EKG: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks: Bundle Branch Blocks

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

When normal conduction is present, the electrical impulse travels from the AV node to the bundle of His into the bundle branches. As the current sweeps through the ventricles, contraction of the ventricles occurs and the QRS complex is created. This happens extremely quickly producing the narrow complex that is less than 0.10 seconds in width. The left ventricle is dominant due to the larger mass of this ventricle as opposed to the right ventricle, thus presenting the normal leftward axis of 0° to +90°. When a bundle branch block occurs, these normalities change (Normal bundle branches).

In normal conduction both ventricles are depolarized at the same time. When either of these branches is blocked, the conduction will be normal for the branch that was spared. The branch that is blocked will have a conduction pause as it comes around the blocked area and then proceeds normally again once it is past the block. Depolarization of the unblocked branch is then more rapid than that of the blocked branch. This asynchronous depolarization creates a widened QRS (greater than 0.10 seconds or 2 1/2 small squares) and two QRS complexes in one. This creates the R1 (R prime) type of complex discussed in Chapter 5. R1 is produced by the ventricle that has delayed depolarization. The QRS width will be at least 0.12 seconds or 3 small squares. The limb leads are the best leads to interpret this variance in width. If the measurement is 0.10 to 0.12 seconds, it is called an incomplete bundle branch block. Therefore, the R-R1 may be present but the width is not greater than 0.12 seconds (incomplete BBB). Greater than 0.12 seconds will generate the term complete bundle branch block. The chest leads will provide better information regarding the R-R1.

Axis, Hypertrophy, and Bundle Branch Blocks: Normal bundle branches

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Clinical Alert

If a patient has a bundle branch block (BBB) with tachycardia that is derived from an area above the ventricles (supraventricular), the complexes can be very wide and can be misdiagnosed as ventricular tachycardia. Treatment is different for each of these rhythms.

Axis, Hypertrophy, and Bundle Branch Blocks: Right Bundle Branch Block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

In right bundle branch block, the left ventricle depolarizes normally. Therefore, this first depolarization is the “R” and constitutes the activity of the left ventricle. The R1 is the right ventricle as it is the delayed conduction. This is best noted in the right chest leads, V1, and V2. Usually a small R wave will be produced first, followed by an S wave and then the second R wave. This may be written as rSR1. This is also known as an “M” pattern or “rabbit ears” (Fig. 6–14). Right bundle branch block can be a normal variation for some individuals. It can also be caused by conduction defects.

Axis, Hypertrophy, and Bundle Branch Blocks: RR1 in right bundle branch block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Another aspect of right bundle branch block is that deep S waves are found in the left lateral chest leads, V5 and V6, known as reciprocal changes. These can also be present in leads I and aVL (Chest leads depicting right bundle branch block). Repolarization is also affected and will be noted as ST segment depression and inversion of the T wave in the right precordial leads (V1 and V2).

Axis, Hypertrophy, and Bundle Branch Blocks: Chest leads depicting right bundle branch block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Axis, Hypertrophy, and Bundle Branch Blocks: Left Bundle Branch Block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

In left bundle branch block, the left ventricular impulse is delayed. The first R wave seen is the right ventricle which will produce a smaller R wave first. The R prime (R1) is not always present in this block. Sometimes it is noted as a wide, notched R wave with an increase in the final upswing. The leads to see this in are V5 and V6. In this variation, the widened QRS will again be seen. ST segment depression and T wave inversion can also be seen in the left chest leads (V5 and V6). Reciprocal deep S waves will be seen in the anterior leads (V1 and V2) (RR1 in left bundle branch block and Chest leads depicting left bundle branch block). Left axis deviation can also occur.

Clinical Alert

There are times when a BBB is not apparent unless the rate increases and a critical rate is achieved. This reflects that the conduction to the right and left branches moves at a normal pace when the rates are slower, however, when the heart rate increases, the conduction defect becomes apparent. This will cause an intermittent type of BBB with the wide complex and rabbit ear morphology only present when the rate surpasses the “critical rate.”

