EKG: Axis, Hypertrophy, and Bundle Branch Blocks

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis is a term that is used to help the health care professional glean a greater amount of information from the 12-lead EKG. Determining whether a patient has a normal axis or not can help to diagnose increases in muscle mass of the left ventricle (hypertrophy), dilation (enlargement) of chambers, proper diagnosis of wide QRS tachycardias, hemiblocks, identification of accessory pathways, and myocardial infarctions (MI). The identification of bundle branch blocks (BBB) can also assist the health care provider in the correct analysis of certain conditions for the patient.

Contents

Axis, Hypertrophy, and Bundle Branch Blocks: Normal Axis

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

The term axis takes into consideration the vectors that illustrate the direction of depolarization of the ventricles. A vector is, in essence, an arrow that shows the direction of the electrical current. It also depicts the strength of the impulse that is traveling. In the normal heart, the greatest amount of current is headed downward and to the left in the direction of the left ventricle because it is thicker and larger. The sum or average of all the vectors that are occurring simultaneously as the impulse travels through the heart muscle is called the mean vector. The vectors originate in the AV node so vectors are always drawn from the site of the AV node as the beginning point of the arrow (Mean vector).

The mean electrical axis is then the direction that the mean vector is taking when it is looked at from a perspective of a circle of degrees in the frontal plane (Mean electrical axis). This is known as the hexaxial reference system and is derived from Einthoven’s Triangle. A circle representing 360° surrounds the heart. It is then divided into four segments. The upper segments are negative degrees and the bottom portion carries positive degrees.

Axis, Hypertrophy, and Bundle Branch Blocks: Mean vector

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks
Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks
The mean electrical axis is important in that it demonstrates whether or not ventricular depolarization is occurring in a normal fashion. Normally the mean electrical axis should fall within the scope of 0° to +90°. (Some cardiologists may recognize the scope of the mean electrical axis to extend up to −30°.) By determining the axis, abnormalities can be recognized.

Axis, Hypertrophy, and Bundle Branch Blocks: Mean electrical axis

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

The frontal planes are the limb leads comprising leads I, II, III, aVR, aVF, and aVL. The two leads that are commonly used to determine axis deviations are leads I and aVF. These two leads are perpendicular to each other with lead I lying at 0° on the plane and aVF lying at +90° on the hexaxial reference system. Lead I’s positive electrode is on the left arm. aVF has the positive electrode on the left “foot” or the left lower portion of the body (Leads I and aVF on the hexaxial reference system). The most powerful period of time within the depolarization process is illustrated by the tip of the R wave, thus the mean electrical axis is noted through the R wave. Positive R wave deflections in these two leads will indicate that the axis is normal. (This is a simplified manner of reading axis deviation. Other aspects of the complete 12-lead EKG can be considered which will render exact measurements of degrees of variance.)

Axis, Hypertrophy, and Bundle Branch Blocks: Leads I and aVF on the hexaxial reference system

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Clinical Alert

Axis deviation can be important in helping to diagnose a variety of conditions as well as hypertrophy of the ventricles and an actual infarction of the heart. In general, the mean vector will point toward hypertrophy and away from the infarcted portion of the heart wall.

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

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Right axis deviation (RAD) is signified when lead I displays a negative QRS complex and aVF continues to show a positive QRS complex. This shows that the mean vector is pointing in the direction +90° to +180° (Right axis deviation).

Clinical Alert

RAD may be normal for some patients who are tall and slender. Their heart can actually be displaced vertically causing this variation. Also RAD would be present (along with decreasing R wave progression in the chest leads V1-V6) in patients with dextrocardia where the heart is in the right side of the chest. In this situation a right-sided EKG would produce a normal axis and proper R wave progression across the chest leads.

Axis, Hypertrophy, and Bundle Branch Blocks: Right axis deviation

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Axis, Hypertrophy, and Bundle Branch Blocks: Extreme Right Axis Deviation

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Extreme RAD is said to occur when both lead I and aVF have negative QRS deflections. This would be represented on the hexaxial reference system in the range of +180° to −90° (Extreme right axis deviation). This is exactly opposite of the normal conduction through the heart muscle and is considered to be a rare occurrence. It is also known as “indeterminate.”

