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ORIGINAL RESEARCH
published: 29 June 2021
doi: 10.3389/fphys.2021.619722
Edited by:
Julian F. Thayer,
The Ohio State University,
United States
Reviewed by:
Katja Weimer,
University of Ulm, Germany
DeWayne P. Williams,
University of California, Irvine,
United States
*Correspondence:
Jan D. Huizinga
huizinga@mcmaster.ca
Specialty section:
This article was submitted to
Autonomic Neuroscience,
a section of the journal
Frontiers in Physiology
Received: 21 October 2020
Accepted: 24 May 2021
Published: 29 June 2021
Citation:
Ali MK, Liu L, Chen J-H and
Huizinga JD (2021) Optimizing
Autonomic Function Analysis via
Heart Rate Variability Associated With
Motor Activity of the Human Colon.
Front. Physiol. 12:619722.
doi: 10.3389/fphys.2021.619722
Optimizing Autonomic Function
Analysis via Heart Rate Variability
Associated With Motor Activity of the
Human Colon
M. Khawar Ali1,2 , Lijun Liu2, Ji-Hong Chen2and Jan D. Huizinga1,2*
1Faculty of Engineering, School of Biomedical Engineering, McMaster University, Hamilton, ON, Canada, 2Division
of Gastroenterology, Department of Medicine, Faculty of Health Sciences, Farncombe Family Digestive Health Research
Institute, McMaster University, Hamilton, ON, Canada
The parameters of heart rate variability (HRV) can non-invasively assess some autonomic
activities, and HRV is influenced by many bodily actions. Although parasympathetic
activity is the primary driver of colonic propulsive activity, and sympathetic activity a
major inhibitor of colonic motility, they are rarely measured and almost play no role
in diagnosis of colon motor dysfunction or in standard treatments. Here we set out
to optimize HRV analysis of autonomic nervous system changes related to human
colon motility. The electrocardiogram and impedance were recorded in synchrony with
colonic motor patterns by high-resolution manometry. Respiratory sinus arrhythmia
(RSA), root mean square of successive differences of beat-to-beat intervals (RMSSD),
the Baevsky Index or Sympathetic Index (SI), and the ratios of SI/RSA and SI/RMSSD
were shown to indicate a marked increase in parasympathetic and withdrawal of
sympathetic activity during the high-amplitude propagating pressure waves (HAPWs).
Strong associations were seen with HAPWs evoked by a meal and rectal bisacodyl
indicating a marked increase in parasympathetic and withdrawal of sympathetic activity
during the gastrocolic reflex and the defecation reflex. When HAPWs occurred in quick
succession, parasympathetic activation (RSA and RMSSD) occurred in a rhythmic
fashion. Hence, during propulsive motor patterns, an overall shift in autonomic activity
toward increased parasympathetic control was shown to be reflected in HRV. HRV
assessment may therefore be valuable in the assessment of autonomic dysfunction
related to colonic dysmotility.
Keywords: high-amplitude propagating pressure waves, RMSSD, RSA, Baevsky’s Stress Index, autonomic
nervous system, colonic motility
INTRODUCTION
Measurement of autonomic function does not yet play a significant role in colon dysmotility
diagnosis, despite the fact that propulsive contractions in the human colon are orchestrated by
the parasympathetic nervous system (De Groat and Krier, 1976;Browning and Travagli, 2019).
Some studies have linked gastrointestinal activity such as the postprandial state (Lu et al., 1999)
Abbreviations: HAPW, High-Amplitude Propagating Pressure Wave; HAPW-SPW, High-Amplitude Propagating Pressure
Wave followed by a Simultaneous Pressure Wave; HRV, Heart Rate Variability; RSA, Respiratory Sinus Arrythmia; SI,
Baevsky’s Stress Index or Sympathetic Index; RMSSD, Root Mean Square of Successive Differences; PEP, Pre-Ejection Period;
HF, High Frequency; LF, Low Frequency; SD1, Standard deviation of minor axis of Poincare’ Plot; SD2, Standard deviation
of major axis of Poincare’ Plot.
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Ali et al. Optimizing ANS Assessment for Motility
and gastric hypersensitivity (Ouyang et al., 2020), to high
frequency (HF) and low frequency (LF) parameters. Autonomic
activity associated with Irritable Bowel Syndrome (IBS)
(Bharucha et al., 1993) and chronic intestinal pseudo obstruction
(Camilleri et al., 1993) were studied using heart rate interval
parameters, heart rate response to deep breathing and other
tests. Autonomic function associated with functional dyspepsia
was studied using HF power and root mean square of successive
differences (RMSSD) (Lorena et al., 2002).
Heart rate can react momentarily to changes in nervous input
from the autonomic nervous system to the sinoatrial node, and
this property establishes heart rate variability (HRV) (Thayer
et al., 2012;Baevsky and Chernikova, 2017;Shaffer and Ginsberg,
2017). Several time and frequency domain analyses and non-
linear methods have been developed to analyse HRV. Especially
spectral analysis of beat-to-beat intervals, assessing the band
power of low-frequency (LF; 0.04–0.15 Hz) and high-frequency
(HF; 0.15–0.4 Hz), are used as matrices of the sympathetic and
parasympathetic nervous systems (Hayano and Yuda, 2019). The
HF band power is considered a measure of parasympathetic
nervous system activity, the effect of activity of the final vagal
fibers innervating the sinoatrial node, a culmination of vagal
innervation that was influenced by a myriad of factors, primarily
breathing but also activity from regulatory nuclei such as the
nucleus tractus solitarius (NTS) that orchestrate coordination
between respiratory, cardiac and gastrointestinal activities to
optimize responses to metabolic demands and hence influence
the autonomic outflow to the heart (Grossman and Taylor,
2007;Browning and Travagli, 2014;Singh and Jaryal, 2020).
Dendritic projections from efferent vagal motor neurons to the
colon extent throughout the NTS and intermingle within the
various subnuclei so as to co-ordinate homeostatic reflexes across
autonomically controlled organs (Browning and Travagli, 2014).
The NTS is a key structure for autonomic and neuroendocrine
integration (Jean, 1991). The coordination of gastrointestinal,
respiratory and cardiac function is dramatically seen in cases of
emergency. The afferent information from the airways is first
processed at the level of NTS and results in various reflexes
that are required for modification of ongoing breathing along
with modulation of autonomic output to the cardiovascular
and respiratory systems (Singh and Jaryal, 2020). It may be
assumed that similar processes are involved in the central
control of colon motility where the NTS plays a critical role
(Browning and Travagli, 2014).
Effects of organ activities on HRV are difficult to predict
and sometimes counterintuitive (La Rovere et al., 2003).
The major motor pattern of the human colon, the high-
amplitude propagating pressure wave is associated with the
autonomic nervous system in two ways. It is orchestrated by the
parasympathetic and enteric nervous system and its occurrence,
due to increased intraluminal pressure and distention of the
colon, will activate stretch sensitive neurons. The role of
the autonomic nervous in orchestrating colonic motility is
exemplified by the sacral defection reflex (Bharucha and Brookes,
2018) that starts with rectal sensation, which activates the
sacral sensory nerves. Then information is signaled into the
sacral parasympathetic nucleus (also called the sacral defecation
center) from where information travels into the brain stem
and frontal cortex to either prevent or initiate a defecation.
