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Abstract

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

The two most commonly used strategies to evaluate dialysis patients' blood pressure (BP) level are 44-hour and 24-hour ambulatory blood pressure monitoring (ABPM). The objective of this study was to find an appropriate 24-hour period that correlated well with the 44-hour BP level and determine the differences between these strategies. In a group of 51 dialysis patients, the authors performed 44-hour ABPM and extracted data for a fixed 24-hour ABPM. The fixed 24-hour ABPM started at 6 am on the nondialysis day. A strong correlation was found between all parameters of 44-hour and the fixed 24-hour ABPM, with paired sample t test showing only small magnitude changes in a few parameters. Both 24-hour ABPM and 44-hour ABPM were superior to clinic BP in predicting left ventricular mass index (LVMI) by multiple regression analysis. It was found that 44-hour ambulatory arterial stiffness index (AASI), but not 24-hour AASI, had a positive association with LVMI (r=0.328, P=.021). However, after adjustment for 44-hour systolic blood pressure, this association disappeared. Fixed 24-hour ABPM is a good surrogate of 44-hour ABPM to some extent, while 44-hour ABPM can provide more accurate and detailed information.

Hypertension, as a major complication of end-stage renal disease, is a significant financial and medical burden on hemodialysis patients. Accurate blood pressure (BP) measurements are needed for daily clinical practice. Ambulatory BP monitoring (ABPM) is the gold standard in the diagnosis of hypertension and has been widely used in dialysis patients.[1] It has been suggested that ambulatory BP (ABP) is a stronger predictor of target organ damage (TOD) and cardiovascular outcomes than predialysis or postdialysis BP.[2, 3] It can also provide detailed information that clinic BP cannot, such as BP rhythm, load, and variability. These indices were also found to be associated with TOD and prognosis in previous studies.[4-6]

In dialysis patients, there are two strategies to perform ABPM. Twenty-four–hour (24H) monitoring has been the conventional choice and is widely accepted and applied in the general population. In dialysis patients, it is also superior to clinic BP. But since dialysis patients have little preserved renal function, it has become common practice to perform 44-hour (44H) monitoring. The loss of renal function leads to the accumulation of sodium and water and impairs the normal day-by-day BP pattern in dialysis patients.[7] A previous study by Kelley and colleagues[8] demonstrated that BP increased continuously at a rate of about 1 mm Hg every 4 hours following the end of dialysis. Therefore, 44H ABPM in dialysis patients might have particular advantages. It provides complete information about interdialytic BP and reveals the real BP pattern. The disadvantages are also obvious: it is inconvenient, time-consuming, expensive, and uncomfortable for patients.

According to the characteristic BP pattern of dialysis patients, it is important to choose an appropriate fixed 24H period when performing 24H ABPM. The fixed 24H should correlate well with the 44H BP level while at the same time be clinically feasible. Since BP continually rises, the first 24H and the last 24H period would be unsuitable to represent the average level of the interdialytic 44H BP. We hypothesized that the 24H period of the nondialysis day, which covered the relative “middle-term” interdialytic period, would be better to predict the whole 44H BP level. This period was also chosen for considerations of clinical feasibility and evidence from a previous study.[9]

Methods

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

Patients

This cross-sectional study was performed at the Center for Kidney Disease of the Second Affiliated Hospital of Nanjing Medical University in Jiangsu, China. Patients enrolled were older than 18 and had been undergoing chronic hemodialysis 3 times a week for more than 3 months. A bicarbonate buffered dialysate with sodium concentration of 138 mmol/L were used for all patients. The dry body weight was estimated in all patients evaluated by routine clinical examination. Exclusion criteria were: (1) previous history of cardiovascular complications including acute coronary syndrome, heart failure, transient ischemic attack, and stroke; (2) history of dilated cardiomyopathy or amyloid degeneration; and (3) unstable condition caused by infection, malignant hypertension, or tumor. Informed consent was obtained from each participant before the study, and the study was approved by the institutional review board at the Second Affiliated Hospital of Nanjing Medical University.