Axis, Hypertrophy, and Bundle Branch Blocks: RR1 in left bundle branch block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Axis, Hypertrophy, and Bundle Branch Blocks: Chest leads depicting left bundle branch block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Variations in EKG tracings are noted in Bundle Branch Block Features for both right and left bundle branch blocks. When determining the presence of bundle branch block, first look at the width of the QRS complex. If it is greater than 0.10 seconds, a block is present. Establish whether it is a right or left block by noting which leads carry the “rabbit ears” or RR1. Right bundle branch block will distinguish itself in leads V1 and V2. Left bundle branch block will be characterized in leads V5 and V6.

Axis, Hypertrophy, and Bundle Branch Blocks: Bundle Branch Block Features

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Clinical Alert

Due to the effects that are present with the R waves, it is difficult to diagnose right ventricular hypertrophy in the presence of right bundle branch block. Left ventricular hypertrophy is also precluded by left bundle branch block. Also, when a patient presents with a left bundle branch block, the diagnosis of acute myocardial infarction becomes very difficult based on EKG changes alone. Other challenges with diagnosing BBB occurs with junctional rhythms, Wolff-Parkinson-White (WPW) syndrome, hyperkalemia, and other disease processes that can widen the QRS complex.

Axis, Hypertrophy, and Bundle Branch Blocks: Hemiblocks

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

A conduction block can also occur in any of the three fascicles or subcomponents of the left bundle branch (Fascicles of the left bundle branch). When a block occurs in any of these sets of fibers, axis deviation will manifest. The anterior and posterior fascicles are the most common and can be important in the diagnosis of acute myocardial infarction. (The third fascicle of the left bundle is the septal fascicle. The right bundle branch does not have separate fascicles.) This is dependent on which coronary vessel is occluded. It is important to compare a present EKG to a previous EKG in order to correctly diagnose a hemiblock. Since an anterior hemiblock will present with a left axis deviation, other causes of this axis deviation (left ventricular hypertrophy, inferior myocardial infarction, or an overweight individual with a “horizontal” lying heart) must be included in the differential diagnosis. Neither anterior nor posterior hemiblocks will have a widened QRS. No ST or T wave changes will appear. Only axis deviation will arise and other causes of this axis deviation must be ruled out. For the anterior hemiblock, left axis deviation of −30° to −90° will be presented. The posterior hemiblock will have right axis deviation. Other EKG changes that can occur include a wide S in lead I and Q wave in lead III with a posterior hemiblock and a Q wave in lead I and wide S wave in lead III in the anterior hemiblock.

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Axis, Hypertrophy, and Bundle Branch Blocks: Fascicles of the left bundle branch

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Clinical Alert

When blocks are combined, serious outcomes can take place. A posterior hemiblock in association with a right bundle branch block can lead the patient into atrioventricular blocks. These blocks in the AV node or the Bundle of His cause serious bradycardias and immediate interventions must occur.

The manifestations of a combination of the right bundle branch and the posterior fascicle would include a widened QRS and an RSR1 in the anterior leads (V1 and V2), which would indicate the right bundle branch was involved and a RAD to provide information regarding the left posterior hemiblock. When the right bundle branch is blocked along with a hemiblock, the term “bifascicular” is used. Blocks of the right bundle branch and both anterior and posterior fascicles are termed “trifascicular” (Bifascicular block).

Axis, Hypertrophy, and Bundle Branch Blocks: Bifascicular block

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

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Axis, Hypertrophy, and Bundle Branch Blocks: Conclusion

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Understanding axis, hypertrophy, and BBB can be difficult concepts, but are important in identifying potentially lethal situations for the patient. Hypertrophy and enlargement of chambers can cause problems in cardiac output and are therefore significant pieces of the puzzle of cardiac disease.

