Principles of Peritoneal Dialysis


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Principles of Peritoneal Dialysis

  1. 1. Principles of Peritoneal Dialysis Ramesh Khanna Karl D. Nolph P eritoneal dialysis is a technique whereby infusion of dialysis solu- tion into the peritoneal cavity is followed by a variable dwell time and subsequent drainage. Continuous ambulatory peri- toneal dialysis (CAPD) is a continuous treatment consisting of four to five 2-L dialysis exchanges per day (Fig. 4-1A). Diurnal exchanges last 4 to 6 hours, and the nocturnal exchange remains in the peritoneal cavity for 6 to 8 hours. Continuous cyclic peritoneal dialysis, in real- ity, is a continuous treatment carried out with an automated cycler machine (Fig. 4-1B). Multiple short-dwell exchanges are performed at night with the aid of an automated cycler machine. Other peritoneal dialysis treatments consist of intermittent regimens (Fig. 4-2A-C). During peritoneal dialysis, solutes and fluids are exchanged between the capillary blood and the intraperitoneal fluid through a biologic membrane, the peritoneum. The three-layered peritoneal membrane consists of 1) the mesothelium, a continuous monolayer of flat cells, and their basement membranes; 2) a very compliant interstitium; and 3) the capillary wall, consisting of a continuous layer of mainly non- fenestrated endothelial cells, supported by a basement membrane. The mesothelial layer is considered to be less of a transport barrier to fluid and solutes, including macromolecules, than is the endothelial layer [1]. The capillary endothelial cell membrane is permeable to water through aquaporins (radius of approximately 0.2 to 0.4 nm) [2]. In addition, small solutes and water are transported through ubiquitous small pores (radius of approximately 0.4 to 0.55 nm). Sparsely popu- lated large pores (radius of approximately 0.25 nm, perhaps mainly venular) transport macromolecules passively. Diffusion and convection CHAPTER move small molecules through the interstitium with its gel and sol phases, which are restrictive owing to the phenomenon of exclusion 4 [3,4]. The splanchnic blood flow in the normal adult ranges from 1.0 to 2.4 L/min, arising from celiac and mesenteric arteries [5]. The lym- phatic vessels located primarily in the subdiaphragmatic region drain fluid and solutes from the peritoneal cavity through bulk transport.
  2. 2. 4.2 Dialysis as Treatment of End-Stage Renal Disease The extent of lymph drainage from the peritoneal cavity is a sub- fraction of glucose absorbed from the dialysate at specific times can ject of controversy owing to the lack of a direct method to mea- be determined by the ratio of dialysate glucose concentrations sure lymph flow. at specific times to the initial level in the dialysis solution. Tests Dialysis solution contains electrolytes in physiologic con- are standardized for the following: duration of the preceding centrations to facilitate correction of acid-base and electrolyte exchange before the test; inflow volume; positions during abnormalities. High concentrations of glucose in the dialysis inflow, drain, and dwell; durations of inflow and drain; sam- solution generate ultrafiltration in proportion to the overall pling methods and processing; and laboratory assays [7]. osmotic gradient, the reflection coefficients of small solutes Creatinine and urea clearance rates are the most commonly used relative to the peritoneum, and the peritoneal membrane indices of dialysis adequacy in clinical settings. Contributions hydraulic permeability. Removal of solutes such as urea, creati- of residual renal