Poor Agreement Between Dialysis Unit Blood Pressure and Interdialytic Ambulatory Blood Pressure

Abstract

Although ambulatory blood pressure (ABP) monitoring (ABPM) is often considered to have an advantage over dialysis unit blood pressure (BP) in dialysis patients, the exact relationship between these two measurements of BP has not been well determined. In a cohort of 90 Chinese dialysis patients, agreement between dialysis unit BP (predialysis and postdialysis BP) and interdialytic ABP was evaluated using Bland‐Altman plots and Lin's concordance correlation coefficient for quantitative analysis, and inter‐rater agreement (κ) for qualitative analysis. Limits of agreement between dialysis unit BP and ABP were wide (predialysis systolic BP: −33.5 to 20.9 mm Hg; predialysis diastolic BP: −22.2 to 11.9 mm Hg; postdialysis systolic BP: −25.4 to 26.5 mm Hg; postdialysis diastolic BP: −18.4 to 12.3 mm Hg). Lin's concordance correlation coefficient exhibited a poor agreement with concordance correlation coefficients of 0.75, 0.81, 0.64, and 0.75 for predialysis systolic BP, postdialysis systolic BP, predialysis diastolic BP, and postdialysis diastolic BP, respectively. When BP level was classified into quartiles, an increasing trend for incorrect classification rate was observed with deleterious hypertension, with the highest value in grade 3 hypertension (100% and 75.0% for predialysis and postdialysis BP, respectively). Therefore, these data suggest that agreement between dialysis unit BP and interdialytic ABP is poor in Chinese dialysis patients, and the bias for patients with higher degree of hypertension is more prominent.

Hypertension is a major comorbidity in dialysis patients, and tight constraint of blood pressure (BP) is always one of the most important issues in the management of end‐stage renal disease (ESRD).1, 2 Although numerous approaches have been taken to improve the control of hypertension in dialysis patients, the current state in the clinical setting is still far from being optimistic.3 A critical issue in the management for dialysis patients is the accurate evaluation of its level. Dialysis unit blood pressure (BP), usually referred to as predialysis and postdialysis BP, serves as a traditional reference for clinical decision‐making. However, this approach was found to be unreliable in practice.4, 5 Instead, the importance of ambulatory BP (ABP) monitoring (ABPM) for dialysis patients has been well recognized and widely accepted as the “golden standard.”6

There are quite a few studies confirming that ABP is superior in predicting end organ damage and mortality in dialysis patients. In a cohort of 140 dialysis patients, Agarwal and colleagues7 found that ABP had a stronger correlation with left ventricular hypertrophy than dialysis unit BP. The authors further demonstrated that ABP was a superior predictor of mortality compared with BP measured in the dialysis unit in their subsequent studies.3 These results are in line with the observations reported by other groups.8, 9

Despite a thorough understanding of the value of ABPM, information on the magnitude of the difference between dialysis unit BP and ABP is insufficient. Although there are studies evaluating the agreement between clinic BP and ABP in the general population,10, 11 comparable work in dialysis patients is relatively scarce. Thus, the aim of the current study was to address this issue in a cohort of Chinese dialysis patients.

Materials and Methods

Patients

This was a cross‐sectional observational study conducted at the following dialysis centers: Second Affiliated Hospital of Nanjing Medical University (Jiangsu, China), Quanjiao Tongren Hospital (Anhui, China), and Dainan People's Hospital (Jiangsu, China). Patients who were older than 18 and had been undergoing maintenance hemodialysis 3 times a week for more than 3 months were recruited into the study. Patients with a change of dry body weight or antihypertensive regimen during the previous 2 weeks were excluded. Patients unsuitable for ABPM including acute infection, malignant hypertension, and overweight were also excluded. Signed informed consents were obtained from all participants and the study was approved by the institutional review board of each respective center.

Ambulatory BP Monitoring

ABPM was performed after a midweek dialysis session for 44 hours using a SpaceLabs 90217 monitor (SpaceLabs Medical Inc, Redmond, WA). The monitor was placed on the non‐access arm, and BP was measured every 20 minutes in the daytime (6 am–10 pm) and every 30 minutes in the nighttime (10 pm–6 am). Patients were instructed to keep their arm immobile during measurement and follow their daily activity. Patients with <70% recordings were excluded from the analysis.

