The Effect of High Temperature on Cardiovascular Autonomic Function Tests in Steel Plant Furnace Worker.

BACKGROUND: The high temperature in the working environment could be one of the risk factors for cardiovascular diseases in steel plant workers. The excessive high temperature at the working place leads to altered autonomic activity and is related to more cardiovascular risk. AIMS AND
OBJECTIVES: The effect of high temperature on cardiovascular autonomic function tests in steel plant furnace worker. MATERIAL AND
METHOD: This was a case-control study for which 50 steel plant furnace workers and 50 controls were selected. The cardiovascular sympathetic function status in worker and control were analyzed by three tests: (i) Blood pressure (BP) response to sustained isometric handgrip test, (ii) BP response to the cold pressor test, and (iii) BP response to standing from a supine position (orthostasis). Statistical analysis was done by using an independent t-test.
RESULTS: Diastolic BP (DBP) response to sustained isometric handgrip and systolic BP (SBP) and DBP response to the cold pressor test showed that the increase BP in workers were more than controls but the difference was not statistically significant (P > 0.05). SBP response to standing from the supine position showed statistically significant (P < 0.001) greater fall in steel plant workers.
CONCLUSION: The sympathetic autonomic dysfunction was seen in steel furnace workers in response to environmental stressor-excessive heat. Copyright:

INTRODUCTION

Occupational exposure to excessive heat at the workplace is a well-known hazard and has been associated with an increase in both morbidity and mortality.

An annual incidence of occupational morbidity as 42,40,700–1,00, 10,800 cases per year, and an annual mortality of 583,700-704,200 per year.

Workers working near furnaces are exposed to very high temperatures. This high temperature stresses the cardiovascular system among workers.

Morbidity pattern in Iron and steelworkers of central India showed 24.4% of workers had hypertension.[1]

Workers of steel plants exposed to high temperature showed a prevalence of metabolic syndrome (18.8%) and reported a significantly higher abnormal rate of ECG (29.43%).[2]

Altered autonomic activity and exposure to high temperatures for a varied duration of exposure will most likely multiply cardiovascular mortality and morbidity.

We have studied cardiovascular sympathetic function among workers of the steel plant furnace.

METHODS

A case-control study was carried out on 50 steel plant furnace workers and 50 healthy age-matched controls from the general population. Institutional Ethical clearance was obtained (20/4/2019). Cases were male subjects, age between 25 and 50 years who were working in steel plant furnace, hot rolling area, casting platform, reheating furnace for more than 1 year. Controls were age-matched male 25-50 years, apparently healthy. Exclusion criteria were those having any cardiovascular diseases, any respiratory illness, Diabetes mellitus, smokers, tobacco chewers, alcohol consumption, obesity, on medication that can alter cardiovascular functions.

Written informed consent was taken from the cases and controls. Physical examination was carried out with the help of a pre-designed Performa which included name, age, sex, body mass index. All observation was taken between 8 am to 10 am.

Before the start of work for cases and controls, following parameters were measured: room air temperature, core body temperature, pulse rate, and blood pressure (BP). It was followed by three tests to evaluate cardiovascular sympathetic function: BP response to sustained isometric handgrip test, BP response to cold presser test, and BP response to the orthostatic test.

BP response to sustained isometric handgrip test

The participant was asked to sit comfortably on a chair. The baseline BP was measured from the left arm with the help of an automatic BP monitor. The cuff was deflated but not removed. The participant was asked to hold the dynamometer in the right hand and compress it with maximum effort, to find maximum voluntary contraction. The handgrip was then maintained at 30% of that maximum for as long as possible up to 5 min. BP was measured from the left arm at 1-min intervals during handgrip. The result was expressed as the difference between the highest diastolic blood pressure (DBP) during handgrip exercise and DBP reading before the handgrip test. The value of more than 15 mm rise in DBP is taken as normal response and below 10 or less is abnormal.

BP response to cold pressor test

The participant was asked to sit comfortably in a chair for 5 min. The baseline BP was measured from the left arm with the help of an automated BP monitor. Cuff was deflated and kept in position. Then the participant was asked to immerse the hand in cold water maintained at 4–6°C for 1 min. BP was determined every 30 s for 1 min. At the end of 1 min hand was removed. Immediately, the BP was recorded at 30 s interval until the BP returned to normal. The pre-immersion pressure was subtracted from the highest reading obtained during or after immersion. Participants whose SBP increases by 25 or more mmHg or whose DBP increases by 20 or more mmHg are considered hyperreactive.

BP Response to standing from a supine position (orthostasis)

The participant was asked to lie supine on the examination table for 5 min. The baseline BP was recorded from the left arm with the help of an automated BP monitor and the cuff is deflated and kept in position. The participant was asked to stand up in 3 s and immediately BP was be recorded again from the same arm. The difference between the baseline SBP and SBP after standing was be noted. The value of <10 mmHg fall in SBP was taken as normal.