Ambulatory BP Monitoring

ABPM was performed after the midweek hemodialysis session for 44 hours using an ABP monitor (SpaceLabs 90217; SpaceLabs Medical Inc, Redmond, WA). The monitor was placed in the non-access arm and was programmed to measure BP every 20 minutes during the daytime period (6 am–10 pm) and every 30 minutes during the nighttime period (10 pm–6 am). Patients were instructed to follow their daily routines during examination. Data were downloaded using the manufacturer's software (SpaceLabs report manager system) and was further analyzed by SPSS 13.0 (SPSS Inc, Chicago, IL). 24H ABP data were extracted from the 44H ABP. The 24H period was set to begin at 6 am on the nondialysis day to the next day at 6 am. Patients who had recordings <70% were excluded from the analysis.

Clinic BP

Predialysis and postdialysis BPs were measured by the dialysis center personnel. The BP recordings were averaged over 2 weeks (6 times) before ABPM was performed.

Echocardiography

Echocardiography was performed within 1 week after the ABPM. It was performed immediately after a midweek hemodialysis session, as suggested by another study for the reason associated with the least intravascular volume.[2] For each patient, the following measurements were taken: end-diastolic interventricular septum thickness (IVSD), posterior wall thickness (PWD), and left ventricular diameter (LVDD). Left ventricular mass (LVM) was calculated using the following formula and corrected for height2.7 measured in meters,[10] as previous studies suggest that correcting LVM for height2.7 minimizes the effect of sex, race, age, and obesity and correlates better with long-term outcomes in dialysis patients.[11, 12]

  • display math

Blood Chemistry

Blood samples were drawn from the dialysis access before dialysis on the day ABPM was performed.

Statistical Analysis

Data were expressed as the mean±standard deviation for normally distributed variables. The paired-sample t test was used to compare 44H ABP and 24H ABP. The strength of associations between normally distributed continuous variables was measured using Pearson's correlation coefficient and followed by multiple linear regressions to assess the independent predictors of LVM index (LVMI). P<.05 was considered statistically significant.

Results

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

From March to July 2013, we conducted ABPM in 63 dialysis patients from our dialysis center. Twelve patients were excluded from this study because of insufficient BP recordings (<70%). In the remaining 51 patients in our report, 49 had adequate echocardiographic evaluation, which constituted the sample to test the relationship between BP and LVMI. Twenty-four patients were willing to have their KT/V assessed for this study.

Clinical characteristics, laboratory tests, echocardiographic parameters, and clinical BP measurements are summarized in Table 1. The majority of the patients were men and the average age was 53.4 years. Average dry body weight was 60.8 kg and body mass index (BMI) was 21.9 kg/m2. Mean duration of dialysis was 58.7 months. Twelve patients had diabetes mellitus and diabetic nephropathy, which, when combined with other glomerulonephritides, constituted the major cause of end-stage renal disease in this cohort. Average dosage of erythropoietin used by each patient was 5556.5 IU per week. Calcium channel blockers, angiotensin-converting enzyme inhibitors/angiotensin receptor blockers, and β-blockers were the most widely used antihypertensive agents in this population, each being used by 58.8%, 43.1%, and 47.1% of the patients, respectively. Evaluation by echocardiography revealed mean values of LVDD, IVS, and left ventricular PWD as 50.9, 11.5, and 10.6 mm, respectively. Ejection fraction was well preserved in the study population, with an average of 65.7%. However, diastolic function was impaired in most patients, represented by a mean E/A of 0.8. The prevalence of left ventricular hypertrophy was 70.6% (30 of 49 patients) with a mean LVMI of 55.7 g/m2.7. The mean predialysis BP was 146.0/88.5 mm Hg, with an average heart rate of 75.8 beats per minute. Both systolic BP (SBP) and diastolic BP (DBP) were lower after dialysis, while postdialysis heart rate (HR) did not differ from predialysis HR.

Table 1. Basic Characteristics
Clinical CharacteristicsMean±SD or No. (%)
  1. Abbreviations: ACE, angiotensin-converting enzyme inhibitor; ARB, angiotensin receptor blocker; BP, blood pressure; CCB, calcium channel blocker; DBP, diastolic blood pressure; EF, ejection fraction; HDL, high-density lipoprotein; HR, heart rate; LDL, low-density lipoprotein; LVH, left ventricular hypertrophy; LV, left ventricular; LVMI, left ventricular mass index; PP, pulse pressure; SBP, systolic blood pressure; SD, standard deviation.