Dialysis Unit Blood Pressure

Patients with ≥5 minutes rest had their predialysis BP measured just before their dialysis session. Postdialysis BP was measured 3 to 5 minutes after dialysis when the patients were stable. All measurements were operated by the dialysis center staff using a validated automated sphygmomanometer (Panasonic EW3106; Panasonic Electric Works, Ltd, Osaka, Japan).12 Recordings were averaged over 2 weeks before ABPM was performed.

Statistical Analysis

Data were expressed as mean±standard deviation for normally distributed data and median (interquartile range) for non‐normally distributed data. For quantitative analysis, Bland‐Altman plots and Lin's concordance correlation coefficient (CCC) were used.13, 14 Interpretation of agreement by CCC is based on the descriptive scale suggested by McBride: poor (CCC<0.90), moderate (0.90≤CCC<0.95), substantial (0.95≤CCC<0.99), and almost perfect (CCC>0.99).15 For qualitative agreement analysis, inter‐rater agreement analysis (κ) was performed.16 Interpretation was as follows: poor (κ<0.20), fair (0.20≤κ<0.40), moderate (0.40≤κ<0.60), good (0.60≤κ<0.80), and very good (0.80≤κ<1.00).17

The following are the definitions of BP level. For predialysis BP: normotension (systolic BP [SBP]<140 mm Hg); grade 1 hypertension (140 mm Hg≤SBP<160 mm Hg); grade 2 hypertension (160 mmHg≤SBP<180 mm Hg); and grade 3 hypertension (SBP≥180 mm Hg). For postdialysis BP: normotension (SBP<130 mm Hg); grade 1 hypertension (130 mmHg≤SBP<150 mm Hg); grade 2 hypertension (150 mmHg≤SBP<170 mm Hg); and grade 3 hypertension (SBP≥170 mm Hg). For ABP: normotension (SBP<135 mm Hg); grade 1 hypertension (135 mm Hg≤SBP<155 mm Hg); grade 2 hypertension (155 mmHg≤SBP<175 mm Hg); and grade 3 hypertension (SBP≥175 mm Hg).

A P value <.05 was considered to be statistically significant. Statistical analyses were performed using SPSS version 13.0 (SPSS Inc, Chicago, IL) and Medcalc version 12.7 (Medcalc Software, Mariakerke, Belgium). Figures were generated using Graphpad Prism 5.0 (Graphpad Software Inc, San Diego, CA).

Results

A total of 112 patients were recruited from March 2013 to October 2013. After excluding patients without sufficient ABP recordings, 90 participants constituted the study sample. Clinical features and BP measurements were presented in Table 1. There were 58 (64.4%) male patients and the mean age was 53.7 years. Nearly one third of the participants were current smokers and one fifth had diabetes. The average body mass index (BMI) was 21.6 kg/m². Mean interdialytic weight gain during the session with ABPM was 2.3 kg. Average duration of dialysis was 32 months. Average values of laboratory tests, including hemoglobin, albumin, triglyceride, total cholesterol, high‐density lipoprotein cholesterol, low‐density lipoprotein cholesterol, calcium, phosphorus, and parathyroid hormone reflected a general dialysis population. As in other studies, predialysis BPs were higher than ABPs in this cohort of patients. The mean value of postdialysis SBP was lower than ambulatory SBP, but this reversed in respect to diastolic BP (DBP).

Table 1.

Clinical Features and Blood Pressure Parameters

	Mean±SD/Median (IQR) or No. (%)
Male	58 (64.4)
Age, y	53.7±14.3
Current smoker	28 (31.1)
Diabetes	21 (23.3)
BMI, kg/m2	21.6±3.1
Interdialytic weight gain, kga	2.3±0.8
Duration of dialysis, mo	32.0 (15.0–68.8)
Hemoglobin, g/L	103.3±16.0
Albumin, g/L	40.9±3.5
Triglyceride, mmol/L	1.75±0.97
Total cholesterol, mmol/L	3.82±0.91
High‐density lipoprotein cholesterol, mmol/L	1.05±0.26
Low‐density lipoprotein cholesterol, mmol/L	2.08±0.54
Calcium, mmol/L	2.32±0.25
Phosphorus, mmol/L	1.66±0.57
Parathyroid hormone, pg/mL	136.0 (53.8–254.1)
Predialysis SBP, mm Hg	143.4±17.5
Predialysis DBP, mm Hg	86.6±9.9
Postdialysis SBP, mm Hg	136.6±19.0
Postdialysis DBP, mm Hg	84.5±10.7
Ambulatory SBP, mm Hg	137.2±23.8
Ambulatory DBP, mm Hg	81.5±12.8

Abbreviations: BMI, body mass index; DBP, diastolic blood pressure; IQR, interquartile range; SBP, systolic blood pressure; SD, standard deviation. ^aThe interdialytic weight gain during the session with ABPM.

