Large-magnitude, transient, bradycardic events in rabbits.

We propose that heart period sequences are organized similarly to sentences, with a lexicon of recurrent, similarly shaped words. These words should fulfill four criteria: universality, nonrandomness, central statistical tendencies, and specific associated physiology. Here we describe a large-magnitude, transient bradycardia (LMTB) and assess whether it constitutes a word. LMTBs were seen in 11 of 12 adult female rabbits. All shape parameters were different than those of the beat-randomized and phase-randomized surrogate sequences ( P < 0.05-0.001). LMTBs were 8.4 ± 2.9 beats and 2.64 ± 0.87 s long and were characterized by bradycardia of 77 ± 49 ms over 1.09 ± 0.49 s with a recovery to baseline over 1.56 ± 0.61 s. The LMTBs had a slower recovery than onset in 9 of 11 rabbits and were highly peaked in 10 of 11 rabbits ( P< 0.05). Scalar, magnitude, and shape parameters had values with central statistical tendencies. About 76% of LMTBs were accompanied by hypotension (mean -6.1 ± 3.9 mmHg) that lagged 2 beats behind the onset of the bradycardia and that correlated with the bradycardia (-10.5 ± 4.1 ms/mmHg). Thus transient bradycardic events are a distinct "word" in the lexicon of heart rate variability.