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

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Left axis deviation can be noted when lead I shows a positive deflection and lead aVF is now negative. This corresponds to −90° to 0° on the hexaxial reference system (Left axis deviation). This can be normal in older individuals or those who are obese. The heart in obese patients can become more horizontally placed due to pressure from the diaphragm pushing upward and shifting it in this more level or horizontal position.

Axis Deviations shows the changes that occur in the leads and some of the etiologies for each of the axis deviations.

Axis, Hypertrophy, and Bundle Branch Blocks: Extreme right axis deviation

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Axis, Hypertrophy, and Bundle Branch Blocks: Left axis deviation

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Axis, Hypertrophy, and Bundle Branch Blocks: Axis Deviations

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

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

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Hypertrophy and enlargement of the chambers of the heart are two somewhat synonymous words. However, there is a difference between the two and they very often exist simultaneously.

  • Enlargement reflects an expansion or dilation of a chamber. This can be caused by situations such as volume overload where the chamber has stretched to accommodate the extra volume of blood. This can also be an acute or chronic situation. Aortic insufficiency could cause left ventricular enlargement as it sits at the point where blood should be expelled into the aorta and therefore, causes an overload of blood volume in the left ventricle. Another potential area of enlargement would be in the left atrium due to mitral valve problems as the diseased valve does not allow good emptying of the left atrium into the left ventricle.
  • Hypertrophy relates to muscle mass itself. This is caused by an increase in pressure rather than volume. As the heart attempts to work against increased pressures that can be caused by hypertension or aortic stenosis, the muscle gains mass and becomes thicker. This makes the wall of the ventricle stronger and more powerful.

The 12-lead EKG can provide clues to these changes in chamber walls. In general, the term enlargement is utilized when the atria are examined and the term hypertrophy is used to designate ventricular changes. The P wave is an expression of atrial activity. Consequently, changes are noted in this waveform for atrial enlargement. The QRS complex correlates to ventricular activity and is therefore used to seek information regarding ventricular hypertrophy.

Axis, Hypertrophy, and Bundle Branch Blocks: Atrial Enlargement

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Atrial enlargement is seen through increased amplitude of the P wave. In order to visualize this amplitude correctly, it is important that calibration of the machine is checked. 1 mV should be equal to 10 mm (10 small squares) in height. As stated in previous chapters, the P wave should be less than 0.12 seconds in duration and the amplitude should be 2.5 mm or less. The best leads to determine atrial enlargement are leads II, III, aVF, and V1.

Right atrial enlargement is seen in the upswing of the P wave since this is the portion of the P wave that represents right atrial contraction. This initial portion of the P wave will be tall and peaked. The duration of the P wave does not change because the latter portion of the P wave signifies left atrial activity, which is unchanged. This type of P wave is also known as P pulmonale because of etiologies of the enlargement which originate from pulmonary problems such as chronic obstructive pulmonary disease, pulmonary hypertension, congenital heart diseases, or failure of the right ventricle. The P wave in V1 will be biphasic with the largest portion being the first component of the waveform. This can indicate right atrial hypertrophy or increase in muscle mass as well if it is enlarged and peaked. The other aspect that can be noted in right atrial enlargement is that of axis deviation of the P wave to the right. It may extend beyond +90°. When this happens, the P waves with greatest amplitude will be seen in leads III and aVF (Right atrial enlargement and Right atrial enlargement on EKG).

Axis, Hypertrophy, and Bundle Branch Blocks: Right atrial enlargement

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Left atrial enlargement will be exhibited by prominence in the final portion of the P wave which is indicative of activity of the left atrium. In V1 the second portion of the biphasic P wave will be deep and extend at least 1 mm past the isoelectric line. In this situation, the P wave is widened, as opposed to right atrial enlargement. The enlarged left atrium requires more time to depolarize and is greater than 0.12 seconds in width. Notching of the P wave is seen in leads I, II, aVL, V4, V5, and V6. Causes of left atrial enlargement include mitral stenosis, hypertension, failure of the left ventricle, and mitral regurgitation.

Axis, Hypertrophy, and Bundle Branch Blocks: Right atrial enlargement on EKG

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

The notched P wave that can be seen with left atrial enlargement is also known by the term P mitrale. There is usually no axis deviation associated with left atrial enlargement (Left atrial enlargement).

Clinical Alert

Always correlate the P wave abnormalities with patient history and clinical assessment. Some EKG changes are normal for the patient and may simply be a sign of a nonspecific conduction irregularity.

