This population-based study showed an association between low BP and suicidal ideation in a large general population representing Korean adults. This is consistent with several earlier studies supporting a correlation between low BP and psychological symptoms, such as depression and anxiety, even though outcome variables are distinct [9, 26, 27]. In this study, four cut-off values were used to define low BP: SBP < 110, < 100, < 95, and < 90 mmHg. All of these hypotensive groups except the SBP < 110 mmHg group showed meaningful correlations with suicidal ideation. As this is the first report to explore the relationship between low BP and suicidal ideation, it was not possible to compare our results with those of other studies assessing the same outcome. Instead, we compared our results with those of Hildrum et al. [9], who evaluated the correlation between low BP and depression and anxiety, using a similar study design. ORs for the comorbidities of anxiety and depression in the lowest fifth percentile of SBP in their study were similar with ORs for suicidal ideation in the SBP < 95 mmHg group in our study; the proportion of subjects corresponding to the lowest fifth percentile of SBP in Hildrum et al. is similar to those in the SBP < 95 mmHg group in this study, suggesting that low BP might affect suicidal ideation to similar extent as it affects typical psychiatric symptoms.
Because there is little research on the factors that influence low BP, we used five multiple regression models including different covariates to assess the effects of the potential confounding variables more closely. The analyses showed stronger correlations in models II-V, which included more potential confounding variables compared with Model I, in which only basic adjustments were made for sex, age, BMI, and total cholesterol level. Not presented in the results here, but in an additional multivariate model that included occupation, LDL-cholesterol level, and hemoglobin level, this association was still significant. These additional findings further support a correlation between low BP and suicidal ideation. We adjusted for several major diseases, such as DM, stroke, MI/angina pectoris, and depression, with little change in the outcome (models IV and V). Other underlying diseases may be confounding variables, but a previous study suggested no difference in baseline diseases between subjects with hypotension and those with a normal BP. A longitudinal follow-up study of 1337 patients conducted between 1958 and 1999 revealed no differences in underlying diseases between patients with low BP and those with normal BP, including malignant diseases, pulmonary diseases, gastric ulcers, mental disorders, blindness, and valvular heart diseases, except anemia [28]. Medication of antidepressants, antipsychotics, anticonvulsants, antihypertensives, and others may be potential compounding variables. Among these, antidepressants, antidiabetes, treatment for stroke and MI/angina pectoris were available in KNHANES data and considered. However, multi-collinearity with the corresponding diseases was apparent and excluded from the final analysis. Subjects who were using antihypertensive medications were included among those defined as having hypertension.
There is no consensually accepted definition for low BP. Most experts consider SBP ≤ 90 mmHg and/or DBP ≤ 60 mmHg as hypotensive [29]. The World Health Organization defines low BP as SBP < 110 in men and SBP < 100 mmHg in women, regardless of DBP [30], whereas a German study stated that values of 100/60 mmHg are hypotensive [31] (quoted in [32]). Some studies have used various criteria, such as SBP < 120 or DBP < 75 (for elderly subjects) [4], SBP < 110 [2], and SBP < 100 mmHg [3], whereas other studies have used the lowest 5th centile [9] or tertile [6]. These different standards for low BP make it even more difficult for comparisons and evaluations of the effects of low BP. The present study offers a methodological advantage in this regard, as the relationship between low BP and suicidal ideation was checked for each SBP category (< 110, < 100, < 95, and < 90 mmHg), which allowed us to identify changes in the results according to different BP cut-off values and to verify which BP level was clinically significant. Patients with the lower BP cut-off showed a more pronounced tendency toward suicidal ideation in all models, suggesting the possibility that lower BP is associated with suicidal ideation. The OR for SBP < 110 mmHg did not differ from that for normal BP for suicidal ideation. In contrast, the lower BP groups, with cut-off values of SBP < 100, < 95, and < 90 mmHg, showed significantly higher levels of suicidal ideation in all covariate models, suggesting that SBP < 100 mmHg is relevant to the relationship between low BP and suicidal ideation. A considerable number of people (11% of the adult population in Korea) have SBP < 100 mmHg. The association between suicidal ideation and this BP level raises the assumption that not only quite low but relatively low BP can be associated with suicidal ideation.
A large size sample that represents the general population of adults is another strength of this study. Although the prevalence of low BP is higher in younger people [3, 33], the majority of studies on the association between low BP and psychiatric disturbances have paid attention to specific age groups [34, 35], particularly to the elderly [4, 7, 26, 27, 36], making it impossible to know the influence throughout all ages of adults.
