The effect of two different protocols of potassium haemodiafiltration on QT dispersion
Michele Buemi1,
Emanuele Aloisi1,
Giuseppe Coppolino1,
Saverio Loddo2,
Eleonora Crascì1,
Carmela Aloisi1,
Antonio Barillà1,
Vincenzo Cosentini1,
Lorena Nostro1,
Chiara Caccamo1,
Fulvio Floccari1,
Adolfo Romeo1,
Nicola Frisina1 and
Diana Teti2
1 Chair of Nephrology, Department of Internal Medicine and 2 Department of Pathology and Experimental Microbiology, University of Messina, Italy
Correspondence and offprint requests to: Professor Michele Buemi, Via Salita Villa Contino, 30. 98100 Messina, Italy. Email: buemim{at}unime.it
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Abstract
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Background. The risk of developing cardiovascular diseases is higher in patients on haemodialysis than in the general population. These patients may develop arrhythmias that depend on the extra- and intracellular concentrations of potassium. ECG findings, particularly the QT interval and its dispersion (QTd) and the QTc (QT interval corrected for heart rate according to Bazett's formula) and its dispersion (QTcd), may be direct indicators of the risk of developing arrhythmia.
Methods. Our cohort comprised 28 patients who were dialysed for 3.54 h three times per week, first with haemodiafiltration with a constant potassium concentration (HDF) in the dialysis bath then with haemodiafiltration with variable concentrations of potassium (HDFk). ECGs were done at different time intervals: at the start of dialysis (T0), at 15 (T15), 45 (T45), 90 (T90) and 120 min (T120) after the beginning of the session, and at the end of treatment (Tend). ECG-derived data (QT, QTd, QTc and QTcd) were measured. At the same time points, plasma electrolytes, intra-erythrocytic potassium and the electrical membrane potential at rest (REMP) of the erythrocytic membrane were measured.
Results. Plasma potassium concentration diminished more gradually in HDFk than in HDF, the difference being statistically significant at T15 and T45 (P<0.05), and T90 (P<0.01). The intra-erythrocytic potassium concentration remained constant throughout the observation period. In both HDF and HDFk, REMP was lower at all points after T0 (P<0.05), but the reduction was greater and more significant in HDF than in HDFk at T15 and T120 (P<0.05). ECG revealed a statistically significant diminution in HDFk vs HDF in the measures of dispersion of QT and QTc at T15, T90, T120 and Tend (P<0.01) and of QTcd at T45 (P<0.05). The mean of QTd, adjusted for plasma potassium, increased over time in HDF with large alternate mean increase and decrease peaks and error intervals. In HDFk, instead, there was a progressive and constant diminution with minor error intervals. QTcd adjusted for plasma potassium had the same trend. A marked difference was found between the final values in standard HDF and those in HDFk.
Conclusions. HDF and HDFk have significantly different effects on QTc. ECG data demonstrate that the risk of arrhythmia could be lower, with a variable removal of potassium during haemodialysis. With HDF but not HDFk, hyperpolarization of the cell membrane is detected, and this could have a destabilizing effect on different types of cardiac cell, giving rise to retrograde circuits.
