6. DISCUSSION Role of genetics in causation of essential hypertension in children with parental history of hypertension is reflected in the form of raised blood pressure and increased adiposity is well documented fact. The variations in study population in the form of confounding factors like age, gender, environmental factors and lifestyle makes it a difficult task to prove. The present study found that students with parental history of hypertension scored high on resting PR, SBP, DBP and BMI when compared to children of non hypertensive parent’s, though only resting pulse rate was statistically significant.
Longitudinal studies with 10 years of follow up in the normotensive children with genetic susceptibility demonstrated features of a permanent abnormality in the form of prolonged elevation in resting PR ; BP ; early metabolic syndrome with higher plasma insulin levels, increased triglycerides .The underlying mechanism was explained on the basis of high sympathetic activity ; associated elevation in plasma norepinephrine levels by 30% of normal, instead of old concept of rise in casual BP due white coat hypertension reaction.
In present study students BMI classification, which was statistically insignificant when compared to control group students. High BMI i.e., obesity is commonly seen feature in these subjects. It acts as an additional stimulating factor to already existing physiological changes due to sympathetic over-activity like vasoconstriction, retention of sodium, insulin resistance, endothelial dysfunction ; increased level of insulin, leptin ; free fatty acids in these subjects resulting in high BP.
The study group in present research exhibited statistically significant rise in DBP and SBP during IHG-test. Lindhand et al study was first one to describe rise in blood pressure on static exercise in 1920.The increased afterload to the heart due to constriction of blood vessels in exercising muscles and increased peripheral vasoconstriction via adrenergic receptors of sympathetic nervous system causes rise in DBP during IHG exercise .Also the metabolites that accumulate in the exercising muscle like lactic acid and adenosine are sensed by metabo-sensitive nerve endings, present in interstitium of skeletal muscle, causing release of metabo-receptor afferent discharge of group IV nerves. Thus increases sympathetic reflex activity leading to vasoconstriction and further rise in BP during isometric exercise.
This finding is supported by many studies like Lopez et al, Dayanand et al, which also noted that normotensive children of hypertensive parents showed high resting PR and BP. The underlying mechanism behind this was explained on the basis of increased sympathetic activity in these children with high plasma catecholamine levels before exercise.
Normally the baroreflex inhibits muscle sympathetic nerve activity, but in normotensive children of hypertensive parents this reflex activity is reduced. As a result the sympathetic vasomotor tone is high in them and chances of developing hypertension are hence increased in them. Further explanation for increased peripheral resistance and in turn DBP during IHG exercise is due to sustained muscle contraction decreasing blood flow to the exercising muscle, thus increasing pressure response and perfusion pressure to active muscles. It is also seen that static isometric exercise increases endothelin-1 secretion along with reflex release of nor-epinephrine that is mediated via hypothalamus at vascular smooth muscle cells. This finding supports the concept of inherited vascular reactivity in these genetically susceptible individuals ; is an early predictor of development of neurogenic hypertension in them.
The neurogenic hypertension has harmful effect on cognitive functions due to elevated blood pressure. In present study AVRT test was performed to assess cognition. It was we found that response time to ART ; VRT in both groups students ; in students of hypertensive parents ART ; VRT both were more than students of non hypertensive parents, in which difference on ART was statistically significant. Many studies have used audiovisual reaction time as a tool to measure sensory motor system intactness, its coordination ; quickness of processing information in diagnosed cases of hypertension in middle aged individuals. Selvaa R et al study observed finding similar to present study, where in hypertensive patients had longer VRT ; ART. They also found positive correlation between RT ; duration of hypertension, reflecting the harmful effect of elevated BP on nerve conduction velocity in long run.
The underlying pathology explained was endoneural hypoxia following inflammatory nerve cell injury ; micro thrombi associated with high BP, inturn reducing O2 supply to them. This mainly causes axonal degeneration of both slow ; fast conducting fibres, along with other complications like thickening of basement membrane ; axonal atrophy. The peripheral nerves are more vulnerable to early hypertensive changes causing hypertensive neuropathy.Many clinical trial studies have found improvement in neuropathy ; cognitive functions with proper anti-hypertensives medication in these patients.Also the studies have found that auditory stimuli reaches motor cortex within 8-10 ms hence has faster RT than visual stimuli, that takes around 20-40ms to reach the cortex.
Telang K et al study also supported the finding of delayed ART ; VRT in hypertensive patients. The reduced blood supply ; metabolism to brain tissue, mainly in fronto-temporal ; subcortical areas affect exective ; attentive cognitive functions in hypertensive patients. The elevated BP causing endothelial damage, thus disrupting the blood brain barrier ; allowing the toxins to easily get access into the brain tissue. The duration of hypertension is also associated with atherosclerotic changes in blood vessels causing hypoxic/ischemia related complications. Though these students with parental history of hypertension are normotensives now, but they are genetically more vulnerable with high risk of developing hypertension in future. These individuals are found to exhibit high BMI ; raised baseline BP as per many researches due to sympathetic imbalance in the form of overactivity, but these studies could not come up with the earliest onset of hypertensive complications. High BP ; BMI with other risk factors are found to be associated with poor response time ; predispose these subjects to neurocognitive complications. The vascular changes associated with proinflammatory cytokines ; tissue necrotising factors induce vascular damage ; myelination abnormalities causing axonal degeneration is the underlying mechanism given in many studies.