ANALYSIS OF THE HEART RATE VARIABILITY BEFORE

AND AFTER ASPHYXIA

Federico Cardona Rocha and Fernando S. Schlindwein

Department of Engineering, University of Leicester, Leicester, LE1 7RH, U.K.

Keywords: Heart rate variability, Autonomic nervous system, Injury, Asphyxia.

Abstract: Over the last two decades there has been a widespread interest in the study of variations in the beat-to-beat

timing of the heart, known as heart rate variability (HRV). The studies of heart rate variability have allowed

access to useful information about disturbances in autonomic regulation, which are a promising marker to

quantify autonomic activity. Heart rate variability has become the conventionally accepted term to describe

variations of both instantaneous heart rate and RR intervals (the RR interval is the time interval between two

consecutive R-points of the QRS complex) (D. Bajic et al., 2006). The objective of this paper is the analysis

and comparison of the HRV before and after asphyxia using data from previous studies where 24 adult

Wistar rats were anesthetised and subjected to controlled asphyxia for specified durations (Boardman et al

2002). Preliminary results of our work show a depression of this parameter after long periods of asphyxia,

indicating that HRV might be a good marker for assessing injury to the autonomic nervous system due to

asphyxia.

1 INTRODUCTION

Cardiac activation is an electrical propagation that

spreads over the structure of the heart in a

coordinated pattern leading to an effective systole.

This results in a measurable change in potential

difference on the surface of the body of the subject.

The resultant amplified and filtered electrical signal

is the electrocardiogram and it is widely used to

measure the heart rate and heart rate variability

(Azuaje et al., 2007).

The base line variability of the heart rate time

series is determined by many factors. However, not

only does the mean beat to beat interval (the heart

rate) change on many scales, but the variance of this

sequence of each heartbeat interval does so too. On

the shortest scale, the time between each heartbeat is

irregular. These short term oscillations reflect

changes in the relative balance between the

sympathetic and parasympathetic branches of the

autonomic nervous system (ANS), the

sympathovagal balance. This heart rate irregularity

is a well studied effect known as heart rate

variability (HRV) (Azuaje et al., 2007). A reduction

of HRV has been reported accompanying many

cardiac and cerebral conditions including foetal

brain stem injury; severe neonatal respiratory

distress syndrome is accompanied by a reduction in

low-frequency heart rate variability, if the

respiratory distress improves heart rate variability

increases (Buerk et al., 1997).

The purpose of the current study is to assess the

heart rate variability before and after asphyxia

periods and determine its correlation with the

severity of the insult. To do this, data arising from

several experiments where 24 adult Wistar rats were

anaesthetised and subjected to controlled asphyxia

for specified durations were used (Boardman et al.,

2002).

2 METHODS

2.1 Statistical Method

The application of statistics to medical data is often

used to design experiments and clinical studies, to

summarize, explore, analyse, and present data, to

draw inferences from data by estimation or

hypothesis testing, to evaluate diagnostic

procedures, and to assist clinical decision making

(Azuaje et al., 2007). Since changes in the ECG are

498

Cardona Rocha F. and S. Schlindwein F. (2009).

ANALYSIS OF THE HEART RATE VARIABILITY BEFORE AND AFTER ASPHYXIA .

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing, pages 498-501

DOI: 10.5220/0001546104980501

 SciTePress

quasi-periodic, on beat to beat basis, the frequency

can be quantified in statistical terms (Azuaje et al.,

2007)Simple linear time domain statistics such as

standard deviation (SD) and root mean square of the

successive R-R intervals have been often employed

to analyse HRV fluctuations providing insights into

the autonomic cardiac regulation (Bezerianos et al.,

2004).

In this study the assessment of the HRV is

carried out / determined by the standard deviation of

normal RR intervals (figure 1).

Figure 1: Points and Intervals in the ECG.

Often the RR interval will oscillate periodically,

shortening with inspiration (and lengthening with

expiration). This is known as ‘respiratory sinus

arrhythmia’ and is mediated by the Bainbridge

reflex.

2.2 Geometrical Method

Time-Domain analysis of heart rate variability is a

statistical approach describing the magnitude of the

variability around a mean, but it does not provide

information about the characteristics of these

variations (Acanfora et al., 1998). Beat to beat

variation can also be displayed by plotting each RR

interval against that of the preceding RR interval,

and allows the identification of beat to beat cycles

and patterns in data that are difficult or maybe not

observable by other methods of analysis.

This Poincaré plot analysis is a quantitative

visual technique taken from nonlinear dynamics,

whereby the shape of the plot is categorized into

functional classes (Smith et al., 2003), (Brennah et

al., 2001). Poincaré plots provide summary

information as well as detailed beat to beat

information on the RR variability (Buerk et al.,

1997).

3 RESULTS

Using the SDNN (figure 2) for the RR intervals we

take the absolute value of each deviation before the

summation otherwise the positive and negative

terms would average to zero. The important

parameter is not the deviation from the mean, but the

power represented by the deviation from the mean.

(Smith et al., 2003)

()

∑

−

μσ

()()() ( )

...

−

−++−+−+−

−

xxxx

μμμμ

Figure 2: Calculation of the standard deviation of the

signal.

We apply this method to the first 5 minutes and

the last 3.5 minutes for each group, to measure the

behaviour of HRV before the asphyxia is present

and after the insult. The results are summarised in

table 1.

Table 1: Comparison before / after asphyxia.

Group Before After

One Minute 3.47 ms 1.87 ms

Three Minute 0.42 ms 0.14 ms

Five Minute 0.68 ms 0.32 ms

Seven Minute 0.34 ms 0.12 ms

Another comparison of the HRV was performed

using Poincaré plots for each group, plotting the RR

intervals against the immediate value (figures 3, 4, 5

and 6).

Figure 3: Comparison before/after asphyxia for one

minute.

400

100 150 200 250 300 350 400

100

150

200

250

300

350

RR Vs RRn-1 Values

RRms

RRn-1

Before asphyxia

After asphyxia

ANALYSIS OF THE HEART RATE VARIABILITY BEFORE AND AFTER ASPHYXIA

499

Figure 4: Comparison before/after asphyxia for three

minutes.

Figure 5: Comparison before/after asphyxia for five

minutes.

Figure 6: Comparison before/after asphyxia for seven

minutes.

4 CONCLUSIONS

The results showed that for both methods the heart

rate variability had a marked increase during the

asphyxia. After the asphyxia occurs the HRV had a

marked decrease for longer periods of asphyxia,

while for shortest periods the drop of the HRV was

not so severe (figure 7).

Figure 7: Decrease of The HRV after longer periods of

asphyxia.

We observed that the relation between the

increase and decrease of the heart rate variability

before and after asphyxia could be used as an

indicator of the severity of the injury to the

autonomic nervous system; HRV might then be a

promising marker to estimate the severity of the

asphyxia. In the future it will be interesting to

investigate longer periods of asphyxia to see if this

observation is confirmed.

ACKNOWLEDGEMENTS

To the National Council of Science and Technology

(CONACYT) of Mexico.

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