Association of pulsatile stress in childhood with subclinical renal damage in adults: A 30‐year prospective cohort study

Abstract The pulsatile stress in the microcirculation may contribute to development or progression of chronic kidney disease. However, there is no prospective data confirming whether pulsatile stress in early life affect renal function in middle age. The authors performed a longitudinal analysis of 1738 participants aged 6–15 years at baseline, an ongoing Adolescent Prospective Cohort with a follow‐up of 30 years. The authors evaluated the association between pulsatile stress in childhood and adult subclinical renal damage (SRD), adjusting for related covariates. Pulsatile stress was calculated as resting heart rate × pulse pressure. Renal function was assessed with estimated glomerular filtration rate (eGFR) and urine albumin‐to‐creatinine ratio (uACR). The results showed that pulsatile stress in childhood was associated with adult SRD (Relative Risk, 1.43; p = .032), and the predictive value of combined pulse pressure and heart rate for SRD was higher than either of them alone. The high pulsatile stress in childhood increased the risk of adult SRD in males (RR, 1.92; p = .003), but this association was not found in females (RR, 0.91; p = .729). Further, the participants were categorized into four groups on the basis of pulsatile stress status in childhood and adulthood. Male patients with high pulsatile stress during childhood but normal pulsatile stress as adults still had an increased risk of SRD (RR, 2.04; 95% CI, 1.18–3.54), while female patients did not (RR, 0.96; 95% CI, 0.46–1.99). The study demonstrated that high pulsatile stress in childhood significantly increased the risk of adult SRD, especially in males. Adequate control of pulse pressure and heart rate from childhood, in the long‐term, is very important for preventing kidney damage.


INTRODUCTION
The chronic kidney disease (CKD) has become an important public health problem in the world. 1 It has been reported that the prevalence of adult CKD was 10.8% in China and 13.0% in the United States. 2,3 Hemodynamic studies have shown that if the resistance of the glomerular efferent arterioles is higher than that of the afferent arterioles, the glomerular capillaries are in a high pressure pulsatility.
The high pulsatile stress (PS) can distort the material of the glomerular arterial wall, 4 affect the normal structure and function of renal microvascular, and lead to the occurrence and progression of CKD. 5,6 Many previous studies have shown that pulsatile stress, may lead to strained elastic fibers, followed by smooth muscle cell hypertrophy, proliferation, and, eventually, arterial stiffness. [7][8][9] And the change of pulsatile stress was a good predictor of arterial stiffness over time. 10 Thomas and coworkers investigated 125 513 men and 96 301 women aged 16-95 years and observed that a combined elevation of pulse pressure (PP) and heart rate (HR) increased the risk of cardiovascular mortality in men. 10 In addition, the Strong Heart Study also reported that pulsatile stress was very important in causing atherosclerosis and vascular hypertrophy. 11 Direct effects of the pulsatile stress on heart, arterial stiffness, and ultimately cardiovascular survival have been documented whereas little data exist relating to renal damage, especially in adolescents. Therefore, the aim of this study was to investigate the association between pulsatile stress in childhood and adult subclinical renal damage in an ongoing adolescent prospective cohort.

Study participants
This study is based on the Hanzhong Adolescent Hypertension Cohort, an ongoing prospective cohort study. The study began in 1987, when and migration. The detailed study design and procedures have been published previously. [12][13][14] Individuals who were pregnant or lactating women, had no blood samples and/or missing measurements and were unable to provide informed consent at follow-up were excluded, and finally, 1738 individuals were included in the analysis. The participant selection process is described in Figure S1.
This study followed the principles of the Helsinki Declaration. This study was approved by the Academic Committee of the First Affiliated Hospital of Xi'an Jiaotong University (XJTU1AF2015LSL-047) and was clinically registered (NCT02734472). All participants in this study signed informed consent forms at baseline and during follow-up. For participants younger than 18 years of age at baseline, informed consent from a parent/guardian was obtained.

Data collection
Personal basic information, family medical history, smoking status, and alcohol consumption history were collected using a unified question- After these pediatric patients had rested for 5 min in a quiet environment, their pulse times was measured manually by our staff during 30 s.
The value is then multiplied by two to obtain an individual's heart rate.
At follow-up, the individuals' heart rate was obtained simultaneously by Omron sphygmomanometer. Pulse pressure was calculated as systolic BP minus diastolic BP. Pulsatile stress was calculated as resting heart rate × pulse pressure. We determined the pulsatile stress quartiles according to sex-specific and age-specific population, and considered the 4th quartile as high pulsatile stress.

Assessment of renal function
Renal function was assessed with estimated glomerular filtration rate (eGFR) and urinary albumin creatinine ratio (uACR). 1 eGFR was calculated using the formula as follows: eGFR = 175×serum creatinine −1.234 ×age −0.179 (×0.79 for girls/women), where serum creatinine concentration is in milligrams per deciliter and age is in years. 2 17 uACR was calculated as urine albumin in milligrams divided by the urine creatinine in millimole (milligrams per millimole). Presence of SRD was defined as eGFR between 30 and 60 ml/min per 1.73 m 2 18 or elevated uACR of at least 2.5 mg/mmol in men and 3.5 mg/mmol in women 19 as previously described.

