Hypertension is among the major risk factors for premature disability and death in both Western [1, 2] and Asian [3] populations. Especially, Asian Indians have been reported to be at increased risk of diabetes, hypertension and heart diseases [4, 5]. Though the increased risk of cardiovascular events in Indian population has been attributed to genetic predisposition [6], there is a strong correlation between changing lifestyle and increasing incidence of hypertension and heart diseases [7, 8]. Studies have reported that sustained sympathetic overactivity, which increases vasoconstrictor tone of the systemic vasculature acts as a major mechanism for genesis of essential hypertension [9‐11]. Though, several studies have revealed autonomic imbalance in hypertensive patients [12, 13], there is paucity of data on the nature of autonomic imbalance that slowly leads to the progression from normotensive state into the state of prehypertension. Nevertheless, prehypertension has recently been reported to be associated with damage to the coronary vasculature and adverse cardiovascular events [14‐16].

Recent reports from our laboratory based on the spectral analysis of heart rate variability (HRV) reveal that sympathovagal imbalance in the form of sympathetic overactivity and vagal withdrawal contributes to the development of prehypertension and hypertension in Indian population [17‐19]. Our previous studies have revealed prehypertension is more prevalent among males and vagal withdrawal is more prominent compared to sympathetic overactivity in male prehypertensives [20, 21]. We have also reported from the studies on young prehypertensives with parental history of hypertension that sympathovagal imbalance (SVI) is more intense in offspring of two parents hypertensive compared to the offspring of one parent hypertensive [22]. Yet, till date the exact mechanism of alteration in sympathovagal imbalance in prehypertensives has not been fully ascertained. Recently, contribution of obesity to the genesis of hypertension has been fully established [23]. However, no study has yet thoroughly evaluated the link of body mass index (BMI) with the causation of prehypertension. Spectral analysis of HRV has been used as a sensitive tool for assessment of autonomic dysfunctions in various clinical disorders [24]. Therefore, in the present study we have attempted to assess the nature and magnitude of autonomic imbalance and the role of BMI as an independent contributor to the genesis of SVI in prehypertensives.

There was no significant difference in age among all the three groups (Table 1). Though there was no significant difference in body weight and BMI between groups 1 and 2, body weight and BMI of group 3 were significantly increased compared to that of groups 1 and 2. Though the difference in BHR between groups 1 and 2 was not significant, BHR was significantly increased in group 3 compared to BHR of groups 1 and 2 (p < 0.001). SBP and DBP of groups 2 and 3 were significantly higher (p < 0.001) compared to that of group 1, though the difference was not significant between groups 2 and 3. RPP of groups 2 and 3 was significantly more compared to that of group 1 and RPP of group 3 was significantly more (p < 0.001) than that of group 2.

Total power (TP) of HRV spectrum of groups 2 and 3 was significantly decreased compared to that of group 1. Also, TP of group 3 was significantly less (p < 0.001) than that of group 2 (Table 2). Though there was no significant difference in LF power between groups 1 and 2, it was significantly less (p < 0.01) in group 3 compared to that of both groups 1 and 2. However, LF power expressed as % of TP was more in group 2 and 3 compared to that of group 1 (Figure 1). The HF power of groups 2 and 3 was significantly less (p < 0.001) than that of group 1, and HF power of group 3 was significantly less (p < 0.001) than that of group 2. Also, HF power expressed as % of TP was less in groups 2 and 3 compared to that of group 1 (Figure 1). Though LF_nu of group 2 (p < 0.05) and group 3 (p < 0.001) was significantly increased compared to that of group 1, change in LF_nu was not significant between groups 2 and 3. HF_nu of groups 2 and 3 was significantly low compared to that of group 1. Also, HF_nu was decreased significantly (p < 0.05) in group 3 compared to that of group 2. LH-HF ratio was increased significantly in groups 2 and 3 compared to that of group 1 and the LH-HF ratio of group 3 was significantly higher (p < 0.001) than the group 2 (Table 2).

Mean-RR, RMSSD, SDNN, NN50 and pNN50 were decreased significantly in group 3 compared to that of groups 1 and 2, though the difference in these parameters was not significant between groups 1 and 2 (Table 2).

Pearson correlation analysis was done to assess the strength of association of various factors with LF-HF ratio or BMI. In group 1, the correlation of LH-HF ratio was not significant with any of the parameters, and in groups 2 and 3, LF-HF ratio had significant correlation only with DBP (Table 3). As the association of BMI with LF-HF ratio was not found to be significant in groups 2 and 3, their correlation was assessed in the entire prehypertensive group. In all the prehypertensive subjects taken together, LF-HF ratio showed significant correlation with all the parameters except for age and SBP and correlation of BMI was significant with all the parameters except for age (Table 4).

