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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?

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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.

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