Gender Differences in Response to Cold Pressor Test in the Age Group of 18-30 Years

Introduction: The Cold Pressor Test (CPT) was first introduced by Hines and Brown in 1932. It was designed to measure the reactivity of the blood vessels to a standard stimulus 2 . Cold Pressor Test is an established challenge test of sympathetic vascular regulation. Sympathetic nervous system activity varies in males and females. Aim: To study the response of Cold Pressor Test on blood pressure in normal healthy young adult males and females. Materials and Method: After Institutional ethics committee approval, a total of 104 subjects in the age group of 18-30 years were recruited with informed consent for the study, after considering inclusion and exclusion criteria. Subjects were explained about the test procedure. Basal blood pressure (pre-test BP) was recorded after 20 min. of rest. Systolic and diastolic blood pressure was measured in mmHg (pre-test). Subject was asked to dip left hand till the wrist in cold water (4 0 -8 0 C) for 1 min. (minute).Blood pressure was recorded from right arm during the test, 1 minute, 2 minutes, 3 minutes and 4 minutes after the test. Result: Basal BP was significantly higher in males than in females. Systolic BP(SBP) response, Diastolic BP response (DBP) to CPT were statistically higher in males compared to females. Conclusions: The gender variations are seen due to differences in the sympathetic nervous system activity and due to the effect of sex hormones. Males are more prone to develop hypertension and other cardiovascular disorders when compared to females due to higher sympathetic activity. introduced by Hines and Brown in 1932. It was designed to measure the reactivity of the blood vessels to a standard stimulus 2 . (CPT) is an established challenge test of autonomic vascular regulation. This study aims to study the gender variation in response to CPT, giving insight into level of sympathetic activity, and subsequent risk of developing hypertension in future and other cardiovascular disorders.

which could be the major mechanism in causation of pre hypertension 3 . Sustained sympathetic overactivity has been reported as among the primary mechanisms for genesis of essential hypertension 2 . The subjects at risk of developing hypertension can be identified by performing the CPT.
The Cold Pressor Test (CPT) was first introduced by Hines and Brown in 1932. It was designed to measure the reactivity of the blood vessels to a standard stimulus 2 .
(CPT) is an established challenge test of autonomic vascular regulation. This study aims to study the gender variation in response to CPT, giving insight into level of sympathetic activity, and subsequent risk of developing hypertension in future and other cardiovascular disorders.

Introduction
The subjects prone to develop hypertension in later life have a period of underlying sympathetic overactivity 1 . Increased sympathetic and decreased parasympathetic activities in young adults alters sympathovagal balance,

Instructions:
Subjects were asked Statistical analysis: Unpaired t test was used to find the significance between various parameters. p value <0.05 was considered as statistically significant (*).    BP measurement continued after the removal of hand from the cold water in order to detect delayed pressor effect and duration of elevated blood pressure.