The results of this study can be generalized to all Korean adults, as the statistics reflect the complex sampling design, non-response to the survey, and post-stratification in the analyses. When weighted, the risk of suicidal ideation in the low BP group increased compared with the unweighted results (OR = 1.20, 95% CI, 1.02 to 1.40; OR = 1.22, 95% CI, 0.98 to 1.50; and OR = 1.39, 95% CI, 0.96 to 1.96 for SBP < 100, < 95, and < 90 mmHg, respectively; unweighted results, model V). It is assumed that the relationship between low BP and suicidal ideation would be more evident by applying weights during the analyses, as the sample reflected a higher population of younger people in urban areas than in rural areas.
There are no published studies investigating the impact of low BP on somatic or psychiatric symptoms in a general adult population in Asia, although the prevalence of hypotension is much higher in younger adults. This study is the first in Asia to investigate the relationship between low BP and substantive psychiatric symptoms in a large general population which includes young adults.
Unlike low BP, high BP was not associated with suicidal ideation in this study. Despite not adjusting for all comorbid cardiovascular diseases but several diseases, which are far more common in hypertensive patients and which affect mental health negatively [37], no association with suicidal ideation was seen in the hypertensive group, further reinforcing the suspicion that low BP itself is related to suicidal ideation. These results are consistent with previous studies showing no significant correlation between high BP and suicidal ideation [22, 23]. However, because a large-scale study suggested a high risk of suicidal ideation in patients with hypertension [21], further studies are needed.
In all five covariate models, the ORs in the prehypertensive group consistently did not differ from those in the normal BP group. These results further support that only low BP, among the different BP levels, is related to suicidal ideation. In addition, although prehypertension may be a risk factor for cardiovascular diseases and should be managed, it does not have a negative impact on mental health [see Additional file 1]. It is assumed that low BP poses quite different health issues from those of higher BP.
The results of this study are in accordance with the BP-emotional dampening hypothesis which suggests that BP has inhibitory effect on overall negative emotional experience and pain perception [38]. Baroreflex sensitivity (BRS) is strongly doubted as a potential mechanism [39]. In high BP, where baroreceptor stimulation dominates, increased BRS results in stronger cortical inhibition [39]. By contrast, in individuals with low BP, who predominantly exhibit baroreceptor inhibition, high BRS is accompanied by reduced central nervous inhibition and thus increased cortical arousal [39]. In several studies supporting this hypothesis, higher BP was associated with dampened responses to negative emotional stimuli [40]. On the contrary, high level of anxiety, hostility, and worry were clearly prevalent in people with low BP who were prevailed with reduced BRS [38, 41]. Psychophysiological approaches to explore whether BRS mediates the link between low BP and suicidal ideation will help to understand the mechanism.
The biological mechanism for the relationship between low BP and negative health effects is not well established. However, studies have suggested that reduced cerebral perfusion in a patient with low BP might be related with depression. Reduced microvascular circulation and oxygen transfer have been hypothesized to be a cause for the physical fatigue in a patient with a low BP. Erythrocyte velocity decreased at very low BP (< 70 mmHg) in a capillary dynamics study using TV microscopy at different BP levels [42]. The hypothesis that inappropriate energy production and accumulation of metabolites are causes of physical fatigue has also been suggested; [32] However, no evidence supports it. Conversely, studies suggesting that depression causes low BP have indicated that overexpressed neuropeptide Y in a patient with low BP is likely to mediate depression [13, 43]. More in-depth biological studies are needed to explain the mechanism.
Although it is clear that mental disorders, such as depression, are related to suicidal ideation, physical conditions are not. The present study is the first to investigate the association between low BP and suicidal ideation, which is a concrete indicator of a negative psychiatric state, thus driving the need to re-evaluate the health implications of low BP. Mental health conditions including suicidal ideation needs to be carefully monitored among those with low BP.
Limitations
Our study had some limitations. First, it was cross-sectional, so a two-way relationship is possible. Although Paterniti et al. [27] showed in a longitudinal study that baseline high depressive symptomatology was not a risk factor for low BP, supporting the low likelihood of a reverse correlation, subsequent prospective studies have found a reverse association between low BP and depression [13, 44, 45]. It is possible that depression may result in low BP by way of weight loss and reduced activities [8]. Although the likelihood of suicidal ideation to induce low BP seems intuitively low, future prospective studies should be conducted to investigate causality. Second, although the quality of the information collected from the hypotensive group, which is normally regarded as normal BP, was not expected to be much different from that of the normotensive group, the possibilities of information bias and recall bias cannot be ruled out, as this study used survey data. Third, the questions pertaining to suicidal ideation used in this study might not to be a sufficient assessment. However, the questionnaire used to screen for suicidal ideation followed the definition of suicidal ideation [46] and considering that the proportion with suicidal ideation in this study was similar to those in other studies [47], the results are considered reliable. Finally, in this study, we only used the levels of SBP to define low BP, and did not consider DBP. Because some previous studies showed different outcomes depending on whether the cut-off values to define hypotension are based on SBP or DBP [9, 48], it is also necessary to explore how suicidal ideation is related to low DBP.
