1Renal and Dialysis Unit, Desio Hospital, Milan 2Department of Medicine, Division of Nephrology and Dialysis, Bassini Hospital, Cinisello Balsamo, Milan, Italy
Correspondence and offprint requests to: Renzo Scanziani, MD, Renal Unit, Desio Hospital, I-20033 Desio, Milan, Italy. Email: juscanz{at}tin.it
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Abstract |
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Methods. We studied 20 patients who received APD, with the standard fill volume (2 l, A), or individualized fill volumes based on the patients body surface area (2.5 l/BSA/1.73 m, B) or on body weight (40 ml/kg body weight, C). The patients tolerance to a given fill volume was evaluated by measuring IPP, and catheter flow characteristics were evaluated by an automated machine.
Results. IPP increased with the increase of the infused volume of dialysate (P < 0.05) and tended towards a positive relationship with the patients body mass index (BMI: A vs IPP: R = 0.39, P = 0.0019; B vs IPP: R = 0.66, P = 0.0012; C vs IPP R = 0.55, P = 0.009). We also found a relationship between fill volume, BMI and IPP: IPP = 1.0839 + 0.53 (ß) x BMI + 0.211 (ß) x fill volume (R = 0.65; r2 = 0.40 P < 0.01). The mean IPP with different dialysate fill volumes tended to be related to the volume of dialysate drained at the transition point (R = 0.37; P < 0.05). The pre-transition flow rate/mean IPP ratio tended towards a positive relationship with the volume of dialysate drained at the transition point (R = 0.35, P < 0.05), the transition time (R = 0.34; P < 0.05) and a negative one with the transition volume (R = 0.35, P = 0.05).
Conclusion. It is possible to customize APD, where the tidal percentage coincides with the transition point for a given catheter and a specific initial dialysate fill volume, the tolerance of which can be measured by assessing IPP.
Keywords: automated peritoneal dialysis; fill volume; intraperitoneal hydrostatic pressure
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Introduction |
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In classic non-tidal APD, the fill and drain of the dialysate is complete for each exchange, and involves large volumes of dialysate. In this procedure, the time required for fluid infusion and drainage can account for 3555% of the total treatment time, resulting in a reduced dialytic efficiency if treatment time is kept constant [9]. This has led to attempts to optimize APD either by identifying the optimal fill volume or by using the tidal exchange mode, which involves a series of partial drainages and infusions after the initial complete fill [10]. The modulation for optimum efficacy of the fill volume per cycle of APD is based on its relationship with the body surface area (BSA): this can lead to an increase in the mass transfer area coefficient (MTAC), modifying the useful surface of the peritoneal membrane [1]. Larger volumes of dialysate are better tolerated in nocturnal treatments because the i.p. pressure (IPP) is lower in the supine patient compared with the upright position [11]. Finally, an increase in the per-exchange volume of the dialysate (with a constant total volume used per treatment) allows for longer dwell periods, and produces better clearances in patients with medium-low or low peritoneal transport characteristics.
In the tidal mode, reducing the fill and drain times, usually used at 50% (exchange volume equal to half the initial fill volume), allows an increase in the dwell period, the efficient part of the dialysis treatment, and takes advantage of the uniform high dialysate outflow portion of the drainage through the peritoneal catheter [12]. It also allows increasing the hourly dialysate flow rate, which has considerable influence on the clearance of small molecules [8], allowing greater volumes of dialysate to be used during each session. It is possible to optimize the tidal mode by studying dialysate outflow during the drainage phase, for the initially high, uniform dialysate outflow is followed by a characteristic slow, irregular outflowthese two distinct phases being separated by the so-called transition point or break point [12,13].
This study is aimed at examining the relationships between IPP, dialysate flow characteristics and different dialysate fill volumes: the standard fill volume of 2000 ml, that calculated on the basis of the BSA, and that calculated on the basis of the body weight (BW).
