Assessment of the cardiovascular and gastrointestinal autonomic complications of diabetes

Table 2 Time domain analysis variables and their physiological relevance

Variable (units)	Description	Physiological relevance
SDNN (ms)	Standard deviation of the normal RR (NN) interval reflecting all of the cyclic components responsible for variability in the period of recording	An overall estimate of HRV, but does not indicate the contribution of any particular influence
SDANN (ms)	Standard deviation of the averages of NN intervals calculated over a short period of time, usually less than five minutes	Reflects the influence of circadian rhythms on autonomic function
pNN50 (%)	The proportion of NN intervals having a difference of > 50 ms	Reflects predominant vagal influence on variability
Triangular index (ms)	The integration of the density distribution of all the NN intervals as a function of the maximum density	Overall estimate of HRV similar to SDNN
RMSSD (m/s)	The square root of the means squared differences in successive NN intervals	Estimate of the short-term components of HRV

SDNN: Standard deviation of normal-to-normal; NN: Normal-to-normal; HRV: Heart rate variability.