Multiple regression analysis was carried out to assess the independent contribution or link of various parameters to LF-HF ratio in prehypertensive population. BMI and BHR were found to have significant impact on LF-HF ratio (Table 5). BMI as an independent factor had maximum contribution (p = 0.001) to LF-HF ratio. Multiple regression analysis was not carried out for normotensive population, as the correlation of LF-HF ratio was not significant with any of the parameters in group 1 (Table 3).

In the present study, significant decrease in total power (TP) of HRV spectrum in prehypertensive population (groups 2 and 3) compared to that of normotensive population (group 1) represents a substantial decrease in heart rate variability, which indicates decreased power of vagal drive in these subjects as TP in general reflects the vagal potency of cardiac modulation [24, 30]. This was supported by a decrease in absolute HF power and HF_nu in prehypertensive subjects, as HF and HF_nu are the indices of parasympathetic drive to the heart [24, 30]. Furthermore, TP, HF and HF_nu were significantly low in prehypertensive subjects with higher BMI (group 3) compared to the subjects of normal BMI (group 2) (Table 2) indicating the greater decrease in vagal drive in obese prehypertensive subjects.

A report by Wang et al has revealed increased sympathetic activity in prehypertensives [31], which was confirmed by our recent study [17]. In the present study we have observed a similar change in the form of increased LF_nu in prehypertensives compared to their normotensive counterparts (Table 2). Though there was an apparent decrease in absolute LF power in groups 2 and 3 (Table 2), which is due to the decrease in TP of HRV, LF when expressed as % of TP was much increased in these subjects (Figure 1). Thus, these findings confirm the increased state of sympathetic drive in prehypertensive subjects as LF represents sympathetic modulation [24, 30]. LF-HF ratio is a sensitive measure of sympathovagal balance [24, 30]. Increase in this ratio indicates increased sympathetic activity [24, 30]. LF-HF ratio was significantly increased (p < 0.001) in prehypertensive subjects (groups 2 and 3) compared to the normotensive subjects (group 1), which indicates a heightened sympathetic discharge in prehypertensives. In the present study, in spite of the modest sample size, we found a big difference in LF-HF ratio between normotensive and prehypertensive groups, which was not observed in previous studies [32‐34]. This difference in observation could be due to the racial difference in basal LF-HF ratio and the difference in altered cardiovascular autonomic tone in response to rise in blood pressure, supported by previous report on ethnic variation in HRV indices and the autonomic responses to stress [35]. However, till date, no report is available on the nature of alteration in sympathovagal balance that tilts towards augmented sympathetic activity in prehypertensives. Moreover, the contribution of altered vagal activity in the causation of prehypertension and hypertension has not yet been fully elucidated.

HRV has been used as a noninvasive tool to quantitatively estimate cardiac autonomic activity and it has proved to be of prognostic significance in hypertension [32‐34]. HRV has been reported to be decreased in hypertension and the magnitude of decrease in HRV predicts the severity of hypertension [32]. HRV also has emerged as a cardiovascular risk marker [30]. It has been observed that among normotensive men, lower HRV has a greater risk for developing hypertension, and autonomic dysregulation has been documented in the early stage of hypertension [33]. Thus, decreased HRV in prehypertensives in the present study supports these earlier reports and could also be used as a predictive tool for the future development of hypertension in these subjects. Moreover, it was reported earlier that changes in cardiac autonomic function is associated with prevalent hypertension and reduced vagal activity along with the imbalance of sympathovagal function is associated with the risk of developing hypertension [34]. Therefore, the major findings of HRV analysis in the present study depicting decreased HRV, increased sympathetic and decreased parasympathetic drive indicate a similar pattern of alteration in sympathovagal balance in prehypertension as observed in hypertension earlier.

From the reports of the present study it appears that decreased vagal tone plays a critical role in the shift of this sympathovagal balance from the transition of normotensive state to prehypertensive state, as evident from decrease in TP and HF_nu in prehypertensives (Table 2). This is because the intensity of sympathovagal imbalance (degree of increase in LF-HF ratio) was more prominent in prehypertensive subjects with higher BMI compared to prehypertensive subjects with normal BMI. In prehypertensive subjects with higher BMI, though there was increased sympathetic activity (increased LF_nu) and decreased parasympathetic activity (decreased HF_nu), the magnitude of change in vagal drive was more than the change in sympathetic drive as there was significant difference in HF_nu, but not LF_nu between the subjects of groups 2 and 3 (Table 2). Moreover, there was significant decrease in the all time domain indices (TDI) in group 3 subjects compared to that of both group 1 and 2 subjects, but not in group 2 subjects compared to the group 1 subjects. This indicates a profound decrease in vagal tone in prehypertensive subjects with higher BMI, as TDI in general reflect vagal modulation of cardiac activities [24, 30]. Also, the basal heart rate was significantly more in prehypertensive subjects with higher BMI (group 3) compared to both the normal BMI subjects (groups 1 and 2), and there was no difference in BHR between normotensive subjects with normal BMI and prehypertensive subjects with normal BMI (Table 1). This indicates a significantly lower vagal tone in prehypertensive subjects with higher BMI as higher basal heart rate is an index of poor vagal tone [36]. Thus, these findings suggest that vagal withdrawal plays an important role in the alteration of sympathovagal balance with increase in BMI in prehypertensive subjects. Reports of our recent studies indicate significant contribution of vagal inhibition in the genesis of prehypertension in the young siblings of hypertensive parents [18, 19]. Hence, we assume that vagal withdrawal could also be important in the causation of prehypertension in adults.

