European Journal of Cardiovascular Medicine

Since 1975, obesity has nearly tripled everywhere Worldwide. Obesity is a global epidemic characterized by significant hemodynamic and metabolic alterations in health, resulting in reduced life expectancy and/or increased health problems.(1)The autonomic nervous system of obese individuals is chronically altered.(2) Since the autonomic nervous system is involved in the energy balance and regulation of the cardiovascular system, its alteration can be strongly implicated in the development of obesity and pathophysiology of varied cardiovascular, metabolic and respiratory physiology complications.(3)There is strong epidemiological evidence indicating reduced FEV1 as a marker for cardiovascular mortality independent of age, gender, and smoking history.(4)

Among the different non-invasive techniques available for the assessment of autonomic cardiovascular status, the cold pressor test is considered being a simple, non-invasive, and validated test of sympathetic activation.(5)  The cold pressor test in healthy subjects triggers a vascular sympathetic activation and an increase in blood pressure. Acute exposure to cold may cause an increase in the respiratory rate due to sympatho-vagal discharge.(6)

In studies using the cold pressor test, it has been seen that there is a significant increase in the lung function parameters induced by pain causing sympathetic and parasympathetic overactivity.(7)(8) These findings indicate to a large extent that stimulation of respiration should form an integral part of cold-induced pain. The heart rate and blood pressure responses can be used as indicators of global sympathetic activation, and thus, of cardiac status. However, there are only a few reports suggesting changes in pulmonary functions during pain-induced autonomic responses. It is also not known which components of pulmonary response show specific changes secondary or concomitant to these autonomic alterations. Hence, the present study was undertaken to answer some of these questions.

Study Design: (Figure 1)

Study Setting: This study is an analytical study conducted One and half year in Department of Physiology UPUMS, Saifai, Etawah after approval of Ethics Committee of the Institute.

Study population (Participants): The current study was planned to estimate differences between cardiovascular autonomic and dynamic lung response in normal BMI, overweight and obese subjects. Since the study involves multiple measurements and three groups, the minimum difference in DBP between any two groups was used for sample size calculation. A previous study has estimated the difference in DBP (mmHg) for lean and obese subjects to be 69.3±2.9 and 72.3±2.4 respectively. If we assume a similar difference in our study population, we would need to study 29 individuals in each group to be able to reject the null hypothesis that the difference in DBP of subjects belonging to these groups is equal with a power of 80%. The type 1 error associated with this study of difference is 0.05. Allowing for 20% loss of subjects owing to withdrawals, etc, we decided to increase the sample size by 20%. Thus, we decided to include 35 subjects in each of the study group.

A written consent was taken from all the participants. They were randomly divided into three groups of 35 subjects each according to their BMI as per the WHO classification. Subjects having BMI between 18.5 to 24.9 were kept in Normal group, BMI between 25.0 to 29.9 kept in overweight group and BMI >30 were kept in obese group.

All the subjects were screened clinically to rule out any comorbidities. Persons with history of smoking, diabetes mellitus, hypertension, angina, arrhythmia, myocardial ischemia, peripheral ischemic disease with documented claudication, respiratory system diseases, neurological diseases, female in menstrual phase, persons with Haemoglobin (Hb) < 10 gm% and deranged thyroid profile were excluded from the study.

The subjects were instructed to avoid caffeine and nicotine for at 3-4 hours and alcohol for 8 hours before testing. Standardization of test conditions was done to make them comparable, especially during the assessment of changes in respiratory parameters. All the subjects were tested upon between 9 am to 12 noon to rule out any alterations due to diurnal variations. Directly before testing, the subjects were laid down for about 30 minutes in a quiet room with room temperature and humidity. Then they were allowed to sit comfortably in a chair and breathe normally. Periscope, a PC based Cardiovascular Analysis System  (include make of instrument) was used for the recording of blood pressure, Heart Rate, pulse pressure and mean arterial pressure.(9) All the recordings of Periscope were done for about 10 seconds and data was stored in the computer for analysis.