A bowel movement may be produced via activation of sacral
parasympathetic nerves and via the enteric nervous system
(Browning and Travagli, 2014;Furness et al., 2014;Brookes et al.,
2016;Bharucha and Brookes, 2018). The primary driver of the
HAPWs is the parasympathetic nervous system (Devroede and
Lamarche, 1974;De Groat and Krier, 1978;Callaghan et al.,
2018). Resection of the parasympathetic pelvic splanchnic nerves
causes loss of the defecation reflex (Devroede and Lamarche,
1974). HAPWs are not observed in ex vivo preparations of
the human colon (Dinning et al., 2016). In the cat, HAPWs
and HAPW-SPWs were identified in vivo and shown to be
associated with firing of parasympathetic efferents (De Groat
and Krier, 1978). Stimulation of sacral extrinsic nerves has also
been shown to be a treatment for constipation (Leblanc et al.,
2015). Interestingly, propulsive motor patterns can be evoked by
injection of a ghrelin agonist in the sacral spinal cord (Shimizu
et al., 2006) or by stimulation of surgically placed electrodes in
the S2 region of the spinal cord (Devroede et al., 2012).
This study was designed to evaluate which of the myriad of
HRV parameters best reflect autonomic nervous system activity
using an established supine to standing protocol, and autonomic
tone and reactivity associated with the high-amplitude pressure
wave that is associated with human colon transit and defecation.
We included HF and LF power, to directly compare RSA and
HF power for statistical analysis, and to compare the disputed
LF power as a measure of sympathetic activity with the Baevsky
Index. We also separated the analysis by intervention, so that
we could assess shifts in autonomic activity during HAPWs in
response to a meal (the gastrocolonic reflex), and in response
to rectal bisacodyl (the sacral autonomic (defecation) reflex,
and in response to distention. We chose a combination of
high amplitude propagating pressure waves (HAPWs) and high
amplitude propagating pressure waves followed by simultaneous
pressure waves (HAPW-SPWs) to incorporate all individual
HAPWs in the statistical analysis, excluding in this analysis
bisacodyl-induced multiple HAPWs since they sometimes are
accompanied by pain and changes in breathing pattern. For
bisacodyl-induced multiple HAPW activity we devised a new
method for continuous assessment of HRV parameters. To study
shifts in autonomic balance we propose new ratios of sympathetic
over parasympathetic parameters.
MATERIALS AND METHODS
Participants
Eleven healthy volunteers (7 males, 4 females, age 30 ±10
years) without any current or prior history of cardiovascular
or gastrointestinal disease and not on any medications affecting
cardiac or gastrointestinal function were recruited by local
advertisement (wall posters) for this study. Each participant was
paid 600 CA$ to complete this study. The study was carried out
at McMaster University with ethics approval from the Hamilton
Integrated Research Ethics Board, and written consent from
all participants.
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Heart Rate and Impedance
Measurements
The electrocardiogram (ECG) was recorded using seven
electrodes on the subject’s torso. Three electrodes formed
a modified Lead II configuration for ECG recording. Four
electrodes were used in a standard tetrapolar electrode
configuration for impedance recording, where two electrodes
supplied a constant current source, and two electrodes registered
the changes in the transfer impedance (reflecting changes in
activity of the sympathetic nervous system). ECG and impedance
were recorded using a MindWare impedance cardio GSC
monitor with a sampling frequency of 500 Hz. (MindWare
Technologies Ltd., Gahanna, OH, United States) and MindWare
BioLab Recording Software. MindWare HRV 3.1 was used for
artifact correction of the ECG signal, to generate beat-to-beat
intervals (RR intervals) and for the calculation of RSA, RMSSD,
HF and LF band powers. PEP was generated by Mindware
Cardiac Impedance software (MindWare Technologies Ltd.,
Gahanna, OH, United States). MATLAB codes were generated to
calculate SD1 and SD2 (Poincare plot) as well as the sympathetic
Index (SI) using the RR interval signal. The breathing frequency
was generated by the Mindware impedance analysis software.
HRV Related to Posture Change
To test general autonomic reactivity using a standard method,
heart rate and HRV changes of all participants were measured
related to posture change. The participants refrained from
smoking, caffeine intake and heavy eating for 2 h prior to the
testing. During the test, they were accommodated in a quiet room
with normal lighting and room temperature. After resting in
supine position for a minimum 10 min, the ECG and impedance
were recorded for 6 min in the supine position, 6 min in
sitting position and immediately upon standing for 6 min. The
HRV parameters tested are shown in Table 1. We calculated
the Baevsky’s Stress Index (SI) (Baevsky and Chernikova, 2017)
according to the formula
SI =AMo×100%
2Mo×MxDMn
where the mode (Mo) is the most frequent RR interval expressed
in seconds. The amplitude of mode (AMo) was calculated, using
a 50 ms bin width, as the number of the RR intervals in the bin
containing the Mo, expressed as a percentage of the total number
of intervals measured. The variability is reflected in MxDMn
as the difference between longest (Mx) and shortest (Mn) RR
interval values, expressed in seconds. The SI is expressed as s−2.
HRV Related to Colonic Motor Patterns
Raw data were obtained from a study that was reported on
previously (Milkova et al., 2020;Yuan et al., 2020). High-
Resolution Colonic Manometry was performed using an 84-
sensor water perfused catheter that detected luminal pressures
at 1 cm intervals from the proximal colon to the anal sphincter.
The catheter was custom-made by Mui Scientific (Mississauga,
ON, Canada) and the acquisition hardware was made by Medical
Measurement Systems (Laborie, Toronto, ON, Canada). The
sampling frequency of the system is 10 Hz. After the catheter
was placed inside the colon with the assistance of colonoscopy,
a 6–8 h high-resolution colonic manometry (HRCM) procedure
was executed. All participants underwent synchronized HRCM,
ECG, and impedance recording during 90 min of baseline,
followed by 20 min of proximal balloon distention, 20 min of
rectal balloon distention using a standard anorectal manometry
balloon assembly, 90 min following intake of a meal, consisting
of organic yogurt fortified by organic milk fat to make it
1,000 kcal (Mapleton Organic, ON, Canada), and 45 min after
administration of rectal bisacodyl. Participants were supine
during all recordings except during the actual intake of the meal.
The manometric analysis was carried out in ImageJ and
MATLAB. All High-Amplitude Propagating Pressure Waves
(HAPWs) with or without an associated SPW, occurring as
single isolated events (Chen et al., 2017) were included in the
present study; the motor pattern needed to have a 2 min quiet
period before and after the motor pattern. All analysis for the
present study was based only on raw data from our studies.
Autonomic reactivity to HAPWs was identified by comparing
the 2 min period prior to the occurrence of an HAPW, during
the occurrence of an HAPW, and the first 2 min immediately
after the HAPW. The HRV signal was divided into segments of 1
min and the HRV parameters were calculated for each individual
TABLE 1 | Autonomic reactivity associated with posture change.
