Assessment of heart rate response to exercise and recovery during treadmill testing in arsenic‐exposed workers

Abstract

Background

Arsenic exposure is associated with various cardiovascular diseases. The aim of the present study was to assess cardiac autonomic function via heart rate response to exercise and recovery period of treadmill testing in arsenic‐exposed workers.

Methods

Sixty‐five (65) occupationally arsenic‐exposed workers and 35 healthy controls were enrolled. Blood and urinary arsenic levels were analyzed and symptom limited maximal treadmill exercise test were performed. Chronotropic response to exercise including age‐predicted maximal heart rate (APMHR), heart rate reserve (HR _reserve), age‐predicted HR _reserve (APHR _reserve) and adjusted HR _reserve and 1st‐, 2nd‐and 3rd‐min heart rate recovery (HRR) indices were calculated.

Results

Baseline clinical and echocardiographic parameters, exercise test duration, resting and maximal heart rate, peak exercise capacity, HR _reserve, APMHR, APHR _reserve, and adjusted HR _reserve were found to be similar between groups. HRR ₁ (22.0 ± 4.3 vs. 24.3 ± 3.1 bpm, p = .003) and HRR ₂ (43.2 ± 6.2 vs. 46.7 ± 6.4 bpm, p = .012) were significantly lower in arsenic‐exposed workers compared to controls. Blood and urinary arsenic levels negatively correlated with HRR ₁ (r = −.477, p < .001 and r = −.438, p < .001, respectively) and HRR ₂ (r = −.507, p < .001 and r = −.412, p < .001 respectively).

Conclusions

Arsenic‐exposed workers had lower HRR indices than normal subjects but chronotropic response were similar. Cardiac autonomic dysregulation may be one of the cardiovascular consequences of arsenic exposure.

1. Introduction

Arsenic exposure is associated with a wide range of cardiovascular disorders including coronary heart disease, hypertension, aortic diseases, and increased risk of cardiovascular mortality (Chen, 2016; Farzan, 2015; Gunduz, 2015; Islam, 2012; James, 2015). Cardiovascular effects of arsenic exposure are not limited to these disorders, but also have a significant influence on cardiac conduction system (Alamolhodaei, 2015; Stea, 2014). In previous studies, relationship between arsenic exposure, cardiac arrhythmias and various electrocardiographic parameters particularly QT interval has been analyzed (Ahmad, 2006; Chen, 2013). These studies suggested that arsenic has an influence on conduction system via calcium and potassium ions. Hereby, arsenic leads to prolongation of action potential duration and delays cardiac repolarization (Mordukhovich, 2009; Mumford, 2007).

Heart rate increases in response to exercise and begins to decrease with recovery period. Exercise is associated with increased sympathetic and decreased parasympathetic activity resulting in an acceleration of heart rate. The attenuated heart rate response to exercise defined as chronotropic incompetence (CI) (Kawasaki, 2010). On the contrary, recovery period is characterized by a combination of sympathetic withdrawal and parasympathetic reactivation resulting in decline in heart rate defined as heart rate recovery (HRR) (Okutucu, 2011). It has been shown that CI and blunted HRR reflect cardiac autonomic nervous system (ANS) activity and predict future cardiovascular events (Zafrir, 2016). Although it has been shown that many heavy metals have a negative effect on cardiac ANS (Feng, 2015; Takeshita, 2004), there are few studies investigated the effects of arsenic on cardiac ANS.

The aim of this study was to investigate the heart rate response to exercise and recovery in arsenic‐exposed workers without known cardiovascular diseases compared with healthy controls.

2. Methods

2.1. Study population

In the present cross‐sectional study, 123 male workers occupationally exposed to arsenic who referred to the clinic between January 2015 and December 2015 were enrolled. Medical history, physical examination findings including height (meter), weight (kg), body mass index calculated as weight/height², blood pressure measurements and laboratory findings were recorded. 12‐lead surface electrocardiogram and transthoracic echocardiography were performed. Those with coronary artery disease, preexisting hypertension defined as systolic blood pressure >140 mm Hg and diastolic blood pressure >90 mm Hg and/or taking antihypertensive medication, valvular heart disease except trivial regurgitations, diabetes mellitus, hyperlipidemia, cigarette smokers, and alcohol consumers were excluded. Also excluded were those with rhythm and/or conduction abnormalities such as bundle branch block that can be very hard or impossible to interpret exercise test and drug use which has effects on autonomic system and exercise performance (including beta‐blocking agents, calcium channel blocking agents, anti‐arrhythmics, tricyclic antidepressants, and antipsychotics). A total of 58 workers were excluded and of the remaining 65 workers included, 32 (49.2%) were employed in metal recycling, 23 (35.3%) were firefighters and 10 (15.5%) were chemical manufacturing industry workers. All worked 8 hr a day, 5 days a week. The majority of workplaces were small or medium sized enterprises. Therefore, environmental exposure/toxic substance testing was rarely performed. Even when performed, accurate values could often not be obtained due to incorrect methods of measurement and inadequate data. The control group included 35 gender and age matched healthy subjects with no previous history of cardiac disease. All study population were over the age of 18 years and men. Written informed consent was obtained from each subject and the institutional ethics committee approved the study protocol.