After the baseline record, the subjects were informed about the cold pressor test i.e he or she will be made to immerse his non - dominant hand in ice cold water at 4 ± 1⁰C contained in a beaker. Mercurial thermometer (make) was used for the temperature measurement of cold water. The subjects were asked to inform when he started feeling the pain and take out his hand when the pain became intolerable and the subject was made to dip his nondominant hand upto the first proximal wrist crease in a beaker filled with tap water at room temperature and dry it and after 5 minutes again cardiovascular parameters were recorded.

The subjects were called again after one week and their baseline FEV1% was recorded by Easy One® spirometer. All the study subjects of the three groups were subjected to go for cold pressure test (CPT) as described. The subject was asked to inform when he started feeling intolerable and simultaneously record FEV1.

As soon as the pain became intolerable and subject took his hand out, the second stopwatch was stopped. The subject's hand was wrapped in a towel and to immerse his hand in the beaker containing tap water at room temperature and FEV1 recorded again after' a gap of 5-10 minutes.

Cold pressor test was employed as the sympatho-excitatory tool to see the changes in the baseline FEV1 in all the groups.

Statistical analysis: STATA software (S13.0) was used to conduct statistical analysis. Two sample t- test with equal variance was conducted to determine the difference between cardiac and respiratory parameters among normal BMI, Overweight and Obese groups. Paired t- test was used to evaluate the difference in the value of parameters during and after the CPT within the group.

Total 105 subjects of age between 25 to 35 years, qualifying the inclusion criteria took part in the study. They were divided into three categories according to their BMI values viz. Normal BMI, Overweight and Obese group, each having 35 subjects. (Figure 2).

Figure 2: Distribution of the participants according to the BMI

Table 1: Comparison of cardiac and respiratory parameters among the three groups before, during and after the CPT

Note: NW=normal weight, OW= overweight, OB = obese; ¹ = Unpaired t-test; ^* = p-value < 0.05 (significant)     

Table 2: Difference of Parameters within the groups

Note: NW=normal weight, OW= overweight, OB = obese; ¹ = paired t-test; ^* = p-value <   0.05   (significant)

Systolic BP

The mean systolic BP before CPT was 120.48±8.86 mmHg, 122.48±6.04 mmHg and 128.94±9.23 mmHg in normal weight, overweight and obese group respectively (Chart1). On determining the pairwise difference in the means of the groups using unpaired t- test, the difference in the mean systolic BP of OB group was statistically significant and higher as compared to NW and OW groups (P-value <0.05). It indicated that the OB had already increased sympathetic tone as compared to others. The mean Systolic BP during CPT was 135.45±13.21 mmHg, 138.17±17.04 mmHg and 134.40±8.89 and after the CPT was 124.48±10.30 mmHg, 121.82±11.41 mmHg and 126.82±9.46 mmHg in normal weight, overweight and obese group respectively. Pairwise analysis revealed no statistical difference in any of the groups during CPT but after CPT was found to be significant when OW were compared to OB (p value =0.050). The net change as a result to CPT was maximum in OW followed by NW and then OB (Table 1).

All the BMI groups encountered a statistically significant (P=0.000) increase in the Systolic BP during CPT, maximally in Overweight followed by normal BMI and then Obese group. However none of the groups have statistically different Systolic BP after CPT (Table 2).

Though statistically insignificant, the net fall in the systolic BP in overweight and obese category after CPT can be due to the premature reversal or weak sympathetic response as compared to vasodilatory mechanisms.

Diastolic BP:

The mean diastolic BP before CPT was 69.97±3.69 mmHg, 72.57±7.06 mmHg and 74.14±6.60 mmHg respectively BMI groups (Chart 2). On determining the pairwise difference in the means of the groups using unpaired t- test the difference in the mean diastolic BP of normal BMI and obese subjects was statistically significant (P=0.007) (Table 1).

The mean diastolic BP during CPT was 77.14±15.47 mmHg, 80.97±11.19 mmHg and 82.62±10.04 mmHg. The mean diastolic BP after CPT was 70.68±3.73 mmHg, 70.51±6.51 mmHg and 73.97±5.85 mmHg respectively. Pairwise analysis revealed no statistical difference in any of the groups during CPT, but after CPT Normal BMI and Obese group (P-value = 0.006) and Overweight and obese group (P-value = 0.022) reflected statistically significant residual increase in diastolic BP (Table 1).

