Vol.7 No.3
Received: March 2017 , Accepted: August 2018 , Available online: September 2018
Heart Rate Variability analysis of chronotropic response to exercise in rickshaw-pullers
Jha A1*, Paudel BH 1
1Department of Basic & Clinical Physiology, BP Koirala Institute of Health Sciences, Dharan, Nepal
ABSTRACT
Heart rate variability (HRV) reflects a healthy autonomic nervous system and is increased with endurance physical training. This study aimed to quantify the cardiovascular system regulation in terms of heart rate variability under a physiological stress-acute graded and controlled exercise in chronic rickshaw pullers. Primary focus of the study was on the recovery of HRV parameters after the exercise. Forty healthy male subjects were included in the study. Twenty of them were rickshaw-pullers, who had been engaged in active cycling for more than 10 years and they constituted the aerobic training model or experimental group. Twenty other healthy male sedentary male constituted the control group. After 15 min supine rest standard limb lead I was used for recording entire epoch of ECG for HRV. ECG was recorded at rest, during exercise and immediately after termination of exercise in supine position for 6 min at spontaneous respiration.
Acute exercise testing was done by automated bicycle ergometer in supine position for 6 minutes. It had automated load increment program. Initial load was 25W and at every 2-min of intervals it was increased by 25W and finally reached up to 100W. Constant frequency of paddling was maintained during the period of six minutes exercise. Along with continuous ECG recording simultaneously blood pressure and heart rate were also measured at every 2 min of intervals. The ECG and respiratory signals were recorded at the sample rate of 4000 Hz for each signal. The ectopic beats or artefacts of ECG were excluded by visual inspection of entire recording. Recovery of SDNN was less in rickshaw pullers than in the controls (p<0.05). Both the groups showed significant difference in power (nu) of HF, when compared the resting vs exercise vs recovery (p<0.001) Significant difference were observed in power (nu) of HF, when compared the exercise-resting vs recovery- exercise vs recovery-resting (p<0.001)
From this study it was concluded that the rickshaw pullers were found to be physically well trained as evidenced by faster recovery in heart rate and heart rate variability (LF and HF). The normalised units of LF or HF are found to be better indicator of recovery from exercise than the SDNN.
INTRODUCTION
Heart rate variability (HRV) is a reliable, non-invasive measure that reflects the balance of sympathetic and vagal neural influences on heart rate [1]. It is defined as the changes in the interval between heartbeats (R-R intervals) over time. HRV is thought to reflect the ability of the autonomic nervous system (ANS) to adapt to changing circumstances by detecting and quickly responding to unpredictable stimuli. Generally a healthy ANS is reliant on dominant vagal modulation. The effect of long term aerobic training have been suggested to be associated with increased heart rate variability, especially vagal mediated respiratory sinus arrhythmia during short term (2-10min) rest recordings and during long term (24 hrs) dynamic recordings [2]. In contrary, some control studies have failed to show any association between aerobic training and heart rate variability during short term [3] or long term recording [4]. However, long duration of training may not necessarily lead to greater enhancement of heart rate variability [5], as prolonged and intense training may restore these changes in HRV back to the baseline level. In the most recent study, the long duration (5 yrs) of regular low to moderate intensity training did not prevent HRV from decreasing in older men population [6]. It is widely assumed that good physical fitness and regular exercise training induce adaptation of the autonomic nervous system, which is most commonly observed in the form of a decrease in a basal heart rate at rest. It is thought to be mediated by an increase in cardiac vagal tone. Although it is commonly assumed that cardiac vagal tone is increased in well-trained individuals that is somewhat controversial matter. Studies [7], [8] comparing athletes and age-matched
sedentary subjects have mostly observed higher HRV in trained individuals than in controls, but Lazogluet et al., 1996 [9] did not find significant differences in 24-h HRV between cyclists and sedentary controls. Controversial results may be due to differences in the baseline characteristics among the study populations and in the methodology of analysis of HRV [10].
Boutcher and Stein 1995 [11] studied sedentary middle-aged men who engaged in moderate-intensity (60% heart rate range) exercise sessions 3 times/week, for a total of 24 sessions. The intervention group showed a marked increase in peak oxygen uptake, whereas the control group showed no change. There was a significant reduction in the resting heart rate, but HRV remained unchanged. Time domain analysis of heart rate variability reveals high values for cyclists compared with control subjects, possibly indicating an increased vagal control in cyclists.