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Subjects and methods |
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The primary diagnoses of their renal failures were: chronic glomerulonephritis (n = 13), diabetic glomerulosclerosis (n = 6), ischaemic nephropathy (n = 4), nephroangiosclerosis (n = 6), pyelonephritis (n = 2), polycystic kidney disease (n = 1), renal tubercolosis (n = 1) and Alports syndrome (n = 1).
Their mean BW was 64.7 ± 9.8 kg, their mean height 160.7 ± 7.8 cm. Their body mass index (BMI), calculated as weight/height2, was 25.3 ± 4.2. The BSA, calculated using the Du BoisDu Bois formula [14], and was 1.68 ± 0.13 m.
All patients were clinically stable and had no peritoneal infections or other abdominal pathologies during the month prior to enrolment in the study.
At enrolment, each patients abdomen was X-rayed to verify the correct positioning of the Tenckhoff catheter in the Douglas cavity. The functionality of the catheter (drainage) was also checked with patients in the upright position.
Measurement of the i.p. hydrostatic pressure
IPP was measured following the method described by Durand et al. [15], using the Y-twin-bag connection system (Baxter Healthcare) and with the patient supine on a rigid horizontal surface. The level of the column of dialysate in the connection tube of the drainage bag was measured using a graduated rod with the zero value fixed at the mid-axillary line. IPP must be measured at atmospheric pressure with no positive or negative counter pressure on the distal part of the measurement line. As shown by Durand et al. [15], in disconnected systems like the one we used, there is no counter pressure. Accordingly, in order to measure IPP, an empty drainage bag was hung above the horizontal plane to allow the dialysate to rise in the drainage line of the Y-twin-bag system. As IPP changes with respiration (movements of the diaphragm), IPP must be measured during inspiration (IPPinsp) and expiration (IPPexp). The IPP value used is the mean of the two values: (IPPinsp + IPPexp)/2. As the density of the dialysate is similar to that of water (1017 g/cm3), the values of IPP are usually expressed in centimeters of water (cm H2O) with a margin of error of <2% [3].
IPP was measured in the morning, immediately after the nocturnal APD on the same day the study was performed. IPP measurements for the sitting and supine positions were done on 2 different days. The abdomen, which was empty after the last APD, was filled with the dialysis solution. In addition to the standard 2000 ml fill volume (A), we also tested the dialysate fill volumes that are frequently recommended, those based either on the patients BSA (B: 2425 ± 192 ml), using the formula 2500 ml/1.73 m/BSA suggested by Blake et al. [16] and Keshaviah et al. [1], or based on the patients BW (C: 2580 ± 371 ml) using the formula (40 ml/kg BW) suggested by Flanigan et al. [4]. A patients tolerance to a given fill volume was evaluated by measuring IPP. Catheter flow characteristics during the drainage phase, with the patient supine, were assessed in order to optimize and customize the i.p. fill volume, the dialysate flow rate and the percentage of the tidal mode.
Evaluation of catheter flow characteristics during drainage
With IPP measured, the flow in the catheter was evaluated during drainage. This procedure was performed using a specific automated exchange machine, Quantum PD (Baxter Healthcare). This equipment operates under closed-system conditions and displays a real-time measurement of the dialysate outflow (ml/min). The drainage bag was positioned lower than the patient and was connected to the machine. The various phases of dialysate outflow through the peritoneal catheter were recorded [12]: an initial phase, with high, uniform outflow (pre-transition flow rate); an abrupt intermediate point, where drainage slows down considerably (usually to 3040 ml/min); and a final drainage phase characterized by a slow irregular flow (post-transition flow rate). The intermediate phase, defined by Brandes et al. [12] and Durand et al. [13], is known as the transition point or break point. In addition to the durations of the initial phase (transition time) and the slower final phase, we measured the maximum initial outflow and the quantity of dialysate drained during the rapid and slower phases (transition volume), and calculated the average flow rate for each phase and the percentage volume drained out at the transition or break.
For each patient and for each fill volume, three consecutive measurements of IPP and drainage flow rates were taken. We also calculated the conductance of the system , that is, the permeability of this system to fluid flow, according to the HagenPoiseuille law [17]. This parameter depends on the bore of the tube and the viscosity of the liquid. Conductance is directly proportional to the fourth power of the radius of the tube and inversely proportional to its length and the viscosity of the liquid.