Male33 (64.7)
Age, y53.4±14.1
Dry body weight, kg60.8±10.8
BMI, kg/m221.9±3.1
Smoker18 (35.3)
Duration of dialysis, mo58.7±53.5
Interdialytic weight gain, kg2.4±0.8
Diabetes mellitus12 (23.5)
Erythropoietin, U5556.5±2693.3
Etiology
Diabetes mellitus12 (23.5)
Hypertension3 (6.3)
Glomerulonephritis14 (27.4)
Other22 (43.1)
Use of antihypertensive drug
CCB30 (58.8)
ACE inhibitor/ARB22 (43.1)
β-Blocker24 (47.1)
Other10 (19.6)
Antihypertensive drugs, No.
014 (27.5)
110 (19.6)
210 (19.6)
≥317 (33.3)
Laboratory tests
Hemoglobin, g/L101.8±12.0
Hematocrit, %30.3±4.0
Total protein, g/L68.8±6.9
Albumin, g/L41.7±3.3
Triglyceride, mmol/L1.77±1.15
Total cholesterol, mmol/L3.72±1.03
HDL cholesterol, mmol/L1.02±0.32
LDL cholesterol, mmol/L2.08±0.60
Calcium, mmol/L2.38±0.25
Phosphorus, mmol/L1.64±0.63
Parathyroid hormone, pg/mL272.9±561.6
Kt/V (N=24)1.39±0.22
Echocardiography findings
LV internal diameter in diastole, mm50.9±5.4
Interventricular septum thickness in diastole, mm11.5±1.9
LV posterior wall in diastole, mm10.6±1.7
EF%65.7±4.7
E/A0.80±0.17
LVMI, g/m2.755.7±18.3
LVH36 (73.5)
Clinical BP
Predialysis SBP146.0±18.1
Predialysis DBP88.5±10.0
Predialysis PP57.5±12.9
Predialysis HR75.8±8.7
Postdialysis SBP138.3±17.7
Postdialysis DBP85.8±10.4
Postdialysis PP52.5±12.8
Postdialysis HR75.8±9.0

As mentioned above, the first 24H and the last 24H BP periods would be unsuitable to represent the average level of the interdialytic 44H period BP because BP continuously rises during this period. In support of this, we calculated the paired differences for SBP between each 24H segment vs the entire 44H period. There were significant paired differences for the first and the last 24H SBP compared with 44H (44H vs first 24H, 2.2±4 mm Hg, P<.001; 44H vs last 24H, −1.5±4 mm Hg, P=.010), while no significant difference was noted for the nondialysis day fixed 24H SBP vs 44H with the smallest paired difference (0.4±3 mm Hg, P=.331).

44H and the fixed 24H ABPs are shown and compared in Table 2. As expected, the overall BPs of both 44H ABP and 24H ABP were lower than predialysis BPs. The majority of the patients exhibited uncontrolled hypertension with an average overall SBP >135 mm Hg, which was the cutoff value for diagnosing hypertension in the ABP setting. A very strong and significant positive correlation was found between all parameters of 44H and 24H ABPs, with the lowest correlation coefficient of 44H and 24H nighttime HR reaching 0.957. Concordance for SBP and DBP between 44H and 24H were demonstrated by the Bland-Altman plot in Figure 1A and 1B. Using paired-samples test to detect the difference between 44H and 24H ABP, significant differences were found only for daytime SBP, nighttime pulse pressure (PP), and nighttime HR, but the degree of the differences was very small: the paired differences for the 3 parameters were 1.2 mm Hg, −1.2 mm Hg, and 1.4 beats per minutes, respectively.

Table 2. Paired Sample Test and Correlation of 44-Hour and 24-Hour ABPM Parameters
 ABPMPaired Samples TestCorrelation
44-Hour24-Hour Paired DifferencesP Value r P Value
  1. Abbreviations: ABPM, ambulatory blood pressure monitoring; DBP, diastolic blood pressure; HR, heart rate; NS, not significant; PP, pulse pressure; SBP, systolic blood pressure.