Heart rate recovery after acute bout of exercise depends on the cardio-respiratory fitness level of the individual and the intensity of exercise [12,13,14] and is greatly affected by autonomic nervous activity during the recovery period [15]. Recently, measurement of HRV has enabled to estimate cardiac autonomic nervous activity [16] to some extent. If cardiac autonomic nervous activity is related to HR recovery, analysis of HR recovery should provide important information in estimating cardiac autonomic function.
26 METHODS
I. Subjects
The study was conducted in 40 healthy male individuals. Twenty of them were rickshaw-pullers (tri-cyclists), who have been engaged in active cycling for more than 10 years and they constituted the aerobic training model or experimental group. Twenty other healthy male sedentary individuals constituted the control group. Rickshaw pullers were regularly engaged in active cycling (rickshaw pulling) for more than 10 years. They had no signs and symptoms of autonomic dysfunctions and their health status were determined on the basis of screening examination & history. For occasional smokers and drinkers, the subjects with the scores of Fagerstrom test for nicotine dependence <4 and alcohol use disorder identification (AUDIT) <8 respectively were accepted. They were normotensive individuals with supine BP <140/90 mmHg and normal body mass index (BMI) <27.
For control, healthy males who were not doing regular physical activity or competitive sports/yoga and other relaxation techniques, without signs and symptoms regarding autonomic function, Medication-free individuals were selected from staffs and students of BPKIHS. The investigation conformed to the principles outlined in the Declaration of Helsinki, and the institutional review board of BP Koirala Institute of Health Sciences approved the study. The participants provided their written informed consent to participate in this study, and the institutional review board approved this consent procedure.
II. Recording Procedure
Participants were refrained from smoking or drinking beverages containing caffeine or alcohol for 24
hours before exercise stress tes All the recordings were done in the forenoon between 8:00 am to 12:00 noon. After the pre–recording procedure was over, the stethograph was tied around the chest and it was connected to the strain gage coupler for recording respiration and then the subjects were given 15 min of supine rest. In the mean time, disposable ECG electrodes were applied on the both wrists and the right forearm, after cleaning the surface adequately which decreases skin impedance and the electrical signal is conducted easily through the electrode. The electrodes were connected to the ECG amplifier via the ECG electrode cable and the standard limb leads were used. Generally, standard limb lead I was used for recording entire epoch of ECG for HRV. The subjects were asked not to close their eyes but to be completely relaxed and stress free.
ECG were excluded by visual inspection of entire recording
STATISTICAL RESULTS
A. General Characteristics of Subjects The study was conducted on healthy male adult individuals of two groups: rickshaw pullers (n=20) and sedentary controls (n=20). The age of rickshaw pullers was significantly higher (32.2±3.29 vs 28.15±2.96, P<0.01) whereas their weight (58.5±8.6 vs 63.55±5.75, p<0.01) was significantly lower as compared to controls. The body height (163.1±8.12 vs 163.6±7.21 P=0.08) was comparable between the groups. The body mass index (22±2.59 vs 23.89±2.26, p<0.05) was significantly lower in rickshaw pullers.
The resting SBP was statistically not different between the groups. The exercise SBPs were also not different between the groups except at 2 min of exercise the rickshaw-pullers exhibited significant increase (13516 vs 1276, p<0.05). The diastolic BP did not show any difference between the groups. The blood pressures during recovery also were not different between the groups. The rickshaw-pullers showed significantly low heart rate as compared to the controls (638 vs 734, p<0.001). The difference in heart rates between the groups was present in 2nd, 4th and 6th min of exercise (9111 vs 989, p<0.001; 10011 vs 11710, p<0.001; 1078 13010, p<0.001 respectively). The differences were also maintained after age or weight adjustment. The recovery heart rates between the groups were also significantly different at all the recorded time points (2nd, 4th, and 6th min: 7616 vs 869, p<0.05; 6710 vs 786, p<0.01; and 648 vs 765, p=0.001 respectively).
B. Effect of Exercise on Time Domain Parameters
Both groups showed significant difference (p<0.01) in all the time domain parameters when compared the resting vs exercise vs recovery. The recovery of the SDNN was less in rickshaw pullers than in the controls (p<0.05) (Table 1).