Figure 1 illustrates the results of Bland‐Altman plots. Agreement limits between dialysis unit BP and ABP were wide, and 95% limits of agreement for predialysis SBP, predialysis DBP, postdialysis SBP, and postdialysis DBP with corresponding ABPs were −33.5 mm Hg to 20.9 mm Hg, −22.2 mm Hg to 11.9 mm Hg, −25.4 mm Hg to 26.5 mm Hg, and −18.4 mm Hg to 12.3 mm Hg, respectively. The width of limits of agreement was nearly the same for predialysis BPs compared with postdialysis BPs.

Figure 1.

Bland‐Altman plots of dialysis unit blood pressure (BP) and ambulatory BP. SBP indicates systolic blood pressure; DBP, diastolic blood pressure.

The results of Bland‐Altman plot were further validated by Lin's CCC for dialysis unit BPs with ABPs, as presented in Figure 2. Although high Pearson's correlation coefficients were noted for each parameter (ranging from 0.73 to 0.83; P<.001 for all), agreement was poor for all BPs (CCC=0.75, 0.81, 0.64, and 0.75 for predialysis SBP, postdialysis SBP, predialysis DBP, and postdialysis DBP, respectively).

Figure 2.

Lin's concordance correlation coefficients for dialysis unit blood pressure (BP) with ambulatory BP.

As a reflection of general clinical practice, we further analyzed the agreement of BP classification between the two approaches of BP measurements (Table 2). Inter‐rater agreement was moderate for both predialysis SBP and postdialysis SBP compared with ambulatory SBP (κ=0.58; 95% confidence interval, 0.44–0.71 for predialysis SBP with ambulatory SBP; κ=0.57; 95% confidence interval, 0.44–0.70 for postdialysis SBP with ambulatory SBP). Incorrect classification rates of dialysis unit BP are shown in Figure 3. In normotensive patients, the rate was lowest (20.0% for predialysis BP and 17.5% for postdialysis BP). An increasing trend for this rate was noted with worsening degree of hypertension, with highest values in patients with grade 3 hypertension (100% and 75.0% for predialysis and postdialysis BP, respectively).

Table 2.

Agreement of Blood Pressure Classification

	Ambulatory BP				Inter‐rater Agreement
	NT	G1	G2	G3	κ	SE	95% CI
Predialysis BP
NT	32	5	1	0	0.58	0.07	0.44–0.71
G1	7	21	6	1
G2	1	1	12	3
G3	0	0	0	0
Postdialysis BP
NT	33	2	1	0	0.57	0.07	0.44–0.70
G1	6	18	5	0
G2	1	6	12	3
G3	0	1	1	1

Abbreviations: BP, blood pressure; CI, confidence interval; G1, grade 1 hypertension; G2, grade 2 hypertension; G3, grade 3 hypertension; NT, normotension; SE, standard error. Bold values indicate the number of patients classified correctly by dialysis unit BP according to ambulatory BP.

Figure 3.

Incorrect classification rate of dialysis unit blood pressure (BP).

Discussion

Agreement between dialysis unit and ABP was assessed in a cohort of Chinese dialysis patients in the current study. In quantitative analysis, wide limits of agreement were noted between dialysis unit BP and ABP in Bland‐Altman plots. This poor agreement was further confirmed by Lin's CCC. These results highlight the imprecision of using peridialysis BP in predicting the average interdialytic BP level. In the following qualitative analysis, predialysis and postdialysis SBP showed moderate agreement with ABP. It is interesting to note that in the higher grades of hypertension, dialysis unit BP showed more marked discrepancy compared with ABP.

The finding of relative poor agreement between dialysis unit BP and ABP is in line with a previous systematic review by Agarwal and colleagues.4 The authors included 18 studies and investigated the agreement between predialysis and postdialysis BP and ABP. Similarly, they conclude that dialysis unit BP has poor agreement with ABP. However, the agreement limits were even larger than those in the current study for both predialysis and postdialysis SBP/DBP with ABP. This may be the result of the heterogeneity of different studies and emphasize the importance for further evaluation in different ethnics.