Keywords: acetate-free dialysis; arrhythmia; haemodiafiltration; haemodialysis; potassium; QT dispersion; QTc dispersion
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Introduction
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The increased risk, compared with the general population, of cardiovascular disease in patients on maintenance haemodialysis (HD) depends on various associated factors, some conventional and others closely related to the renal disease [1]. Moreover, in HD patients, the higher mortality rate is due to the occurrence of serious complications, such as alterations in cardiac rhythm and sudden death [2,3]. HD influences the cardiovascular system, because it induces changes in the chemico-physical characteristics of body liquids, such as pH, temperature and electrolyte concentrations [4]. Regarding the latter, variations of potassium, both serum and intracellular, play a pathogenic role in the development of arrhythmias in patients on HD [5,6]. The removal of potassium during HD is determined by the blooddialysate concentration gradient, which generates diffusiveconvective flows through the HD membrane. The use of dialysis baths with low, and constant, potassium content presents a high gradient in the initial phases of HD both across the dialytic membrane and across the cellular membrane (7). This often leads to rapid potassium depletion and an anomalous intracellular/extracellular potassium ratio (Ki/Ke). This has significant impact on electrochemical processes and on the electric potential of the membranes of excitable cells, such as nerve cells and muscle fibres. Studies on cellular models, such as red blood cells, demonstrate that the intracellular depletion of potassium and the hyperpolarization of cellular membranes increase the risk of arrhythmia in patients on HD [710]. Cardiac arrhythmias may occur during HD, and patients may experience weakness or muscle cramps, which are the direct consequences of the altered electrochemical balance of the cellular membranes [11]. Electrocardiography (ECG) is considered a valid tool for the detection of such alterations. Changes such as SVPCs (supraventricular premature contractions) and VPCs (ventricular premature contractions), alterations in the STT tract (ST elevation and inverted T) and atrial fibrillation are reliable markers of the alterated electric potential of cardiac cell membranes [12,13]. It is also important that HD alters the duration and homogeneity of ventricular repolarization. The QT interval and the QT interval corrected for heart rate (QTc) reflect the duration of ventricular depolarization and repolarization, and Qt dispersion (QTd) and QTc dispersion (QTcd) (defined as the difference between the maximal and minimal QT and QTc, respectively) represent the variability of cardiac repolarization, and they are found to be significantly prolonged during HD. Currently, they are considered important markers of the risk of severe, sometimes fatal cardiac complications [14,15].
The aim of our study was to investigate the effects of two different modalities of potassium removal via haemodiafiltration (HDF) on ECG parameters and on the electrochemical balance of cell membranes.
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Patients and methods
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We enrolled 28 clinically stable patients who were receiving acetate-free dialysis (AFB, Hospal, Italy) for 3.54 h three times a week [13 women, 15 men, mean age 59.8±10.5 years, mean dialysis age 2.7±1.6 years, residual glomerular filtration rate (GFR) 2.3±0.6 ml/min]. The patients data are summarized in Table 1. All patients had been dry-weight-stable for at least 3 months, and had achieved a normotensive, oedema-free state. Throughout the study, each patient complied with fluid and dietary restrictions, and maintained a constant ultrafiltration volume. Patients were on dialysis for primary interstitial nephritis (n = 10), polycystic kidney disease (n = 4), glomerulonephritis (n = 8) or chronic pyelonephritis (n = 6). Before enrolment, patients underwent 24 h Holter ECG monitoring. Criteria for exclusion were ECG-detected signs of cardiac arrhythmias, ECG signs of previous myocardial infarction, diabetes mellitus or other systemic disease. We also excluded patients with pacemakers, those on digitalis therapy or other drugs known to interfere with the QT interval in conformity to the guidelines set by the Minister of Health of the Italian Government [16], and patients who had dilatational heart disease or myocardial fibrosis, or both. All HD patients, except those with polycystic kidney disease (n = 4), were given intravenous erythropoietin three times a week (mean dosage 23±7 UI/kg body weight). Of the cohort, 17 were receiving angiotensin-converting enzyme inhibitors, 16 calcium antagonists, 14 angiotensin receptor antagonists, 10 nitrate agents and seven diuretics; 25 patients received vitamin D analogues, and 15 used calcium-containing phosphate binders. Echocardiography was performed on all patients and their mean left ventricular ejection fraction was 59.7±9.5%. Each patient underwent two different kinds of dialysis: haemodiafiltration with constant (HDF) and haemodiafiltration with variable (HDFk) concentrations of potassium in the dialysis bath. During the two treatments, we evaluated ECG at different time points: the start of dialysis (T0), and 15 (T15), 45 (T45), 90 (T90) and 120 min (T120) after the beginning of the session, and at the end of the treatment (Tend). At the same time points, the following were analysed: (i) blood gases (pH,
, PCO2); (ii) serum electrolytes (Na+, Cl, K+, Ca+, P, Mg2+); (iii) intra-erythrocytic potassium; (iv) pre- and post-dialytic blood urea nitrogen (BUN); and (v) weight loss. All laboratory analyses were performed using routine automated methods.
