Palpitations, i.e. when the heart beat becomes noticable, are quite frequently encountered and often associated with tachycardia. However, not all palpitation need to be associated with fast heart rhythms.
The ECG below was recorded from a young female adult using her smart watch. The cause for her palpitations is nicely depicted.
This is a 25mm/s strip lead I tracing. Smart watches usually record lead I, since the watch body is placed on the left arm wrist and the electrical circuit is closed using the right hand. Therefore, the recorded electrical current flow is pointed right arm to left arm, i.e. lead I of the ECG.
You can see sinus beats (upright P waves in front of every QRS complex, regular and constant PR interval of 160ms). The 5th beat looks different. QRS is wider, depolarisation is pointed into a different direction (QRS negative) and depolarization seems altered (T wave taller and steeper). Also, there is a pause following this extra beat which is twice as long as the prior R-R intervals (1.600ms vs 800ms). No P wave precedes the extra beat. This is called a premature ventricular contraction (PVC).
Several more of these PVCs are seen throughout the strip. All of them show the same morphology. This is called monomorphic PVCs, meaning they all originate from the same region of the heart (e.g. the right ventricular outflow tract, or elsewhere).
PVCs occur in healthy persons of any age, but are more prevalent in the elderly and in men. In a very significant proportion of people they occur spontaneously with no known cause.
Possible underlying causes include catecholamine excess, medications (e.g. tricyclic antidepressants), substances/drugs (caffeine, nicotine, cocaine,...), arterial hypertension, hypercalcemia, or anxiety among several others.
Depending on the "burden" (i.e. the percentage of PVCs in relation to sinus beats within 24 hours), PVCs may cause cardiomyopathy. This effect, however, seems to be more significant in adults than in pediatric patients.
Some cases may require medical therapy (usually beta blockers) or ablation therapy.