Supine Mean
±SEM
Supine Mean
±SEM
p-value (t, df) (t-
test)/
rs (Wilcoxon)
RSA [ln(ms)] 6.76 ±0.28 5.80 ±0.32 ***0.0006 t= 4.958,
df = 10
RMSSD (ms) 57.90 ±3.65 28.49 ±3.65 ***0.001 rs = 0.7671
SD1 (ms) 58.77 ±6.85 29.53 ±2.64 **0.0012 t= 4.459,
df = 10
SD2 (ms) 102.94 ±12.52 82.99 ±4.77 0.1434 t= 1.588,
df = 10
HF Power (ms2) 1552.88 ±537.25 498.55 ±120.61 **0.0020 rs = 0.7455
LF Power (ms2) 1064.53 ±417.59 1122.36 ±240.33 0.5771 rs = 0.1182
PEP (ms) 121.64 ±4.94 122.75 ±6.40 0.8772 t= 0.1585,
df = 10
SI (s−2) 32.85 ±6.96 50.73 ±5.72 *0.0322 rs = 0.4455
LF/HF Ratio 0.69 ±0.13 3.19 ±0.64 **0.0049 rs =
−0.02727
SD2/SD1 1.77 ±0.07 2.97 ±0.21 ***0.0004 t= 5.196,
df = 10
SI/RSA 5.20 ±1.21 9.33 ±1.30 *0.0244 rs = 0.5727
SI/RMSSD 0.80 ±0.22 2.38 ±0.47 **0.0020 rs = 0.5982
HR (bpm) 63.78 ±2.68 80.50 ±3.24 *** <0.0001 t= 9.854,
df = 10
Number of subjects N = 11; t-value and df are reported when the t-test was applied
while the rs (Spearman) value is reported where the non-parametric Wilcoxon
signed rank test was applied. Abbreviations in this all and other tables: RSA,
Respiratory Sinus Arrythmia; SI, Baevsky’s Stress Index or Sympathetic Index;
RMSSD, Root Mean Square of Successive Differences; PEP, Pre-Ejection Period;
HF, High Frequency; LF, Low Frequency; SD1, Standard deviation of minor axis of
Poincare’ Plot; SD2, Standard deviation of major axis of Poincare’x Plot; HR, heart
rate.
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segment. Even if the HAPW lasted 50 sec, the whole segment
of 1 min was taken into account. In case of before and after,
where the time period taken into account was 2 min, the data
was analyzed for each minute separately (using a 1 min window)
and the mean of the results of the two segments was taken to
represent the HRV parameter. RSA, RMSSD, HF power and SD1
were calculated as measures of parasympathetic activity and LF
power, PEP and SI were calculated as measures of sympathetic
activity. LF/HF, SD2/SD1 and SI/RSA and SI/RMSSD ratios were
also calculated for each phase.
Analysis of HRV in Association With
Motor Complexes
Autonomic activity related to motor complexes, more than one
HAPW as a cluster, was assessed graphically by generating time
matched images of the motor complexes in HRCM with the
frequency domain HF band (the RSA band) images of the HRV
data. The process of generating the HF power (RSA band power)
image started by importing the ECG and impedance signal into
ImageJ using the Cardio Images plugins (Parsons, 2019). In
the Cardio Images plugin, the peak detection and correction of
the ECG signal was carried out by a Pan-Tomkins algorithm
as well as by a Neural Networks model generated and trained
in TensorFlow, followed by manually checking and editing the
wrongly detected/edited R peaks. The tachogram of RR intervals
was plotted as a raster image using a sampling frequency of
10 Hz, image width of 5 s with cubic interpolation in Intervals
plugin. The Frequency Win Plugin was used to calculate FFT
spectra of the tachogram raster image using window length of
60 s and intervals of 10 s. The power spectra are collated into
an image with time on the y-axis and frequency on the x-axis
with pixel intensity as amplitude (ms). Similarly, the HRCM data
was converted into an image using the Event Series plugin in
ImageJ. Both the images were then imported, and time matched
in MATLAB as shown in Figure 1. A Win frequency plugin
generated the HRV spectrogram from 0 to 5 Hz, to study the
RSA/HF band only; the lower frequency band (0–0.14 Hz.) as
well as the frequency band above 1Hz was removed in MATLAB,
and the spectrogram with the frequency band of 0.14–1 Hz. was
plotted as an aligned image with the HRCM image as shown
in Figure 1B. Similarly, the raster image of RR intervals was
imported into Matlab and was used to calculate RMSSD and
SI, which were also plotted as aligned images with the HRCM
Figures 1C,D.
Statistical Analysis
Supine to Standing
To evaluate HRV related to posture change, all the HRV
parameters were analyzed independently. Each HRV parameter
was calculated for supine and standing position for all the
participants (n= 11) and tested for normal distribution using
the Shapiro-Wilk Normality test. If the data for both supine and
standing was normally distributed, the comparison was carried
out using the paired t-test. The Wilcoxon Matched Pair Signed
Rank test was used in case one or both of the supine and standing
HRV parameter data was not distributed normally. The change in
each HRV parameter was considered significant between supine
and standing, if the calculated p-value was less than 0.05.
High-Resolution Colonic Manometry (HRCM)
All HAPWs (n= 65) that had a 2 min period before and after
without major motor patterns, in order to obtain a “baseline” and
“recovery” period, from all the participants were investigated. For
each HRV parameter, the results from all HAPWs were averaged
for each subject and were presented as one reading with three data
points (Before-During-After). These averaged results from all the
participants were used for further analysis. Initially, the data was
tested for normal distribution using the Shapiro-Wilk Normality
test. If the data was normally distributed, the parametric test
ANOVA followed by Bonferroni Multiple Comparison test was
used for comparison. While the non-parametric Friedman test
followed by Dunn’s Multiple Comparison test was used for
data sets that were not normally distributed. The p-value was
calculated for before-to-during [p-value (B-D)] and during-to-
after [p-value (D-A)]. A difference was considered significant
when p<0.05. The t-values and degrees of freedoms are reported
with parametric tests while the z-value is reported with non-
parametric tests.
In addition, the HAPW’s were grouped based on the HRCM
condition with 12 HAPW’s observed during baseline, 16 during
meal, 14 during prucalopride, 5 during proximal balloon
distension, 7 during distal balloon distension and 11 during
bisacodyl. The same statistical procedures as mentioned above
were applied to each group separately to identify the effect of
the stimulus conditions on the association of autonomic nervous
system with HAPW’s.
RESULTS
Autonomic Reactivity Associated With
Posture Change
The parasympathetic parameters RSA, RMSSD, SD1, and HF
power all decreased from supine to standing consistent with
a decrease in parasympathetic reactivity. The sympathetic
parameter SI showed a significant increase from supine to
standing. PEP did not show any significant change. The shift from
parasympathetic to sympathetic going from supine to standing
was reflected in the change in LF/HF ratio, SD2/SD1 ratio, SI/RSA
as well as the SI/RMSSD ratio. The posture change resulted in an
increase in heart rate. SD2 and LF power did not change, likely
a reflection of the fact that these parameters are associated with
both sympathetic and parasympathetic changes (Table 1).
Autonomic Reactivity Associated With
HAPWs
A significant increase in RSA indicated activation of the
parasympathetic nervous system during the motor activity as
compared to the period before the motor pattern and the change
recovered within 2 min (Table 2). An increase in RSA during
the HAPWs was seen in all subjects, average 9.3%, with recovery
afterward. Similarly, an increase in RMSSD was seen in all
subjects except one. There was an average increase of 24.6% in
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FIGURE 1 | HRV parameters associated with a single HAPW. (A) 2-min before, during, and 2-min after HAPW recorded by HRCM. (B) HF power/RSA band of HRV
signal time matched with HRCM recording. (C) RMSSD time matched with HRCM. (D) SI time matched with HRCM. Distance at 0 cm is positioned at the proximal
colon, distance at 80 cm is just proximal to the anal sphincter.
TABLE 2 | Autonomic nervous system modulation in association with all individual HAPWs and HAPW-SPWs combined.