SBP139.6±22.7139.1±23.20.4±3NS0.992<.001
DBP81.5±12.481.2±130.3±1.9NS0.989<.001
PP58.2±17.657.8±17.20.4±1.9NS0.994<.001
HR76±9.375.7±9.50.3±2.1NS0.975<.001
Daytime SBP140.2±22.7139±23.11.2±3.6.0210.988<.001
Daytime DBP82.4±12.681.8±13.10.6±2.3NS0.985<.001
Daytime PP57.8±17.557.3±17.40.5±1.9NS0.994<.001
Daytime HR77.9±9.677.5±9.80.3±2.2NS0.974<.001
Nighttime SBP138.5±23.7139.7±24.8−1.1±5.9NS0.972<.001
Nighttime DBP79.8±12.779.9±13.6−0±3.7NS0.963<.001
Nighttime PP58.7±18.259.9±18.5−1.2±3.5.0200.981<.001
Nighttime HR72.1±9.870.7±9.91.4±2.9.0010.957<.001
image

Figure 1. Bland-Altman plots for systolic blood pressure (SBP) and diastolic blood pressure (DBP) of 44-hour ambulatory blood pressure (44H ABP) and 24-hour ambulatory blood pressure (24H ABP). (A) Bland-Altman plots for SBP of 44H and 24H ABP. (B) Bland-Altman plots for DBP of 44H and 24H ABP.

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Considering the rising BP, we also evaluated whether the fixed 24H BP could predict the whole 44H BP in different dialysis shift groups since the time elapsed following the end of dialysis to the beginning of the fixed 24H was different among shifts. We calculated the paired differences grouped by dialysis shift (7:30–11:30 am, 12:30–4:30 pm, 5:30–9:30 pm) (Table 3). The differences were still acceptable for each parameter and no significant intergroup difference was found.

Table 3. Paired Differences of 44-Hour and 24-Hour SBP, DBP, PP, and HR Grouped by Dialysis Shift
 Morning (N=15)Afternoon (N=23)Night (N=13)
Paired DifferencesP ValuePaired DifferencesP ValuePaired DifferencesP Value
  1. Abbreviation: NS, not significant. aIntergroup comparison using Kruskal-Wallis test with P value of .49 and .37 for systolic blood pressure (SBP) and diastolic blood pressure (DBP), respectively. bIntergroup comparison using one-way analysis of variance with P value of .74 and .34 for pulse pressure (PP) and heart rate (HR), respectively.

SBPa0.1±4.3NS0.4±2.3NS0.8±2.4NS
DBPa0.3±2.8NS0.5±1.3NS0.1±1.8NS
PPb0.1±1.9NS0.5±2.2NS0.7±1.3NS
HRb0.9±1.7NS0.3±2.4NS−0.3±1.9NS

The correlation between 44H ABP and 24H ABP in diagnosing hypertension was further evaluated. 29/19 patients were diagnosed as SBP hypertension/normotension by both 24H and 44H ABP. Different diagnoses were found in only 3 of 51 (5.9%) patients. The accuracy was the same when diagnosing DBP hypertension.

The BP parameters correlated significantly with LVMI in univariate analysis, as presented in Table 4. However, no significant association existed between the variants of clinical characteristics and laboratory tests listed in Table 1 with LVMI. This correlation between BP and LVMI was less strong for clinic BPs than both 44H and 24H ABPs. Multiple linear regression analysis was performed in 3 models (Table 5. Since we aimed to determine the major variants associated with LVMI from different BP measurements in the same cohort, we did not control for sex, age, BMI, hemoglobin, parathyroid hormone, and Kt/V. In model 1, in which we compared 44H ABP and clinical BP parameters, the strongest predicator for LVMI was 44H ambulatory nighttime PP. Postdialysis HR was also an independent predicator for LVMI in this model. None of the clinical BP parameters were capable of predicting LVMI. In model 2, comparing 24H ABP and clinic BP, 24H ambulatory PP was the only independent predictor for LVMI. When comparing 44H and 24H ABP in model 3, 44H ambulatory nighttime PP was the only variant that remained significant.

