Effect of high bicarbonate hemodialysis on ionized calcium and risk of metastatic calcification

Clinica Chimica Acta 343 (2004) 231 – 236 www.elsevier.com/locate/clinchim

Effect of high bicarbonate hemodialysis on ionized calcium and risk of metastatic calcification Barry Kirschbaum * Division of Nephrology, Department of Internal Medicine, Virginia Commonwealth University, P.O. Box 980160, Richmond, VA 23298, USA Received 3 September 2003; received in revised form 2 February 2004; accepted 4 February 2004

Abstract Background: Disturbances in calcium and phosphate metabolism among chronic hemodialysis patients result in renal osteodystrophy and vascular calcification. Even though it is the ionized fraction of calcium that is metabolically active, this measurement is generally not available and decisions are made on the basis of total calcium. Formulae to predict ionized calcium concentrations are available. Methods: The OPTI Critical Care Analyzer with E-Ca cuvettes was used on-site to measure acid – base parameters, electrolytes, and ionized calcium. Additional assays included total calcium, phosphate, and albumin. Results: Using a dialysate with 1.25 or 1.5 mmol/l calcium and 40 mmol/l bicarbonate, we observed a statistically significant increase in pH and total CO2 concentrations in post-dialysis blood. Total and ionized calcium increased significantly only in the patients with central venous catheters but not in those with fistulas or grafts. All patients experienced a decrease in phosphate concentrations. Conclusions: The metabolic alkalosis induced by high bicarbonate dialysate was not associated with a decrease in ionized calcium or a change in the calculated concentration product ratio for hydroxyapatite formation in the immediate post-dialysis period. However, if a 40% phosphate rebound were to occur 2 h after termination of dialysis, the calculated risk of metastatic calcification would increase 2.8-fold compared to pre-dialysis conditions. Formulae to calculate ionized calcium are not useful in this population. D 2004 Elsevier B.V. All rights reserved. Keywords: Calcium; Ionized calcium; Hemodialysis; Acidosis; Alkalosis; Critical care analyzer

1. Introduction Maintenance of normal blood concentrations of phosphate and calcium among patients with advanced renal failure remains an elusive goal [1,2]. Therapy consists of prescribing oral phosphate binders such as calcium or magnesium salts or sevelamer hydrochloride to block intestinal phosphate absorption and potent

* Tel.: +1-804-828-1756; fax: +1-804-828-7567. E-mail address: [email protected] (B. Kirschbaum). 0009-8981/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cccn.2004.02.003

vitamin D analogs to promote calcium absorption [3,4]. While this approach is effective, a high percentage of end-stage kidney failure patients do not achieve satisfactory, long-term control of the phosphate concentration. A consequence of hyperphosphatemia is secondary hyperparathyroidism, due in part to a decrease in ionized calcium [5,6]. At the same time, increases in the concentration of hydrogen ions and numerous other solutes which bind to albumin may displace calcium from its albumin-binding sites, thereby maintaining ionized calcium in the near normal range. Aggressive therapy with high efficiency dia-

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lyzers to improve solute clearance and dialysate high in bicarbonate (40 mmol/l) and relatively low in calcium (1.25 – 1.5 compared to 1.75 mmol/l) may impact on the concentration of ionized calcium [2]. The free calcium fraction is believed to regulate parathyroid hormone release. In general, measurement of free calcium is not available to assist with the management of dialysis patients except as derived from equations which have not been well validated. Our study was designed to measure ionized calcium and blood gas parameters on-site in a typical hemodialysis population. The goals were to document changes in ionized calcium concentration associated with the metabolic alkalosis of high bicarbonate dialysis and to assess the risk of metastatic calcification attributable to these changes. A secondary goal was to compare measured and computed ionized calcium concentrations using published equations.

