Hi Malcolm, i get a few of these and for me they seem to coincide with working fairly hard on intervals, and on occasions i may feel a palpitation – and more often than not, i see the marker symbol on the HR graph. If i click on that point, it also then lines up the 20 sec ECG trace,and the unusual heart beat tends to stand out – like the example below at the 11 sec point of the trace approx ie