Assessing a 24 hour ECG tracing of a 12-year old girl you find the following:
The automatic interpretation algorithm reports this as a couplet of premature atrial complexes (PACs). You can spot the annotations at the bottom of the picture stating "N" for normal beat and "S" for supraventricular beat. Also, the timing of the two beats labeled as "S" is definitely earlier - 300ms S to S compared to 520ms N to N.
However, looking closer at the leads you find a perfect sinus rhythm maintained throughout all the tracing in lead 2! The "PACs" only appear in leads 1 &3, thus representing artifacts. The actual diagnosis is regular sinus rhythm, no extrasystoles.
Next you take a look at this tracing from an implantable loop recorder (ILR), which states "Tachycardia", HR 250 bpm...
Now, you do see the annotations at the bottom stating "FS" for Fast Sense witch a cycle lengths of 220-250ms. That would support the automatic annotation of Tachy around 250 bpm. However, looking at the ECG tracing you find quite small (low amplitude) narrow complexes followed by larger (higher amplitude) and wider complexes. The latter actually represent the T waves. The smaller signals represent the QRS complexes. This becomes more clear at the end of the tracing where QRS complexes gain amplitude and become clearly visible while T waves remain more or less the same.
This phenomenon tends to appear in ILR tracings when the device is placed in rather soft tissue that allows for movement of the device as the patient moves (e.g. raises his/her arms).
The correct diagnosis therefore is sinus rhythm at a heart rate of about 125 bpm. Heart rate would fit for physical activity, as in this case the patient was doing walking sports. The "FS" is due to T wave oversensing due to low amplitude QRS complexes.
For those concerned, T wave oversensing is not a true artifact of course.
Another ILR tracing with tachycardia detection, HR about 270 bpm...
Again, the annotations state "FS" with irregular cycle lengths of 190 to 250ms. Looking at the ECG tracing you notice some quite sharp high frequency signals that seem to be interpreted as beats by the device. However, you can circle through the regular QRS complexes at the beginning and end of the tracing to find the "true" sinus beats (green). The other signals are artifacts (red), most likely caused by muscular interference.
At the next inpatient appointment the signal artifacts were indeed reproducible with exercise.