2. Methods Twenty-one chronic hemodialysis patients agreed to provide a pre- and post-dialysis blood sample. Twelve patients had arteriovenous grafts or fistulas, nine had internal jugular venous catheters. The median length of the dialysis session and blood flow were 4 h and 400 ml/min. Median dialysate K+ was 2 mmol/l (range 1 –3 mmol/l) and dialysate Ca2 + was 1.25 mmol/l (range 1.25 – 1.5) and 1.5 mmol/l (range 1.25– 1.5) in the graft and catheter groups, respectively. Other characteristics of the patients are listed in Table 1. Three milliliters of blood was drawn from the arterial line into syringes coated with 80 units of calcium balanced lithium heparin (BD Vacutainer Systems).1 The syringes were capped to minimize exposure to air and assayed within 60 min using ECa cuvettes on an OPTI CCA Blood Gas Analyzer as described previously [7]. The analyzer measured whole blood pH, pCO2, pO2, sO2, Na+, K+, ionized calcium (iCa2 +), hemoglobin. Total CO2, bicarbonate, and base excess were calculated. pH 7.4-normalized

Table 1 Patient characteristics according to type of vascular access

Number Age (year) Weight (kg), pre Weight (kg), post Weight (kg), delta Epogen U Calcijex, n Paricalcitol

Graft/fistula

Catheter

12 56.5 (39 – 66) 87.4 (72.2 – 102.6) 84.2 (69.3 – 99.1) 3.24 (2.2 – 4.21) 6800 (0 – 16,000) 0 3

9 53.5 (32 – 76) 72.5 (62.7 – 82.2) 69.4 (60.1 – 78.7) 3.07 (2.27 – 3.87) 10350 (2300 – 20,000) 3 2

Age and epogen dosage (per treatment) provided as median with range. Weight provided as mean with 95% confidence limits of the mean. Calcijex and paricaltrol provided as number of patients treated.

ionized calcium (nCa2 +) was also calculated from the equation nCa2þ ðpH 7:4Þ ¼ iCa2þ  100:22ðpH7:4Þ This formula was taken from the CCA OPTI operator’s manual. The remaining blood was centrifuged and the plasma saved for measurement of phosphate by an ultraviolet method, albumin by bromcresol green, and total calcium (tCa2 +) by arsenazo III utilizing kits and standards from Pointe Scientific (Lincoln Park, MI). Total protein was measured on diluted plasma using a Coomassie blue reagent with albumin as standard. Ionized calcium was calculated from the equation of Harris et al. [8] iCa2þ ¼ tCa2þ  ð0:019  albuminÞ  ð0:0091  HCO3 Þ  0:10 with Ca2 + expressed as mmol/l and albumin as g/l. Two additional formulae for calculating ionized calcium were also evaluated. iCa2þ ¼ ððtotal Ca2þ  6Þ  ðtotal protein=3ÞÞ =ðtotal protein þ 6Þ (Ref. [9]) iCa2þ ¼ 1  ð8  albuminÞ þ ð2  globulinÞ þ 3  total unadjusted calcium

1

Syringe suitability for ionized calcium determination was affirmed by Leslie Magee of BD Vacutainer Systems Preanalytical Solutions division.

(Ref. [10]) with Ca2 + expressed as mg/dl, proteins as g/dl, and globulin as total protein minus albumin.

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Protein bound calcium (pbCa2 +) was calculated from the equation pbCa2þ ¼ :0211  albumin  ð0:42  albumin  ð7:42  pHÞ=47:3Þ (Ref. [11]) where Alb is g/l. Complexed calcium (cCa2 +) was computed as total minus the sum of ionized and protein bound fractions. The concentrations of phosphoric acid, monobasic, dibasic, and tribasic phosphate were calculated using K 1 = 7.107  10 3, K2 = 7.99  10 8, and K3 = 4.8  10 13 as dissociation constants [12]. The concentration product ratio (CPR) for hydroxyapatite formation was calculated using measured values for iCa2 + and derived values for PO43  and OH and the formula: CPR  Ksp=[iCa2 +]5  [PO43 ]3  [OH] [13]. Results are presented as mean or median + 95% confidence limits. Differences between pre-and postdialysis blood samples as well as between the graft/ fistula and catheter groups were sought using paired or unpaired two-sided t-tests as appropriate with p < 0.05 accepted as significant. Linear regression analysis and Bland – Altman graphic presentation [14] were used to compare measured and formuladerived free calcium concentrations. Correlation anal-

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ysis was employed to establish linear associations between pairs of variables. Statistics and graphs were prepared with the Quattro Pro program of Word Perfect suite.