clearances are significant in determining the nine, phosphate, and other metabolic end products from the body adequacy of dialysis. The mass-transfer area coefficient (MTAC) depends on the development of concentration gradients between represents the clearance rate by diffusion in the absence of ultrafil- blood and intraperitoneal fluid, and the transport is driven by the tration and when the rate of solute accumulation in the dialysis process of diffusion. The amount of solute removal is a function solution is zero. Peritoneal clearance is influenced by both blood of the degree of its concentration gradient, the molecular size, and dialysate flow rates and by the MTAC [8]. Therefore, the membrane permeability and surface area, duration of dialysis, and maximum clearance rate can never be higher than any of these charge. Ultrafiltration adds a convective component proportion- parameters. At infinite blood and dialysate flow rates, the clearance ately more important as the molecular size of the solute increases. rate is equal to the MTAC and is mass-transfer–limited. Large mol- The peritoneal equilibration test is a clinical tool used to charac- ecular weight solutes are mass-transfer–limited; therefore, their terize the peritoneal membrane transport properties [6]. Solute clearance rates do not increase significantly with high dialysate flow transport rates are assessed by the rates of their equilibration rates [9]. In CAPD, blood flow and MTAC rates are higher than is between the peritoneal capillary blood and dialysate (see Fig. 4-8). the maximum achievable urea clearance rate. However, the urea The ratio of solute concentrations in dialysate and plasma at specif- clearance rate approximately matches the dialysate flow rate, sug- ic times during the dwell signifies the extent of solute transport. The gesting that the dialysate flow rate limits CAPD clearances. Peritoneal Dialysis Regimens FIGURE 4-1 Continuous peritoneal dialysis regimens. Day Night Day Night A, Continuous ambulatory peritoneal dialy- sis (CAPD); B, continuous cyclic peritoneal Left 2.0 dialysis (CCPD) is shown. Multiple sequen- tial exchanges are performed during the day 1.0 and night so that dialysis occurs 24 hours a day, 7 days a week. 0.0 A Day Night Day Night Right 2.0 1.0 0.0 B Exchanges, n
  3. 3. Principles of Peritoneal Dialysis 4.3 FIGURE 4-2 Day Night Day Night Intermittent peritoneal dialysis regimens. Left Peritoneal dialysis is performed every day 2.0 but only during certain hours. A, In daytime ambulatory peritoneal dialysis (DAPD), 1.0 multiple manual exchanges are performed during the waking hours. B, Nightly peri- 0.0 toneal dialysis (NPD) is also performed while patients are asleep using an automated A cycler machine. One or two additional day- time manual exchanges are added to enhance solute clearances. Left Day Night Day Night 2.0 1.0 0.0 B Solute Removal 24 Blood urea nitrogen, mg/dL 100 20 80 Creatinine, mg/dL 16 60 40 12 20 8 0 Dialysate Blood 4 Dialysate –20 Blood 0 80 160 240 320 400 480 560 0 0 40 80 120 160 200 240 280 320 360 A Time, min B Time, min FIGURE 4-3 Solute removal. Solute concentration gradients are at maximum at rate by either increasing the intraperitoneal dialysate volume per the beginning of dialysis and diminish gradually as dialysis progress- exchange or increasing the frequency of exchange. By convection es. As the gradients diminish, the solute removal rates decrease. or enhanced diffusion, solutes are able to accompany the bulk flow Solute removal can be enhanced by increasing the dialysate flow of water. (From Nolph and coworkers [10]; with permission.)