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Table 1. Clinical and dialytic characteristics and haemodynamic and echocardiographic parameters of patients studied
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Intra-erythrocytic potassium was measured with an indirect technique based on the measures of whole blood potassium (Kwb), serum potassium (Ks) and haematocrit (PVC). The following formula was then used for the determination of red blood cell potassium (Krbc):
in which Kwb and Ks are given as mmol/l, PVC as a percentage value and Krbc as mmol/l of packed cells. Potassium was measured using a flame spectophotometer (Instrumentation Lab), and haematocrit using a microhaematocrit centrifuge. Whole blood was obtained by heparinization. To measure the mean concentration of whole blood potassium (Kwb), 1 ml of blood was mixed with 4 ml of distilled water; after haemolysis, the solution was centrifuged, and, in the measurements, the 1:5 dilution was taken into account. The membrane potential was calculated according to Nerst's formula [17,18]:
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Haemodialysis techniques
All the patients, regularly treated with HDF, were studied on two successive midweek sessions. Each patient served as his or her own control, and had first HDF and then HDFk. In the HDF method, the dialysis bath was contained in one bag (SAFEBAG 93G, Hospal Spa, Italy) and the composition was NaCl (284.31 g/l), KCl (5.22 g/l), CaCl (10.29 g/l), MgCl2 (2.63 g/l) and glucose (35 g/l). In HDFk, on the other hand, the dialysate was contained in two bags (AF and B), each separately connected to the machine and tapped concomitantly. Both compartments (AF and B) had identical amounts of NaCl, CaCl, MgCl2 and glucose, but different KCl contents (19.57 g/l in compartment B and 0 g/l in compartment AF). The dialysis was the same for the two methods as to: duration (3.54 h), blood flow (300 ml/min), bicarbonate infusion (2000 ml/h with BIOSOL sacks AFB145; Hospal), dialysis membrane (AN69) and mean weight loss, set at 2.5 kg. The main difference between the two methods was that, in HDF, the constant and low content of potassium (2 mEq/l) determined a high dialysateplasma gradient, at least initially. In HDFk, however, the initial concentration of potassium in the dialysate was 1 mEq/l less than that in the plasma, and diminished until it reached 2 mEq/l at the end of HD. Consequently, the plasmadialysate potassium concentration gradient in the second treatment was constant, with a better removal of potassium during dialysis. All the HD sessions were conducted using the Integra machine (Hospal, Bologna).
ECG measurements
ECGs were obtained during HD by means of a 12-channel recorder (Esaote Biomedica, Florence, Italy) at a paper speed of 25 mm/s (gain 10 mm/mV). To analyse the QT interval, the 12-lead tracings were enlarged, always on the same photocopier, by a factor of three; and a minimum of 10 leads were studied in each patient. For each lead, three consecutive cardiac cycles were measured and averaged. The QT intervals were measured, manually with callipers by one observer, from the beginning of the QRS complex to the end of a T wave. If the T wave was indistinct, the reading was excluded from the analysis. Moreover, for each derivation, the lengths of the minimum and maximum QT intervals and the difference between them were measured. Each QT interval was corrected for heart rate using Bazett's formula: QTc = QT/RR1/2.
