Chapter 2

Depolarization happens in many directions, but the mean vector points __________ because ____________

1. Left and inferior

2. LV dominates EKG

The direction of the mean vector is called

Mean electrical axis

Normal QRS axis is between

+90º to 0º

If the QRS complex is positive in both ___________ and __________, the QRS axis must be normal

Lead I and aVF

How to find the precise axis for an EKG wave

1. Find the limb lead where the wave is nearly biphasic,

sometimes appears isoelectric

2. Axis will be oriented 90° (perpendicular) to this lead

3. Use info from other leads to match positive direction

In R-axis deviation, the axis is between

+90º and + 180º

How will an R-axis deviation affect the QRS complex deflection in aVF and Lead I?

I → Negative

aVF → Positive

Cases for R-axis deviation

1. Right ventricular hypertrophy (RVH)

2. Chronic lung disease (COPD)

3. Lateral myocardial infarction (MI)

4. Acute RV strain (Ex: pulmonary embolism)

5. Left posterior hemiblock

Extreme R-axis deviation is called

Superior axis or northwest axis

In extreme R-axis deviation, the axis is between

+180º and -90º

How will an extreme R-axis deviation affect the QRS complex deflection in aVF and Lead I?

I → Negative

aVF → Negative

Causes for extreme R-axis deviation

1. Extreme RVH

2. Ventricular rhythms → Rhythm is not coming from the SA node

3. Hyperkalemia

In L-axis deviation, the axis is between

0º and -90º

How will an L-axis deviation affect the QRS complex deflection in aVF and Lead I?

I → Positive

aVF → Negative

Causes for L-axis deviation

1. Left ventricular hypertrophy (LVH)

2. Inferior myocardial infarction (MI)

3. Left bundle branch block (LBBB)

4. Left anterior fascicular hemiblock

Increase in cardiac muscle mass

Hypertrophy

Dilation of a cardiac chamber

Enlargement

Cardiac hypertrophy is caused by

Pressure overload

Cardiac enlargement is caused by

Volume overload

How can hypertrophy/enlargement affect the ECG?

1. Increase in amplitude

2. Increase in duration of depolarization

3. Axis deviation

Left ventricular hypertrophy can be caused by

1. Hypertension

2. Aortic stenosis

Right ventricular hypertrophy can be caused by

1. Pulmonary hypertension → Seen in COPD

2. Uncorrected congenital heart disease

What leads are used to assess atrial enlargement and why?

1. Lead II → Oriented parallel to normal atrial depolarization (positive)

2. Lead V1 → Oriented perpendicular to depolarization (biphasic)

Right atrial enlargement (RAE) is also known as

P pulmonale

How are the amplitude and duration of the wave affected by RAE?

→ The amplitude of the first portion of the P wave increases

→ No effect on duration

How is RAE diagnosed using leads?

A P wave greater than 2.5 mm (0.25 mV) in leads II, aVF, and III

Left atrial enlargement (LAE) is also known as

P mitrale

How are the amplitude and duration of the wave affected by LAE?

→ The amplitude of the second half of the wave increases

→ Duration increase

How is LAE diagnosed using leads?

V1: A terminal deflection at least 1 mm (0.1 mV) below the isoelectric line and at least 0.04 sec wide

Lead II: Increase in the amplitude of the second half of the P wave

Effects on limb leads and precordial leads due to RVH

Limb leads

→ QRS axis exceeding +100º

→ Lead I QRS must be more negative than positive

Precordial leads

→ V1 R wave > S wave (amplitude)

→ V6 S wave > R wave

Effects on limb leads and precordial leads due to LVH

Limb leads

→ aVL R-wave exceeding 11 mm (1.1 mV)

Precordial leads

→ (V5 or V6) R wave + S wave (V1 or V2) > 35 mm

→ R wave amplitude increase in leads overlying the LV

→ S wave amplitude increase in leads overlying the RV

True or false: It is Possible to have both RVH and LVH present, but the effects of the dominant LV obscure the RV

True

True or false: Secondary repolarization abnormalities are present in hypertrophy and enlargement

False

Secondary repolarization abnormalities are present only in hypertrophy

Indicators of secondary repolarization abnormalities

1. Down-sloping ST segment depression

2. T-wave inversion (i.e. T wave axis no longer aligned

with that of QRS)

RV repolarization abnormalities can be seen in leads

V1 and V2

LV repolarization abnormalities can be seen in leads

I, avL, V5, and V6

What ECG pattern is associated with secondary repolarization abnormalities, showing gradual descent followed by abrupt ascent (asymmetric)?

Strain pattern