3. Results A summary of all measured and calculated parameters is presented in Table 2. As previously reported [15], there were statistically significant differences of pH, pCO2, pO2, sO2, tCO2, and HCO3 between blood samples taken from grafts compared to venous catheters. Among patients with grafts, dialysis was associated with increases in pH, BE, tCO2, HCO3, hemoglobin, and albumin and decreases in K+ and phosphate. Patients with catheters showed post-dialysis increases in pH, pCO2, BE, tCO2, and HCO3 and decreases in pO2, K+, and phosphate. Ionized and total calcium were unchanged in the graft/fistula group and increased significantly in the catheter group. The ratio of ionized to total calcium remained unaffected. nCa2 + increased significantly in both groups during the dialysis treatment. In the combined groups, dialysis was associated with an increase in protein-bound calcium from 0.73 (95% CI 0.69– 0.76) to 0.83 (95% CI 0.78– 0.85) mmol/l

Table 2 Pre-dialysis and post-dialysis values by type of vascular access Graft

Catheter

Pre pH pCO2 (mm Hg) pO2 (mm Hg) sO2 (%) BE (mmol/l) tCO2 (mmol/l) HCO3 (mmol/l) Na (mmol/l) K (mmol/l) iCa (mmol/l) nCa (mmol/l) HGB (g/dl) Albumin (g/l) Phosphate (mmol/l) Calcium (mmol/l)

7.44 43.9 88.1 96.2 4.5 30.5 26.0 139 4.0 1.09 1.12 10.4 34.9 1.5 2.11

Post (7.42 – 7.46) (41.1 – 46.8) (79.2 – 97.1) (95.3 – 97.1) (2.8 – 6.1) (28.7 – 32.3) (24.4 – 27.7) (138 – 141) (3.76 – 4.23) (1.03 – 1.15) (1.05 – 1.18) (10.1 – 10.8) (32.9 – 37.0) (1.26 – 1.74) (1.94 – 2.26)

7.53*** 44.5 85.9 96.5 12.2*** 37.5*** 35.8*** 141 2.94*** 1.14 1.22** 11.4* 38.5** 0.82*** 2.19

Pre (7.50 – 7.56) (42.0 – 46.9) (78.5 – 93.4) (95.8 – 97.2) (10.5 – 13.8) (36.3 – 38.8) (34.7 – 37.0) (140 – 142) (2.79 – 3.10) (1.11 – 1.16) (1.18 – 1.25) (10.5 – 12.3) (36.4 – 40.5) (0.72 – 0.91) (2.14 – 2.24)

7.34+ + + 49.7+ + + 39.2+ + + 71.5+ + +  0.21+ + + 27.6+ 29.1+ 141 4.24 1.10 1.06 11.0 34.7 1.86 2.01

Post (7.31 – 7.36) (47.3 – 52.0) (36.0 – 42.4) (68.0 – 75.0) (  2.0 – 1.5) (25.9 – 29.2) (27.4 – 30.8) (138 – 144) (4.05 – 4.44) (1.06 – 1.13) (1.03 – 1.09) (10.3 – 11.7) (32.3 – 37.1) (1.42 – 2.29) (1.92 – 2.1)

7.46*** 51.4**/+ + + 34.7*/+ + + 68.8+ + + 9.8***/ + 37.6*** 36.2*** 142 3.12*** 1.16*** 1.18*** 11.8 35.9 0.88*** 2.15*

(7.44 – 7.48) (49.2 – 53.6) (31.0 – 38.4) (64.6 – 73.0) (9.2 – 10.3) (36.1 – 39.0) (34.9 – 37.4) (141 – 144) (2.99 – 3.26) (1.13 – 1.20) (1.13 – 1.23) (10.3 – 13.2) (33.5 – 38.4) (0.63 – 1.14) (2.04 – 2.26)

Pre- vs. post-dialysis p-values: *V 0.05, **V 0.01, ***V 0.001; graft vs. catheter p-values: + V 0.05, + + + V 0.001; BE = base excess = (1  0.014  HGB)  [(1.43  HGB + 7.7)  24.8+[HCO3]]; nCa = normalized calcium at pH 7.4 = iCa2 +  100.22(pH  7.4).