Before Mean ±SEM During Mean ±SEM After Mean ±SEM p-value (B-D) (t, df) or (z-value) p-value (D-A) (t, df ) or (z-value)
RSA [ln(ms)] 6.38 ±0.27 6.90 ±0.26 6.46 ±0.24 *0.0182 (3.420, 8) *0.0220 (3.290, 8)
RMSSD (ms) 53.24 ±6.73 66.35 ±10.35 52.03 ±8.34 0.1609 (1.862, 8) 0.1608 (1.973, 8)
SD1 (ms) 45.19 ±4.66 56.44 ±6.93 40.93 ±4.42 0.1201 (2.189, 8) 0.0593 (2.641, 8)
SD2 (ms) 100.02 ±7.64 126.93 ±12.76 79.44 ±0.18 *0.0069 (4.094, 8) *0.0166 (3.481, 8)
HF Power (ms2) 1675.88 ±715.99 1594.33 ±425.33 1074.31 ±319.33 0.1979 (1.650) 0.1187 (1.886)
LF Power (ms2) 1477.46 ±241.43 1223.29 ±200.60 588.49 ±143.83 0.8796 (0.8128, 8) *0.0128 (3.661, 8)
PEP (ms) 115.04 ±3.02 114.91 ±4.14 118.06 ±7.25 >0.9999 (0.053, 7) 0.4309 (1.362, 7)
SI (s−2) 94.9±29.1 51.92 ±18.43 88.81 ±25.62 *0.0190 (2.593) **0.0044 (3.064)
LF/HF Ratio 2.75 ±0.75 1.21 ±0.24 0.88 ±0.20 0.4772 (1.179) 0.1187 (1.886)
SD2/SD1 2.62 ±0.19 2.53 ±0.23 2.15 ±0.18 >0.9999 (0.00) 0.1542 (1.768)
SI/RSA 18.76 ±6.92 8.69 ±3.69 16.13 ±5.27 **0.0094 (2.828) ***0.0008 (3.536)
SI/RMSSD 5.53 ±2.60 1.97 ±1.24 3.52 ±1.72 **0.0094 (2.828) ***0.0008 (3.536)
HR (bpm) 69.39 ±3.98 66.73 ±3.79 64.33 ±3.64 0.1434 (2.075, 8) 0.4711 (1.283, 8)
The number of subjects, N = 9; the number of HAPW’s, n = 65; t-value and df are reported for the parametric test (ANOVA) and z-value is reported in case of the
non-parametric Friedman test. *P ≤0.05; **P ≤0.01, ***P ≤0.001.
the RMSSD during the HAPW (Table 2). Due to one outlier, the
change in RMSSD did not reach statistical significance. Similarly,
SD1 also increased numerically in all volunteers but one, and did
not reach statistical significance (Table 2).
Although RSA showed a significant increase, the HF power,
derived from the same data set as the RSA, did not show a
significant change (Table 2). RSA is the natural log (ln) of HF
power and taking a natural log will remove the effect of large
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outliers. Indeed, when we removed 4 out of 65 values from the
HF power data set that showed more than 3 SD units off the mean
value, the HF power changed from 993.07 ±300.47 before the
motor pattern to 1769.64 ±546.76 (p= 0.0485) and recovered to
1135.88 ±403.85 (p= 0.0485); the increase in RMSSD during the
HAPW and its decrease afterward, also became significant.
The change in sympathetic index (SI) indicated a decrease in
sympathetic activity during the motor patterns that recovered
within 2 min (Table 2). The PEP did not show significant
changes (Table 2).
The SI/RSA decreased 42% during an HAPW and
recovered within 2 min, consistent with activity shifting
toward parasympathetic activity during the motor activity.
Similarly, SI/RMSSD showed a 64.4% decrease. Both the LF/HF
and SD2/SD1 ratios did not change significantly with motor
activity. The heart rate did not show any significant change with
motor activity (Table 2).
Since RSA is sensitive to respiratory rate changes and to
respiratory tidal volume changes, the breathing frequency and
volume were calculated before during and after all HAPWs. The
breathing frequencies before and during all HAPWs, were 15.9
±0.4 and 15.4 ±0.5 breaths/min (P= 0.225), and 15.20 ±0.47
(p>0.9999) after the HAPW. The values for volume were 0.0123
±0.0113 and 0.0131 ±0.0008 V2(P>0.999), respectively, and
it was 0.008 ±0.007 V2(p= 0.7420) after the HAPW. Hence,
no significant change in breathing frequency was observed in
response to an HAPW.
Autonomic activity associated with HAPWs may arise from
the activity that initiates the HAPW and from potential
mechanoreceptors activated by the actual HAPW. Although
rectal bisacodyl almost always evoked HAPWs in the present
cohort of healthy subjects, in one subject, two low amplitude
simultaneous pressure waves were associated with an increase in
RSA from 4.83 to 5.88 ln(ms) with a concomitant decrease in SI
from 159 to 84 s−2; consistent with the notion that the initiating
autonomic activity is seen by HRV and that the change may not
solely dependent on the strong HAPW evoking distention.
Autonomic Nervous System
Associations With HAPW’s in the
Different Conditions
Activity of autonomic nervous system activity during an HAPW
may be different in different conditions, hence we assessed HRV
parameter changes separately under each condition: baseline,
meal, prucalopride, proximal balloon distension, distal balloon
distension and bisacodyl, The dramatic shift in autonomic
balance toward a dominant parasympathetic activity that was
described above was observed during the HAPWs that were
evoked by a meal (Table 3A) and by rectal bisacodyl (Table 3B)
as reflected by RSA, RMSSD, SI and SI/RSA and SI/ RMSSD.
During baseline, the mean values of all the HRV parameters
during HAPW changed in the expected direction (5.03% increase
in RSA, 6.53% increase is RMSSD, 24.79% increase in SD1,
38.68% increase in HF power, 30.44% decrease in SI), but the
changes did not reach statistical significance (Table 3C).
The 90 min period after oral prucalopride, where we
hypothesize that prucalopride stimulates the gastric mucosa to
evoke HAPWs as a gastrocolic reflex, both RSA and RMSSD
increased significantly, and SI decreased significantly during
the HAPW’s. and recovery afterward in both RMSSD and SI
was also significant. A significant shift in autonomic balance
toward parasympathetic activity was indicated by a decrease in
SI/RMSSD (Table 3D).
The periods of balloon distention had low n numbers,
nevertheless, distal balloon distension was accompanied by a
significant increase in RSA and recovery after the HAPW
(Table 3E), but changes in response to proximal balloon
distention did not reach significance (Table 3F).
Autonomic Reactivity Associated With
Motor Complexes
Motor complexes are defined here as more than one HAPW
and/or HAPW-SPW that occurred close together such that they
could not be analyzed separately. In order to assess HRV during
the motor complexes, a continuous assessment procedure was
developed as outlined in the methods section. The major finding
was that motor complexes were associated with an increase in
HF power that was not continuous but rhythmic. The average
duration of RSA reactivity, measured at 0.14–0.5 Hz, was 50 ±10
s and the frequency of occurrence was 0.8 ±0.2 cycles/min which
was similar to the HAPW frequency within motor complexes
(Figure 2). However, with long HAPWs, more than one RSA
band occurred, giving the RSA activity a distinct rhythmic
appearance (Figure 3). 37 out of a total 40 motor complexes
studied, had RSA bands associated with them. Although there
was complete synchronization of individual HAPWs and bursts
of RSA activity, with motor complexes (n= 34), rhythmic RSA
activity sometimes (n= 6) continued after the HAPW to slowly
die out. Sometimes (n= 3), the RSA activity started prior to the
measured HAPW, but the HAPW likely originated earlier at a
more proximal site, beyond the reach of the catheter. RMSSD also
increased during the HAPWs and motor complexes. There was
complete synchronization between RSA and RMSSD. SI changes
were observed as more or less reciprocal to the RMSSD and
RSA bands (Figures 1,2). During all 90 min baseline periods,
when HAPWs are rare, there was never rhythmic HF activity
although very low amplitude HF activity was continuously
observed (Figure 4).