All the BMI groups encountered a statistically significant (p<0.05) increase in the Diastolic BP during CPT, maximally in Obese followed by overweight and then Normal BMI group. However none of the groups have statistically different Diastolic BP after CPT (Table 2).

Though statistically insignificant, the net fall in the Diastolic BP in overweight and obese category after CPT can be due to the premature reversal or weak sympathetic response as compared to vasodilatory mechanisms.

Pulse pressure:

The mean PP before CPT was 50.51±10.68 mmHg, 50.11±4.99 mmHg and 54.80±11.13 mmHg respectively BMI groups (Chart 3). On determining the pairwise difference in the means of the groups using unpaired t- test the difference in the mean PP of Overweight and Obese subjects was found statistically significant (P-value = 0.026) (Table 1).

The mean PP during CPT was 58.31±12.09 mmHg, 57.20±14.06 mmHg and 51.77±7.76 mmHg respectively. The difference in mean was found statistically significant in the Normal BMI and obese group (P-value = 0.008) and Overweight and Obese group (P-value = 0.049) (Table 1).

The mean PP after CPT was 53.80±10.72 mmHg, 51.31±10.40 mmHg and 52.85±9.33 mmHg respectively. Pairwise analysis revealed no statistical difference in any of the groups after CPT (Table 1).

The Normal BMI group and Overweight group encountered a statistically significant (p<0.05) increase in the Pulse pressure during CPT (Normal BMI > Overweight) whereas the Obese group encountered statistically significant (P-value = 0.041) fall in PP. However none of the groups have statistically different Pulse pressure (PP) after CPT (Table 2).

Mean Arterial Pressure:

The mean MAP before CPT was 86.80±3.14 mmHg, 89.27±6.31 mmHg and 92.40±5.47 mmHg respectively BMI groups (Chart 4). On determining the pairwise difference in the means of the groups using unpaired t- test the difference in the mean MAP in all the paired group was found statistically significant in normal BMI and Obese group (P-value = 0.000) and overweight and obese groups (P-value = 0.029) (Table 1).

The mean MAP during CPT was 96.58±13.16 mmHg, 100.03±11.68 mmHg and 99.88±8.95 mmHg respectively. The difference in mean was found statistically insignificant in all the comparison groups (Table 1).

The mean MAP after CPT was 88.61±4.37 mmHg, 87.61±6.90 mmHg and 91.59±5.77 mmHg respectively. Pairwise analysis revealed significant statistical difference in Normal BMI and Obese group (P-value = 0.017) and overweight and obese groups (P-value = 0.011) (Table 1).

All the BMI groups encountered statistically significant increase in the MAP during CPT (p<0.05) but none of them had significant differences after CPT (p>0.05) (Table 2). Maximum changes were encountered, during CPT, in overweight group but the normal group had a sustained and prolonged response in the after CPT time zone.

Heart Rate (HR)

The mean HR before CPT was 77.7±6.3 /min, 72.7±11.1 /min and 73.8±12.3 /min respectively BMI groups (Chart 5). On determining the pairwise difference in the means of the groups using unpaired t- test the difference in the HR of normal BMI and overweight group subjects was statistically significant (P-value = 0.025) (Table 1).

The mean HR during CPT was 85.0±6.9 /min, 75.0±6.9 /min and 75.9±8.5 /min. Pairwise analysis revealed significant difference in the Normal BMI and Overweight (P-value = 0.000) comparison group and Normal BMI and Obese (P-value = 0.000) comparison group, but not in overweight and obese group (Table 1).

The mean HR after CPT was 79.0±8.9 /min, 71.0±4.0 /min and 73.0±12.5 /min respectively. Pairwise analysis revealed significant difference in the Normal BMI and Overweight (P-value = 0.000) comparison group and Normal BMI and Obese (P-value = 0.024) comparison group (Table 1).

