pressure can be reduced by doing meditation, yoga, or deep breathing exercises to reduce.

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5. Regular health checkups

Get routine health checkups to detect any issues with the QRS complex early in the ECG.

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Conclusion

A QRS complex is essential to see heart health. Its duration, shape, and amplitude provide information about the heart's electrical activity. Abnormalities in the QRS complex indicate conditions like bundle branch block, ventricular hypertrophy, or previous heart attacks.

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By doing regular exercise, eating a nutritious diet, managing stress, and avoiding smoking and alcohol, we can have a healthy heart. For more insights on heart health and related topics, visit Gauze.md, which is a dedicated resource for heart health.