Right and left leg block

The femoral block is a condition characterized by a delay or obstruction of conduction of the electrical impulses that make the heart beat.

The heart is formed from 4 cavities:

  1. Two atria in the upper area,
  2. Two ventricles in the lower area.

The heart is a very reliable pump, because it beats at a constant pace throughout life, but it can also vary according to the needs of the body.

It requires an electrical signaling system that follows a precise sequence to coordinate muscle contractions of the atria and ventricles.

In the right atrium there is an area with a specialized tissue called the sinus node, which transmits electrical signals to other centers in the heart:

  1. Atrioventricular node,
  2. bundle of His,
  3. Purkinje fibres

The atrioventricular block is a disturbance of impulse conduction from the atrium to the ventricle at the level of the atrioventricular node or the bundle of his.

Sinus node: In the right atrium, the electrical impulse originates and is transmitted:

  1. To the right atrial myocardium via James fibers,
  2. To the left atrial myocardium and via the Buckmann bundle.

Atrioventricular node: In the lower part of the right atrium. Receives atrial impulses and transmits them to the bundle of His.

Left and right leg: They originate from the bundle of His and run in the endocardium of the respective ventricle.

The left leg is divided into two half legs:

  1. Anterior
  2. Posterior.

Purkinje fibers: They are a complex of a nerve network that conduct the signal to the ventricular endocardium.

Activation of the atrial myocardium occurs in the right atrium.

The direction of the electrical impulse is to the right and down, generating the p-wave in the electrocardiogram.

Activation of the ventricular myocardium occurs in the left part of the septum.

The direction of the pulse is downwards and to the left.

Since the muscle mass of the left ventricle is larger, the QRS complex is formed.

The delay or obstruction can only block the electrical impulses that propagate right or left in the heart.


Classification of the femoral block

  1. First degree, if the block causes a slight line delay and the PR segment is more than 200 ms. It is a benign disease, therapy is not required.
  2. Second degree, when the conduction of excitation partially fails. The atrio-ventricular block of the second degree is divided into:
  3. Type Mobitz I (Wenckebach) – in the electrocardiogram there is a progressive extension of the p-Q path and every now and then an impulse to the ventricles is lost, practically a p-wave is created, which is not followed by a QRS complex. The impulse conduction is continued with this rhythm at the next heartbeat.
  4. The number of strokes between the first p and the one that does not lead is variable and can range from 3 to 6.
  5. With this doctor of femoral block, no therapy is required.
  6. Type Mobitz II – in the electrocardiogram, a QRS complex does not always appear after the p-wave. This can be a sporadic phenomenon (for example, nocturnal vagus overactivation causes line slowdown).
  7. Atrioventricular block 2:1 – A wave with a QRS complex forms every two p-waves, which causes a significant reduction in heartbeat.
  8. Third degree, complete failure of excitation conduction. When both legs are completely blocked (bilateral or trifascicular block), the electrical impulse is unable to reach the heart chambers.

Complete thigh block (3rd degree)

The electrical impulse does not go beyond the atrioventricular junction.

This is a form of the complete thigh block. If there were no spontaneous electrical impulses within the ventricles (idioventricular flight rhythm), the ventricles would stop contracting.

This event is dangerous and could lead to death.

Fortunately, this form of block is rare.

As a rule, both bundles can be damaged on the right and left and conduct the electrical impulses slower than normal, but neither is completely blocked.

The p-wave does not transmit the impulse to the ventricles. In the ECG, the p-waves are not connected to the QRS complex, but independently of each other.

The frequency of ventricular contractions is much slower, a rhythm ranges from 20 to 40 beats per minute and can have severe consequences.

The main clinical symptom of grade III atrioventricular block is syncope (loss of consciousness).

In this case, the doctor advises implanting an artificial pacemaker to ensure adequate blood supply to the body.

This problem leads to a wider QRS complex (because the activation of the ventricles is slowed down), but this model does not reflect either the right or left femoral block.

When one leg does not work, the impulse continues through the normal contractile tissue. However, this is not specialized for a fast transfer and thus slower.

The result is a slowdown of the line and a widening of the QRS complex.

Other conduction defects can affect the areas below the bundle of His and then affect the right and left legs.

Right leg block (RSB)

In the case of the right leg block, the electrical signal reaches the left ventricle, activates the ventricle, and then continues to the right ventricle.

The QRS complex is therefore wider and divided in two, because it has 2 waves: R and R’.

Due to the delay in the conduction of the impulse to one ventricle, two ventricular systoles occur.

