One quantification of the heart rate variability of an ECG is pNNx, i.e. the proportion of consecutive NN distances that differ by at least 50 ms. When a premature beat occurs, the preceding and following RR intervals are much shorter and longer than the mean. Are these intervals considered NN for the purpose of calculating the pNN50?
No, the beats originating outside of the sinoatrial node are not considered for pNN50. Moreover, this metric cannot be applied to the rhythm featuring any type of ectopic or non-sinoatrial activity.
NN50 has its name from an acronym "normal-to-normal", this acronym is used instead of RR ("from R to R") to emphasize that only the normal, e.g. sinoatrial beats are considered for evaluation.