Statistical analysis
The statistical analysis was performed using the statistics software package SPSS edition 8 for Windows (SPSS Inc., Chicago, IL, USA).
The findings were expressed as the mean ± SD and as percentages. Patients were assessed when receiving the standard 2000 ml fill volume (A) in the sitting or supine positions, and for the different fill standards (A, B or C) when supine. The findings for the peritoneal outflow phases, in patients in the sitting and supine positions, were compared using the Students t-test for unpaired data.
Using ANOVA one-way variance analysis and Sheffeès test, we compared the IPP values and the above-described fill standards A, B or C.
Using Pearsons simple linear regression, we searched a relationship between IPP values, the patients BMIs and BSAs and the data characterizing the peritoneal drainage phases. The mean flow of the high outflow phase was related to the drainage volume at the transition point and at the transition time, in absolute terms and as percentages.
We also searched for relationships between the conductance of the drainage system, calculated according to the HagenPoiseuille law [17], and the volume of dialysate drained at the transition point (in absolute terms and as percentages), the transition time (in absolute terms and as percentages), and the transition volume, using Pearsons simple linear regression. Pearsons R coefficient was considered when necessary. P < 0.05 was considered significant.
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Results |
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Dialysate volumes and IPP
The volume of dialysate infused showed a significant increase from the standard fixed volume of 2000 ml (A) to a volume (B) calculated according to the formula (2500 ml/1.73 m/BSA) or to a volume (C) calculated according to the formula 40 ml/kg (2059 ± 40 cc vs 2425 ± 42.84 cc vs 2580 ± 82.9 cc; P < 0.05).
Table 1 shows the mean value ± SD of IPP during inspiration (IPPinsp) and expiration (IPPexp), their mean value [(IPPinsp + IPPexp)/2] and their difference (Delta IPP = IPPinsp IPPexp).
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Furthermore, in all the groups we found a relationship between mean IPP, fill volume and BMI according to this regression equation: IPP =1.0839 + 0.53 (ß) x BMI + 0.211 (ß) x fill volume (R = 0.65; r2 = 0.40, P < 0.01).
No significant relationships were observed with the duration of dialysis, the patients age, sex or BSA. IPP was not different between males and females.
During IPP measurements, the mean IPP of four patients exceeded 18 cm H2O, considered to be the limit of tolerance. Of them, two patients exceeded 18 cm H2O when their intake volumes were calculated on the basis of BSA (2500 ml/1.73 m/BSA) or according to BW (40 ml/kg/BW). The other two exceeded the 18 cm H2O when their intake volumes were calculated according to BW alone (40 ml/kg/BW). These four patients showed clinical signs of intolerance (discomfort, malaise, lower back or abdominal pain, or both, mild dyspnea). In two of these patients we calculated mean IPPs in excess of 20 cm H2O.
Evaluation of peritoneal catheter flow rates during the drainage phase
Catheter functionality (infusion and drainage) was assessed for all patients in the sitting and supine positions before measuring the catheters dialysate outflow ratesaccording to the different fill volumes in patients, in the supine position. The mean flow rate was 268 ± 35 ml/min with a mean drain time of 7.57 ± 0.59 min for a fill volume of 2059 ± 40 ml, with the dialysate bag positioned 65 cm from the mid-axillary line.
Figure 1 shows catheter drainage characteristics: an initial phase with high regular dialysate outflow from the peritoneal cavity, a point where the flow rate suddenly drops, and a phase characterized by slow irregular drainage, this both in the sitting or supine positions.
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Discussion |
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Our IPP volumes are comparable with those measured by other authors [18,19]; the linear increase in IPP of 1.0 cm H2O for every 500 ml increase of dialysate volume was reported by other authors [18]. We observed this phenomenon only when the increase of the dialysate volume was related to the BSA. However, we observed an increase of 1.8 cm H2O for every 500 ml of increase in the volume of the dialysate when the volume was calculated on the basis of BW, using the formula 40 ml/kg BW. This is probably due to the observed positive relationship between IPP and BMI.