A statistically significant change (increase) was observed in the Normal BMI (P-value = 0.000). However the Overweight and obese group during CPT and all the groups after CPT didn’t reveal any statistically significant change (Table 2). 

FEV1%:

The mean FEV1 % before CPT was 88.68±6.59, 87.37±6.16 and 83.54±3.59 respectively BMI groups, during CPT was 88.57±6.77, 86.82±5.97 and 83.17±3.48 respectively and after CPT was 88.57±6.73, 87.25±6.08 and 83.48±3.48 respectively (Chart 6). Normal BMI and overweight group revealed no statistical difference in all the time frames, but Normal BMI and Obese and Overweight and Obese comparison groups revealed statistically significant differences before, during and after CPT (Table 1).

Overweight group encountered a statistically significant (p<0.05) decrease in the   FEV1 % during CPT (P-value = 0.009), whereas rest all the groups have no difference statistically in before CPT and after CPT values (Table 2).

The Present study assesses the effect of CPT to establish the relation of autonomic cardiovascular and respiratory parameters.   Heart rate (HR) increased significantly in the normal BMI and Obese groups (p = 0.000^*) however the overweight group didn’t reveal any statistically significant change. Systolic BP encountered a statistically significant (p<0.05) increase in overweight subject as compared to normal weight and obese. Regarding diastolic BP, maximally change (increased) was observed in Obese followed by overweight and then Normal BMI group.  Pulse pressure (PP) significantly (p<0.05) increased in Normal BMI > Overweight whereas the Obese group encountered statistically significant fall in PP. All the groups encountered statistically significant(p<0.05) in the Mean arterial pressure (MAP) during application of CPT. Among respiratory parameters Overweight group encountered a statistically significant (p<0.05) decrease in the   FEV1 % during CPT, whereas rest all the groups have no difference statistically.

Sympathetic nervous system plays an important role in the metabolic homeostasis. It has been related to numerous metabolic and cardiovascular disorders. Obesity associated sympathetic activation  is  a potential mechanism to increase cardiovascular events. Landsberg hypothesised that the increase in the sympathetic activity is homeostatic response, for stimulating thermogenesis, with weight gain.(10)

Increased sympathetic activity induced by  CPT causes elevation of blood pressure more in obese subjects which might be contributed by more release of norepinephrine, endothelin’s, prostaglandins and angiotensin II .(11) Weight loss following low calorie diet can cause improvement in autonomic function as is demonstrated by a study in which there was improvement in blood pressure to stress test as cold pressor test.(12) Cardiac autonomic neuropathy is associated with weight gain, a 10 % increase in the body weight is associated with decline in parasympathetic tone and increased heart rate.(13)

In laboratory research on hypertensionCPT is  used to elicit pressor response and adrenergic vasoconstriction.(14)(15)(16) In our study results increased in  HR, SBP and DBP due to acute pain induced  CPT are same  as observed by Wolf that a sharp rise in blood pressure occurred after immersion of the hand ten to 60 seconds in cold water which reached its maximum at about the point of maximum pain(17) and Fagius et al in which both SBP and DBP were increased by the end of the minute of immersion, and  returned to the initial values within 1 min. BP  elevation correlated linearly with increase in total outflow of sympathetic activity, the relationship being stronger for systolic than for diastolic blood.(18)

Present study revealed that weight gain induces proportionate increase in the resting  SBP which is significant in the higher BMI groups and application of CPT causes significant changes in the mean Systolic BP of all the individual groups with a maximal response in the Overweight group (15.48±2.60 mmHg), followed by the Normal BMI group (14.97±2.87mmHg) and then the Obese group (5.54±1.33 mmHg). This is in accordance to the Landsberg Hypothesis of baseline increase in the blood pressure with weight gain. Park et al evaluated MSNA activity in response to the CPT and found significant MSNA augmentation in the Overweight individuals predominantly despite insignificant increase in the systolic BP.(5) Similarly we found hyperresponsiveness reflected by change in mean BP of the overweight individuals.. The DBP, though was insignificantly different before CPT, but was directly proportional to the BMI of the groups. CPT induced significant increase in the diastolic BP, maximum in the obese category. Study of Srivastava et al(19) and Kuniyoshi et al(20) indicated significantly high resting diastolic BP and significant increase during CPT in obese individuals , as depicted in our study. Gentile et al proposed significant increase in the systolic BP but not diastolic BP with weight gain.(21)

Our study proposes a small shift in the Systolic BP in the Obese group as a response to CPT.