Axis, Hypertrophy, and Bundle Branch Blocks: Ventricular Hypertrophy

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

The ventricles can become both hypertrophied as demonstrated by an increase in the muscle mass due to an increase in the pressure and enlarged from a continuous volume surplus. The term used with the ventricles is usually hypertrophy. The QRS complex provides information about the work of the ventricles and is therefore impacted when ventricular hypertrophy occurs. The changes in the QRS have to do with amplitude and axis deviation.

Clinical Alert

The QRS can exhibit changes due to factors other than hypertrophy. Lung disease, age, and weight can also create these changes. Very thin and/or young adult patients may have increased QRS amplitude simply because the chest leads are geographically closer to the heart.

Axis, Hypertrophy, and Bundle Branch Blocks: Left atrial enlargement

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Right Ventricular Hypertrophy (RVH) is noted on the 12-lead EKG through right axis deviation. Usually the left ventricle is the stronger of the two and carries the axis. When the right ventricle increases in size, it can then become more prominent and the left ventricle becomes subservient. The normal axis should be between 0° and +90° and will now demonstrate a deviation toward the right, between +90° and +180°. Lead I will become more negative and lead V1 will have a much larger R wave than usual. The normal R wave progression will also reverse. V1 lies over the right ventricle and it will now take the lead in R wave height rather than the left ventricle (leads V5 and V6). R waves will be larger in leads V1 and V2 and smaller in leads V5 and V6. The S waves also reverse being smaller in V1 and larger in V6. Etiologies of right ventricular hypertrophy are chronic pulmonary disease processes such as chronic obstructive pulmonary disease (COPD), pulmonary hypertension, valvular problems, and congenital heart disorders. When the right ventricle is performing its duty of delivering blood to the pulmonary circulation, anything (such as pulmonary hypertension) that causes it to pump against a strong resistance can become an etiology for hypertrophy in the right ventricle (Right ventricular hypertrophy).

Left Ventricular Hypertrophy (LVH) is a more difficult diagnosis to make on EKG tracings because the left ventricle normally predominates. At times the axis will deviate beyond −15°. Changes in the chest leads that might be present include

  • Large S wave in V1
  • Large R wave in V6 (greater than 18 mm)
  • Large R wave in V5 (greater than 26 mm)
  • Combined amplitude of S wave in V1 or V2 and R wave in V5 or V6 is equal to greater than 35 mm in height
  • R wave height inV6 is greater than that in V5
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Axis, Hypertrophy, and Bundle Branch Blocks: Right ventricular hypertrophy

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

When assessing for left ventricular hypertrophy, the more of the above criteria that are met, the greater the chance that it is actually present (Left ventricular hypertrophy). Limb lead changes can also occur, though the chest lead changes are more common. In the limb leads, the R wave will be greater than 13 mm in lead aVL, 21 mm in aVF, and 14 mm in lead I. Also the R wave in lead I and S wave in lead III combined will surpass 25 mm. Etiologies for left ventricular hypertrophy include systemic hypertension, valvular disease processes, cardiomyopathies, and aortic insufficiency and stenosis. When looking at the possible causes, consider the anatomy and physiology of the heart. The left ventricle is responsible for delivering blood to the systemic circulation; therefore, if something is causing the left ventricle to exert more pressure in order to deliver, such as systemic hypertension, it can cause hypertrophy.

Axis, Hypertrophy, and Bundle Branch Blocks: Left ventricular hypertrophy

Focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

 

Clinical Alert

Both ventricles can possess hypertrophy. When this happens, changes for left ventricular hypertrophy would be present in the chest leads as well as RAD in the limb leads.

Another feature that might present on the EKG related to right and left ventricular hypertrophy is strain. This is also known as secondary repolarization abnormality. The T wave will have a gradual dipping or downstroke and a quick upswing on the return to the isoelectric line. The T wave then demonstrates both inversion and asymmetry. This can also be seen with a depressed ST segment that is also “humped.” Right ventricular strain would obviously be present in the initial chest (V) leads, V1 and V2, while left ventricular strain would be manifested in leads V5 and V6 (Strain or secondary repolarization abnormality).

Axis, Hypertrophy, and Bundle Branch Blocks: Strain or secondary repolarization abnormality

focus topic: Axis, Hypertrophy, and Bundle Branch Blocks

Axis, Hypertrophy, and Bundle Branch Blocks

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