The difference between the maximum and minimum QTc provided the QTcd. Inter- and intra-observer reliabilities of QTd and QTcd measurements were also assessed. To ensure intra-observer reliability, 80 randomly selected ECGs were re-measured by the principal investigator. The mean differences between the first and second reading for QTd and QTcd was 2.95 ms (SD 5.85 ms; range 7.9 to 10.6) and 2.70 ms (SD 5.36 ms; range 8.4 to 10.65) (intra-class correlation coefficient: r = 0.99; P<0.001). To ensure inter-observer reliability, a second blinded observer re-measured 80 ECGs. The mean differences between the measurements made by the two observers for QTd and QTcd were 1.67 ms (SD, 16.12 ms; range, 35.3 to 39.1) and 2.06 ms (SD, 17.80 ms; range, 36.9 to 42.3); the intra-class correlation coefficient was r = 0.88 (P<0.01). The local Ethics Committee approved the study protocol.
Statistical analysis
The means and SDs of all variables were calculated. The Student t-test for paired samples was used to determine statistical significance, which was set at P<0.01. For each ECG parameter, the differences between the mean values obtained for each of the two modalities of HDF treatment were calculated and applied to a bilateral hypothesis test. This test was performed to verify if the differences were casual or related to the different dialysis techniques. We also calculated the weighted means of every ECG parameter and serum potassium value observed at the same time points. The use of these means has the advantage of considering simultaneously, and linking closely, the values of the electrolyte and the ECG parameter as the ECG data per se partly depend on the potassium concentration. The SPSS 11.0 statistical package and Microsoft Excel were used for tabulation and analysis.
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Results
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Diastolic and mean arterial blood pressures, body weight and total ultrafiltration were in line with our dialysis prescription in both sessions.
Plasma electrolytes
As both of our techniques provided equal dialysate compositions, we did not find statistically significant differences between the two techniques for any of the following parameters: BUN, Ca2+, P, Na+, Cl, P and Mg2+.
Serum potassium
Both pre-dialytic and post-dialytic values of serum potassium between the beginning and the end of treatment diminished in a statistically significant manner with each of the two HDF methods. A comparison between its plasma concentrations at the different observation points showed a more gradual reduction during HDFk than with HDF, the difference attaining statistical significance at T15 and T45 (P<0.05), and at T90 (P<0.01, Figure 1).

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Fig. 1. Plasma potassium concentrations. Potassium was measured in plasma (mEq/l) during HDF (grey) and HDFk (black) at each observation point (at the start of dialysis, 15, 45, 90 and 120 min later, and at the end of the treatment). Significant differences in the variations of plasma potassium were found between HDF and HDFk at 15 and 45 min (P<0.05), and at 90 min (P<0.01). Plasma potassium diminished more gradually during HDFk than during HDF). Values are presented as means±SD.
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Intra-erythrocytic potassium
Intra-erythrocytic potassium was not significantly changed at any point using either of the dialysis methods utilized. The electrical membrane potential at rest (REMP) was significantly reduced at all observation points and with both types of dialysis (HDF and HDFk) compared with T0 (P<0.05). The comparison of data from both dialysis methods revealed a greater, significant reduction in REMP in HDF vs HDFk only at T15 and T120 (P<0.05, Table 2).