Grewal S and Gupta V reported similar results in 2011 summarising that the  mean change in the systolic BP of the obese individuals was less responsive to the CPT due to blunting of the sympathetic nervous system, both sympathetic and parasympathetic branches.(22) With weight gain, there is a decrease in sympathetic activity and reduction in baroreflex functioning because of central sympatho-inhibition.(23)Garg et al concluded a lesser increase in the blood pressure after the cold water immersion points towards sympathetic insufficiency in obese subjects.(11) Obesity impairs autonomic control of heart rate and blood pressure. Obese subjects exhibit lower sympathetic response on exposure to cold. Similar results were proposed by Bedi et al concluding that the afferent fibres for this response are the pain fibres (which are stimulated by placing the hand in cold water) and the efferent fibres are the sympathetic fibres resulting lesser increase in the DBP after the cold water immersion thus points towards sympathetic insufficiency in obese children.(24)

During the recovery phase, after CPT, the normal individuals have a sustained residual increase in the systolic and diastolic BP as compared to the overweight and obese individuals, though statistically insignificant as compared to before CPT records; and can be attributed to blunted and weak sympathetic response in overweight and obese individuals as compared to normal BMI individuals.

Simultaneously the reactivity to CPT was positively correlated to BMI in the studies by Kuniyoshi et al, Gentile et al, Jaju et al and Mourot et al.(25)(20)(21)(26) . Mourot et al reported two responses of CPT in the normal individuals viz. increase in HR and decrease in HR higher BMI groups. The autonomic cardiac neuropathy is the possible cause of the baseline decrease in the HR and less responsiveness to the CPT in higher BMI groups. Again, in the after test time zone the normal weight individuals were the group having sustained sympathetic response.

In our study the obese group had a high baseline PP as compared to other BMI groups. And it was significantly higher as compared to the overweight individuals. CPT response caused significant increase in the PP but a net decrease was found in the obese individuals as stated by Mourot et al.(25) After test findings were insignificant.

 Pain is a complex phenomenon which has a sensory discriminatory, motivational-affective and cognitive components.(27) Pain may cause either respiratory stimulation or inhibition, depending on its character, origin (visceral or somatic) and intensity. Nociceptive afferents can reflexly stimulate the respiratory centres. This stimulation of respiration could be secondary to the excitatory inputs of the higher centres to cardiorespiratory centres in the lower brainstem.

Tandon et al were of opinion that the forced vital capacity and the vital capacity showed significant increase during cold induced acute pain. The forced vital capacity expressed as a percentage of the predicted value also increased. This could be due to a better muscular and voluntary effort by the subject, coming into picture as a general arousal response to pain during cold stress. Another possibility could be that a change in the bronchomotor tone i.e., relaxation and dilatation of large airways. Interestingly FEV1%, is not effort dependent as FVC and VC and is affected by type calibre of large airways, this again points towards the possibility of bronchodilatation in response to cold induced acute pain. The FEVI % showed a significant decrease which points towards the fact that FEV1 did not increase to the extent to which FVC increased. The decreased in FEV1% in spite of an increased FEV1 indicates that the effect of increase in FVC which accompanied bronchodilatation nullified' that due to an increase in airway calibre. Thus, the reason for this bronchodilatation could be due to the following: It could be in response to one of the components of Herring Breuer inflation reflex which brings about a relaxation of tracheobronchial tree smooth muscle. Possibly this may modulate airway calibre. The reason for bronchodilatation could be due to sympathoadrenal discharge leading on to an increase in the circulating catecholamine levels during the cold pressor test, which is a well-documented fact. The increase in circulating Epinephrine which acts on the adrenoceptors in the airway may be responsible for bronchodilatation and the type of responses which we got during cold induced acute pain.(7)

Nabil et al  studied 42 subjects were categorized into underweight, normal weight, overweight and obese  patients with COPD and evaluate autonomic dysfunction and found that there was no corelation between BMI and autonomic dysfunction response in patient with COPD and also found non - significant corelation for FEV1 and PEF with increasing BMI.(28) This may be the reason that COPD patients have enhanced sympathetic tone at rest and are less able to respond to sympathetic and parasympathetic stimuli in comparison to healthy persons.