Key points of this chapter include:

  • A vector is an arrow that shows the direction of the electrical current.
  • The mean vector is the sum or average of all vectors that are occurring simultaneously.
  • The mean electrical axis is the direction in which the mean vector is traveling.
  • The normal mean electrical axis would be in a downward and left direction.
  • The hexaxial reference system is derived from Einthoven’s Triangle and is the circle of degrees in the frontal plane that is used to determine the mean electrical axis.
  • The mean electrical axis helps to identify that normal ventricular depolarization is occurring.
  • The mean electrical axis should lie within the scope of 0° to +90°.
  • The two leads most commonly used to identify the mean electrical axis are leads I and aVF.
  • Leads I and aVF are perpendicular to each other.
  • Lead I lies at 0°.
  • Lead aVF lies at +90°.
  • The R wave is used to assess the mean electrical axis. Positive R wave deflections in leads I and aVF means that the axis is normal.
  • The mean vector will point toward hypertrophy and away from infarction.
  • Right axis deviation (RAD) is signified on the 12-lead EKG by a negative QRS complex in lead I and a positive QRS complex in lead aVF.
  • Tall slender individuals may have a normal right axis deviation.
  • Dextrocardia would have a right axis deviation.
  • A right-sided EKG would produce a normal axis and normal R wave progression across the chest leads in a patient with dextrocardia.
  • Extreme RAD is in the range of +180° and −90° and would show negative QRS deflections in both lead I and aVF.
  • Left axis deviation will be manifested with a positive deflection in lead I and a negative deflection in lead aVF.
  • An obese patient might have a normal left axis deviation.
  • Enlargement indicates an expansion or dilation of a chamber.
  • Enlargement is caused by volume overload.
  • Hypertrophy indicates an increase in muscle mass.
  • Hypertrophy is caused by an increase in pressure.
  • Enlargement is usually used to discuss the atria. Hypertrophy is used to describe changes in the ventricles. These can also happen simultaneously.
  • Enlargement is noted on the EKG in the P wave, an indicator of atrial activity.
  • Right atrial enlargement is seen as a tall peaked first upswing of the P wave. Another name for this is P pulmonale.
  • Axis deviation of the P wave to the right is also seen in right atrial enlargement.
  • Left atrial enlargement will be shown on the 12-lead EKG as a prominence in the final portion of the P wave. This is also known as P mitrale.
  • The QRS complex is used to visualize hypertrophy of the ventricles.
  • Right ventricular hypertrophy is noted by RAD on the 12-lead EKG.
  • Left ventricular hypertrophy is noted by several aspects of the 12-lead EKG including the presence of an S wave in V1, and the amplitude of the R wave in the lateral leads as well as the combined amplitude of the S wave in the anterior (V1 or V2) leads and the R wave in the lateral (V5 or V6) leads.
  • Strain is noted on the 12-lead EKG in the T wave and the ST segment.
  • Strain is also known as secondary repolarization abnormality.
  • Right ventricular strain would be seen in leads V1 and V2.
  • Left ventricular strain would be seen in leads V5 and V6.
  • A bundle branch block can occur in either the right or left bundle branch.
  • A widened QRS would indicate a bundle branch block.
  • The presence of an R1 (R prime) is also indicative of a bundle branch block.
  • RR1 is also known as “rabbit ears.”
  • Reciprocal changes, seen as deep S waves will also be seen in bundle branch blocks.
  • ST segment depression and T wave inversion can also be seen with bundle branch blocks.
  • Right bundle branch block is noted in the right chest leads, V1 and V2.
  • Left bundle branch block is noted in the lateral chest leads, V5 and V6.
  • A left bundle branch block may have a notching on the R wave.
  • Intermittent bundle branch blocks (BBB) can also be seen.
  • Left bundle branch block will cause difficulty in diagnosis of acute myocardial infarction.
  • The left bundle branch is subdivided into three fascicles.
  • When one of these fascicles becomes blocked, the patient has a hemiblock.
  • Hemiblocks will not have QRS widening. Only axis changes are indicative of hemiblocks.
  • Other causes of axis deviations must be ruled out to diagnose a hemiblock. EKGs must be compared to prior EKGs.
  • Left axis deviation can indicate an anterior hemiblock.
  • RAD can indicate a posterior hemiblock.
  • Bifascicular blocks are a combination of a right bundle branch block and one of the fascicles of the left bundle branch.
  • Trifascicular blocks are a combination of a right bundle branch block and
    both the anterior and posterior fascicles of the left bundle branch.
  • A posterior hemiblock in the presence of a right bundle branch block is a dangerous combination and can lead the patient to an atrioventricular block which can cause a profound bradycardia.
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