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Table 2. The concentrations of intra-erythrocyte potassium and erythrocyte membrane potentials during HDF and HDFk
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ECG parameters
Mean values and SDs for QT, QTd, QTc and QTcd are shown in Table 3 for each type of treatment (HDF vs HDFk). The comparison of data, using the Student t-test, showed values that were, on average, lower at all times with HDFk than with standard HDF. A statistically significant diminution in QT was found at T45 and T120. In HDFk, QTd values were always lower than those in HDF, attaining statistical significance at T15, T45, T90, T120 (P<0.05) and at Tend (P<0.01). QTc values were significantly lower with HDFk than HDF at T45 and T120 (P<0.01), and at T15 (P<0.05). With HDFk, QTcd values were significantly lower than with HDF at all observation points (P<0.01). A statistically significant difference was found between the QT means at T45, and for QTc at T45 and T120. More important was our finding of differences in the QTd and QTcd means, which were significant at all the observation points. QTd, adjusted for plasma K+, was on average increased in HDF. We observed growth and reduction peaks and mean error intervals that were quite wide (Figure 2). On the other hand, not only were these error intervals smaller in HDFk, shifting little from the mean, but their mean values showed a constant, progressive diminution. Plasma potassium, for which QTcd was adjusted, showed the same trend (Figure 3). Moreover, there was a wide difference between its final values in standard HDF (110.5) and in HDFk (33.4). Starting at 15 min, all its values were negative, indicating a reduction in the weighted means obtained with HDFk compared with HDF, and the mean values for the differences were correspondingly lower. The results of the Student t-test were significant (P<0.05) for QT and QTc at T15, T90 and Tend and for QTd and QTcd at T45, T90 and T120 (see Figures 2 and 3). The means, over the entire observation period, of each ECG parameter adjusted for plasma potassium concentration were markedly lower for HDFk than for HDF.

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Fig. 2. Graphic presentation of QT dispersion (QTd) adjusted for plasma potassium values in HDF and HDFk. Significant differences between HDF (grey) and HDFk (black) and error intervals are shown. Significant differences were found at 45, 90 and 120 min (HDF vs HDFk, *P<0.05). QTd, adjusted for plasma K+, was on average increased in HDF. We observed growth and reduction peaks and mean error intervals that were quite wide. Values are presented as means±SD.
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Fig. 3. Graphic presentation of QTc dispersion (QTcd) adjusted for plasma potassium values in HDF and HDFk. HDF is in grey and HDFk in black. Significant differences between HDF and HDFk are shown along with interval errors. Significant differences were found at 45, 90 and 120 min (HDF vs HDFk, *P<0.05). Error intervals are smaller in HDFk than HDF, shifting little from the mean, but their mean values showed a constant, progressive diminution. Plasma potassium, for which QTcd was adjusted, showed the same trend. There was a wide difference between final values in standard HDF (110.5) and in HDFk (33.4). Values are presented as means±SD.
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Discussion
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Patients on HD present different cardiovascular diseases and have a higher mortality rate, which is often due to the high incidence of events such as arrhythmias, which can cause sudden death [1]. HD appears to cause ECG changes [12]. Dysrhythmias and arrhythmias frequently occur after the start of an HD session, and persist for at least 5 h after dialysis. This phenomenon appears to have different causes, including dialysis-induced electrolyte alterations. In a recent study, we demonstrated how using a dialysis bath with higher calcium concentration reduces the dispersion of QT and QTc intervals [19]. These intervals and their increased dispersions have been linked to the occurrence of arrhythmias in ESRD patients on HD [20,21]. This effect could have various causes: regional differences in ventricular wall stress (mechano-electric or contractionexcitation feedback) caused by ventricular dilation, fibrosis and calcification, autonomic failure caused by uraemic autonomic neuropathy, decreased circulatory volume, rapid correction of metabolic acidosis and rapid changes in serum K+. Most patients on HD present morphological and structural cardiac alterations that cause a predisposition to the alterations of ECG parameters [22]. To obviate the interference of such conditions with our findings, we enrolled patients without evident cardiac disease, as determined by an ECG and a 24 h Holter ECG.