After 2 h of exposure to the hot environment in the steel plant furnace the room air temperature, core body temperature, pulse rate was measured and all the cardiovascular sympathetic function tests were repeated for the cases.

After 2 h of daily work, the room air temperature, core body temperature, pulse rate was measured and the entire cardiovascular sympathetic function test was repeated for the controls.

RESULT AND ANALYSIS

Study was conducted on total 50 numbers of cases and 50 age and BMI - matched controls [Table 1].

Table 1

Demographic data

	CASE	CONTROL	P
Numbers	50	50
Age	35.67±4.74 Years	34.72±3.46 Years	>0.5
Height	158.25±4.16 cm	159.97±7.7 cm	>0.5
Weight	56.4±4.21 kg	57.77±7.91 kg	>0.5
BMI	22.81±1.75	21.60±1.55	>0.5

The room temperature in working area before start of work was 32±0.75 °C and after 2 hours the temperature raised to 55±0.98 °C. The change in the room temperature after 2 hours of starting of work was statistically significant (<0.05) as compared to the controls. The core body temperature of the cases at start of work was 37+ 0.440C and after two hours of work the core body temperature of cases raised to 39 + 0.34 °C. The change in the core body temperature of cases after 2 hours of starting of work was statistically significant (<0.05) as compared to the controls [Table 2].

Table 2

Comparison of Room temp and Body core temp in 0c

	Room temp		P	Body core temp		P
	Before	After 2 hrs		Before	After 2 hrs
Cases	32±0.75	55±0.98	<0.05	37±0.44	39±0.34	>0.05
Controls	32±0.89	35±1.02	>0.05	37±0.13	37±0.65	<0.05

The change in the heart rate of cases after two hour of work was 32.58±0.96 per minute which is statistically significant as [Table 3]. The rise in heart rate in cases were statistically greater as compared to the controls.

Table 3

Comparison of heart rate in cases before exposure to heat and after 2 hr of exposure to heat

Group	Before start of work	After 2 hr of work	Change in HR	P	P
Cases	76.83±6.19	108.41±7.15	32.58±0.96	<0.001*	<0.5
Controls	79.77±3.29	91±4.37	12±1.08	>0.5

The rise in the diastolic blood pressure in response to sustained isometric handgrip test after 2 hr [Table 4] was increased in cases but difference was not statistically significant in the cases as compared to the controls.

Table 4

Comparison of diastolic blood pressure rise in response to sustained isometric handgrip test after 2 hr of working

Group	Pre-test DBP‡ (mmhg)	DBP During handgrip. (mmhg)	Change in DBP (mm hg)	P
Cases	76.83±6.19	93.41±7.15	16.58±0.96	>0.05
Controls	80.29±5.30	93.5±6.06	15.21±0.76

The rise in the systolic blood pressure [Table 5] and diastolic blood pressure [Table 6] in cold pressor test after 2 hr of working was not statistically significant in the cases as compared to the controls.

Table 5

Comparison of systolic blood pressure rise in cold pressor test after 2 hr of working

Group	Sitting SBP (mmhg)	SBP after 1 min of cold immersion (mmhg)	Change in SBP (mmhg)	P
Cases	111±9.21	133.3±9.56	22.21±2.82	>0.05
Controls	120.1±9.24	140.4±9.06	20.30±2.97

Table 6

Comparison of diastolic blood pressure rise in cold pressor test after 2 hr of working

Group	Sitting DBP (mmhg)	DBP after 1 min of cold immersion (mmhg)	Change in DBP (mmhg)	P
Cases	76.83±6.19	97.41±7.18	20.58±3.73	>0.05
Controls	80.29±5.30	99.52±6.06	19.23±2.98

The fall in systolic blood pressure in orthostatic test after 2 hr of working [Table 7] was statistically significant in the cases as compared to the controls.

Table 7

Comparison of SBP fall in orthostatic test after 2 hr of working

Group	Supine SBP (mm hg)	SBP immediately after standing (mm hg)	Change in SBP (mm hg)	P
Workers	126.92±9.27	119.7±9.50	7.22±2.98	<0.001*
Controls	122.1±9.28	109.7±10.4	12.4±2.93

DISCUSSION

Society for Occupational Health indicates the Occupational Exposure Limits of heat stress for working people in hot environments.[3] In our study Wet Bulb Globe Thermometer (WBGT) is above 35°C and after 2 h exposure to working in a hot environment, heart rate (HR) of workers increased significantly. Elevated heart rate did not reach baseline even after rest because of thermal stress on workers similar results noted in glass bangle workers.[4]

Heart rate variability (HRV) showed that that high frequency (HF %) significantly decreased and the LF/HF ratio significantly increased after 30 min exposure under the heated condition at WBGT 35°C.[5]

Norepinephrine amount under 30-min exposures under heated condition was greater[67] and repeated exercise in heat increased norepinephrine and epinephrine levels.[8]

In heat stress circulating blood volume has to do additional work to transport oxygen and also act as cooling fluid. Nitric oxide has a vasodilator effect which is increased and shifts heated blood from central circulation to the periphery to dissipate heat.[9]

As because of heat, dehydration, and hypovolemia stroke volume can't be increased beyond the limit so this is achieved by increasing HR.