The correlation between IPP and BMI, which was not found by other authors [20], introduces a new element useful for calculating the optimal fill volume of dialysate in peritoneal dialysis, particularly APD. In fact, several authors [12,21,22] have observed increases of the MTAC (K0 A) that were related to the BMI.
It stands to reason that the relationship between the surface of the peritoneal membrane and BMI, and between BMI, fill volume and IPP, could point towards the dialysate fill volume that optimizes the efficacy of the surface of the peritoneal membrane, and still is tolerated by the patient. In fact, even though a patients tolerance of the i.p. volume of the dialysate is subjective and clinically variable [3,19,23], IPP measurements demonstrate that values in excess of 18 cm H2O when supine must be carefully evaluated, because of the negative impact on respiratory capacity and the potential of respiratory disturbances during sleep [3].
When identifying the optimal fill volume, the effect of IPP on ultrafiltration and, therefore, on fluid balance, should not be ignored. An increase of 1000 ml in the volume of the dialysate leads to an increase of 2.2 cm H2O in the mean IPP when the patient is supine. This leads to a net increase in the reabsorption of the fluids of 36 ml/h/cm H2O [3,24,25].
Our findings with respect to IPP and the observations of clinical tolerance would indicate that, for adult patients, the tolerated fill volume should be calculated on the basis of the BSA (2.5/1.73 m2 BSA) as suggested by Keshiaviah et al. [1]. However, further study it is necessary to verify an increase of dialytic efficiency when a prescribed fill volume is based on BMI.
In our patients, the time and mean flow rates at infusion were similar to those reported by other authors [12]. Greater flow rates are observed when patients are upright rather than supine [12], largely due to lower IPPs in supine patients [26]. This effect becomes of major importance for the clinician who has to determine the real times needed for dialysate infusion when scheduling the various cycles of APD.
The measurements of dialysate flow during the drainage phase through a Tenckhoff peritoneal catheter with the patient upright or supine, using different fill volumes, show the repeatability of the various phases of the outflow itself: an initial phase, with high, uniform outflow of most of the dialysate load, a sudden drop in the dialysate flow rate and a final phase characterized by slow, irregular drainage (Figures 1 and 2). During the high outflow phases, irrespective of whether the patient is upright or supine, and even when fill volumes are different, the average flow rate is six to seven times greater than the mean flow rate during the slower drainage phase. According to Brandes et al. [12], the explanation for this sudden drop in the drainage rate is the convergence of distented intestinal loops around the peritoneal catheter at a time when there is a certain residual volume of dialysate in the abdominal cavity (transition volume), that restricts drainage considerably.
In our opinion, the transition or break point may be due to a sudden drop in the conductance of the hydraulic system [17]when a critical i.p. volume has been reached, outflow from the abdomen becomes limited. The conductance, which in our case is the relationship between the pre-transition flow rate and the mean IPP, with P2 = 0, is usually constant. Its decrease during the drainage phase may be due to a sudden change occurring when the amount of i.p. dialysate reaches a certain volume (transition volume)the equilibrium between that i.p. volume and IPP, which is the force behind the drainage phases, causing a sudden diminution of dialysate outflow.
Further studies undoubtedly are necessary to confirm our hypothesis and give us a better understanding of the critical mean IPP value correlated to a critical i.p. volume, studies which use an on-line pressure/flow detector system during the drainage segment.
The relationship between IPP and the volumes, drained at the transition points (Figure 3), and the mean flow during the high outflow phase and the transition time show the importance of identifying the optimal fill volume prior to calculating the drainage times. The optimization of the drainage time is tightly linked to the clinicians knowledge of the various drainage phases. At the transition time with the patient in a supine position, the mean percentages of dialysate drained with the different fill volumes were between 77 (A) and 84% (C) of the total volume, while the mean percentage of the transition time was between 39 (A) and 48% (C) of the total drainage time.