2.2. Collection of biological samples and analysis methods

Blood samples were obtained at the end of workshift and urinary arsenic levels in morning spot urine samples were determined using Inductively Coupled Plasma Mass Spectrometry (ICP‐MS) (7700 series; Agilent Technologies, Inc., Santa Clara, CA, USA). Blood samples were digested by the microwave induced acid digestion method. Urine samples were collected in sterile plastic pots and then diluted 1 in 10 with 5% nitric acid solution. Standard solution of arsenic was prepared by dilution of certified standard solutions (High Purity Standards, Charleston, SC, USA). Two levels of quality control materials were used (Seronorm; Sero AS, Billingstad, Norway). Arsenic calibration curve ranged from 0 to 100 μg L⁻¹.

2.3. Treadmill exercise testing

Treadmill exercise testing was conducted using modified Bruce protocol. Mason‐Likar modification of 12‐lead electrocardiogram was continuously recorded at 25 mm s⁻¹ paper speed (Papouchado, 1987). A progressive, exhaustive and symptom limited exercise test were performed. Heart rate were recorded immediately before exercise (HR_rest) and at maximum exercise (HR_peak). Exercise capacity was measured by metabolic equivalent level at peak exercise. Dynamic exercise related parameters including age‐predicted maximum heart rate (APMHR), heart rate reserve (HR_reserve), age‐predicted heart rate reserve (APHR_reserve) and adjusted HR_reserve were calculated (Brubaker, 2011):

$$\text{APMHR (bpm)}=220-\text{Age (year)}$$

$${\text{HR}}_{\mathrm{reserve}}\text{(bpm)}={\text{HR}}_{\mathrm{peak}}-{\text{HR}}_{\mathrm{rest}}$$

$${\text{APHR}}_{\mathrm{reserve}}\text{(bpm)}=\text{APMHR}-{\text{HR}}_{\mathrm{rest}}$$

$$\text{Adjusted}{\text{HR}}_{\mathrm{reserve}}(\%)={\text{HR}}_{\mathrm{reserve}}/{\text{APHR}}_{\mathrm{reserve}}$$

Chronotropic incompetence diagnosed when subjects failed to achieve 85% of APMHR and 80% of adjusted HR_reserve (Brubaker, 2011). After achieving peak workload, all subjects spent at least 3 min of “active” recovery by walking slowly. HRR indices were calculated by subtracting the 1st‐(HRR₁), 2nd‐(HRR₂), and 3rd‐min (HRR₃) heart rates from the maximum heart rate obtained during stress test. Abnormal HRR is defined as a decrease in HRR₁ of <18 bpm or in HRR₂ of <42 bpm (Cole, 2000; Ghaffari, 2011).

2.4. Electrocardiography

12‐lead electrocardiography was recorded at a paper speed of 50 mm s⁻¹ (Nihon Kohden, Tokyo, Japan) at rest in supine position. The following ECG features were studied by a cardiologist who was blind to participant data and verified by a second physician to avoid any error in measurements: Resting heart rate, QT interval, corrected QT interval (QTc), Tp‐e interval and Tp‐e/QT ratio. QT interval was measured manually from the onset of QRS complex to the point at which the tangent of maximal down‐slope of the descending limb of T wave crossed isoelectric baseline. QT interval was corrected for heart rate using the Bazett formula: $\text{QTc}=\text{QT}/\sqrt{\text{RR interval}}$ (Funck‐Brentano, 1993). Tp‐e interval was defined as the interval from peak to end of T wave. Measurements of Tp‐e interval were performed from precordial leads (Castro Hevia, 2006). Finally, the Tp‐e/QT ratio was calculated from these measurements.

2.5. Transthoracic echocardiography

Standard TTE imaging was performed according to recommendations of the recommendations of the American Society of Echocardiography (Lang, 2005). Images were obtained using a 2.5–3.5 MHz transducer in left lateral decubitus position in parasternal and apical views. Left ventricular end‐diastolic and end‐systolic diameters were determined with M‐mode echocardiography under 2‐dimensional guidance in parasternal long‐axis view. Left ventricular ejection fraction was calculated in apical 4‐chamber view, according to modified biplane Simpson's rule.