This indicates that the HR increased because there is a substantial withdrawal of parasympathetic activity and an increase in sympathetic activity along with that increase in the level of catecholamine.[10] This entire act as cardiovascular stress.

Muscle metaboreceptor stimulation and central command may contribute to a decrease in cutaneous venous conductance (CVC) in heat stress workers after handgrip.[11]

Heat stress attenuates the increase in arterial blood pressure during isometric handgrip exercise.[12] But, isometric handgrip exercise increase in BP was more compared with normothermic.[13]

BP response to systemic infusion of α-adrenergic agonists was decreased in heat-stressed rats.[14] So heat stress may impair neurovascular transduction and attenuates blood pressure response.

Under heat stress, 50% of cardiac output is distributed to the skin to disseminate heat as compared to normothermic. Cardiac output going to the skeletal muscle will be less and an increase in sympathetic activity to skeletal muscle during heat stress is not sufficient to elevate blood pressure and no cutaneous vasoconstriction during post-exercise isometric handgrip test.

In the hyperthermia, metaboreceptor stimulation increases conductance to nonhairy skin but nonglabrous cutaneous vessel is not altered.[15]

Our results are similar to a study conducted by Crandall CG that pressure increased by metaboreceptor during high temperature was less than normothermia.[16]

Attenuated blood pressure response to isometric handgrip exercise is because elevated temperature directly affects the sensitivity of muscle afferent by increasing blood flow and alters the chemical composition of interstitial fluid with low k level and decreased sensitivity of mechanoreceptor.[17]

CPT mainly increases vascular resistance and an increase in muscle sympathetic nerve activity (MSNA) via nonbaroreflex mechanisms.[1819] Postsynaptic alpha-adrenergic vasoconstriction response in muscle and other beds is impaired. So elevation of vascular resistance in response to CPT will be decreased and it attenuates BP rise.

We didn't get significant differences between control and subjects because when a heat-stressed hand with high temperature was immersed in cold water, it would alter the temperature of cold water, and the expected difference between the last minute of CPT would be minimal. So painful stimuli that was responsible for sympathoexcitation was expected to identical in both subject regardless of thermal condition.

As much of cardiac output is going to skin and so increase in noncutaneous vascular resistance to CPT may be important to elevate blood pressure. The effectiveness of increases in vascular resistance from noncutaneous beds would be minimized relative to when subjects are normothermic.

We found a greater fall in Systolic Blood Pressure (SBP) in the orthostatic test after 2 h working as compared to controls.

Orthostasis reduces ventricular filling and thus stroke volume. To overcome this baroreflex and sympathetic nervous system activate various cardiovascular mechanisms to maintain cardiac output in the form of an increase in HR, cardiac contractility, and vascular resistance.[2021]

Inadequate cardiac output is a key mechanism by which heat stress alters orthostatic tolerance.

By reducing systemic vascular resistance and increasing blood volume in cutaneous vasculature, stroke volume is compromised.[22]

Cardiac preload affects stroke volume and the relationship between these is not linear. Heat stress shift Frank starling curve close to steep portion.[23]

This will cause a more profound decrease in stroke volume when compared with normothermic.

To maintain stroke volume, heat stress increases cardiac contractility independent of frank starling law.[24]

Cardiac contractility is greater during the combined effect of heat and orthostatic stress.[25]

An increase in cardiac contractility with heat stress occurs via an increase in sympathetic activity and parasympathetic withdrawal.[26]

HR also increases due to primarily due to baroreceptor unloading in response to fall in blood pressure.[20]

Heat stress impairs orthostatic intolerance and to overcome this again there is an increase in her rate and contractility by parasympathetic withdrawal and sympathoexcitation.

Limitation

We used only baseline tests to evaluate the sympathetic function and a more detailed study needed with continuous monitoring of HR with spectral analysis, catecholamine levels, ECG changes, and follow-up for long-term basis to know cardiovascular diseases.

CONCLUSION

High temperature in the working place is harming the cardiovascular system. It increases cardiovascular sympathetic outflow and because of that cardiovascular system is under continuous strain during high temperature. This sympathoexcitation could increase mortality and morbidity among workers having cardiac disease also it could increase the risk of cardiovascular disease in the future among workers working in high temperatures.

Implications

1. This study indicates that further research is needed to find the outcome of excessive temperature and Thrombo-embolic and cardiovascular events by measuring vascular autonomic function test, radiotracer technique, plasma and urinary catecholamine, coagulation parameters and Echocardiography.

2. Preventive measures such as Yoga training, shift work, cool coats, proper cooling at the workplace can be suggested to modulate autonomic activity.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.