The U waves ! On ECG

 

The U waves ! 

Generally Not seen or difficult to be seen on surface However Whenever seeable , the following needs to be known 

In normality , the following will be its like morphological appearances ! 

a. small, rounded deflection just after the T! 

b.low amplitude , less than one fourth of the height of the preceding T wave 

c.Usually same polarity as the before T however Negative U can appear, even With upright T !!!

d. usually they will be either a monophasic positive or negative deflection, however sometimes they can be diphasic (positive-negative & negative-positive).

      

Best surface ecg leads to visualise 

a. frequently absent in the limb leads and mostly easily seeable in chest leads V2 and V3. 

b. As U vector is aligned similarly to the T vector ! So the normal U wave will sometimes be negative in limb leads III , AVF and isoelectric in leads I, aVR, and aVL! .

c. better visualized in the precordial/ chest leads! timing of the U will however remain same in all leads. 

d. the increase in amplitude will be a reflect of the overall increase in ECG amplitude. 

Relation of U amplitude to the heart rate

Generally amplitude is usually less than 0.2 mV but this will be strongly heart rate dependent. 

a.U can be easily seen in more than 90 percent of cases when the heart rate is less than 65 beats/min. 

b. Becomes less seeable in about two thirds of cases when the heart rate was 80 to 95 beats/min

c. and even very less easily seeable when the heart rate raises to 80 to 95 beats/min. 

d. And almost non seeable Ldr the heart rate exceeds 95 beats/min.

Trick / clue to still try to see U !! 

A very low-amplitude U wave during rapid heart rate can still be seeable by enlarging/ zooming the tracing.

Abnormal U waves !!

Alarm be raised when the U is either inverted, or when it is merged with the T ,or when its amplitude is greater than that of the T ! 

An abnormal U will often be quite subtle and will very rarely be an isolated ECG finding ! Hence easily overlooked by ECG readers other than keen eyes of a cardiac electrophysiologist !

An inverted U in leads V2 to V 5 will be abnormal !! This may appear transiently during acute Ischemia or in the presence of significant hypertension !

Sometimes the amplitude of the U wave may become more like In Hypokalemia Or with use of Some anti arrhythmic drugs or In some LQ T types ! 

Difficulty in differentiation from second Peak of T 

In some cases it is difficult to differentiate the U from the second peak of a notched T wave. 

Various models have been proposed to differentiate however still its very difficult 

Mechanism of causation ! Origin not yet clear and so is the Significance which 

its not exactly known.

theory 1 ! The U may reflect the relatively late repolriazation process of His-Purkinje cells and certain LV myocytes . 

Theory 2. This may be a mechanoelectric phenomenon

Theory 3. M cells due to their delayed repolarization properties and large mass within the ventricles! 

However still to my knowledge no definitive single cause.

Thanks