Table 4. Univariate Analysis: Variants Associated With LVMI
  r P Value
  1. Abbreviations: HR, heart rate; LVMI, left ventricular mass index; PP, pulse pressure; SBP, systolic blood pressure.

44-H ambulatory SBP0.539<.001
44-H ambulatory PP0.531<.001
44-H daytime ambulatory SBP0.520<.001
44-H daytime ambulatory PP0.513<.001
44-H nighttime ambulatory SBP0.551<.001
44-H nighttime ambulatory PP0.554<.001
Predialysis SBP0.475.001
Predialysis PP0.492<.001
Postdialysis SBP0.471.001
Postdialysis DBP0.313.029
Postdialysis PP0.411.003
Postdialysis HR−0.370.009
24-H ambulatory SBP0.520<.001
24-H ambulatory PP0.526<.001
24-H daytime ambulatory SBP0.504<.001
24-H daytime ambulatory PP0.510<.001
24-H nighttime ambulatory SBP0.523<.001
24-H nighttime ambulatory PP0.518<.001
Table 5. Multiple Regression Analysis
 βSEP Value
  1. Abbreviations: HR, heart rate; PP, pulse pressure; SE, standard error. Model 1: 44-hour ambulatory blood pressure (44-H ABP) and clinic blood pressure (BP) parameters associated with left ventricular mass index (LVMI) were included. Model 2: 24-hour ambulatory blood pressure (24-H ABP) and clinic BP parameters associated with LVMI were included. Model 3: 44-H ABP and 24-H ABP parameters associated with LVMI were included.

Model 1
Constant64.8521.26.004
44-H ambulatory nighttime PP0.500.12<.001
Postdialysis HR−0.510.24.041
Model 2
Constant23.417.94.005
24-H ambulatory PP0.550.13<.001
Model 3
Constant22.777.54.004
44-H ambulatory nighttime PP0.560.12<.001

We also evaluated the association between ambulatory arterial stiffness index (AASI) derived from 44H and the fixed 24H ABP with LVMI. This index was calculated by 1 minus the slope of diastolic on systolic pressure during ABPM.[13] The mean 44H AASI was 0.48±0.15 and correlated significantly with LVMI (r=0.33, P=.021) (Figure 2A). The mean 24H AASI was 0.50±0.18 and failed to show a significant correlation with LVMI (r=0.26, P=.073) (Figure 2B). After adjustment for ambulatory SBP, neither of the two AAS indices showed a significant correlation with LVMI (44H AASI, r=0.307, P=.303; 24H AASI, r=0.279, P=.471).

image

Figure 2. Association of ambulatory arterial stiffness index (AASI) with left ventricular mass index (LVMI). (A) Association of 44-hour (44H) AASI with LVMI. (B) Association of 24-hour (24-H) AASI with LVMI.

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Discussion

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

In a group of 51 dialysis patients whose disease was relatively well-controlled, as reflected by the average hemoglobin, albumin, and Kt/V, we compared the data of 44H and a fixed 24H ABP. The BP parameters showed a very strong correlation between these two strategies with relatively small differences. According to the paired-samples t test, there were significant differences only in daytime SBP, nighttime, and nighttime HR, and the magnitude of their differences were very small, at 1.2 mm Hg, −1.2 mm Hg, and 1.4 beats per minute, respectively. These small differences suggest the utility of the fixed 24H ABPM in evaluating the BP level of dialysis patients compared with 44H monitoring. This was confirmed by evaluating the ability to discriminate hypertension against normotension in both 44H and 24H monitoring. A majority of patients had the same BP definition according to 24H and 44H monitoring. However, it is important to note that a small percentage of patients (5.9%) were still wrongly diagnosed by 24H data compared with 44H.

In the current study, we demonstrate that the correlation between ABPs and LVMI was stronger than predialysis and postdialysis BPs. This notion was also supported by other studies.[2, 14] We confirmed at the same time that both the fixed 24H and 44H ABPs were superior in predicting LVMI than clinic BP. Using stepwise regression analysis comparing 44H ABP and 24H ABP, 44H nighttime PP remained the sole variant associated with LVMI, indicating an advantage of 44H ABP in association with hypertensive end organ damage.