The block of the right leg is manifested in the right derivatives of the heart: V1, V2 and V3.

V1 and V2 are the anterior and right derivatives, so they show well the front wall of the right heart. The block of the right leg is found relatively often on the ECG and can be caused by various diseases.

It can also be found in people with a healthy and normal heart, in which case it is not serious.

Reasons for this can be:

  1. pulmonary embolism (blood clots in the lungs),
  2. a heart attack (myocardial infarction),
  3. Myocarditis – A viral or bacterial infection of the heart muscle,
  4. High blood pressure (hypertension),
  5. Chronic obstructive airway disease (COPD),
  6. coronary heart disease – ischemia of the heart,
  7. Congenital disease (present in newborns from birth), for example, a septal defect.

The most common cause of the right leg block in children arises from surgery to restore an atrial septal defect or in another congenital heart disease such as:

  1. Ebstein’s anomaly,
  2. Atrioventricular canal,
  3. Tetralogy of Fallot.


  1. Postoperative right bundle branch block: identification of three levels of block. – Horowitz LN, Alexander JA, Edmunds LH Jr – Circulation. 1980 Aug; 62(2):319-28.
  2. The mechanism of apparent right bundle branch block after transatrial repair of tetralogy of Fallot.

The tetralogy of Fallot is a malformation of the heart, in which 4 anomalies occur simultaneously:

  1. narrowing (or stenosis) of the pulmonary valve,
  2. ventricular septal defect (opening between the two ventricles of the heart),
  3. Riding aorta,
  4. Right heart hypertrophy.

In a normal heart, this ventricular contraction delay does not cause any impairment of the heart’s function.

An isolated right leg block usually has no medical significance and can be neglected.

The complete leg block is a slowdown in the conductivity of the affected side of at least 0.12 sec., while the incomplete leg block has a delay between 0.10 and 0.12 sec.

This period of time is measured in the ECG.

Incomplete right leg block

If an ECG curve resembles a femoral block, but the width of the QRS complex is less than 0.12 seconds, the problem may be an incomplete femoral block.

The incomplete right leg block occurs more frequently and can be regarded as a normal variant of the QRS complex.

It is often observed in adolescents and does not represent a serious clinical picture.

Left leg block

The left leg block is less common than the right femur block and usually indicates heart disease.

The left leg block can be caused by:

  1. dilated cardiomyopathy,
  2. Hypertrophic cardiomyopathy,
  3. Hypertension
  4. Myocarditis
  5. Aortic valve disease and other heart diseases.
  6. Arrhythmogenic dysplasia of the right ventricle – This is a condition that mainly affects young people and begins with palpitations. It can lead to death. Arrhythmogenic dysplasia is characterized by the transformation of the myocardium into fibro-obese tissue.
  7. The result is a right ventricular arrhythmia and extrasystoles in the form of a left femoral block on the ECG.
  8. For diagnosis, a Holter ECG and a stress test are needed to see if extrasystole persists. An ultrasound is also used to make a diagnosis.

If cardiac examination does not reveal underlying heart disease, the problem of a left femoral block (especially in adolescents) is minimal.

However, the delay of left ventricular contraction can cause a decrease in cardiac function, especially in patients with:

  • Heart disease
  • Decreased left ventricular ejection fraction (percentage of blood volume pumped out of the ventricle compared to that located inside).

Doctors recommend therapy of cardiac resynchronization to patients with heart failure and left femoral block.

This treatment helps to recoordinate left ventricular contraction and improve cardiac function.

Patients with complete left leg block can only engage in sports with a cardiovascular reduced activity (sport fishing, sailing, horseback riding).

In a left leg block, the right side of the heart is activated first, and then the left.

On the ECG, a split R-R’-wave is formed in the left leads of the heart (V5 and V6).

The QRS complex is over 120 ms wide.

The left leg is divided into:

  1. Anterior
  2. Posterior.

If the block affects only one of these legs, then one speaks of a hemiblock. A hemiblock may occur:

  1. Front left,
  2. Left rear (rare).

Also in this case, the QRS complex is wide (between 80 and 120 ms).

The patient may suffer from a right femoral block and a left hemiblock at the same time.

Simultaneous blockage of the right and left legs is not possible, since ventricular systole does not occur. However, the femoral blocks can occur alternately.

frequency-dependent left anterior hemiblock may occur, for example in elderly patients who have a well-functioning conduction system at rest, but in whom, as soon as the frequency increases, a femoral block appears.

Altered leads in femoral block:

  1. The leads in aVL and I show the left side of the heart,
  2. III, aVF and II demonstrate the lower heart area,
  3. V4, V5 and V6 show pathologies of the left ventricle.