Most of the drainage time is used up removing a small percentage of the dialysate volume. It is obvious that with APD, the intermittent technique requires longer time spans, due to the times needed to fill and particularly to the drain the dialysate. These intervals are relatively inefficient in terms of clearance. In total, the dialysate fill volume, i.e. the maximum i.p. volume per cycle, remains in contact with the peritoneal membrane for too short a time compared with the time during which there is a constantly decreasing amount of dialysate in the peritoneal cavity. Keshaviah et al. [1] showed how the decrease in the coefficient of mass transfer area (K0A) is directly proportional to the decrease in the i.p. volume of the dialysate.
Under these conditions, the increase of the dialysate flow does not improve the clearance of small molecules, and once a certain flow has been reached, there actually is deterioration. In 1961, Boen [27] had already highlighted this phenomenon, and reported that the maximum possible urea clearance was obtained with a dialysate flow rate of around 3 l/h and that greater flow rates in fact reduced the clearance. Following the characterization of peritoneal transport by Twardowski et al. [2], improvements were made in the determination of the flow that leads to maximum creatinine clearance. That lies between 1.4 and 1.6 l/h for the low transporters and 2.2 and 2.3 l/h for the other transporters [7,8].
The tidal exchange mode involves a series of drains and partial fills following the initial complete fill, and for this reason it would appear to be a rational method for improving the efficiency of dialysis [10]. Only the tidal mode will allow an increase in the hourly dialysate flow and at the same time have a considerable effect on the clearance of the small molecules [8]therefore allowing the use of greater dialysate volumes during each session. In this mode, the flow values that produce the maximum creatinine clearances were found to lie between 1.8 l/h for the low transporters and 4.2 l/h for the other transporters [8].
The efficiency of dialysis with the tidal mode, normally used at 50% (exchange volume equal to half the initial fill volume), and the hourly dialysate flow as well as the type of peritoneal transport, depend on the initial fill volume, the reserve volume, the tidal exchange volume and the total duration of the exchanges [9].
By reducing the fill and drain times to 15% of the total duration of APD, and by increasing the dwell period, Flanigan [10] reported better results in terms of clearance with the 50% tidal treatment where the initial fill volume was 40 ml/kg BW, with a reserve volume of 15 ml/kg BW.
Our findings allow us to suggest that further reductions in drain times are possible. If a tidal percentage is calculated on the basis of the time to the transition point or break point (transition from the high outflow to the slower outflow), it is possible to use the process to the full, and not just partially, as is usual in the standard tidal processes (at 50%). As a result, the rapid drainage phase will allow further optimization of the treatment times.
The correspondence of the high outflow phase with the drain time during APD allows a more rapid exchange of a greater dialysate volume, with the possibility of increasing the dwell period. Brandes et al. [12], using only the initial high outflow part of the drainage phase (pre-transition flow rate), suggested that an increase of up to 10% in urea clearance occurs during an 8-h APD with a total dialysate volume of 12 l.
Further studies are necessary to confirm that a real improvement in dialytic efficiency is obtainable with modifications of the tidal mode. In particular, the dialysate flow that produces the maximum clearance values and the number of cycles required for the different types of peritoneal permeability [7,8] must be identified to obtain the target values of urea and creatinine clearance [28], particularly in those patients with no residual renal clearance or in those for whom it is difficult to perform additional exchanges during the day. As the time required for the various drainage phases and the percentage volume drained at the transition or break point are unique to each catheter [12,13], it is possible, and it may be necessary, to customize the tidal mode according to the initial fill volume best tolerated by each patient and the fill and drain times of each type of catheter, in order to achieve an optimum hourly dialysate flow rate that takes into consideration the patients peritoneal transport characteristics.
Our data show that it seems possible to optimize APD either by increasing the dialysate volume or by improving the efficiency of the treatment through a reduction in the exchange time using only the high outflow phase of the drainage. Measurement of IPP and flow rates through the peritoneal catheter during drainage involve such simple manoeuvres that the process can be used in routine clinical practice, allowing greater precision in the prescription of peritoneal dialysis.
Conflict of interest statement. None declared.
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References |
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