Question

Why might a condition such as cardiac hypertrophy(growth of the heart) change the mean frontal axis of the heart?

Answer 1

Cardiac hypertrophy
Thickening of the heart muscle (hypertrophy) refers to the compensatory hypertrophy of the ventricular myocardium due to physiological or pathophysiological events. Or, to put it in simpler terms: it is an increase in cardiac muscle mass when cardiac muscle fibers thicken, or cells become enlarged, due to chronic and increased stress on the heart.

Left ventricular hypertrophy-left axis deviation
Right ventricular hypertrophy- right axis deviation
Left Ventricular Hypertrophy (LVH)
General ECG features include:

≥ QRS amplitude (voltage criteria; i.e., tall R-waves in LV leads, deep S-waves in RV leads)
Delayed intrinsicoid deflection in V6 (i.e., time from QRS onset to peak R is ≥ 0.05 sec)
Widened QRS/T angle (i.e., left ventricular strain pattern, or ST-T oriented opposite to QRS direction)
Leftward shift in frontal plane QRS axis
CORNELL Voltage Criteria for LVH
(sensitivity = 22%, specificity = 95%)

S in V3 + R in aVL > 24 mm (men)
S in V3 + R in aVL > 20 mm (women)
Other Voltage Criteria for LVH:

Limb-lead voltage criteria:
R in aVL ≥ 11 mm or, if left axis deviation, R in aVL ≥ 13 mm plus S in III ≥ 15 mm
R in I + S in III > 25 mm
Chest-lead voltage criteria:
S in V1 + R in V5 or V6 ≥ 35 mm
Right Ventricular Hypertrophy
General ECG features include:

Right axis deviation (> 90 degrees)
Tall R-waves in RV leads; deep S-waves in LV leads
Slight increase in QRS duration
ST-T changes directed opposite to QRS direction (i.e., wide QRS/T angle)
May see incomplete RBBB pattern or qR pattern in V1
Specific ECG features (assumes normal calibration of 1 mV = 10 mm):

Any one or more of the following (if QRS duration < 0.12 sec):
Right axis deviation (> 90 degrees) in presence of disease capable of causing RVH
R in aVR ≥ 5 mm, or
R in aVR > Q in aVR
Any one of the following in lead V1:
R/S ratio > 1 and negative T wave
qR pattern
R gt; 6 mm, or S < 2mm, or rSR' with R' > 10 mm
Other chest lead criteria:
R in V1 + S in V5 (or V6) 10 mm
R/S ratio in V5 or V6 < 1
R in V5 or V6 < 5 mm
S in V5 or V6 > 7 mm

The electrical heart axis is an average of all depolarizations in the heart. The depolarization wave begins in the right atrium and proceeds to the left and right ventricle. Because the left ventricle wall is thicker than the right wall, the arrow indicating the direction of the depolarization wave is directed to the left.
To determine the heart axis you look at the extremity leads only (not V1-V6). If you focus especially on leads I, II, and AVF you can make a good estimate of the heart axis. An important concept in determining the heart axis is the fact that electricity going towards a lead yields a positive deflection in the electric recording of that lead. Imagine the leads as cameras looking at the heart. Lead I looks horizontally from the left side. Lead II looks from the left leg. Lead III from the right leg and lead AVF from below towards the heart. A positive deflection here is defined as the QRS having a larger 'area under the curve' above the baseline than below the baselin
Positive (the average of the QRS surface above the baseline) QRS deflection in lead I: the electrical activity is directed to the left (of the patient)
Positive QRS deflection in lead AVF: the electrical activity is directed down.
This indicates a normal heart axis. Usually, these two leads are enough to diagnose a normal heart axis! A normal heart axis is between -30 and +90 degrees.

A left heart axis is present when the QRS in lead I is positive and negative in II and AVF. (between -30 and -90 degrees)
A right heart axis is present when lead I is negative and AVF positive. (between +90 and +180)
An extreme heart axis is present when both I and AVF are negative. (axis between +180 and -90 degrees)
largest vector in the heart is from the AV-node in the direction of ventricular depolarization. Under normal circumstances, this is directed left and down.(towards leads I and AVF).
The direction of the vector can changes under different circumstances:

When the heart itself is rotated (right ventricular overload), obviously the axis turns with it.
In case of ventricular hypertrophy, the axis will deviate toward the greater electrical activity and the vector will turn toward the hypertrophied tissue.
Infarcted tissue is electrically dead. No electrical activity is registered and the QRS vector turns away from the infarcted tissue
In conduction problems, the axis deviates too. When the right ventricle depolarizes later than the left ventricle, the axis will turn to the right (RBBB). This is because the right ventricle will begin the contraction later and therefore will also finish later. In a normal situation the vector is influenced by the left ventricle, but in RBBB only the right ventricle determines it.
Left axis deviation
Causes of left axis deviation include:
Mechanical shifts, such as expiration, high diaphragm (pregnancy, ascites, abdominal tumor)
Left ventricular hypertrophy
Left bundle branch block
left anterior fascicular block
Congenital heart disease (e.g. atrial septal defect)
Emphysema
Hyperkalemia

Right axis deviation
Causes of right axis deviation include:
Mechanical shifts, such as inspiration and emphysema
Right ventricular hypertrophy
Right bundle branch block
Left posterior fascicular block
Dextrocardia
Ventricular ectopic rhythms
Preexcitation syndromes

Lateral wall myocardial infarction