Significant difference was observed in all the time domain difference variables when compared the exercise-resting vs recovery-exercise vs recovery-resting. After age adjustment mean R-R difference and SDNN difference between recovery-resting were significantly different between the groups (p<0.001 and p<0.05 respectively) (Table 2). The widest RR intervals were significantly logner in rickshaw pullers than in controls during recovery (p<0.001) (Table 3).
C. Effect of exercise on frequency domain analysis
Both the groups showed significant difference in power (ms2) of LF and HF, frequency domain parameters when compared the resting vs exercise vs recovery (p<0.001) (Table 4). Significant difference was observed in power (%) of VLF and HF difference, when compared the exercise-resting vs recovery-exercise vs recovery-resting, p values were p<0.001 and p<0.001 respectively (Table 5).
DISCUSSION
This study was carried out with the aim of evaluating the effect of long-term rickshaw-pulling on exercise-induced changes in heart rate variability parameters. The rickshaw pullers were considered chronically trained individuals because one of the selection criteria was profession of rickshaw pulling for 10 or more years. This was verified from their city organisation (Rickshaw Pullers’ Union, Dharan). The control subjects were from the institute’s employees and other subjects randomly taken. The number of subjects were put 20 in each group according to the approved revised protocol.
As human activities such as changing body posture, doing physical exercise, mental work cause changes in nerve activities, the heart rate is easily altered by those daily activities. Therefore, this study was carried out. The HRV change in response to physical training or work is a relevant issue in work physiology. And, the rickshaw pullers were presumed to be the best model of the long-term training or “heavy work”. Therefore, physiological training (daily work) has been chosen the model of increased aerobic capacity.
Rickshaw pullers are unique in the south Asia where they not only carry load but also do cycling of wheel. Therefore they were taken as trained subjects for this study. Rickshaw pullers who were involved in their work for more than ten years were included. Subjects of the control group were taken from the staff and students of BPKIHS. They were not involved in any exercise, yoga, and any relaxation practices. All the subjects committed in protocol were fulfilled. The comparisons were made between and within the groups. Since it is well known that acute bout of exercise is
known to change a number of cardio-respiratory parameters, the differences between the groups was the concern of the study, and are highlighted more in this discussion than the findings of the within-group differences across the three physiological states of testing (resting, exercise and recovery).
The data between the groups were compared by non-parametric method (Mann-Whitney) using SPSS (version 10) because some of the variables showed skewed distribution. Since the groups also showed difference in age and the body weight, which are known to affect the high frequency of HRV [10], both of these variables were adjusted while commenting on differences between the groups whenever required. The adjustment was done by regression analysis using EPI-Info (version 3.1) software.
32
exercise where the rickshaw pullers exhibited its higher value as compared to the controls. The SBP increase could have been contributed by increase in heart rate and better cardiac filling in the beginning of the exercise. However, HR increase was not larger in this group than in the controls. The rickshaw-pullers exhibited less fluctuation in mean RR intervals during the recovery period that the controls. It was evident from the fact that they had smaller SDNN difference between recovery and the resting condition than the controls. In other words, they had less fluctuation in vagal input to the heart. They had gradual recovery than the control group leading to smaller SD of the RR intervals. Moreover, the control group reached relatively high level of HR at the peak of exercise and their RR difference from the resting was larger (Tables 1,2, 3).
There were many differences in frequency domain parameters between the groups. The rickshaw pullers exhibited significantly higher VLF and LF than the controls during the period of 6 min exercise. The percent of VLF and HF power was significantly high and low during exercise respectively in within group comparison in both the groups. This clearly indicates that the vagal input to the heart decreased and the slow frequency factors like renin-angiotensin system came into play. These findings were further confirmed by powers of these frequencies in terms of millisecond squared. Furthermore, the rickshaw pullers exhibited faster recovery in LF (Table 5). This picture is also seen in
percent of power of LF (Table 6). Simultaneously, the rickshaw pullers had better recovery in HF (Table 7).