Mitral inflow velocities were measured by pulsed wave Doppler from apical 4‐chamber view positioned at the tips of mitral leaflets at end‐expiration. Peak early filling velocity (E wave), peak filling velocity during atrial systole (A wave), E/A ratio, and deceleration time of early filling velocity were calculated. Isovolumic relaxation time was derived using continuous wave Doppler by placing the cursor in left ventricular outflow and mitral inflow simultaneously and recorded as time interval between aortic valve closure and mitral valve opening. Peak early diastolic tissue velocity (Eʹ) of mitral annulus was measured from 4‐chamber view. Sample volume was positioned on septal (medial) and lateral insertion sites of mitral valve. At least three consecutive measurements were obtained and Eʹ wave was recorded as the average of septal and lateral measurement. E/Eʹ ratio, was calculated as E wave divided by Eʹ.

2.6. Statistical analysis

Statistical analysis was performed using SPSS software (version 20.0; SPSS Inc., Chicago, IL, USA). Variables with normal distribution were analyzed using Kolmogorov–Smirnov test and presented as mean ± SD, while those without normal distribution were presented as median with minimum and maximum range. Categorical variables were presented as number and percentage. Comparison of continuous variables was performed with t test for independent variables with normal distribution, and with Mann–Whitney U test for those without normal distribution. Pearson correlation analysis was used to test normalcy of distribution, and Spearman correlation analysis was used for variables without normal distribution. p < .05 was considered statistically significant.

3. Results

Baseline demographic, clinical and laboratory characteristics are shown in Table 1 No statistically significant difference was found between the groups regarding age, body mass index, systolic and diastolic blood pressures, and laboratory findings. Mean age of arsenic‐exposed and control groups were similar (44.3 ± 9.8 vs. 43.9 ± 9.0 years, p = .830). Median blood arsenic (24 μg dl⁻¹ [10–48] vs. .3 μg dl⁻¹ [.1–1.1], p < .001) and urinary arsenic levels (73 μg L⁻¹ [35–246] vs. .9 μg L⁻¹ [.1–3.2], p < .001) were found to be significantly higher in arsenic‐exposed workers, compared to the control group. In arsenic‐exposure group, median duration of exposure was 26 months (6–60 months).

Table 1.

Baseline demographic, clinical, laboratory and serologic data of the groups

Variables	Arsenic‐exposed group (n = 65)	Control group (n = 35)	p
Age (years)	44.3 ± 9.8	43.9 ± 9.0	.830
Body mass index (kg m−²)	24.6 ± 4.9	23.8 ± 4.1	.413
Systolic blood pressure (mm Hg)	125.3 ± 8.1	125.7 ± 8.1	.840
Diastolic blood pressure (mm Hg)	75.3 ± 7.2	74.7 ± 7.0	.691
Hemoglobin (g dl−1)	14.5 ± 1.7	14.9 ± 1.2	.146
White blood cell count (/μL)	7268 ± 1605	6995 ± 1455	.391
Fasting blood glucose (mg dl−1)	87.5 ± 8.7	88.9 ± 9.6	.476
Creatinine (mg dl−1)	.88 ± 0.18	.90 ± .11	.534
Total cholesterol (mg dl−1)	187.6 ± 36.0	185.31 ± 36.5	.761
Low density lipoprotein (mg dl−1)	119.0 ± 23.5	117.9 ± 26.7	.827
High density lipoprotein (mg dl−1)	46.4 ± 9.7	44.3 ± 8.9	.276
Triglyceride (mg dl−1)	155.9 ± 56.8	158.6 ± 60.8	.828
Blood arsenic level (μg dl−1)	24 [10–48]	.3 [.1–1.1]	<.001
Urine arsenic level (μg L−1)	73 [35–246]	.9 [.1–3.2]	<.001

Transthoracic echocardiographic and electrocardiographic measurements of the groups are shown in Table 2 All echocardiographic parameters and QT, QTc, and Tp‐e intervals and Tp‐e/QTc ratio were similar between the groups.

Table 2.