Another important finding is that the bias between dialysis unit BP and ABP became more apparent at higher grades of hypertension. In grade 3 hypertension, according to ambulatory SBP, 100% and 75% of patients were mistakenly classified by predialysis SBP and postdialysis SBP, respectively. It is worthy to point out that there was a limited number of patients with grade 3 hypertension and this may lead to overestimation of the incorrect classification rate. However, the increasing trend (Figure 3) does exist and is supported by the results of the Finn‐HOME study.11 The Finn‐HOME study was conducted in a general population in Finland and its findings reveal that the difference between clinic and home BP becomes larger at higher levels of BP. The exact mechanism responsible for this phenomenon remains elusive and needs to be addressed in further studies. This should be taken with caution in clinical practice, however, since it indicates that dialysis patients with more severe hypertension are more often underestimated by dialysis unit BP.

Lack of agreement between clinic BP and ABP was largely due to the white‐coat effect in the general population.18, 19 Nevertheless, it is obviously more complex for dialysis patients. In a previous study, Kelley and colleagues20 developed a trended cosinor model to describe the interdialytic BP changes. They noticed that interdialytic SBP increases at a rate of approximately 1 mm Hg per 4 hours elapsed postdialysis. Thus, the big difference between peridialysis BP and ABP may be an inherent characteristic for dialysis patients to some extent according to this model. Since predialysis and postdialysis BPs are measured at the end and the beginning of the interdialytic period, it is reasonable for the mean interdialytic ABP to be lower/higher than predialysis/postdialysis BP, as observed in our study. However, this is not consistent with other studies, which suggest that the model could not account for all the differences and that there are possibly other determinants involved.5, 21, 22 In our opinion, factors contributing to the differences between peridialysis BP and ABP may include the effect of hemodialysis on hemodynamic and cardiac function, interdialytic weight gain, white‐coat effect, daily activity, and inappropriate clinic BP measurements.

Although there were attempts to improve the accuracy of dialysis unit BP,22, 23, 24 it is not yet appropriate for dialysis unit BP to substitute interdialytic ABP, with the latter being expensive, inconvenient, and uncomfortable. Alternative approaches, including shortening the interval of ABP measurement and home BP monitoring, have been tested and the results are promising and of clinical importance.25, 26, 27 More studies are warranted to address this issue.

There is a growing population of dialysis patients with ESRD in China.28 However, ABPM is highly underutilized in this population, as reflected by few studies on ABPM in Chinese dialysis patients. Dialysis patients with hypertension are treated mostly based on predialysis BP. Possible reasons for this include limited access to ABPM, unaffordable healthcare, and relative lower grade of education. Thus, in our opinion, it is of great importance to evaluate the agreement between dialysis unit BP and interdialytic ABP in Chinese dialysis patients. We hope to provide clinical evidence for practice and highlight the importance of ABPM in China.

Study Limitations

It should be pointed out that there are some limitations in this work. First, patients enrolled had various levels of BP, which leads to a limited number of patients in subgroups. Therefore, the increasing trend of poor agreement between dialysis unit BP and ABP at higher levels of BP needs to be validated in further studies with larger sample sizes. However, this limitation is unlikely to lead to different trends, as our results have been confirmed to some extent elsewhere.11 Second, ABPM with a fixed measurement interval was used in our study. However, it was suggested that sampling rate and different measurements during the daytime and nighttime may lead to bias of mean ABP.29, 30 Third, as mentioned above, our results showed a difference in degree of agreement limits with Agarwal's.4 Whether and how the agreement limits vary across different ethnic groups remain elusive. Meanwhile, our study addresses only the magnitude of difference between ABP and dialysis unit BP. The underlying cause leading to the poor agreement has not been revealed and needs to be addressed in further research.

Conclusions

In a cohort of Chinese dialysis patients, the agreement between dialysis unit BP and interdialytic ABP is poor, and the bias for patients with higher degrees of hypertension is more prominent. We suggest that physicians exercise caution in using predialysis and postdialysis BP in the BP management of Chinese dialysis patients. Home BP and ABP, even in short duration, should be considered in clinical practice.

J Clin Hypertens. 2014;16:701–706. DOI: 10.1111/jch.12395. © 2014 Wiley Periodicals, Inc.