 Lad U et al found in their study that there was positive correlation in underweight male and female with FEV1 and overweight male, and female had negative correlation with FEV1.(29)

Nageswari et al concluded  that reduced in dynamic lung response and derangement of sympathetic cardiovascular function in obese as compared to non – obese school children of  12 – 16 years of age group and results indicated significant increase in base line diastolic blood pressor ( P<0.004) in obese children corelate positive with BMI and increased in diastolic blood response to cold pressor test (P<0.001) and borderline response to isometric exercise (p<0.002) in obese children indicated autonomic cardiovascular instability and the dynamic lung function were significantly decreased ( p< 0.04) in obese children, which correlate negative with increase BMI.(30)

Bijit Dutta and Bonti Bora studied 30 male healthy subjects (age group 19 -20 years old) and   introduced localized cold stimuli in externally controlled environment by immersing both feet up to ankle in a bucket full cold water with maintained temperature between 8 – 10⁰ C and parameters were noted after 2 minutes. They  observed that the tidal volume and Inspiratory capacity showed a significant increase (p<0.05) whereas the Inspiratory reserve volume, expiratory reserve volume and forced vital capacity showed a significant decrease (p<0.05) in response to the cold stimuli. Cold stimulates the efferent vagal fibers either directly or reflexively which increases airway resistance and decreases anatomic dead space secondary to airway constriction which is responded by the decrease in the Inspiratory reserve volume, expiratory reserve volume, forced vital capacity, forced expiratory volume in 1^st sec.(31)

Obesity is associated with detrimental effects on the pulmonary functions. To test this concept on otherwise healthy young individuals of different BMI groups and its correlation to it, we conducted this study and found significant differences (reduction) in the FEV1 % of predicted FEV1 among the overweight and obese group and normal and obese group. The data analysis reveals that the weight gain significantly reduces the FEV1% proportionately. We observed no change in the FEV1 % due to application of the CPT or after CPT values except in the overweight group.

Cross-sectional and longitudinal studies have demonstrated that a rise in BMI lowers FEV1, forced vital capacity, functional residual capacity and the expiratory reserve volume.  Central obesity  and excess weight on the anterior chest wall due to  obesity lowers chest wall compliance and respiratory muscle endurance with increase in work of breathing and airway resistance.(32)(33) Furthermore, adipose tissue in the anterior abdominal wall and in  intra-abdominal visceral tissue hinders diaphragmatic movement, diminishes basal lung expansion during inspiration.  Studies have demonstrated that changes in lung volume can occur at early stages of obesity and are not limited to the morbidly obese individuals.(33) In a study by Leone et al impaired lung function (FEV1, FVC, FEV1/FVC) was associated with the components of the metabolic syndrome: most strongly with abdominal obesity (higher WC) and with elevated low density lipoproteins, hypertension, and insulin resistance. But none of the studies show any correlation or effect of sympathomimetic CPT on the lung volumes.(34)

We conclude that:

1. The highest shoot-ups in the blood pressures and decrease in r-r intervals in overweight group itself indicates that the overweight group, despite away from the established obesity, are more prone to the cardiovascular events. The blunted sympathetic response in obese group may reflect a little protection against these events.
2. Higher BMI groups have decrease in the FEV1% proportionately to the weight gain but no correlation to the application to CPT could be established.
3. Cold pressure test can be employed as a preliminary test in the early stages in the susceptible individuals to cardiovascular risks, after establishing their cardiovascular fitness, to know their sympathetic reactivity.