Transthoracic echocardiographic and electrocardiographic parameters of the groups

Variables	Arsenic‐exposed group (n = 65)	Control group (n = 35)	p
LV end‐diastolic diameter (cm)	4.6 ± .3	4.6 ± .2	.682
LV end‐systolic diameter (cm)	2.8 ± .3	2.8 ± .2	.299
LV ejection fraction (%)	64.4 ± 3.7	64.9 ± 2.8	.513
E wave (cm s−1)	109.3 ± 11.5	111.7 ± 8.5	.233
A wave (cm s−1)	86.9 ± 12.0	85.1 ± 10.8	.453
E/A ratio	1.2 ± .2	1.3 ± .2	.164
Eʹ wave (cm s−1)	13.0 ± 1.2	13.2 ± 1.0	.306
E/Eʹ ratio	8.5 ± 1.3	8.4 ± .9	.970
Deceleration time (ms)	181.8 ± 15.6	176.7 ± 15.1	.118
Isovolumic relaxation time (ms)	81.7 ± 7.4	80.7 ± 6.8	.471
QT interval (ms)	357.0 ± 22.4	363.4 ± 22.0	.172
QTc (ms)	397.1 ± 23.1	403.8 ± 20.4	.140
Tp‐e interval (ms)	78.2 ± 13.0	80.5 ± 10.5	.342
Tp‐e/QTc ratio	.19 ± .03	.19 ± .02	.739

LV, Left ventricular.

All subjects had normal resting electrocardiogram and completed the exercise stress test without complication. No rhythm abnormalities or ischemic changes were observed during the tests. Exercise test parameters of the groups are shown in Table 3 arsenic‐exposed workers and control subjects had similar exercise test duration, HR_rest, HR_peak, peak exercise capacity, HR_reserve, APMHR, APHR_reserve, and Adjusted HR_reserve. The number of subjects failed to reach 85% of APMHR and adjusted HR_reserve below 80% were also similar between the groups. Although HRR₃ did not show difference, mean HRR₁ values (22.0 ± 4.3 vs. 24.3 ± 3.1 bpm, p = .003) and mean HRR₂ values (43.2 ± 6.2 vs. 46.7 ± 6.4 bpm, p = .012) were significantly lower in the group exposed to arsenic than in control group.

Table 3.

Exercise test parameters, chronotropic incompetence, and heart rate recovery indices of arsenic exposed workers and control subjects

Variables	Arsenic‐exposed group (n = 65)	Control group (n = 35)	P
Duration of exercise test (min)	9.0 ± 1.3	8.9 ± 1.2	.647
HRrest (bpm)	74.9 ± 9.5	74.6 ± 8.6	.901
HRpeak (bpm)	163.6 ± 13.5	165.1 ± 14.1	.610
Peak exercise capacity (METs)	12.2 ± 1.5	12.6 ± 1.3	.269
HRreserve (bpm)	88.7 ± 16.8	90.4 ± 14.7	.595
APMHR (bpm)	175.7 ± 9.8	176.1 ± 9.0	.830
<85% of APMHR (n)	14	7	.858
APHRreserve (bpm)	100.8 ± 13.6	101.4 ± 13.0	.814
Adjusted HRreserve (%)	88.7 ± 16.3	90.1 ± 16.3	.693
<80% of Adjusted HRreserve (n)	20	10	.820
HRR1 (bpm)	22.0 ± 4.3	24.3 ± 3.1	.003
HRR2 (bpm)	43.2 ± 6.2	46.7 ± 6.4	.012
HRR3 (bpm)	57.1 ± 5.7	59.0 ± 5.8	.115

APHR_reserve, Age predicted heart rate reserve; APMHR, Age predicted maximum heart rate; bpm, Beats per minute; HR, Heart rate; HRR, Heart rate recovery; METs, Metabolic equivalents.

In correlation analysis, blood arsenic level was found to be negatively and significantly correlated with HRR₁ (r = −.477, p < .001) (Figure 1a) and HRR₂ (r = −.507, p < .001) (Figure 1b) in arsenic‐exposed workers. Likewise urinary arsenic level was found to be negatively and significantly correlated with HRR₁ (r = −.438, p < .001) (Figure 2a) and HRR₂ (r = −.412, p < .001) (Figure 2b). Additionally, HR_rest showed a weak correlation with blood (r = .249, p = .046) and urinary (r = .332, p = .010) arsenic level.

Figure 1.

Graphics of correlation between blood arsenic level and (a) first minute and (b) second minute heart rate recovery values. Solid lines represent the correlation slope, and dotted lines represent 95% confidence intervals

Figure 2.

Graphics of correlation between urinary arsenic level and (a) first minute and (b) second minute heart rate recovery values. Solid lines represent the correlation slope, and dotted lines represent 95% confidence intervals

4. Discussion

The present study is the first to evaluate chronotropic response and HRR by using treadmill exercise test which is more simple, easy to use, inexpensive and widely available tool for assessment of cardiac autonomic functions in arsenic‐exposed workers. While dynamic exercise and chronotropic parameters were similar to control group, HRR₁ and HRR₂ were found to be lower. In addition, blood and urinary arsenic levels were evaluated and occupational exposure was the primary source of arsenic in present study. Furthermore, decline of HRR₁ and HRR₂ indices were correlated with blood and urinary arsenic levels.