  4. 4. 4.4 Dialysis as Treatment of End-Stage Renal Disease 1.0 1.0 0.9 0.9 Dialysate to plasma ratio 0.8 Dialysate to plasma ratio 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 Urea 0.4 Urea 0.3 Creatinine 0.3 Creatinine Uric acid Uric acid 0.2 Phosphorus 0.2 Phosphorus 0.1 Inulin 0.1 Inulin Calcium Calcium 0 100 200 300 400 500 0 100 200 300 400 500 A Dwell time, min B Dwell time, min FIGURE 4-4 Solute removal. The rates of change of solute concentrations are is near 1.0. Smaller size solutes (ie, urea and creatinine) diffuse similar for 1.5% dextrose dialysis solutions (panel A) and 4.25% across the membrane faster, equilibrate sooner, and are influenced dextrose dialysis solutions (panel B). Hypertonic exchanges enhance more by exchange frequency as compared with larger size solutes solute removal owing to larger drain volumes. Net solute diffusion (ie, uric acid, phosphates, inulin, and proteins). (From Nolph and ceases at equilibration when the dialysate to plasma solute ratio (D/P) coworkers [10]; with permission.) Total dialysate volume (V) Creatinine clearance per exchange (Ccr) Creatnine dialysate to D/P=1 plasma ratio (D/P) High transport 1.0 2600 Ccr=V 2 Low transport 2300 0.5 1 2000 NIPD DAPD CAPD Ccr=V × D/P 1700 NTPD CCPD CCPD 0 (NE) (DE) 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 1 2 3 4 5 6 7 A Dwell time, h B Dwell time, h C Dwell time, h FIGURE 4-5 Solute removal. In a highly permeable membrane, smaller molecules Consequently, intraperitoneal volume peaks later. Ultrafiltration in (ie, urea and creatinine) are transported at a faster rate from the a low transporter peaks late during dwell time. Therefore, a low blood to dialysate than are larger molecules, enhancing solute removal. transporter continues to generate ultrafiltration even after 8 to 10 Similarly, glucose (a small solute used in the peritoneal dialysis solution hours of dwell. The solute creatinine dialysate to plasma ratio to generate osmotic force for ultrafiltration across the peritoneal mem- (D/P) increases linearly during the dwell time. Patients with average brane) is also transported faster, but in the opposite direction. This solute transfer rates have ultrafiltration and mass transfer patterns high transporter dissipates the osmotic force more rapidly than does between those of high and low transporters. NIPD—nightly inter- the low transporter. Both osmotic and glucose equilibriums are mittent peritoneal dialysis; NTPD—nighttime tidal peritoneal dialy- attained eventually in both groups, but sooner in the high trans- sis; DAPD—daytime ambulatory peritoneal dialysis; CAPD—con- porter group. Intraperitoneal volume peaks and begins to diminish tinuous ambulatory peritoneal dialysis; CCPD (NE)—continuous earlier in the high transporter group. When the membrane is less cyclic peritoneal dialysis (night exchange); CCPD (DE)—continu- permeable, solute removal is lower, ultrafiltration volume is larger ous cyclic peritoneal dialysis (day exchange). (From Twardowski at 2 hours or more, and glucose equilibriums are attained later. [11]; with permission.)
  5. 5. Principles of Peritoneal Dialysis 4.5 FIGURE 4-6 150 Solute sieving. A, Dialysate sodium concentration is initially reduced and tends to return Serum and dialysate to baseline later during a long dwell exchange of 6 to 8 hours. B, Dialysate sodium con- Sodium, mLq/L 140 130 centration decreases, particularly when using 4.25% dextrose dialysis solution, because of 120 1.5% dextrose dialysis solutions the sieving phenomenon. Removal of water during ultrafiltration unaccompanied by sodium, 110 in proportion to its extracellular concentration, is called sodium sieving [7,12]. The peri- 100 toneum offers greater resistance to the movement of solutes than does water. This probably 90 Inflow relates to approximately half the ultrafiltrate being generated by solute-free water movement 0 100 200 300 400 500 through aquaporins channels. Therefore, ultrafiltrate is hypotonic compared with plasma. A Dwell time, min Dialysate chloride is also reduced below simple Gibbs-Donnan equilibrium, particularly during hypertonic exchanges. Patients with a low peritoneal membrane transport type tend to reduce dialysate sodium concentration more than do other patients. Therefore, during a short dwell exchange of 2 to 4 hours, net electrolyte removal per liter of ultrafiltrate is well below the extracelluar fluid concentration. As a result, severe hypernatremia, excessive 150 thirst, and hypertension may develop. This hindrance can be