U-wave

ECG: Sinus rhythm with a rate of 60 bpm; normal P-wave: positive in leads I, II, V5, V6 and negative in aVR, with no sign of atrial enlargement; normal PR-interval; narrow QRS (98 ms); wide U-wave, maximal in V4 to V5, positive in all leads except for aVR;

ECG 2: 17-year-old adolescent with no prior history, asymptomatic;

ECG 2: This tracing shows an irregular atrial sinus activity with successive accelerations and slowdowns; probable physiological sinus arrhythmia in an adolescent patient; a supplementary positive deflection of low voltage (< 1 to 2 mm) and maximum in V2, V3, V4 is noted following the T-wave and after a return to the isoelectric line; this is a physiological U-wave without pathological significance;

Comments: On a normal electrocardiogram, an additional deflection is often seen at the end of repolarization. The U-wave is an electrical signal of low amplitude and low frequency which occurs after the T-wave. Different hypotheses have been proposed to explain the presence of this wave although its origin remains controversial. It can be observed in a healthy heart with well-defined normality criteria as well as in a number of diseases with evidence of prominent or reversed U-waves.

• A physiological U-wave displays certain features: it immediately follows the T-wave after a brief return to the isoelectric line and is of short duration (between 160 and 200 ms); it is generally asymmetrical and of small amplitude; it does not exceed 1 to 2 mm and is generally proportional to the amplitude of the preceding T-wave (5 to 25% on average of the height of the T-wave); a physiological U-wave can be identified in all leads, but is essentially visible in precordial and especially mid-precordial (V2-V3) leads; the U-wave is generally of the same polarity as the T-wave and is therefore positive in the majority of the leads except for aVR and occasionally for V1; the presence of a U-wave is more frequent after a slowing of the heart rate; it often has the same polarity as the P-wave and, in some leads, it can be mistaken for a P-wave and lead to the misdiagnosis of a 2/1 atrio-ventricular block;
• The origin of the physiological U-wave remains a subject of debate; various hypotheses have been proposed: 1) delayed repolarization of the Purkinje cell network; it would indeed appear that the duration of repolarization of Purkinje cells is longer than that of ventricular myocardial cells; this is the oldest but likely the least probable hypothesis, the repolarization of these cells being an integral component of the T-wave and therefore unable to generate this additional wave; 2) prolonged repolarization of a specific group of cells, the M (midmyocardium) cells, which are located in the subepicardial third of the ventricular wall; this hypothesis appears equally improbable; 3) post-potentials generated by a mechanical factor corresponding to myocardial relaxation; these low-amplitude potentials identified on the electrocardiogram thus correspond to a delayed post-depolarization generated in diastole; this is the most probable hypothesis;

An abnormal U-wave can be demonstrated in different diseases with 2 types of patterns: a prominent (large amplitude) U-wave and a negative inverted U-wave.

•  Positive and prominent U-wave: this pattern is relatively frequent in the context of a slowing of the heart rate and a bradycardia; indeed, there appears to be an inverse relationship between heart rate and U-wave amplitude (the amplitude increases with the decrease in heart rate); a highly prominent U-wave should invariably evoke a hypokalemia which is a relatively frequent cause (association between ST segment depression, weak T-wave amplitude, very tall U-wave); a prominent U-wave can also be observed in patients with left or right ventricular hypertrophy, hypothyroidism, metabolic disorder (hypocalcemia, hypomagnesemia), mitral valve prolapse (rare cases described), hypothermia, increased intracranial pressure or receiving antiarrhythmic therapy (quinidine, amiodarone, sotalol); it should be noted that the presence of a prominent U-wave and an alternation of the U-wave can be observed immediately before the occurrence of a torsade de pointes;
• Negative U-wave: this may represent a criterion of myocardial ischemia which sometimes appears very early in the context of a severe lesion of the left coronary trunk or proximal LAD; a U-wave can also be observed during an episode of Prinzmetal’s angina; other non-ischemic causes can be associated with a negative U-wave: right or left ventricular hypertrophy (negative U-wave often seen in leads with tall R wave), aortic valve diseases, congenital heart diseases, hyperthyroidism;

Take-home message: A positive, low amplitude U-wave is most often physiological. On the other hand, the presence of a prominent U-wave should evoke bradycardia or hypokalemia, while the presence of a negative T-wave is evocative of an ischemic origin.