Statistical analyses
Continuous variables were shown as mean ± SDs if normally dis- Strengths of associations were determined by estimating the relative risks (RR) and their 95% confidence intervals (95%CI). Furthermore, we conducted a sensitivity analysis by excluding individuals who had diabetes or received antihypertensives therapy (n = 82) to eliminate the influence of these factors on the results. All statistical analyses were performed using SPSS 25.0 (SPSS, Inc., Chicago, Illinois, USA). Statistical significance was set as a two-tailed p value of less than .05.

Participants' clinical characteristics
The participants' clinical characteristics in childhood and adulthood by sex are summarized in Table 1 Table S1.
In adulthood, male patients had higher height, weight, BMI, waist, hips, systolic BP, diastolic BP, pulse pressure, triglycerides, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), fasting glucose, serum uric acid, serum creatinine, urine albumin and urine uric acid than female patients, and females had higher heart rate, high-density lipoprotein cholesterol (HDL-C), eGFR and uACR than male patients.
Males were more likely to smoke and drink than females.

Association between pulsatile stress in childhood and adult SRD
The differences of eGFR and uACR between NPS group and HPS group are shown in Figure 2. There were no differences in eGFR values between NPS group and HPS group, regardless of sex (all p > .05). The HPS group had higher uACR values than NPS group (p = .002). In male patients, HPS group had higher uACR values than NPS group (p < .001), but there was no difference between the two groups in female patients (p = .327). We used multivariable adjusted logistic regression to examine the association of pulsatile stress in childhood with SRD in adults (

F I G U R E 2
The levels and differences of the eGFR (A) and uACR (B) in normal pulsatile stress group and high pulsatile group, overall and by sex group. eGFR, estimated glomerular filtration rate; uACR, urinary albumin-to-creatinine ratio; NPS, normal pulsatile stress group; HPS, high pulsatile stress group F I G U R E 3 Relationship between pulsatile stress and eGFR and uACR by correlation analysis. Relationship between pulsatile stress and eGFR in overall patients (A), male patients (B) and female patients (C). Relationship between pulsatile stress and uACR in overall patients (D), male patients (E) and female patients (F). R, correlation coefficient. eGFR, estimated glomerular filtration rate; uACR, urinary albumin-to-creatinine ratio operating curve (ROC). As shown in Figure 4, we found that the R 2 value of pulsatile stress for predicting SRD was higher than R 2 value of PP (0.568 vs. 0.545, p = .023) and heart rate (0.568 vs. 0.536, p = .031) for predicting SRD.

Association between pulsatile stress groups and SRD in adults
Furthermore, for the comparison of outcomes, the participants were categorized into four groups on the basis of pulsatile stress status in childhood and adulthood. Group I included participants with NPS in childhood and adulthood; group II, those who had NPS in childhood but HPS as adults; group III, those who had HPS in childhood but NPS as adults; and group IV, those with HPS in childhood and adulthood.
We used a logistic regression to examine the association between pulsatile stress groups and adult SRD. Age, sex, BMI and busts at baseline, BMI, waist, hips, smoking, drinking, fasting glucose, serum uric acid, triglycerides, total cholesterol, HDL-C and LDL-C at followup were all adjusted. As shown in Table 3

Sensitivity analyses
We performed additional analyses after excluding individuals who had diabetes or received antihypertensive drugs (n = 82) to eliminate the effect of these factors on the results. As shown in Tables S2 and S3, the results of the sensitivity analyses remained the same. Furthermore, we performed a sensitivity analysis after excluding individuals with eGFR > 200 ml/min per 1.73 m 2 (n = 10), and the results remained the same (Table S4). We performed an additional analysis of the relationship between pulsatile stress and subclinical renal damage using a threshold of 30 mg/g of uACR, and the results remained the same (Table S5). In addition, we also performed an analysis of the relationship between pulsatile stress and renal damage by adjusted BMI z-score rather than BMI at baseline. The results of the two are consistent (Table   S6).