Further studies and similar experiments with larger study in different BMI groups and longer duration of study along with gender and ethnic comparison should be conducted to form a basis of the acute response to cold generated by the body to counteract it in all age, sex, religion, diet and environment. Future studies for further finer vision of mechanical properties of vessels, as wall thickness and the atherosclerotic components must be taken in account along with sympathetic response tests.

Funding: Non funded.

1. Haslam, David W., and W. P. T. James. "Obesity." Lancet, vol. 366, no. 9492, 2005, pp. 1197–1209.
2. Valensi, Philippe, et al. "Cardiac Autonomic Function in Obese Patients." International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity, vol. 19, no. 2, 1995, pp. 113–118.
3. Bray, George A. "Medical Consequences of Obesity." Journal of Clinical Endocrinology & Metabolism, vol. 89, no. 6, 2004, pp. 2583–2589.
4. Sin, Donald D., Li Wu, and S. F. P. Man. "The Relationship Between Reduced Lung Function and Cardiovascular Mortality: A Population-Based Study and a Systematic Review of the Literature." Chest, vol. 127, no. 6, 2005, pp. 1952–1959.
5. Park, J., et al. "Abnormal Sympathetic Reactivity to the Cold Pressor Test in Overweight Humans." American Journal of Hypertension, vol. 25, no. 12, 2012, pp. 1236–1241.
6. Bijlani, R. L., and M. S. Understanding Medical Physiology: A Textbook for Medical Students. 4th ed., Jaypee Brothers Medical Publishers; McGraw-Hill, 2011, https://lib.ugent.be/catalog/rug01:001492605.
7. Tandon, O. P., Himani A., and S. Singh. "Pulmonary Responses During Cold Induced Acute Pain." Indian Journal of Physiology and Pharmacology, vol. 41, 1997, pp. 16–22.
8. Manhas, M., et al. "A Study of Cardiovascular and Pulmonary Responses During Cold Pressor Test (CPT) in Healthy Volunteers." JK Science, vol. 13, no. 3, 2011, pp. 145–149.
9. Naidu, M. U. R., et al. "Validity and Reproducibility of Arterial Pulse Wave Velocity Measurement Using New Device with Oscillometric Technique: A Pilot Study." Biomedical Engineering Online, vol. 4, 2005, p. 49.
10. Landsberg, L. "Diet, Obesity and Hypertension: An Hypothesis Involving Insulin, the Sympathetic Nervous System, and Adaptive Thermogenesis." QJM: An International Journal of Medicine, vol. 61, no. 236, 1986, pp. 1081–1090.
11. Garg, R., et al. "A Study of Autonomic Function Tests in Obese People." International Journal of Medical Research and Health Sciences, vol. 2, no. 4, 2013, pp. 750.
12. Ashida, T., et al. "Effects of Short-Term Hypocaloric Diet on Sympatho-Vagal Interaction Assessed by Spectral Analysis of Heart Rate and Blood Pressure Variability During Stress Tests in Obese Hypertensive Patients." Hypertension Research, vol. 30, no. 12, 2007, pp. 1199–1203.
13. Klein, S., et al. "Clinical Implications of Obesity with Specific Focus on Cardiovascular Disease: A Statement for Professionals from the American Heart Association Council on Nutrition, Physical Activity, and Metabolism: Endorsed by the American College of Cardiology Foundation." Circulation, vol. 110, no. 18, 2004, pp. 2952–2967.
14. Pickering, T. G., and W. Gerin. "Cardiovascular Reactivity in the Laboratory and the Role of Behavioral Factors in Hypertension: A Critical Review." Annals of Behavioral Medicine, vol. 12, no. 1, 1990, pp. 3–16.
15. Sherwood, A., et al. "Evaluation of Beta-Adrenergic Influences on Cardiovascular and Metabolic Adjustments to Physical and Psychological Stress." Psychophysiology, vol. 23, no. 1, 1986, pp. 89–104.