Heart rate is under tight control of cardiac ANS that has a dynamic balance between sympathetic and parasympathetic activation. During rest, heart rate is normally between 60 and 100/bpm. Exercise results in an acceleration of heart rate caused by sympathetic dominance and vagal withdrawal. Chronotropic response to exercise has prognostic significance. CI described as failure to achieve 85% of the APMHR or 80% of adjusted HR_reserve (Brubaker, 2011) and has been associated with increased cardiovascular mortality (Lauer, 1999). In the present study, the percentages of subjects failed to reach 85% of the APMHR and 80% of adjusted HR_reserve were found to be approximately 20% and 30%, respectively, and this difference was not significant. Additionally exercise duration, HR_peak, and achieved peak exercise capacity were also similar in arsenic‐exposed workers and control subjects. These findings suggested that sympathovagal balance during exercise did not change during dynamic exercise in case of arsenic exposure.

Heart rate recovery indicates decreased heart rate in recovery phase and attenuated HRR reflects reduced parasympathetic nervous system activity (Okutucu, 2010; Vivekananthan, 2003). Although suggested threshold for normal response to recovery are still controversial, most widely used thresholds are ≥12 bpm at 1 min for supine recovery (passive), ≥18 bpm at 1 min for upright position (active) and ≥42 bpm at 2 min (Cole, 2000; Ghaffari, 2011). In the present study, all subjects completed recovery period by walking slowly. Although within normal limits in arsenic‐exposed workers and control subjects, HRR₁ and HRR₂ indices were significantly lower in workers. Beyond determined cut‐off value, significant decline in HRR values is important result due to providing information about autonomic functions in this population. Moreover, study group consisted of individuals without known cardiovascular diseases or risk factors. Thus, mean HRR values should not be expected below the cut‐off values in such a low‐risk group. As it is seen in CI, there are “individuals” with abnormal HRR values, but “mean” HRR values of “groups” are within normal limits. At this point, interpretation of results “compared to control” will be more accurate. Upon cessation of exercise, augmentation of parasympathetic effects on HR occurred rapidly within the first minute. Therefore, analyzing the HRR₁ is more important when evaluating cardiac autonomic function. Higher statistical difference observed in HRR₁ and the lack of difference in HRR₃ may be due to more dominant role of autonomic functions in the first minutes of recovery phase.

Results of the present study suggest that arsenic exposure might impair heart rate response to recovery period after exercise but not to dynamic exercise. Although HRR that mainly under parasympathetic dominance was blunted, chronotropic response that dominated mainly by sympathetic activity did not differ. Thus, it could be hypothesized that arsenic has mainly influenced on parasympathetic activity rather than sympathetic activity. There are some experimental studies suggested this hypothesis in literature. It has been demonstrated that arsenic could inhibit cholinergic system in rat brain (Kobayashi, 1987; Srivastava, 2014). In an another study, Fonseca et al. reported that arsenite inhibits release and synthesis of acetylcholine and exhibits anticholinergic effects within central and peripheral nervous system (Fonseca, 1991).

4.1. Study limitations

In the present study, only blood and urinary arsenic levels were analyzed. Because arsenic has a high affinity for sulfhydryl groups, it accumulates in keratin‐rich tissues such as toenails and hair and arsenic in these tissues may better predict long‐term exposure than does blood and urinary levels. Furthermore, due to the cross‐sectional design of the present study, no follow‐up data were available, and no cardiovascular outcomes were defined. Finally, arsenic related health problems can vary as a result of working conditions and exposure route. However, measurements of workplace environment and further analysis of working group related end‐points have not been performed. Thus, findings of the present study need confirmation in larger studies.

5. Conclusion

The present study showed that arsenic exposure was associated with blunted HRR, sympathovagal imbalance in recovery period after exercise and blood and urinary arsenic concentrations were negatively correlated with HRR₁ and HRR₂. However, chronotropic response and dynamic exercise related parameters did not differ compared to control subjects. These finding may be the consequence of negative effects of arsenic on cardiac ANS.

Conflict of Interest

None.

Karakulak UN, Gunduzoz M, Ayturk M, Ozturk MT, Tutkun E, Yilmaz OH. Assessment of heart rate response to exercise and recovery during treadmill testing in arsenic‐exposed workers. Annals of Noninvasive Electrocardiology. 2017;22:e12437 10.1111/anec.12437