This study is new in this part of the world and very pertinent in terms of the work physiology. The rickshaw pullers were found to be physically well trained as evidenced by faster recovery in heart rate and heart rate variability (LF and HF). The difference between the recovery values and the resting values of the HRV parameters clearly showed that the normalised units of LF or HF are found to be better indicator of recovery from exercise than the SDNN. It would have been better to measure the oxygen consumption to quantify the level of exercise in the study. Standard and automated bicycle was used in the study and quantification of the exercise was done on the basis of increment of heart rate during exercise as an evident of increase heart rate in both groups between 55 to 75% of HRmax. The HRmax is widely accepted index for quantification of exercise.
This study should be considered as a study during incremental phase of exercise and not at steady state. It has given a new insight into the HRV changes during incremental phase of exercise and the HRV recovery has been unique in this study.
ACKNOWLEDGEMENT
REFERENCES
1. Acharya U, Paul Joseph K, Kannathal N, Lim CM, Suri JS. Heart rate variability: a review. Med Biol Eng Comput. 2006;44:1031–1051.Stein PK, Ehsani AA, Domitrovich PP, Kleiger RE, Rottman JN.
Effect of exercise training on heart rate variability in healthy older adults.Am Heart J 1999; 138(3 Pt
1): p. 567-576.
2. Maciel BC, Gallo Junior L, Marin Neto JA, Limo Filho EC, Terra Filho J, Manco JC.
Parasympathetic contribution to bradycardia induced by endurance training in man.Cardiovasc Res
1985; 19(10): p. 642-648.
3. Loimaala A, Huikuri H, Oja P, Pasanen M, Vuori I. Controlled 5-mo aerobic training improves heart
rate but not heart rate variability or baroreflex sensitivity. J Appl Physiol 2000; 89(5): p. 1825-1829.
4. Uusitalo AL, Laitinen T, Vaisanen SB, Lansimies E, Rauramaa R. Effects of endurance training on
heart rate and blood pressure variability.Clin Physiol Funct Imaging 2002; 22(3): p. 173-179.
5. Uusitalo AL, , Laitinen T, Vaisanen SB, Lansimies E, Rauramaa R. Physical training and heart rate
and blood pressure variability-a Five-Year Randomized Trail. Am J Physiol Heart Circ Physiol 2004;
286: p. 1821–1826.
6. Jose AD. Effect of combined sympathetic and parasympathetic blocked on heart rate and cardiac
function in man. Am J Cardiol 1996; 18: p. 476-478.
7. De Meersman, R. E. Heart rate variability and aerobic fitness. Am Heart J 1993; 125:p. 726-731. 8. Lazoglu AH, B. Glace, G. W. Gleim, N. L. Coplan. Exercise and heart rate variability. Am Heart J
1996; 131: p. 825-827.
9. Byrne EA, J. L. Fleg, P. V. vaitkevicius, J. Wright and S. W. Porges. Role of aerobic capacity and
body mass index in the age-associated decline in heart rate variability. J Appl Physiol 1996; 81: p.
743-750.
10. Boutcher SH, Stein P. Association between heart rate variability and training response in sedentary
middle aged men. Eur J Appl Physiol Occup Physiol 1995; 70(1): p. 75-80.
11. Savin WM, Davidson DM, Haskell WL. Autonomic contribution to heart rate recovery from exercise
in humans. J Appl Physiol 1982; 53: p. 1572-1575.
12. Hagberg JM, Hickson RC, Ehsani AA and Holloszy JO. Faster adjustment to and recovery from
submaximal exercise in the trained state. J Appl Physiol 1980; 48: p. 218-224.
13. Darr KC, Basset DR, Morgan BJ and Thomas DP. Effect of age and training status on heart rate
recovery after peak exercise. Am J Physiol 1988; 254: p. 340-343.
14. Baraldi E, Cooper DM, Zanconato S and Armon Y. Heart rate recovery from one minute exercise in
children and adults. Pediatr Res 1991; 29: p. 575-579.
15. Savin WM, Davidson DM and Haskell WL. Autonomic contribution to heart rate recovery from
34 16. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation
and clinical use. Circulation 1996; 93: p. 1043-1065.
17. Jansen MJA, De Bie J, Swenne CA, et al,. Supine and standing sympathovagal balance in athletes and
controls. Eur J Appl Physiol 1993; 67: p. 164-167.
18. Darr KC, Basset DR, Morgan BJ, Thomas DP. Effect of age and training status on heart rate recovery