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Table 3 Pre- and post-dialysis calcium composition

albumin tCa2 + iCa2 + nCa2 + pbCa2 + cCa2 +

Pre-dialysis (mmol/l)

Post-dialysis (mmol/l)

p

0.52 2.06 1.09 1.09 0.73 0.24

0.56 2.17 1.15 1.20 0.82 0.21

0.009 0.07 0.009 < 0.001 < 0.001 0.47

(0.5 – 0.54) (1.96 – 2.17) (1.06 – 1.13) (1.05 – 1.13) (0.69 – 0.76) (0.16 – 0.33)

(0.53 – 0.58) (2.12 – 2.23) (1.13 – 1.17) (1.17 – 1.23) (0.78 – 0.85) (0.17 – 0.25)

Total and ionized calcium were measured. Normalized (n), proteinbound (pb), and complexed (c) calcium values were calculated using equations listed under Methods.

(p < 0.001) (Table 3). The increase in the ratio of protein bound to total calcium was not significant. Complexed calcium showed a nonsignificant reduction from 0.24 (95% CI 0.2– 0.33) to 0.21 (95% CI 0.17– 0.25) mmol/l. As expected, there was a reduction in total phosphate during dialysis. The calcium  phosphate product, a crude estimate of the risk of calcification, declined from 3.39 (95% CI 2.9 – 3.88) to 1.83 (95% CI 1.6– 2.06) mmol2/l2 ( p < 0.001). All of the ionic forms of phosphate declined during the treatment (Table 4), but as percent of the total phosphate, significant increases in dibasic and tribasic anions and decreases in monobasic and undissociated phosphoric acid were observed. Dialysis was associated with decreases in the concentration of H3PO4 from 3.2  10 9 to 1.95  10 9 mol/l and PO43  from 13  10 9 to 9.2  10 9 mol/l (Table 4). Both species comprise a tiny fraction of the total plasma phosphate concentration, but since tribasic phosphate is a component of hydroxyapatite, it has been regarded as a gauge of the risk of metastatic calcification [8]. The concentration product ratio (CPR) for hydroxyapatite formation was calculated using the

Fig. 1. Bland – Altman plot of measured and calculated ionized calcium. Abscissa—average ionized calcium, mmol/l. Ordinate— difference between measured iCa2 + and that calculated from the formula of Harris et al. [8], mmol/l. Broken horizontal line identifies the mean of the differences. Solid horizontal lines outline the mean F 1 standard deviation. Linear regression equation: y =  0.79x + 0.96, r2 = 0.28, p < 0.001.

formula described by Meyer [13] for kidney stones. The variables are ionized calcium to the 5th power, tribasic phosphate to the 3rd power, and hydroxyl ion. The ratio of post-to pre-dialysis CPR for all patients was 0.882 (95% CI 0.518 – 1.25). Thirteen patients had a decrease in CPR and eight an increase. Movilli et al. [16] reported a decrease in iCa2 + and PTH concentrations in dialysis patients supplemented with oral sodium bicarbonate sufficient to raise mean serum HCO3 values from 19 to 24 mmol/l. In their study, iCa2 + was calculated using an equation from the paper by Harris et al. [8]. The equation was not derived from direct measurement of iCa2 + but rather from data reported by Nordin et al. [17] who did not evaluate patients with kidney failure. In this study, we found a weak but significant correlation between measured

Table 4 Pre- and post-dialysis phosphate composition Pre-dialysis

Post-dialysis

mol/l H3PO4 1 H2PO 4 2 HPO 4 3 PO 4

3.2  10 9 0.56  10 3 1.09  10 3 13  10 9

% total (2.6 – 3.8) (0.47 – 0.64) (0.94 – 1.25) (11 – 15)