DISCUSSION
Assessment of Sympathetic Activity
During Posture Change
In the assessment of sympathetic increase in the supine to
standing protocol, the Baevsky Stress Index or Sympathetic Index
increased 52%, whereas SD2, PEP and the LF power did not
show significant changes. In order to maintain a near constant
blood pressure, in response to the postural changes from supine
to standing when blood is pooled in the legs and blood pressure
decreases, the baroreceptor reflex increases sympathetic activity
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TABLE 3A | HRV parameters associated with HAPWs in response to the meal (n= 16).
Before ±SEM During ±SEM After ±SEM p-value (B-D) (t, df) or (z-value) p-value (D-A) (t, df ) or (z-value)
RSA [ln (ms)] 6.22 ±0.24 6.72 ±0.20 6.21 ±0.17 *0.0473 (2.54, 14) **0.0013 (4.47, 13)
RMSSD (ms) 48.15 ±10.45 47.93 ±6.90 38.35 ±5.13 *0.0352 (2.37) **0.0038 (3.10)
SD1 (ms) 40.24 ±6.68 46.34 ±4.85 35.53 ±4.69 0.0569 (2.19) **0.002 (3.29)
SD2 (ms) 94.45 ±15.96 112.38 ±10.387 79.72 ±9.3107 *0.0352 (2.37) *0.0212 (2.56)
HF Power (ms2) 1930.41 ±1100.6 1199.00 ±295.64 641.37 ±112.73 0.1358 (1.83) **0.0038 (3.10)
LF Power (ms2) 1239.22 ±405 1296.89 ±355.38 667.15 ±135.33 0.4025 (1.28) **0.0212 (2.56)
PEP (ms) 120.91 ±2.29 123.27 ±2.36 124.00 ±1.72 0.2575 (1.58, 13) 0.9107 (0.11, 13)
SI (s−2) 77.34 ±10.17 55.30 ±7.01 89.57 ±12.22 *0.0467 (2.27) **0.0092 (2.84)
LF/HF Ratio 2.06 ±0.84 1.26 ±0.27 1.43 ±0.37 >0.9999 (0.36) 0.9304 (0.73)
SD2/SD1 2.67 ±0.60 2.40 ±0.22 2.63 ±0.34 0.4025 (1.79) >0.9999 (0.36)
SI/RSA 11.60 ±1.82 8.12 ±1.27 13.87 ±2.03 *0.0123 (2.78) **0.002 (3.29)
SI/RMSSD 2.33 ±0.44 1.48 ±0.32 2.69 ±0.46 *0.0123 (2.74) **0.002 (3.29)
HR (bpm) 71.50 ±1.96 70.33 ±1.80 70.35 ±1.80 >0.9999 (0.18) >0.9999 (0.09)
t-value and df are reported for parametric tests (ANOVA) and the z-value is reported with non-parametric Friedman tests. *P ≤0.05; **P ≤0.01.
TABLE 3B | HRV parameters associated with HAPWs in response to rectal bisacodyl (n= 11).
Before ±SEM During ±SEM After ±SEM p-value (B-D) (t, df) or (z-value) p-value (D-A) (t, df) or (z-value)
RSA [ln (ms)] 5.67 ±0.57 6.13 ±0.48 5.68 ±0.47 *0.0407 (2.35, 10) 0.2529 (1.23, 8)
RMSSD (ms) 32.95 ±7.45 46.00 ±9.52 33.79 ±7.90 ***0.0007 (3.58) *0.0278 (2.46)
SD1 (ms) 26.87 ±4.38 36.90 ±6.70 22.48 ±4.22 *0.0268 (2.99, 10) 0.0582 (2.21, 8)
SD2 (ms) 75.42 ±9.70 78.24 ±7.89 62.71 ±8.77 0.2896 (1.12, 10) 0.1159 (2.19, 8)
HF Power (ms2) 694.90 ±226.5 1037.75 ±296.52 544.23 ±142.21 0.3594 (1.34) 0.0883 (2.01)
LF Power (ms2) 1181.99 ±409.5 1079.44 ±231.31 465.79 ±108.27 0.7422 (0.98) *0.0278 (2.46)
PEP (ms) 108.00 ±3.25 109.23 ±3.19 111.11 ±3.00 0.5032 (1.09, 13) 0.5032 (1.09, 9)
SI (s−2) 222.29 ±75.10 146.39 ±58.42 223.09 ±72.76 *0.0278 (2.46) **0.0073 (2.91)
LF/HF Ratio 2.57 ±0.49 2.21 ±0.80 1.36 ±0.42 0.5271 (1.12) 0.5271 (1.12)
SD2/SD1 2.95 ±0.31 2.44 ±0.22 2.68 ±0.43 0.2544 (1.59, 13) 0.6402 (0.48, 9)
SI/RSA 54.98 ±21.95 29.75 ±13.02 45.09 ±15.53 *0.0146 (2.68) **0.0016 (3.35)
SI/RMSSD 23.32 ±7.98 11.53 ±4.61 18.85 ±6.58 **0.0094 (2.83) **0.0094 (2.83)
HR (bpm) 86.22 ±5.18 84.00 ±5.50 82.98 ±4.93 0.0883 (2.01) >0.9999 (0.34)
t-value and df are reported for parametric tests (ANOVA) and the z-value is reported with non-parametric Friedman tests. *P ≤0.05; **P ≤0.01, ***P ≤0.001.
TABLE 3C | HRV parameters associated with HAPWs during baseline (n= 12).
Before ±SEM During ±SEM After ±SEM p-value (B-D) (t, df) or (z-value) p-value (D-A) (t, df) or (z-value)
RSA [ln (ms)] 7.01 ±0.30 7.37 ±0.33 6.81 ±0.33 0.6419 (1.25,21)0.0695 (1.91,21)
RMSSD (ms) 72.23 ±9.83 76.94 ±11.77 60.62 ±9.37 >0.9999 (0.43)0.066 (2.12)
SD1 (ms) 64.65 ±7.83 80.68 ±8.91 66.18 ±7.31 0.0562 (2.31,11)**0.0086 (3.67, 10)
SD2 (ms) 111.94 ±8.06 152.09 ±10.06 110.47 ±9.33 *0.0136 (2.93, 11) **0.001 (5.06, 10)
HF Power (ms2) 1964.75 ±519.63 2724.68 ±1146.1 1766.25 ±629.52 >0.9999 (0.21)0.5728 (1.06)
LF Power (ms2) 1649.09 ±397.7 1629.77 ±369.21 970.19 ±402.49 >0.9999 (0.61) > 0.9999 (0.61)
PEP (ms) 120.50 ±2.41 125.33 ±2.11 125.45 ±2.26 0.1181 (2.09,11)0.9486 (0.07,10)
SI (s−2) 34.00 ±5.89 23.65 ±3.41 57.88 ±12.46 0.3316 (1.39)**0.004 (3.09)
LF/HF Ratio 2.04 ±0.77 1.08 ±0.30 1.04 ±0.23 0.7845 (0.85) > 0.9999 (0.21)
SD2/SD1 2.04 ±0.25 2.11 ±0.23 1.83 ±0.19 0.7648 (0.31,11)0.2662 (1.59,10)
SI/RSA 4.99 ±0.94 3.35 ±0.52 9.27 ±2.25 0.4017 (1.28)**0.0028 (3.20)
SI/RMSSD 0.63 ±0.16 0.39 ±0.08 1.39 ±0.41 0.2712 (1.49)***0.0006 (3.62)
HR (bpm) 56.92 ±2.18 54.42 ±1.93 54.36 ±1.50 0.0897 (2.25,11)0.9206 (0.10,10)
t-value and df are reported for parametric tests (ANOVA) and the z-value is reported with non-parametric Friedman tests. *P ≤0.05; **P ≤0.01, ***P ≤0.001.