With the presence of a left leg block, it is not possible to diagnose a heart attack on the ECG.

Causes of femoral block

  1. Heart disease
  2. Compensated heart failure
  3. Heart muscle thickens, stiffens or weakens (cardiomyopathy)
  4. High blood pressure (hypertension)
  5. A congenital heart defect (existing from birth)
  6. A heart attack (myocardial infarction)
  7. A viral or bacterial infection of the heart muscle (myocarditis)
  8. Scar tissue after heart surgery
  9. Blood clots in the lungs (pulmonary embolism)

Symptoms of femoral block

In most people, the femoral block causes no symptoms.

Sometimes people with this condition don’t even know they have a femoral block.

Possible symptoms include:

  1. Fainting (syncope)
  2. Fainting (presyncope)
  3. Slow heart rate (bradycardia)

Diagnosis and examinations of the femoral block


An electrocardiogram (ECG) is an examination used to diagnose the femoral block.

This disease can also be detected when recording a Holter ECG.

ECG for a femoral block

The figure shows the electrocardiogram in a femoral block.

A widened QRS complex (beginning of the Q wave to the end of the S wave) is typical for a complete right femoral block.

An incomplete (or focal) right leg block exists when the width is between 0.10 and 0.12.

If there were a left femoral block, it would be opposite, with a broad QRS complex, but negative in the right and positive in the left derivatives.

Risks and complications of the femoral block

The main complication of a femoral block is the low heart rate, which is why a pacemaker is sometimes required.

People who have a heart attack and develop a femoral block are more likely to develop complications such as sudden cardiac death.

The complications of a femoral block are similar regardless of whether the block is on the right or left side of the heart.

Complications include:

  1. Slow heart rate (bradycardia), some of these people need a pacemaker.
  2. Arrhythmia.
  3. Cardiac arrest or sudden cardiac death.

Therapy and medication for femoral block

Most people with femoral block have no symptoms and do not need therapy.

You can get pregnant with a thigh block.

Nevertheless, in the case of heart disease that causes the femoral block, treatment of the underlying disorder is advisable.

Treatment for heart disease involves taking medications to:

  1. lowering blood pressure,
  2. Reducing the effects of heart failure.

If the patient shows signs or symptoms of a femoral block (for example, fainting), the doctor may recommend a pacemaker to regulate the heartbeat.

When should surgery be performed on a femoral block?

Does the patient have symptoms and a block of this type:

  1. alternating right and left leg block,
  2. Bifascicular block with and without AV block (His bundle), only if the patient had a TIA.

Inserting a pacemaker

For some people who suffer from fainting and femoral block, doctors may recommend implanting an artificial pacemaker (stimulator). This cardiac stimulator is a compact device that is implanted under the skin of the chest.

The internal pacemaker is placed near the collarbone in an operation performed under local anesthesia.

The pacemaker’s generator is connected to wires that lead to the heart.

The pacemaker emits electrical impulses when needed to regulate the heartbeat.

These devices have sensors that can detect when the heart is beating too slowly, and a pulse from the pacemaker serves to speed up the heartbeat.

The biventricular defibrillator (resynchronizing therapy) is indicated for:

  1. Left leg block,
  2. QRS greater than 150 ms.

It consists of three catheters:

  1. One in the right atrium,
  2. One to the right ventricle,
  3. One to the left ventricle (in the coronary sinus).

This device simultaneously stimulates the right atrium, right ventricle and left ventricle to synchronize the heart correctly.

Some patients also experience an improvement in heart failure and heart volume normalizes.

Another device that can be used is the Left Ventricular Assist Device Jarvik2000, which pumps blood from the ventricle into the aorta.

It is a device outside the body that has a connecting wire to the heart.

This wire enters the body at the level of the mastoid.

The patient concerned must put on a belt with this device.

It is indicated:

  1. In children born with this condition, for example, aortic stenosis.
  2. In inpatients with end-stage heart failure who do not respond to treatment.
  3. In this way, the risk of death before a heart transplant is reduced.

Prognosis for patients with femoral block

In healthy people, the left leg block does not increase the risk of disease or death.

The risk/mortality ratio is only 1.3 times higher than in other people. In patients who have recently developed a femoral block, mortality is 10 times higher than normal.

In patients with heart failure, a left leg block slightly increases the risk of cardiovascular and mortality.

Source: Dmitriy Scherbak e Gregory J. Hicks (Rhythm, Bundle Branch Block, Left (LBBB).

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