Heart Electrical Axis (MEA)

Estimated reading time: 5 minute

What is the Mean Electrical Axis (MEA) of the heart?

The electrical axis of the heart, or cardiac axis (MEA), is a concept that describes the direction of net ventricular depolarization of the heart. This refers to the direction in which the electrical activity of the heart propagates in a two-dimensional plane (the frontal plane). This direction is usually left-right ve head-tail are evaluated on the axes.

The normal electrical activity of the heart spreads in an orderly manner throughout the heart muscle, starting from the right atrioventricular node (SA node). During this spread, the electrical axis of the heart is in a specific direction. However, ventricular enlargement (hypertrophy) or ventricular conduction abnormalities Conditions such as (bundle branch block, ectopic focus) may cause this axis to deviate from its normal direction.

---

What are the methods for determining MEA?

There are three primary methods of measuring or estimating MEA. Each of these methods uses only the 6 standard limb leads (I, II, III, aVR, aVL, aVF).

1. Graphic Method (Most Precise But Most Time-consuming Method)

In this method, two derivations (usually Leads I and aVF) to calculate the MEA.

Steps:

1. Determine the net positive or negative deviation for each lead.

• Measure the sum of the positive and negative components of the QRS complex. (For example, if the R wave is +5 mm and the S wave is -2 mm, the net deflection is +3 mm.)

2. Plot these deviations on an axis chart.

• Plot the deviation in Lead I on the horizontal line on the axis chart.

• Plot the deviation in aVF on the vertical line on the axis graph.

3. Draw perpendicular lines through these deviations and find the point of intersection.

4. Draw a line from this intersection to the center of the graph.

• The angle formed by this line gives the MEA.

This method gives the most accurate results, but is time consuming as it requires more calculations and drawings.

2. Use the Lead with the Highest Clear QRS Deviation

In this method, to determine the MEA, the maximum R wave height has a derivation.

Steps:

1. Examine the QRS complexes in the 6 limb leads.

2. Identify in which lead the R wave is highest.

3. The axis of this derivation is generally parallel to the MEA.

This method is faster because you only need to compare the height of the QRS complexes. However, it may be less sensitive.

3. Using the Most Isoelectric Derivative

In this method, to find MEA the lead in which positive and negative QRS deflections are equal located.

Steps:

1. Examine the QRS complexes in the 6 limb leads.

2. Find in which lead the positive and negative QRS components are closest to each other.

3. The MEA will be parallel to the axis perpendicular to this lead.

4. To determine if the MEA is positive or negative, look for the QRS on this vertical axis to be positive or negative.

This method is used to rapidly detect ventricular axis changes.

---

What are the reference ranges for MEA deviations for cats and dogs?

The normal range of MEA in humans is generally -30° to +90° However, these limits are different in dogs and cats.

• MEA in cats: Generally 0° to +160° is accepted between.

• MEA in dogs: It may vary depending on the breed and body type of the dog, but generally +40° to +100° It is accepted between. This range may vary in large breed dogs.

---

Why do we need to calculate MEA?

Knowing the MEA helps diagnose many heart conditions. The following conditions can cause misalignment:

• Ventricular hypertrophyIncreased ventricular muscle mass shifts the electrical axis toward hypertrophy.

• Ventricular conduction abnormalitiesConduction disorders such as left bundle branch block (LBBB) or right bundle branch block (RBBB) alter the electrical propagation between the ventricles, causing the axis to shift.

• Ectopic rhythmsAbnormal impulses from the ventricles, such as ventricular tachycardia, can change the axis.

---

MEA Changes and Reasons

1. Left Axis Deviation

• Possible Causes:

• Left ventricular hypertrophy: An increase in left ventricular muscle mass causes the electrical axis to shift to the left.

• Left anterior fascicular block: A blockage in the conduction pathway of the electrical impulse in the heart causes the axis to shift to the left.

• Rhythms originating from ectopic fociElectrical impulses from an abnormal focus outside the ventricles may cause the axis to shift to the left.

2. Right Axis Deviation

• Possible Causes:

• Right ventricular hypertrophy: Increased right ventricular muscle mass causes the electrical axis to shift to the right.

• Right bundle branch block: A block in the conduction of electrical impulses in the right ventricle causes the electrical axis to shift to the right.

• Pulmonary hypertensionIncreased pressure in the pulmonary vessels can cause the right ventricle to enlarge, shifting the electrical axis to the right.

• Ventricular tachycardia: An abnormal electrical impulse originating from the ventricles may cause the axis to shift to the right.

3. Flat Axis (Axis Close to 0°)

• Possible Causes:

• Ventricular hypertrophyWhen both ventricles are equally enlarged, the electrical axis is not shifted exactly to one side and the axis may remain close to 0°.

• Ventricular conduction disordersWhen there are conduction abnormalities in both ventricles, the electrical axis may remain in a neutral position.

These changes may indicate many cardiac conditions, including increases in heart muscle mass (hypertrophy), conduction system abnormalities (bundle-branch block), and abnormal impulses originating from the ventricles (ventricular tachycardia). Accurate evaluation of the MEA is important in the diagnosis of these diseases.

---

---

---

---

---

Other Articles You May Be Interested In

• Electrical Activity of the Heart
• Determining Heart Rate and Rhythm
• Loss of Appetite (Anorexia) and Its Causes
• Cardiac Arrhythmias in Cats and Dogs
• Tetralogy of Fallot (TOF)

---

“The only impossible journey is the one you never start.”

Tony Robbins

---