Potassium and electrical membrane potential at rest
The present study confirms the findings of Santoro et al. [23] that rapid K+ removal in the initial phases of dialysis is associated with ECG alterations. Our findings suggest that serum potassium diminished between the beginning and the end of dialysis identically with each of the two HDF methods. A comparison of data obtained at the initial times revealed a significant reduction of serum potassium in HDF vs HDFk. The advantage of HDFk consists of profiling, in that initially using a higher concentration of K+ in the bath induces a low blooddialysate gradient and a more gradual removal of the cation. This could have important consequences on electrochemical imbalances in cellular membranes. According to Redaelli et al., the arrhythmogenic effect of dialysis induction depends mainly on the electrochemical imbalances of cellular membranes. A rapid removal of K+ through the dialysis membrane is reflected in the flows across the cell membrane. This induces a critical ratio between intracellular and extracellular K+(Ki/Ke); the depletion of serum and intracellular K+ and the resultant alterations in the electrical potentials of the membrane [24]. In order to study the intracellular variations of K+ during the two HDF sessions, we utilized the model proposed by Rombolà et al. [7]. We evaluated intra-erythrocytic K+, based on the premise that the transmembrane shifts of K+ and, therefore, the electric potentials of the membrane of red blood cells, are analogous to those of cardiac cells. In our protocol, intra-erythrocytic K+ was not significantly different between the two methods. However, our calculation of electric potentials made using Nerst's model based on the Ki/Ke ratio demonstrated that, 120 min after the beginning of the dialysis session, there was a significantly greater cellular hyperpolarization in HDF than in HDFk. Moreover, in the first 15 min of HDF, the electric potential of the membrane was reduced significantly with respect to its initial value. This diminution did not occur in HDFk. These data appear to suggest that the too rapid dialytic removal of K+, unlike the gradual lowering of K+, causes a destabilization in the electric balance of the membrane. The analysis of the electric potentials demonstrated that in HDF, cellular membranes had a slight tendency to hyperpolarization compared with HDFk. The hyperpolarization of the membrane of a single cardiac cell would by itself be destabilizing and have a facilitatory effect on the genesis of arrhythmias, because it alone would involve the distancing of the electric potential at rest from the threshold potential of activation. However, as different cellular types (nodal, conduction and muscular) co-exist in the myocardium, arrhythmia occurs secondary to their desynchronization. The differences in the polarizations of the three cellular types is the basis for the possible onset of re-entrance circuits [25,26].
ECG data
Our results confirm the findings of Severi et al., who utilized the index of ventricular repolarization (PCA-T) as a tool to evaluate the risk of arrhythmia. They found that PCA-T (calculated using appropriate software) was increased in standard HDF compared with HDFk [27]. Our analysis also demonstrated significant differences between the ECG findings from the two techniques. The QT and QTc values obtained during HDFk were significantly lower than those obtained during HDF. Larger differences were found for their dispersions (QTd and QTcd). The QTd and QTcd values obtained during HDFk were significantly lower than those obtained for HDF at the different observation points. The analysis of our data thus suggests that there is a reduced risk of arrhythmia in HDFk, based on the significant reduction in QTd and QTcd. However, in their prospective study of a cohort of patients, Zabel et al. found that QTd is not a negative prognostic marker for the onset of arrhythmias [28]. Coumel et al., on the other hand, suggest that these markers (i.e. QT, QTd, QTc and QTcd), like the ejection fraction, heart rate variability and heart rate, are good predictors of pathological cardiac events, and that their limitation depends on the variability of the measuring techniques used (manual or automated) [29]. QTd and QTcd, adjusted for the contemporaneous values of plasma potassium, show not only a greater increase in HDF, but also a different pattern: data during the session alternate growth and reduction peaks and the error intervals are on average large. This probably indicates a more marked instability of the membrane electric potential and, therefore, a higher risk of arrhythmias. In HDFk, the data show a constant and progressive diminution, with less marked error time intervals. This study enabled us to demonstrate that the ECG markers of ventricular repolarization are increased during dialysis also in patients without underlying cardiac diseases. This phenomenon could be dependent on the speed of potassium removal. The reduction in the dialysateblood gradient of potassium appears to reduce those markers.
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Acknowledgments
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The authors thank Perspectum S.A.S. and in particular Dr E. Castagna for help with statistical analysis.
Conflict of interest statement. None declared.
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Received for publication: 22. 8.04
Accepted in revised form: 22.12.04