  Human heart rate is known to display complex fluctuations. Evidence of
multifractality in heart rate fluctuations in healthy state has been reported
[Ivanov et al., Nature {\bf 399}, 461 (1999)]. This multifractal character
could be manifested as a dependence on scale or beat number of the probability
density functions (PDFs) of the heart rate increments. On the other hand, scale
invariance has been recently reported in a detrended analysis of healthy heart
rate increments [Kiyono et al., Phys. Rev. Lett. {\bf 93}, 178103 (2004)]. In
this paper, we resolve this paradox by clarifying that the scale invariance
reported is actually exhibited by the PDFs of the sum of detrended healthy
heartbeat intervals taken over different number of beats, and demonstrating
that the PDFs of detrended healthy heart rate increments are scale dependent.
Our work also establishes that this scale invariance is a general feature of
human heartbeat dynamics, which is shared by heart rate fluctuations in both
healthy and pathological states.