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TABLE 3D | HRV parameters associated with HAPWs in response to prucalopride (n= 14).
Before ±SEM During ±SEM After ±SEM p-value (B-D) (t, df) or (z-value) p-value (D-A) (t, df) or (z-value)
RSA [ln (ms)] 6.17 ±0.23 6.80 ±0.28 6.42 ±0.35 *0.0216 (2.55) 0.2333 (1.57)
RMSSD (ms) 43.30 ±6.16 55.97 ±7.42 53.95 ±0.35 *0.0467 (2.27) *0.0467 (2.27)
SD1 (ms) 35.68 ±5.253 45.74 ±6.24 41.2817 ±7.23 0.1025 (2.12,13)0.2165 (1.30,13)
SD2 (ms) 76.897925 ±5.93 93.16 ±9.52 82.4542 ±10.48 0.0731 (2.32,13)0.0731 (1.98,13)
HF Power (ms2) 694.95 ±205.84 1327.93 ±351.56 1304.46 ±472.90 0.0997 (1.96)0.6536 (0.98)
LF Power (ms2) 595.53 ±119.8 911.97 ±274.02 607.13 ±242.66 >0.9999 (0.39)0.5615 (1.08)
PEP (ms) 110.67 ±3.74 113.33 ±3.62 115.17 ±3.60 0.23 (1.65,13)0.2692 (1.15,13)
SI (s−2) 91.06 ±12.60 65.34 ±11.87 87.27 ±19.55 *0.0465 (2.56, 13) *0.0465 (2.34, 13)
LF/HF Ratio 1.20 ±0.23 0.75 ±0.10 0.64 ±0.16 0.2333 (1.57) > 0.9999 (0.39)
SD2/SD1 2.478 ±0.27 2.275 ±0.26 2.31167 ±0.32 0.8994 (0.76) > 0.9999 (0.38)
SI/RSA 15.17 ±2.15 10.43 ±2.17 15.00 ±3.62 0.2611 (1.51)0.1176 (1.89)
SI/RMSSD 2.94 ±0.45 1.65 ±0.37 3.63 ±1.09 *0.0467 (2.27) *0.0092 (2.84)
HR (bpm) 71.75 ±4.19 70.33 ±4.01 70.07 ±3.64 >0.9999 (0.38) > 0.9999 (0.19)
t-value and df are reported for parametric tests (ANOVA) and the z-value is reported with non-parametric Friedman tests. *P ≤0.05.
TABLE 3E | HRV parameters associated with HAPWs in response to distal balloon distention (n= 7).
Before ±SEM During ±SEM After ±SEM p-value (B-D) (t, df ) or (z-value) p-value (D-A) (t, df) or (z-value)
RSA [ln (ms)] 6.56 ±0.41 7.31 ±0.20 6.25 ±0.21 *0.0138 (4.42, 5) *0.0008 (10.88, 4)
RMSSD (ms) 51.73 ±8.29 61.19 ±8.43 43.51 ±3.59 0.7593 (1.85,5)0.4863 (1.35,4)
SD1 (ms) 52.82 ±10.39 49.69 ±5.95 42.03 ±4.57 0.7649 (0.31,6)0.7351 (0.77,4)
SD2 (ms) 105.08 ±11.97 120.76 ±13.77 90.732 ±11.20 0.2277 (1.58)0.4118 (1.26)
HF Power (ms2) 1449.08 ±561.1 1844.86 ±344.43 631.55 ±162.59 >0.9999 (0.53,5)*0.0493 (4.20, 3)
LF Power (ms2) 842.96 ±438.2 1574.24 ±370.03 520.67 ±212 0.3146 (1.41)0.1542 (1.77)
PEP (ms) 123.43 ±2.09 126.00 ±2.68 127.60 ±1.91 0.6296 (0.92,6)0.638 (0.51,4)
SI (s−2) 48.39 ±10.42 43.67 ±7.95 61.04 ±12.76 0.7077 (0.39,6)0.6823 (0.86,4)
LF/HF Ratio 7.92 ±2.02 5.88 ±0.94 10.06 ±2.35 0.5777 (1.06)0.5777 (1.06)
SD2/SD1 2.594 ±0.47 3.31 ±1.15 2.21 ±0.28 >0.9999 (0.32) > 0.9999 (0.63)
SI/RSA 7.92 ±2.02 5.88 ±0.94 10.06 ±2.35 0.4536 (0.88,6)0.4536 (1.31,4)
SI/RMSSD 1.30 ±0.43 0.79 ±0.13 1.52 ±0.40 0.5194 (1.24,6)0.5168 (1.32,4)
HR (bpm) 63.29 ±1.70 64.14 ±2.06 62.31 ±1.07 0.6954 (0.81,6)0.6954 (0.76.4)
t-value and df are reported for parametric tests (ANOVA) and the z-value is reported with non-parametric Friedman tests. *P ≤0.05.
TABLE 3F | HRV parameters associated with HAPWs in response to proximal balloon distention (n= 5).
Before ±SEM During ±SEM After ±SEM p-value (B-D) (t, df ) or (z-value) p-value (D-A) (t, df) or (z-value)
RSA [ln (ms)] 6.63 ±0.36 6.75 ±0.18 7.10 ±0.31 >0.9999 (0.33,4)0.6283 (1.18,4)
RMSSD (ms) 46.44 ±0.36 43.99 ±5.85 48.88 ±3.65 >0.9999 (0.45,5) > 0.9999 (0.64,4)
SD1 (ms) 39.03 ±2.91 40.57 ±7.04 46.17 ±4.60 0.7593 (0.27,5)0.4863 (0.66,4)
SD2 (ms) 87.99 ±14.16 85.74 ±9.28 100.57 ±11.71 >0.9999 (0.32) > 0.9999 (0.32)
HF Power (ms2) 805.16 ±293 818.90 ±134.34 1119.5±444.78 >0.9999 (0.05,5) > 0.9999 (0.60,4)
LF Power (ms2) 1347.52 ±529.7 601.13 ±212.58 879.83 ±444.92 0.6856 (0.95) > 0.9999 (0.00)
PEP (ms) 118.33 ±3.38 111.33 ±6.35 118.40 ±3.68 0.2225 (1.88,5)0.2225 (1.87,4)
SI (s−2) 61.36 ±7.67 67.27 ±15.99 57.17 ±10.36 0.9121 (0.40,5)0.9121 (0.35,4)
LF/HF Ratio 10.20 ±1.87 10.19 ±2.38 8.24 ±1.60 0.3095 (1.42) > 0.9999 (0.47)
SD2/SD1 2.28 ±0.32 2.34 ±0.31 2.39 ±0.49 0.993 (0.11,5)0.993 (0.076,4)
SI/RSA 10.20 ±1.87 10.19 ±2.38 8.24 ±1.60 0.9942 (0.01,5)0.8978 (0.44,4)
SI/RMSSD 1.16 ±0.21 1.61 ±0.57 1.23 ±0.25 0.7325 (1.02,4) > 0.9999 (0.45,4)
HR (bpm) 65.00 ±2.75 65.50 ±3.45 64.58 ±3.77 0.8425 (0.31,5)0.8425 (0.56,4)
t-value and df are reported for parametric tests (ANOVA) and the z-value is reported with non-parametric Friedman tests.