From this study the exact cause of sympathovagal imbalance in prehypertensive subjects can not be definitively ascertained. Nevertheless, BMI could be a potential factor for the causation of SVI as it was highly correlated with LF-HF ratio in prehypertensive subjects (Table 4). Moreover, BMI was strongly correlated with all the cardiovascular parameters including SBP and DBP in all prehypertensive subjects considered together (Table 4). In addition, BMI emerged as an important contributor to SVI in prehypertensives as it was found to have independent correlation with LF-HF ratio (Table 5) as determined by multiple regression analysis. Increased adiposity could be a key determinant for the development of prehypertension in susceptible individuals as obesity has been reported to be associated with increased sympathetic and decreased parasympathetic activity [37‐40]. It was suggested that alteration in plasma levels of leptin, neuropeptide-Y and α-MSH (melanocyte-stimulating hormone) might be involved in activation of sympathetic activity that leads to hypertension in obese patients [41]. Further, there are report on cardiac dysfunctions associated with insulin resistance, oxidative stress and inflammation that are dependent on the quantity of fat mass in obese, but not in overweight children [42]. Therefore, we assume that SVI caused by increased adiposity is among the major predictors of increase in blood pressure in prehypertensives. This was further supported by the report of Schmid et al that increase in BMI is significantly associated with increase in sympathetic tone and increased blood pressure in young healthy overweight subjects [43]. Also, previous studies have demonstrated that sympathetic activity normalizes in overweight subjects following six-month calorie restriction combined with exercise [44].

Though the exact mechanism of increased blood pressure induced by sympathetic activation in obesity is not known, it has been suggested that retrograde inflammation could be the pathophysiologic link as increased sympathetic activity induces a proinflammatory state by IL-6 production, which in turn results in an acute phase response [45]. Also, increase in sympathetic activity has been reported to elevate blood pressure by promoting inflammation-mediated arterial rigidity in obesity [46]. As hyperinsulinemia is known to stimulate adrenergic activity and hyperinsulinemia is observed in obesity, it has been postulated that chronic hyperinsulinemia may lead to enhanced sympathetic tone and cardiovascular risk in obese patient [47]. Therefore, we propose to assess the link of BMI with inflammatory markers and hyperinsulinemia in the causation of SVI in prehypertensives in our future studies.

RPP is an indirect measure of myocardial load and oxygen consumption [29]. As degree of correlation of LF-HF ratio and BMI with RPP was highly significant in prehypertensives (Table 4), myocardial energy expenditure could be more in these subjects. Hence, the risk of cardiovascular dysfunctions associated with prehypertension [9‐11] may possibly be linked to the level of BMI and RPP in these subjects.

Diastolic blood pressure (DBP) is the reflection of peripheral vascular tone and resistance [48]. In the present study, the degree of correlation of LF-HF ratio was maximum with DBP in groups 2 and 3 (Tables 3) and all the prehypertensive subjects taken together (Table 4). Hence, alteration in vascular tone could be directly linked to the degree and nature of SVI and it appears that BMI is an independent contributor to the genesis of SVI in these subjects. Inspite of the limitations that we have not performed direct assessment of sympathetic activity, not measured cardiac functions and there is less sample size of prehypertensive subjects having normal BMI, the present study emphasizes the necessity to improve vagal tone in individuals having blood pressure in prehypertensive range so that the sympathovagal balance is restored in these subjects and they do not progress into the stage of clinical hypertension. As practice of regular aerobic exercises such as morning walk, swimming, cycling etc. and practice of yoga such as asanas, meditation, pranayama etc. have been reported to decrease blood pressure, improve vagal tone and cardiac health, and reduce body weight gain [44, 47, 49‐51], prehypertensive subjects should be encouraged to practice such program to prevent increase in their BMI and progression to hypertension.

In the present study, sympathovagal imbalance (SVI) was observed in prehypertensive subjects, which was mild in subjects with normal BMI and severe in subjects with higher BMI. Vagal withdrawal was significantly associated with sympathetic overactivity in prehypertensive subjects. Magnitude of SVI (alteration in LF-HF ratio) was correlated with BMI and diastolic blood pressure. BMI had an independent contribution to change in LH-HF ratio. It was concluded that BMI could be a major predictor of SVI in prehypertensives. Hence, it was advised that life style modifications should be adopted by prehypertensives for achieving their effective autonomic homeostasis.