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The 'U' wave is a wave on an electrocardiogram. It is the successor to the 'T' wave and may not always be observed as a result of its minute (small) size. 'U' waves are thought to represent re-polarization of the Purkinje fibers. However, the exact source of the U wave remains unclear. The three most common theories for the origin are:
Contents
In a normal heart beat, the 'T' wave represents re-polarization of the ventricles, specifically the repolarisation of the AV node and bundle branches. The U wave occurs when the ECG machine picks up repolarisation of the Purkinje fibers.
Description
According to V. Gorshkov-Cantacuzene, "the U wave is the momentum carried by the blood in the coronary arteries and blood vessels". The resistivity of stationary blood is expressed as (Ht)=|Ht · (1+ αHt), where α – is a coefficient, Ht – is the hematocrit; at that time, as during acceleration of the blood flow occurs a sharp decrease in the longitudinal resistance with small relaxation times.
However, there are a number of factors affecting the resistivity of blood. Erythrocyte aggregation occurs at low shear rates and this leads to the conclusion that to all vessels (with the exception of large veins) the effect of aggregation is irrelevant. The interior of a blood vessel includes a near-wall layer of plasma (referred to as lubricant), the size of which strictly depends on the Reynold's criterion and the shear rate of the flowing blood. Given that the thickness of this layer in all blood vessels (except capillaries) is less than 5 microns, and the resistivity of the plasma is two times less than in blood, then according to the scheme of parallel insertion it is easy to estimate that the contribution of this layer to the resistivity is negligible. By reducing the speed of blood flow profiles the dependence of Ht on the radius of the vessel becomes more elongated. However, at normal values of Ht, the effect is also insignificant. With high enough shear rates, the red blood cells become susceptible to deformation. The contribution of this phenomenon is difficult to assess, because it is present in the background of all the above effects. However, even the sum of all these factors has little effect on the resistivity of blood.
From the above discussion, it follows that at the time of ejection of blood from the left ventricular part of the pulse is carried away, because there is no electrical resistivity of blood, which gradually increases high up in the coronary arteries and blood vessels. Thus, we can conclude that the U wave is the momentum carried by the blood in the coronary arteries and blood vessels. Further, it is possible to taking this momentum back to Purkinje fibers along the vessels of the myocardium. This idea is also proved by the fact that hypertrophy of the left ventricle, myocardial ischemia, coronary insufficiency have momentum there is no possibility to move on to the Purkinje fibers, therefore, the ECG recorded negative U wave.
Interpretation
According to many studies, U wave often fails to register in all leads except the V6, while most often in V2 and V3 with heart rate less than 96 beats per minute. Its amplitude often is 0.1–0.33 mV. Particularly difficult is the allocation of the boundaries of the U wave on the background of the T wave and R wave, which may partial or complete (in the case of T wave) the merger. It is shown that higher values of heart rate or hypocalcemia U wave are superimposed on the T wave and in tachycardia — merges with the R wave of the next cardiac cycle.
Prominent U waves are most often seen in hypokalemia but may be present in hypercalcemia, thyrotoxicosis, or exposure to digitalis, epinephrine, and Class 1A and 3 antiarrhythmics, as well as in congenital long QT syndrome, and in the setting of intracranial hemorrhage.
An inverted U wave may represent myocardial ischemia (and especially appears to have a high positive predictive accuracy for left anterior descending coronary artery disease ) or left ventricular volume overload.
A U-wave can sometimes be seen in normal younger, athletic individuals.