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FIGURE 2 | HRV parameters associated with two consecutive HAPWs. (A) Before-During and After a Motor Complex. (B) Time matched HF Power/RSA band of
the HRV signal before-during and after MC. (C) RMSSD time matched with HRCM. (D) SI time matched with HRCM.
to increase heart rate. Baroreceptor action potentials are relayed
to the nucleus tractus solitarius, which uses action potential
frequency as a measure of blood pressure. The end-result
of baroreceptor de-activation is excitation of the sympathetic
nervous system and inactivation of the parasympathetic nervous
system. Houtveen et al. (2005) recorded PEP at different
breathing frequencies during supine, sitting and standing; PEP
increased from supine to standing hence the sympathetic activity
appeared to decrease. This was also observed by Cacioppo
et al. (1994). PEP is a measure of ventricular contractility,
influenced by beta-adrenergic receptor mediated ventricular
sympathetic innervation (Van Lien et al., 2015). Nitroprusside
causes vasodilation, baroreceptor unloading and a reflex increase
in sympathetic tone, which was associated with a significant
decrease in PEP (Schächinger et al., 2001). Our study indicates
that the sympathetic activity that is increased upon standing is
not reflected in the PEP value.
Sympathetic pathways within the body form a vast network;
only those sympathetic activities that interact with, or directly
or indirectly take part in sympathetic regulation of heart rate
will be seen in HRV. For example, muscle sympathetic nerve
activity measured at the peroneal nerve induces vasoconstriction
and is modulated by the baroreflex but it represents regional
sympathetic neural activity, it does not equal sympathetic
discharge directed to the heart (Schächinger et al., 2001).
Both LF power and SD2 did not significantly increase with
posture change in the present study. Many studies continue
to presume that LF power, especially if adjusted for HF
power, total power, or respiration, provides an index of cardiac
sympathetic “tone” and that the ratio of LF:HF power indicates
“sympathovagal balance,” but strong evidence has been presented
that LF power neither reflects cardiac sympathetic tone at supine
nor in response to standing (Goldstein et al., 2011). There is also
no evidence that the LFa (low frequency area) (Nguyen et al.,
2020) is specific for sympathetic activity (Goldstein et al., 2011;
Rahman et al., 2011). Rahman et al. (2018) showed that SD2
as well as the ratio of SD1 and SD2 are not related to cardiac
sympathetic activity.
Baevsky et al. developed an index of regulation strain, or stress
index, or, relevant to our study, a “Sympathetic Index” which
illustrates the sympathetic or central regulation activity (Baevsky
and Chernikova, 2017). The activation of sympathetic regulation
results in the stabilization of the heart rhythm which causes
a decrease in variation of RR intervals and an increase in the
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FIGURE 3 | HRV parameters associated with a complex of multiple HAPWs (a motor complex). (A) HRCM recording with a motor complex containing long
overlapping HAPWs. (B) HF power/RSA band of HRV signal. (C) RMSSD. (D) SI. (E) HF/RSA power band in 3D.
number of intervals with similar duration. The histogram of RR
intervals becomes narrower and increases in height. Although
the SI is not yet widely applied for sympathetic measurement,
it is used by commercially available HRV data analysis software
as one of the measures of sympathetic activity (Kubios, 2020);
they use the square root of SI to minimize the effect of
outliers. The marked change in SI due to posture change in the
present study suggests it to be a sensitive marker for orthostatic
sympathetic change.
Sympathetic Activity and the Colonic
Propulsive Motor Patterns
The HAPWs, the most significant propulsive motor pattern of
the human colon, were accompanied by a significant decrease
in SI, hence a decrease in sympathetic activity. During motor
complexes, SI was always showing some value above zero, which
indicates that there was continuous sympathetic activity which
was inhibited during HAPWs. We infer that withdrawal of
sympathetic activity is part of the autonomic reflexes that initiate
the HAPW, e.g., in response to a meal or rectal bisacodyl. Could
stretch receptors be activated during the HAPW? Viscerofugal
neurons, connecting to the sympathetic prevertebral ganglia
allow the colon to fill, and when the circular muscle of the colon
wall contracts to empty the segment, the mechanoreceptors are
unloaded and synaptic input decreases (Szurszewski and Miller,
2006). Hence the marked reduction in SI observed in the present
study is consistent with a withdrawal of sympathetic activity,
allowing the HAPW to proceed.
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FIGURE 4 | The RSA band in baseline and with HAPWs. (A) Baseline HRCM recording without any colonic motor pattern. (B) HF power/RSA band during the
baseline. (C) HRCM recording with Motor Complex. (D) HF power/RSA band during the Motor Complex.
Parasympathetic Activity and the
Colonic Propulsive Motor Patterns
All HRV parameters studied that are associated with
parasympathetic activity, were decreased in response to
posture change from supine to standing reflecting the
well-known decrease in parasympathetic activity to prevent
orthostatic hypotension.
Individual HAPWs, were associated with a significant increase
in RSA. Many of these HAPWs occurred during baseline
or in the aftermath of taking a meal where the HAPWs
are not felt and do not cause a sensation and are not
associated with evoked body movements, discomfort or changes
in breathing patterns. We suggest that this may reflect the
activity in the parasympathetic nervous system associated with
the initiation of the HAPW. HAPWs occur most often after
a meal as a result of the gastro-colonic reflex, or in response
to rectal stimulation where they are the result of a sacral
defecation reflex. The sacral defecation reflex involves the sacral
defecation center (the parasympathetic nucleus), Barrington’s
nucleus and the NTS (Taché and Million, 2015). Barrington’s
nucleus and the NTS are also involved in cardiac homeostasis
(Gasparini et al., 2020), and in this way, the neuronal traffic
in association with HAPWs can influence heart rate, similar
to the influence of breathing on the heart rate: “breathing
at different rates within the 9–24 bpm range, which changes
HF power, does not change mean heart rate” (Shaffer and
Ginsberg, 2017). The fact that a significant change occurred in
RSA in association with the motor patterns without a change
in heart rate is consistent with the hypothesis that the origin
of the parasympathetic activity is the neural activity associated
with gut activity and not cardiac activity. The fact that the
heart rate does not change suggests that the vagal tone, the
mean vagal efferent effects to the sinus node, does not change
(Grossman and Taylor, 2007).
