This vignette gives descriptions and references regarding the scientific background of the package. RHRT includes the possibility for non-standard methodology in order to further analyse HRT and its optimal application. The respective sources and explanations of these variations can be found here.
To ensure snippets free of any bias and containing effective VPCs, the VPCSs are filtered based on their interval lengths. The first publication to mention filter rules was Grimm et al.. With little variations these are the criteria that are used in the package as possible VPCSs are only saved as HRT objects if they match the following criteria:
Filter rules for CPI and CMI:
Both intervals are compared to the reference interval
(RFI). This interval is calculated as the mean of the preceding
intervals before the coupling interval.
Filter rules for regular intervals:
How many preceding and following intervals of CPI and CMI are checked
is based on numPreRRs and numPostRRs of
vectorToHRT. The default is 5 and 15, respectively. If any
of the intervals do not fit the rules, the complete set is
neglected.
HRT is influenced by the heart rate. While there is no clear
conclusion for TO, TS values clearly positively correlate with the RR
interval length (reviewed in Blesius
et al. 2020). Therefore, RHRT calculates nTS that is
normalised to a fixed interval length (800 ms per default) in addition
to the common TS.
Beside the heart rate, TS is biased by the number of HRTs used to calculate it (reviewed in Blesius et al. 2020). While physiological reasons were suggested for this phenomenon (Cygankiewicz et al. 2004 and Chen 2009), Hallstrom et al. 2004 reasoned it to be a mathematically induced relation based on the number of VPCSs as well as the number of postRRs to determine TS. They proposed a method to normalise TS in which, firstly, TS is normalised to a HR of 75 bpm (which is 800 ms interval length). Here, it makes no mathematical difference whether TS is normalised or the intervals themselves before assessing TS. Secondly, the following formula is used:
nTS = TS - ( 0.02475 * (numPostRRs-2)^0.9449 * (RMSSD / √#VPCSs) )
RHRT uses this normalisation per default. This can be changed with
the boolean parameter normHallstrom in
vectorToHRT and calcAvHRT.
The HRT parameter values pre se do not give any information about 1) how many VPCSs have been used to determine them and 2) how reliable the values are. However, two identical values are inherently different if one is calculated from a VPCS with a highly varying values and the other from a high amount of VPCS with hardly any variation. Still, HRT classification generally does not take this into account.
RHRT implements a reliability check to give the opportunity to only
use HRT parameter values that are reliable to a desired extent. This
check consists of a one-sided t-test (t.test of the stats
package) off all separate values against the respective cut-off of the
parameter. The resulting p-value implicates the possibility of the
classification being true based on being the combination of average and
variability of the parameter values and therefore the reliability of the
averaged value.
These t-tests are being done automatically during
calcAvHRT which is called by vectorToHRT. The
default values of the cut-offs are 0 for TO, 2.5 for
TS as well as nTS and 10 for TT.
getResults returns the results if reliable. However, it
returns all results ignoring the reliability check via the boolean
parameter safe and changes the p-value cut-off with
pmax (0.05 per default).
Keep in mind that the parameter value cut-offs coTO,
coTS and coTT are only used to compare the
values and classify them. They are not related to the identically named
parameters of calcAvHRT that are used for the t-tests.
The order in which the HRT parameters are calculated has an impact on
the resulting values (Chen
2011). Though Schmidt
et al. 1999 proposed to first calculate an averaged tachogram and
determine TS then and for TO to first assess it from the separate VPCSs
and average the results afterwards, the order gets switched in some
studies as reviewed in Blesius
et al. 2020. Therefore, RHRT gives the opportunity to change the
calculation order for TO and TS through the parameters orTO
and orTS of calcAvHRT. By default the order is
as suggested by Schmidt et al. Additionally with av you can
switch between mean and median as averaging
function.
Further information can be found in the other vignettes: synopsis, objects & functions and example pipelines.