1.95  10 4 33.6 66.4 8  10 4

(1.74 – 2.16) (32.2 – 35) (65 – 67.8) (7.4 – 8.7)

mol/l

% total

1.1  10 9*** (0.9 – 1.3) 0.24  10 3*** (0.2 – 0.28) 0.6  10 3*** (0.52 – 0.69) 9.2  10 9*** (8 – 10)

1.29  10 4*** 28.4*** 71.6*** 11  10 4***

(1.17 – 1.41) (27.3 – 29.6) (70.4 – 72.7) (10.2 – 11.8)

The concentrations of the four phosphate fractions were calculated using the measured total phosphate and published dissociation constants: K1 = 7.107  10 3, K2 = 7.9910 8, K3 = 4.8  10 13. *** Pre- vs. post-dialysis p-value < 0.001.

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and calculated iCa 2 + with r 2 = 0.11 ( p <0.05). Bland –Altman plot showed generally poor agreement between the two values with measured higher than calculated at low iCa2 + concentrations and the reverse at higher concentrations (Fig. 1). Results using two other formulae [9,10] to calculate ionized calcium showed no significant correlation with measured iCa2 +. Of all the variables measured and calculated in this study, pre-dialysis iCa2 + correlated significantly only with tCa2 + and nCa2 + while postdialysis iCa2 + correlated significantly with tCa2 +, nCa2 +, total phosphate, HPO42 , and PO43 . None of these correlations were useful for developing a reliable formula to calculate the ionized calcium concentration.

4. Discussion The dialysate content of calcium and bicarbonate determines the load of these solutes delivered to the patient during treatment. The use of oral calcium salts to bind phosphate in the gut and the availability of potent analogs of vitamin D have led to the widespread usage of lower dialysate calcium concentrations compared to the days when aluminum salts were the principal phosphate binder. Dialysate bicarbonate concentrations have increased in response to the publicity of studies citing the ill effects of acidosis on protein and bone chemistry [18,19]. The composition of the dialysate used in our study was 40 mmol/l bicarbonate and 1.25 or 1.5 mmol/l calcium. High-efficiency dialysis reduces plasma phosphate concentrations quickly but is associated with a rebound in the phosphate concentration within 2 h of terminating the treatment. Harris et al. [8] reported a rebound increase of 40% in patients treated with 40 mmol/l bicarbonate dialysate. Our study did not include measurement of the phosphate rebound. If we were to apply the 40% correction factor to our measured post-dialysis phosphate concentrations and make the additional assumption that pH and ionized calcium did not change during this 2-h interval, the result would be an increase in the post- to pre-dialysis concentration product ratio from 0.88 (95% CI 0.52– 1.24) to 2.45 (95% CI 1.44– 3.46). To the extent that the concentration product ratio is a meaningful measure of the risk for metastatic calcification [8], this

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2.8-fold increase in the ratio might assume pathophysiologic importance. Since the CPR is determined by the 5th power of ionized calcium and the 3rd power of the PO43  concentration which will be greatest at the time of maximum metabolic alkalosis, use of a high dialysate calcium should further provoke the risk for metastatic calcification. Vitamin D-induced hypercalcemia, a common complication which may go unnoticed for weeks in dialysis units that check blood calcium once a month, will add to the problem. Very few of our patients were receiving vitamin D preparations at the time of the study because of difficulties encountered with hypercalcemia, hyperphosphatemia, and high calcium  phosphate products. Several formulae for predicting the value of ionized calcium using total calcium and other variables such as phosphate and plasma protein concentrations have been published [8– 10,20]. Some enjoy a high degree of success when applied to individuals in good health with normal concentrations of the measured analytes. However, these conditions do not apply to the renal failure population. None of the formulae included in this paper provided values for ionized calcium that matched well with the measured values. The OPTI CCA and similar instruments which utilize multi-assay cassettes provide a simple and direct means to measure ionized calcium, other electrolytes, and blood gas parameters quickly on-site. Widespread application of this technology may result in improved management of acid – base and calcium problems within the hemodialysis population.

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