Observation and Results:
The SBP values 1 minute after the test were higher in males (115.91±11.26mmHg) than in females (107.56±11.79mmHg). The SBP values 2 minutes, 3 minutes, 4 minutes after test were significantly higher in males than in females.
The maximum SBP attained in males was higher (127.42±9.48 mmHg) than in females (119.12±10.76mmHg) & the p value was statistically significant (p=0.001). The change in SBP (Maximum SBP minus Basal SBP) was higher in females (16.91±8.19mmHg) than in males (16.18±7.45mmHg) and was not statistically significant (p=0.637). The change in SBP was higher in females correlating with Srivastava et al 8 .
The DBP response to CPT during test was higher in males (86.98±10.50mmHg) than in females (69.29±7.22 mmHg) & the values were statistically significant (p=0.001) correlating with Srivastava et al 8 . Males had higher DBP values 1 minute, 2 minutes, 3 minutes & 4 minutes after the test than females. Maximum DBP attained was higher in males than in females. The change in DBP was more in males (13.73±8.47mmHg) than in females (11.18±6.50 mmHg) in contradiction to Srivastava et al. 8 with a higher change in DBP in females than in males.
Mechanism of CPT induced increase in BP 9 : When the hand is immersed in cold water (4°-8°C), the A & C fibres are activated in response to pain, temperature sensation. These fibres ascend contralaterally as anterolateral system which includes Spinothalamic, Spino-reticular & Spino-mesencephalic tracts. The nociceptive specific neurons ascend in the lateral mesencephalon, while the thermoreceptive specific neurons project to Dorsomedial part of ventro posteromedial nucleus of Thalamus. The ascending fibres of spinoreticular tract project to medullary reticular formation, via the Ionotropic Glutamate receptors activate the Rostral Ventrolateral Medullary (RVLM) neurons resulting in increased sympathetic nervous activity leading to increase in blood pressure, heart rate, total vascular resistance, muscle sympathetic activity 10,11,12 , decrease inforearm blood flow 13 . Thus, CPT increases the sympathetic nervous system activity to increase the BP which depends on the sympathetic tone.
In the present study, males had higher: Gender variations to CPT is due to androgens, estrogen, progesterone.
Androgensplays a crucial role in gender differences in blood pressure regulation. The probable mechanisms by which androgens lead to increased blood pressure are: Androgens lead to increased formation of Angiotensin II, which increasesoxidative stress leading to production of superoxide, quenching of Nitric Oxide, and also reducethe renal vascular response to vasodilators, including residual Nitric Oxide(NO). Androgens also cause production of F2-isoprostanes which potentiate the effect of Ang II as avasoconstrictor, and alsostimulate endothelin-1 production to increase blood pressure further. 14 Estrogens and their receptors play a crucial role in endothelium-dependent maintenance of vascular tone [15][16][17] . Estrogenscause relaxation via endotheliumderived hyperpolarizing factor (EDHF), by inducing vasodilator prostanoids (PGE2, PGI2), and by inhibiting endothelin-1 production 18 . Endothelial progenitor cells (EPCs) also play a crucial role in vascular response. Fertile females havehigher levels of EPC than men. Intrinsic differences within the endothelial cells contribute to differences between men and women. Other factors that modulate autonomic cardiac activity, may influence sex differences, such as inflammation, increased pain sensitivity to cold, and psychological disorders (e.g. depression). Healthy females have a lower central sympathetic neural output to periphery and a lower sympathetic vasoconstrictor drive when compared to healthy men. 19 Studies done at molecular level indicate that centralactions of estrogens include facilitation of cholinergic transmission and activating the synthesis of a vital rate limiting enzyme choline acetyltransferase, involved in the formation of acetylcholine formation, and also have a role in sympathetic outflow inhibititon 20,21 Estrogen 22 decreases the production of cyclooxygenase derived products, reactive oxygen species, angiotensin-II & endothelin-1 which are vasoconstrictor agents. It inhibits smooth muscle cells by activating K+ efflux & by inhibiting calcium influx through inhibition of L-type Ca 2+ channels and decreases myosin light chain phosphorylation and contraction of smooth muscle. Estrogen also inhibits proliferation of vascular smooth muscle cell.
Membrane-impermeant forms of estrogenacts oncell surface estrogen receptors, leading to the activation of mitogen-activated protein kinase (MAPK) and increased cGMP production and nitric oxide (NO) release.
Estrogenacts on the renin-angiotensin system is at the formation of Ang II, at the Ang II receptors level and on Ang II-induced responses 22 . Estrogen increases gene expression and plasma levels of angiotensinogen and antagonizes the AT1 receptor-mediated growthpromoting effects of Ang II in vascular smooth muscle cells. Estrogenacts on the endothelin-1 (ET-1) pathway at different levels: at its formation, at level of the receptors and on ET-1-induced responses.
The luteal phase is associated with a significantly reduced parasympathetic activity. The high progesterone levels during the luteal phase may have an inhibitory role on cardio-vagal activity [22][23][24][25] . Studies showed a higher sympathetic activity and a sympathovagal imbalance in luteal phase with an associated increase in parasympathetic activity in follicular phase,showing sympathetic predominancein luteal phase [26][27][28][29] . In physiological doses, progesterone acts as a sympathomimetic agent which induces nor epinephrine release 30,31 The results corroborates with report by Weitz et al 32 , that the activity of sympathetic nervous system shows gender specific differences with lower sympathetic nerve activity to muscle vascular bed in women as compared with men, the difference vanishes after menopause.
Thus, the female sex hormones together cause lower BP response to CPT in females. The gender variations are seen due to differences in the sympathetic nervous system activity and due to the effect of sex hormones. Males are more prone to develop hypertension and other cardiovascular disorders when compared to females due to higher sympathetic activity.

Conclusion
The Change in SBP & change in DBP values between males and females are not statistically significant.