Dear Rodney,First, please do not confuse heart-rate variability and vagal tone. They are different things, although under very circumscribed conditions, the former is sometimes a reasonably accurate marker of the latter (and only cardiac vagal tone, not general vagal tone, at that); please look at my comments and other ResearchGate traces for clarification.  Secondly, can you more precisely define what you mean by beat-to-beat latency? It is true that cardiac parasympathetic effects upon the heart are much quicker than sympathetically, but this is the basis for HRV respiratory sinus arrhythmia spectral calculations.