- EKG Fundamentals
- Reading EKGs
Hypertrophy refers to an increase in size of a cell, leading to a greater cell mass. In the heart, this primarily occurs secondary to increases in pressure or volume. Hypertrophy differs slightly from hyperplasia, which refers to an increase in the number of cells in an organ. As a terminally differentiated end organ, the heart can not undergo significant hyperplasia as a response to increases in work. Instead, the heart relies on hypertrophy, where it can increase up to to four times the normal size. After completing the section on axis, this should help reinforce how hypertrophy can shift the electrical axis of the heart.
There are two patterns of hypertrophy within the heart, concentric hypertrophy and eccentric hypertrophy. As a response to increases in pressure, such as a stenotic valve, additional contractile elements are recruited and added in parallel. The end result is an increase in wall thickness. Increases in chamber volume, often caused by leaky valves, results in chamber dilation through eccentric hypertrophy, where additional contractile elements are added in series. The end result is an increase in chamber size with relative preservation of wall thickness.
Both atrial and ventricular chambers can undergo hypertrophy, and both can be identified on EKG. This is an important diagnosis to make, as lifestyle changes and pharmacological interventions can reverse hypertrophic changes and the associated morbidity and mortality.
Depolarization is initiated in the SA-node, and transmitted through the RA through the three Internodal tracts. In addition, Bachmann's Bundle carries the wave of depolarization to the LA. Right atrial activation precedes left atrial activation, however on the EKG, a single fused P-wave is typically seen. The first half of the P-wave represents Right atrial activation and the second half of the P-wave represents Left atrial activation.
To analyze P-waves, focus primarily on Leads II and V1. Lead II is parallel to the atrial axis, and as such, has an upright P-wave. Lead V1 rests over the atrium, with the axis running perpendicular to the electrode, resulting in a biphasic P-wave. In any lead, the duration of the P-wave should be less than 0.12s (3 small boxes) and the amplitude of the largest deflection (positive or negative) should be less than 2.5mm.
Atrial enlargement often results in a disruption or slowing of conduction. However, because the right atrium is depolarized earlier than the left atrium, this slowing does not alter the total duration of the P-wave, which is often determined by left atrial activation. Instead, the delayed activation usually results in a summation of the left and right atrium as the two chambers depolarize simultaneously. The net result is that Right atrial enlargement causes an increase in amplitude > 2.5mm in a limb lead. In lead II, you can see a tall, often peaked p-wave, while the first portion of V1 is often taller than the second negative deflection representing the left atrium.
As the atrium expands and conduction is disrupted, P-waves increase in duration. Instead of one smooth wave in lead II, you may see two notches (double peaks) in the wave. A P-wave duration greater than 0.12s is suggestive of left atrial enlargement. Additionally, a downward deflection greater than 1mm or longer than 0.04mm (1box) in lead V1 is also suggestive of left atrial enlargement.
With atrial hypertrophy, understanding the conduction pattern and timing was the key to predicting EKG changes. With ventricular hypertrophy, the anatomic relationships between the anterior chest leads and the ventricles will be the most important piece of information.
The anterior surface of the heart is predominately right ventricle, while a portion of the anterior surface and most of the left border is composed of left ventricle. As a result, we will spend most of the time focused on the anterior chest leads, reserving the limb leads to check for an axis deviation.
Leads V1 and V2 are closest to the right ventricle, while leads V5 and V6 are closest to the left ventricle. Based on what we have learned about axis, it should come as no surprise that most of the electrical forces are oriented towards the LV. As a result, the positive R-wave traveling towards V1 is relatively small, while the negative S-wave indicating electrical flow away from the RV is much larger. The reverse is true in leads V5 and V6. The R-wave traveling towards the lateral leads is larger than the S-wave, indicating that this is the dominant area of ventricular depolarization.
Despite the basic theory outlined above, left ventricular hypertrophy (LVH) is a difficult condition to accurately diagnose on EKG. There are numerous voltage criteria available, each with varying specificity and sensitivity. In general, all of the different criteria have a poor sensitivity, often less than 50%. However, the specificity is frequently high, often greater than 85%. Due to this low sensitivity and high specificity, applying multiple criteria to an EKG may increase the detection rate. When using multiple criteria only one must be positive to be suggestive of LVH.
Some of the commonly used voltage criteria include:
Please note that the voltage criteria should be applied to adults over the age of 35. There are many factors that can alter voltage readings on an EKG. Some common variables include age, weight, gender, and race. With adults, voltage decreases with advancing age, men often have higher QRS voltage readings than women, obesity distorts EKG readings, and ethnicity appears to alter specificity and sensitivity for individual criteria.
In addition to the voltage criteria, LVH is often associated with additional conduction abnormalities. Left axis deviation is a common finding in LVH, although it is not diagnostic. In addition, LVH may lead to distortion and disruption of the conduction system, creating a widened QRS complex and an incomplete left bundle branch block. Another finding that is suggestive but not diagnostic is a slightly prolonged QT-interval.
With right ventricular hypertrophy (RVH) the pattern of R-wave progression in reversed. Now, the right ventricle is the dominant area, resulting in an increased magnitude of depolarization. In lead V1, the R-wave is greater than the S-wave while in V6, the S-wave is greater than the R-wave.
When looking at the limb leads you can see right axis deviation, which is manifest by a negative QRS complex in lead I.
Hypertrophy does not need to be limited to a single chamber, and frequently is not. Unfortunately, there is not a single way to diagnose biventricular hypertrophy (BVH). Instead, you may see a combination of LVH and RVH criteria intermingled. Some common patterns include right axis deviation with LVH criteria or very tall biphasic QRS complexes in multiple leads.
A common finding in ventricular hypertrophy is an alteration in repolarization. Previously, these alterations was believed to be the result of increased work, and was described as a “strain pattern”. However, that theory has fallen out of favor, and with it, the old terminology. Today, these repolarization abnormalities are referred to as secondary ST-T abnormalities or repolarization abnormalities.
These changes in repolarization effect the ST-segment and T-waves, and are characterized by:
Repolarization abnormalities are typically found in leads that are closest to the hypertrophic tissue. With LVH repolarization abnormalities are most common in V5 and V6, while RVH is more likely to have changes in V1 and V2.
After reviewing the section on ischemia, take a moment to review the section on secondary repolarization abnormalities. Until you compare them side-by-side, it is difficult to differentiate the two types of T-wave inversion and ST-depression.