It is interesting to note that PP, especially ambulatory nighttime PP, was a superior determinant of LVMI than SBP. The role of nighttime BP and PP has been investigated in previous studies. In a long prospective study conducted by Jokiniitty and colleagues,[15] the 3 best BP variables identified in predicting LVMI are 24H PP, nighttime PP, and daytime PP. After 10 years of follow-up in their study, the change in LVMI was best predicted by the change in casual PP. In treated hypertensive dialysis patients, Amar and colleagues[3] demonstrated that nocturnal BP and 24H PP are independent predictors of cardiovascular mortality.

AASI, first suggested by Li and colleagues[13] in 2006, was a surrogate index of arterial stiffness derived from ABP measurements. This assumption was based on the opinion that loss of elasticity of the artery influences the height of the diastolic pressure and its relation to systolic pressure.[16] AASI is calculated by 1 minus the slope of diastolic on SBP during ambulatory monitoring. The index is associated with PWV and other end organ damage including left ventricular hypertrophy[13, 17, 18]; however, its use is still controversial.[19, 20] In addition, although this index has been examined in chronic kidney disease,[21, 22] there are no prior studies specifically addressing AASI in dialysis patients. To our knowledge, this is the first report demonstrating AASI and its association with LVMI in this group of patients. We found that 44H AASI, but not 24H AASI, correlated significantly with LVMI. Nevertheless, after adjustment for ambulatory SBP, this association was no longer significant, suggesting that AASI predicts LVMI through BP level.

There are quite a few studies focusing on ABPM in dialysis patients, including both 44H and 24H ABP. However, very few studies have attempted to find an appropriate 24H period representing the whole interdialytic session. Martin and colleagues[7] found that dialysis patients with the same interdialytic BP could have different target organ damage as a result of different BP patterns. They conclude that an increase in BP in the second half of the interdialytic period could have a more important pathophysiologic and prognostic role. In this study, we chose a fixed 24H period on the nondialysis day and demonstrated excellent correlation with 44H ABPM. The prognostic value of 24H ABPM on a nondialysis day has been confirmed by Tripepi and colleagues.[9] This validates the clinical significance for performing 24H ABPM on a nondialysis day in hemodialysis patients. Furthermore, performing 24H ABPM in the fixed period is more feasible and may lead to increased use of ABPM in routine clinical practice. It is notable that the fixed 24H period in our study overlapped with the second half of the interdialytic period, as in study by Martin and colleagues.

Study Limitations

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

It should be pointed out that there are some limitations in our study. First, the sample size is small. Furthermore, the 24H nondialysis day period was fixed in our study. Whether there were more appropriate 24H periods, especially corresponding to different dialysis shifts, remained to be addressed. Even though we did not find any significant intergroup difference for the 3 dialysis shifts, it should be acknowledged that a longer-term ABPM will have advantages over shorter ones as a result of the rising interdialytic BP. Finally, the study was conducted in a group of Chinese dialysis patients whose ABP characteristics had not been previously reported. Whether these data are applicable to other races is uncertain.

Conclusions

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

We have compared the 44H ABP with the fixed 24H ABP on the nondialysis day of the interdialytic session. Both ABPs are superior to clinic BP in correlation with LVMI. PP, especially 44H nighttime PP, appears to be the most important variant for LVMI. 44H AASI, not 24H AASI, may also be correlated with LVMI, but such a correlation disappears after being adjusted for ambulatory SBP. Taken together, while 44H ABP provides accurate and detailed information, the fixed 24H ABP on the nondialysis day serves as a good surrogate of the longer ABP and has the advantage of easier measurement.

Acknowledgments and disclosures

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References

We sincerely thank Dr Roderick J. Tan of the University of Pittsburgh for English editing of this paper. This work was supported by the Foundation for Science and Technology Program in Health of Jiangsu to Junwei Yang.

References

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Study Limitations
  7. Conclusions
  8. Acknowledgments and disclosures
  9. References
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