16. LeBlanc, J., et al. "Autonomic Nervous System and Adaptation to Cold in Man." Journal of Applied Physiology, vol. 39, no. 2, 1975, pp. 181–186.
17. Wolf, S., and J. D. Hardy. "Studies on Pain. Observations on Pain Due to Local Cooling and on Factors Involved in the 'Cold Pressor' Effect." Journal of Clinical Investigation, vol. 20, no. 5, 1941, pp. 521–533.
18. Fagius, J., et al. "The Cold Pressor Test: Effects on Sympathetic Nerve Activity in Human Muscle and Skin Nerve Fascicles." Acta Physiologica Scandinavica, vol. 137, no. 3, 1989, pp. 325–334.
19. Srivastava, R. D., et al. "Influence of Age and Gender on Cold Pressor Response in Indian Population." Indian Journal of Physiology and Pharmacology, vol. 54, no. 2, 2010, pp. 174–178.
20. Kuniyoshi, F. H. S., et al. "Abnormal Neurovascular Control During Sympathoexcitation in Obesity." Obesity Research, vol. 11, no. 11, 2003, pp. 1411–1419.
21. Gentile, C. L., et al. "Modest Weight Gain Is Associated with Sympathetic Neural Activation in Nonobese Humans." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, vol. 292, no. 5, 2007, pp. R1834–R1838.
22. Grewal, S., and V. Gupta. "Effect of Obesity on Autonomic Nervous System." International Journal of Current Biological and Medical Sciences, vol. 1, 2011, pp. 15–18.
23. Laederach-Hofmann, K., et al. "Autonomic Cardiovascular Regulation in Obesity." Journal of Endocrinology, vol. 164, no. 1, 2000, pp. 59–66.
24. Bedi, M., et al. "Assessment of Autonomic Function Activity in Obese Children." Vascular Disease Prevention, vol. 6, no. 1, 2009, https://benthamopen.com/ABSTRACT/VDP-6-139.
25. Mourot, L., et al. "Effects of the Cold Pressor Test on Cardiac Autonomic Control in Normal Subjects." Physiological Research, vol. 58, no. 1, 2009, pp. 83–91.
26. Jaju, D., et al. "Hemodynamic and Autonomic Reactivity to Mental and Physical Stress in Lean, Overweight and Obese Subjects." Journal of Obesity & Weight Loss, vol. 2, no. 1, 2016, p. 105.
27. Melzack, R., and K. L. Casey. "Sensory, Motivational, and Central Control Determinants of Pain: A New Conceptual Model." Skin Senses, vol. 1, 1968, pp. 423–443.
28. Nabil, D., et al. "Evaluation of Autonomic Dysfunction in Underweight, Normal Weight, Overweight and Obese Patients with Chronic Obstructive Pulmonary Disease." Indian Journal of Public Health Research and Development, vol. 10, no. 12, 2019, pp. 487–492.
29. Lad, U., et al. "Correlation Between Body Mass Index (BMI), Body Fat Percentage and Pulmonary Functions in Underweight, Overweight and Normal Weight Adolescents." Journal of Clinical and Diagnostic Research, vol. 6, no. 5, 2012, pp. 350–353.
30. Nageswari, K. S., et al. "Assessment of Respiratory and Sympathetic Cardiovascular Parameters in Obese School Children." Indian Journal of Physiology and Pharmacology, vol. 51, no. 3, 2007, pp. 235–243.
31. Dutta, B., and B. Bora. "Study of Pulmonary Function Tests in Response to Localized Cold Stimuli in Age Group Between 19–30 Years of Guwahati City." International Journal of Research in Medical Sciences, vol. 5, no. 7, 2017, pp. 3107–3110.
32. Ochs-Balcom, H. M., et al. "Pulmonary Function and Abdominal Adiposity in the General Population." Chest, vol. 129, no. 4, 2006, pp. 853–862.
33. Babb, T. G., et al. "Dyspnea on Exertion in Obese Women: Association with an Increased Oxygen Cost of Breathing." American Journal of Respiratory and Critical Care Medicine, vol. 178, no. 2, 2008, pp. 116–123.
34. Leone, N., et al. "Lung Function Impairment and Metabolic Syndrome: The Critical Role of Abdominal Obesity." American Journal of Respiratory and Critical Care Medicine, vol. 179, no. 6, 2009, pp. 509–516.