Which wave is typically not always visible in an ECG?

The U wave is considered the wave that is not always visible on an ECG. While the P wave, QRS complex, and T wave are fundamental components of the cardiac cycle and are typically present in a normal ECG tracing, the U wave is less consistently observed. It generally appears after the T wave and is associated with the repolarization of the Purkinje fibers or the ventricular myocardium. The visibility of the U wave can vary significantly among individuals; it may be absent entirely or may appear only under particular conditions, such as in certain electrolyte imbalances or in specific cardiac conditions.

The presence or absence of the U wave can also be influenced by various factors, including heart rate and the overall health of the heart. Therefore, it's essential for healthcare professionals to recognize that while the first three waves are standard in an ECG reading, the U wave may not always be evident, highlighting its variability and the need for careful interpretation of ECGs.