The electrocardiogram (ECG) provides a non-invasive visual representation of the heart’s electrical activity. This diagnostic tool records the sequence of depolarization and repolarization that occurs with each heartbeat, translating these events into a distinct pattern of waves, segments, and intervals. While most people are familiar with the main deflections (P wave, QRS complex, and T wave), the U wave is a less frequently observed component. This small, subtle deflection appears late in the cardiac cycle, representing a final, minor electrical event following the main ventricular repolarization. Changes in the U wave’s appearance can indicate underlying physiological or pathological changes in the heart.
Identifying the U Wave on an ECG
The U wave is a gentle, rounded deflection appearing immediately after the T wave and before the P wave of the next cardiac cycle. Its position marks the very end of ventricular repolarization and the transition into the heart’s resting phase. A normal U wave is quite small, typically measuring less than one millimeter in height and rarely exceeding 25% of the preceding T wave’s height. Due to its minimal size, the U wave is often undetectable or absent in many healthy individuals. When visible in a normal heart, it is usually upright, follows the same direction as the T wave, and is most consistently observed in the mid-precordial leads (V2 and V3).
The Physiological Basis for the U Wave
The precise cellular origin of the U wave remains a subject of ongoing debate among electrophysiologists. Several theories exist, including the delayed repolarization of the specialized His-Purkinje fiber network. These fibers distribute the electrical impulse throughout the ventricular muscle, and their slow return to the resting state may generate the late electrical signal. Another theory suggests the U wave originates from mid-myocardial M-cells, which repolarize later than the bulk of the ventricular muscle. Finally, some suggest the U wave is a mechanoelectrical phenomenon, where after-potentials are generated by mechanical forces, such as the stretching of the ventricular wall during filling. The consensus leans toward the U wave representing a final, heterogeneous phase of ventricular repolarization.
Conditions Associated with Prominent U Waves
Prominent U Waves and Hypokalemia
When the U wave becomes unusually large, or prominent, it serves as a significant clinical marker. A prominent U wave is defined as one taller than 1–2 millimeters or exceeding 25% of the T wave amplitude. The most frequent cause is hypokalemia (low serum potassium). As potassium levels drop, the U wave prominence increases, often accompanied by decreased T wave amplitude and ST-segment depression. In severe hypokalemia (below 3 mEq/L), the U wave can become taller than the T wave. This change is a classic ECG finding indicating a potentially life-threatening electrolyte imbalance.
Other Causes of Prominence
The U wave’s size is inversely related to heart rate; bradycardia (slow heart rate) naturally makes the U wave more prominent. This occurs because longer pauses between heartbeats allow the electrical activity to fully develop. Certain medications, notably antiarrhythmic drugs like quinidine and digitalis preparations, can also induce or amplify a prominent U wave. Prominent U waves can also be seen in structural heart conditions, such as left ventricular hypertrophy, and in states of increased nervous system activity.
Inverted and Fused U Waves
An inverted U wave is a different, highly specific abnormality suggesting underlying heart disease. It often indicates myocardial ischemia, or lack of blood flow to the heart muscle, and may be an early sign of coronary artery disease. In severe cases, the T wave and the prominent U wave can merge, creating a large, continuous T-U fusion wave. This fusion can obscure the true end of the T wave, giving the false appearance of a prolonged QT interval, a finding often seen in severe hypokalemia or inherited long QT syndromes.