DISCUSSION
Our analysis included 1738 schoolchildren aged 6-15 years who were enrolled from rural China in 1987 and followed for 30 years. In this study, we assessed the association between pulsatile stress in childhood and adult subclinical renal damage in an ongoing adolescent prospective cohort. We found that pulsatile stress in childhood was significantly associated with adult SRD in males, but this association has not been found in females. In addition, we demonstrated that HPS in childhood, completely independent of adult pulsatile pressure, can increased the risk of adult SRD in males, while female patients did not.
In other words, for male individuals, regardless of their adult pulsatile stress, childhood HPS will increase the risk of SRD in middle age.
CKD is associated with increased risks of all-cause mortality, cardiovascular disease, and progression to kidney failure. 20,21 In a crosssectional pediatric chronic kidney disease cohort from the CKiD study, they found that the pulse pressure was associated with lower eGFR in pediatric CKD patients. 22 A community-based cohort study included 1426 participants, followed for a median of 4.8 years and found that central pulse pressure is an independent determinant of renal function. 23 And previous study have demonstrated that high resting heart rate is a potent predictor of renal outcomes. 24 However, there is no prospective data confirming whether pulsatile stress (the combined effect of the two components) in early life affect renal function in middle age. In our study, we investigated the association between pulsatile stress in childhood and adult SRD using an ongoing adolescent prospective cohort. We found the prevalence of adult SRD in HPS group was higher than NPS group. And adolescents with HPS were 1.4 times more likely to have SRD in middle life than normal individuals.
And we found that the predictive value of pulsatile stress, that is, combined pulse pressure and heart rate, for SRD is higher than any one of them alone. Our results were similar with a kidney transplant study.
They revealed an eightfold increased risk for microalbuminuria and 12.2-fold increased risk for macro-albuminuria comparing upper with lower tertile of pulsatile stress. 25 Our study, while different, emphasize the effect of childhood pulsatile stress on adult SRD. In this context, pulse pressure may alone not be relevant for the determination of the microcirculatory damage. Other parameters may be needed, such as increased sympathetic activity, which accelerates heart rate.
After combining pulse pressure and heart rate, continuously enhanced pulsatility down to microcirculatory level may thus distort resistance Further, we found a sex difference in the association between childhood pulsatile stress and SRD in adults. We found that HPS in childhood can increase the risk of adult SRD in males, but this association has not been found in females. Similarly, we revealed that HPS in childhood, independent of adult pulsatile pressure, had 2.0-fold increased risk for adult SRD in males, while female patients did not. Consistent with our findings, a large French population study demonstrated a combined elevation of the two components of pulsatile arterial stress was associated with an important increase in cardiovascular mortality in men. 10 McEniery and coworkers also found pulse pressure heart rate product assessed over the whole of 20 years was independently correlated with aortic pulse wave velocity in males. 27 The mechanism for this sex difference may be that both pulse pressure and heart rate play a more important role in men than in women. 10,28 We also found that childhood pulsatile stress was associated with adult uACR levels but not eGFR. Our results are in line with a finding we have already reported in a previous study that higher BP trajectories were correlated with higher of uACR levels but not eGFR. 14 The failure to reach significance for eGFR was mainly due to the small number of patients with abnormal eGFR range in our study. In addition, the difference between uACR and eGFR may be caused by various characteristics of the analyzed population, such as age. The eGFR values was used to define kidney damage, severe CKD, and end-stage renal disease. As Huang and coworkers has revealed, in old people, aortic stiffness was significantly associated with a decrease in eGFR, but not with uACR levels. 29 On the contrary, micro-albuminuria is an accepted marker for microcirculatory damage, 30 more appropriate in young and middle-aged individuals. In our study, patients were recruited in childhood and followed for 30 years to middle age, and the renal function was assessed by eGFR and uACR jointly to make the result more accurate.
There are several strengths in this study. First, this is a large number of adolescent cohort with the prospective follow up in nature, allowing us to fully explore risk factors in childhood. Second, individuals were followed for 30 years up to adulthood, with a relatively high response rate, which is sufficient to assess the impact of childhood variables on adult target organ damage. Third, this study focused on exploring the association between childhood pulsatile stress and adult SRD.
To our knowledge, this impact has rarely been explored in previous studies. However, our study also has limitations. First of all, our study only included Han people in northern China. Therefore, epidemiological studies of other ethnicities are needed to verify our results. Second, there is a lack of data on physical activity, diet and other risk factors that may affect renal function in our cohort. Third, the eGFR value in this study was estimated based on serum creatinine. Although the value is calculated according to the internationally recognized standard formula, it is still relatively inaccurate. More accurate filtration markers, such as cystatin C or β2-microglobulin, were not examined.
Fourthly, the use of a single morning spot urine sample to evaluate uACR is also a major limitation of this study. However, in this study, every step from the collection, storage and detection of urine specimens was strictly and accurately completed, which can improve its stability and reduce the false positive rate. Finally, the mean age of the patients was 42 years old at the last follow-up. Therefore, the prevalence of SRD is relatively low, especially the number of individuals with abnormal eGFR range. However, the prospective design of our study provides us an opportunity to perform further follow-ups to determine the future risk of CKD.
In summary, this study demonstrated that high pulsatile stress in childhood significantly increased the risk of adult SRD, especially in males. Adequate control of pulse pressure and heart rate from childhood, in the long-term, is very important for preventing kidney damage.

ACKNOWLEDGMENTS
The Hanzhong Adolescent Hypertension Study is a joint effort of many investigators and staff members whose contribution is gratefully acknowledged. We especially thank the children and adults who have participated in this study over many years. This work was sup-