Left ventricular hypertrophy (LVH) describes a thickening of the muscular wall of the heart’s main pumping chamber, the left ventricle, which is responsible for pumping oxygenated blood to the body. A 12-lead electrocardiogram (ECG) is a standard test that records the heart’s electrical activity from twelve different perspectives. By capturing the electrical impulses that travel through the heart muscle with each beat, an ECG provides information about heart rate, rhythm, and the health of the cardiac muscle, including indirect evidence of muscle thickening.
The Electrical Impact of an Enlarged Left Ventricle
An enlarged left ventricle alters an ECG because of changes in how electricity travels through the larger muscle. A primary consequence of LVH is an increase in the mass of the ventricular muscle. This greater amount of tissue generates more electricity during each heartbeat, which is visualized on an ECG as waves with higher amplitude. This is seen particularly in the R waves in leads overlying the left ventricle and S waves in leads over the right ventricle.
The increased thickness of the ventricular wall also affects the speed of the electrical signal. It takes longer for the wave of depolarization, the electrical event that triggers muscle contraction, to travel through the hypertrophied muscle. This delay can result in a widening of the QRS complex, which represents ventricular depolarization. The R wave peak time, the duration from the beginning of the QRS to the peak of the R wave, may also be prolonged in leads V5 and V6.
The process of repolarization, when heart muscle cells electrically reset after contraction, is also disturbed. This is due to the mechanical stress and altered blood flow in the thickened muscle. These changes manifest on the ECG as abnormalities in the ST segment and T wave, often called a “strain” pattern.
Voltage Criteria for Diagnosing LVH
Several voltage-based criteria have been developed to standardize the diagnosis of LVH on an ECG. One of the most widely recognized is the Sokolow-Lyon index. This criterion involves measuring the depth of the S wave in lead V1 and adding it to the height of the R wave in either lead V5 or V6. If the combined sum is greater than 35 millimeters (mm), it is considered a positive indicator for LVH.
The Sokolow-Lyon index focuses on the horizontal plane of the heart’s electrical forces. The S wave in V1 reflects the electrical pull of the enlarged left ventricle moving away from the right side of the heart. Conversely, the tall R wave in V5 or V6 represents the strong electrical force moving towards the left side.
Another commonly used set of criteria is the Cornell voltage criteria, which has different thresholds for men and women. For men, the diagnosis is suggested if the sum of the S wave in lead V3 and the R wave in lead aVL is greater than 28 mm. For women, the threshold is lower, with a sum greater than 20 mm indicating LVH. The Cornell criteria are considered by some to be more accurate in certain populations than the Sokolow-Lyon index.
Other voltage criteria exist, such as an R wave in lead aVL greater than 11 mm used as a standalone marker. Similarly, an R wave in lead I plus the S wave in lead III totaling more than 25 mm can also suggest LVH. Each of these criteria attempts to quantify the increased electrical forces generated by the thickened left ventricular muscle.
Non-Voltage ECG Findings in LVH
Beyond voltage measurements, certain patterns on the ECG provide supporting evidence for LVH. A primary finding is the “left ventricular strain” pattern, characterized by ST segment depression and T wave inversion in the lateral leads such as I, aVL, V5, and V6. This pattern signifies that the thickened heart muscle is not repolarizing normally, a consequence of the increased workload and potential decrease in blood supply to the inner layers of the heart wall.
The heart’s mean electrical axis can also shift due to the increased muscle mass on the left side. This finding, known as left axis deviation, means the general direction of electrical depolarization is more leftward than normal. It reflects the electrical dominance of the hypertrophied left ventricle.
The strain placed on the left ventricle can have a backward effect on the left atrium. The stiff, hypertrophied ventricle makes it harder for the left atrium to empty, causing pressure to rise and the atrium to enlarge. This left atrial enlargement can be detected on an ECG by a change in the P wave, which represents atrial depolarization. The P wave may become broader or notched, a pattern sometimes called “P mitrale.”
Limitations and Diagnostic Confirmation
While the ECG is a useful screening tool, it has limitations in diagnosing LVH. The various criteria have low sensitivity, meaning they can fail to detect LVH even when it is present. The Sokolow-Lyon index, for example, has a sensitivity of only about 20%. Specificity, the ability to correctly identify those without the condition, is higher, but false positives can still occur.
Certain factors can produce ECG patterns that mimic LVH in healthy individuals. For instance, young, thin, and athletic individuals often have ECGs with high QRS voltages because their chest walls are thinner, allowing a stronger signal. This physiological LVH seen in athletes is a normal adaptation to training and is different from the pathological hypertrophy caused by conditions like high blood pressure. This context is important to avoid misinterpretation.
Given the ECG’s limitations, a definitive diagnosis of LVH requires direct visualization of the heart muscle. The standard method for confirmation is an echocardiogram, which uses ultrasound to create detailed images of the heart. This allows a physician to measure the thickness of the left ventricular walls. If an ECG suggests LVH, an echocardiogram is the next step to confirm the diagnosis and guide treatment.