Vagal afferent neurons are likely activated by colonic motor
patterns and their dendritic projections extent throughout
the NTS and intermingle within the subnuclei providing a
potential means to coordinate respiratory and cardiac autonomic
activities (Browning and Travagli, 2014). Sensory nerves in
the pelvic plexus will also convey colonic information to the
spinal cord (Smith-Edwards et al., 2019). Hence neuronal traffic
originating for an HAPW may also influence HRV. Research
is ongoing to distinguish efferent and afferent neuronal traffic
associated with HAPWs and to explore their role in HRV
changes and diagnostic value. It is also possible that HAPWs
induce colonic blood pressure changes that might influence HRV
(Semba and Fujii, 1970).
When HAPWs occur in quick succession within motor
complexes, overlapping in time, RSA was markedly increased.
Strong motor patterns induced by bisacodyl sometimes results
in discomfort and increased breathing frequency; increased
breathing frequency results in decreased RSA but this was
not found, likely superseded by processes that increased RSA.
Importantly, there was never continuous RSA activity even when
HAPWs were continuously present. RSA increases occurred in
bursts that were not synchronous with individual HAPWs within
the burst, and they continued for several minutes and then
diminished gradually (Figure 3). The HF “rhythmicity” suggests
that there is a refractory period in the parasympathetic neural
activity. In some instances, the RSA bands started prior to the
HAPW’s, however, in those cases the HAPWs were seen in the
most proximal sensor hence the true beginning of the HAPW was
not captured by the catheter. Since contractions are ongoing, the
rhythmicity of the parasympathetic activity suggests that it arises
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from activity orchestrating the HAPWs and not from distention,
but this remains to be investigated.
HRV Parameters Associated With
Autonomic Reflexes
When the HRV parameters associated with HAPWs were
analyzed within each intervention separately, strong associations
between HAPWs and sympathetic decrease and parasympathetic
increase were observed in response to a meal, which reflect the
gastrocolic reflex, in response to rectal bisacodyl that reflect the
sacral autonomic (defecation) reflex and in a rapid response to
oral prucalopride which we hypothesize is due to a gastrocolic
reflex mediated by stimulation of gastric enterochromaffin cells
and subsequent activation of vagal sensory nerves. In a subset
of patients with chronic constipation, absence of the autonomic
reflexes is associated with high sympathetic activity (Chen and
Liu, unpublished observations).
RSA vs. RMSSD and HF Power
The RMSSD increased one to one with the increase in RSA
associated with HAPWs and within the motor complexes,
confirming the marked association between parasympathetic
activity and human colon propulsive motor patterns. The
association between RMSSD and single HAPWs occurred despite
the fact that the recording period of 1 min during the HAPW was
too short for an ideal assessment as argued by Baek et al. (2015).
The RSA is the natural log of the HF power; the HF power did
not show a significant change likely because with a few HAPWs
(4/65), the HF power was more than 3 standard deviations from
the mean values. If we took these values out and assessed 61 out
of 65 motor patters, the HF power showed a significant increase
going from baseline to motor pattern and back to recovery.
The Significance of Changes in
Breathing Frequency
The HF band, also known as the respiratory band, is associated
with variation in heart rate due to respiration. The HF band
has been set to range from 0.15 to 0.4 Hz. which corresponds
to the respiration frequency of 9–24 breaths/min, the normal
frequency range in adults. The heart rate increases during
inhalation due to inhibition of vagal outflow and decreases during
exhalation due to the restoration of vagal outflow by release of
acetylcholine (Gasparini et al., 2020). If the breathing rate is
outside the range of 9–24 breaths/min, then the calculated HF
power will be due to noise or harmonics of lower frequency
bands and the respiration frequency power will not be included
in the HF power. If the respiration frequency is lower than
9 breaths/min, as is the case with slow, deep breathing, the
most prominent component of the respiration frequency power
will lie in the LF band, and without adjusting the HF band,
it will indicate low HF power and high LF power and lead to
wrong interpretations of HF power. Similarly, during exercise,
a breathing frequency of over 24 breaths/min, the prominent
component of parasympathetic power will be out of the range
and will wrongly represent low parasympathetic power. If during
experimental conditions the breathing frequency goes outside
the 9–24 breaths/min range, then the HF power band can be
adjusted based on the respiration frequency. This was used in a
recent study by Nguyen et al. where the HF or parasympathetic
band was replaced by “RFa” (respiratory frequency area; Colombo
et al., 2015;Nguyen et al., 2020). In the present study the
subjects were lying in a relaxed supine position throughout the
recording and were not asked to breathe deeply, they were only
asked to change position from supine to sitting for a period
of 15 min while eating. The respiration rate was within the
range of 9–24 breaths/min almost all of the time. In our study,
any incident for which the respiration frequency was out of
the range of 9–24 breaths/min was rejected during analysis;
hence, there appears to be no benefit in using RFa under our
experimental conditions.
The New Parameters SI/RSA and
SI/RMSSD
The two branches of the autonomic nervous system can work
reciprocally but they can also work independently. Hence a
sympathetic over parasympathetic ratio may not reflect an
autonomic “balance” and cannot be used as a singular parameter
of autonomic balance. Furthermore, LF and SD2 are not
considered good parameters for sympathetic activity, making the
LF/HF and SD2/SD1 ratios less useful. These ratios in our study
behaved significantly different from SI/RSA and SI/RMSSD.
Hence, we calculated SI/RSA and SI/RMSSD in conjunction with
the SI, RSA and RMSSD values as additional parameters to
evaluate changes in autonomic activity. The SI/RSA and the
SI/RMSSD decreased markedly with the HAPWs confirming a
shift to parasympathetic activity.
In conclusion, we show that HAPWs are associated
with measurable changes in HRV parameters reflecting
parasympathetic and sympathetic activity. Most of the single
HAPWs reported here were not noticed by the subjects, they
occurred without discomfort or change in respiration pattern.
Under normal conditions, they would just be part of everyday
movement of content into anal direction without urge to
defecate. These motor patterns were not associated with a
change in heart rate, suggesting a physiological correlation
between the HAPW, the gastrocolic reflex, the sacral defecation
reflex, and the autonomic parameters RSA, RMSSD and SI. Our
inference is that these motor patterns and reflexes are directed
by autonomic activity reflected in HRV, hence RSA, RMSSD, SI,
SI/RSA, and SI/RMSSD may develop as biomarkers of autonomic
(dys)regulation of colonic motility.
DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be
made available by the authors, without undue reservation.
ETHICS STATEMENT
The studies involving human participants were reviewed and
approved the by Hamilton Integrated Research Ethics Board. The
Frontiers in Physiology | www.frontiersin.org 12 June 2021 | Volume 12 | Article 619722
fphys-12-619722 June 23, 2021 Time: 17:50 # 13
Ali et al. Optimizing ANS Assessment for Motility
patients/participants provided their written informed consent to
participate in this study.
AUTHOR CONTRIBUTIONS
MKA analyzed all the data, incorporated the Baevsky’s Stress
Index, and wrote the first draft of the manuscript. LL
assisted with data analysis and manuscript writing. J-HC
directed and performed all volunteer HRCM studies and
discussed manuscript writing. JDH oversaw the autonomic
nervous system analysis and manuscript writing. All authors
approved the manuscript.
FUNDING
This study was supported by the Natural Sciences and
Engineering Research Council (NSERC) Grant 386877 to JDH.
MKA was supported by a fellowship from the Farncombe Family
Digestive Health Research Institute and NSERC.
ACKNOWLEDGMENTS
We gratefully acknowledge that Dr. Sean Parsons provided all
ImageJ plug-ins. An abstract of this work was presented at the
2021 Canadian Digestive Diseases Week.
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Conflict of Interest: The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
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