overcome by lowering the Serum and Sodium, mLq/L 140 dialysate dialysate sodium concentration to 132 mEq/L. In patients who use cyclers with short dwell 130 4.25% dextrose exchanges and who generate large ultrafiltration volumes, lower sodium concentrations 120 110 dialysis solutions may need to be used (such as 118 mEq/L for 2.5% glucose solutions or 109 mEq/L for 100 4.25% solutions). In continuous ambulatory peritoneal dialysis with long dwell exchanges 90 of 6 to 8 hours, significant sieving usually does not occur, whereas in automated peritoneal Inflow dialysis with short dwell exchanges, sieving may occur. Sieving predisposes patients to 0 100 200 300 400 500 thirst and less than optimum blood pressure control, especially in those who have low-nor- B Dwell time, min mal serum sodium levels, those with low peritoneal membrane transporter rates, or both. (From Nolph and coworkers [10]; with permission.) FIGURE 4-7 Transcapillary ultrafiltration Fluid removal by ultrafiltration. During peritoneal dialysis, hyperosmolar glucose solution Lymphatic absorption 600 generates ultrafiltration by the process of osmosis. Water movement across the peritoneal membrane is proportional to the transmembrane pressure, membrane area, and membrane hydraulic permeability. The transmembrane pressure is the sum of hydrostatic and osmotic 500 pressure differences between the blood in the peritoneal capillary and dialysis solution in the peritoneal cavity. Net transcapillary ultrafiltration defines net fluid movement from the peritoneal microcirculation into the peritoneal cavity primarily in response to osmotic 400 pressure. Net ultrafiltration would equal the resulting increment in intraperitoneal fluid volume if it were not for peritoneal reabsorption, mostly through the peritoneal lymphatics. mL/h Peritoneal reabsorption is continuous and reduces the intraperitoneal volume throughout 300 the dwell. A, The net transcapillary ultrafiltration rate decreases exponentially during the dwell time, owing to dissipation of the glucose osmotic gradient secondary to peritoneal Peak ultrafiltration volume glucose absorption and dilution of the solution glucose by the ultrafiltration. Later in the 200 exchange net, ultrafiltration ceases when the transcapillary ultrafiltration is reduced to a rate equal to the peritoneal reabsorption. B, When the transcapillary ultrafiltration rate decreases below that of the peritoneal reabsorption rate, the net ultrafiltration rate becomes 100 negative. Consequently, the intraperitoneal volume begins to diminish. Thus, peak ultrafiltra- tion and intraperitoneal volumes are observed before osmotic equilibrium during an exchange. (Continued on next page) 0 1 2 3 A Dwell time, h Peak intraperitoneal volume Dialysate 2800 Intraperitoneal 2600 2400 0 1 2 3 4 B Dwell time, h
  6. 6. 4.6 Dialysis as Treatment of End-Stage Renal Disease FIGURE 4-7 (Continued) 360 Dialysate Serum C, Osmotic equilibrium most likely precedes glucose equilibrium Osmolality, mOsm/L because of both solute sieving and the higher peritoneal reflection 340 coefficient of glucose compared with other dialysate solutes, allow- Osmotic ing net transcapillary ultrafiltration to continue at a low rate even 320 equilibrium after osmotic equilibrium. D, Ultrafiltration can be maximized by measures that delay osmotic equilibrium, which can be accom- plished by using hypertonic glucose solutions, larger volumes, or 300 both, during an exchange. More frequent exchanges shorten dwell 0 1 2 3 4 times and increase the dialysate flow rate and thus avert attaining C Dwell time, h osmotic equilibrium. Additionally, potential exists for enhancing ultrafiltration by measures that reduce the peritoneal reabsorption rate. (From Mactier and coworkers [13]; with permission.) 2000 Dialysate Glucose, mOsm/L Serum Hypothetical 1000 glucose equilibrium 0 1 2 3 4 D Dwell time, h FIGURE 4-8 STANDARDIZED 4-HOUR PERITONEAL EQUILIBRATION TEST Standardized 4-hour peritoneal equilibra- tion test. Dt/D0 glucose—final to initial dialysate glucose ratio. 1. Perform an overnight 8- to 12-h preexchange. 2. Drain the overnight exchange (drain time not to exceed 25 min) with patient in the upright position. 3. Infuse 2 L of dialysis solution over 10 min with patient in the supine position. Roll the patient from side to side after every 400-mL infusion. 4. After the completion of infusion (0 time) and at 120 min, drain 200 mL of dialysate. Take a 10-mL sample, and reinfuse the remaining 190 mL into the peritoneal cavity. 5. Position the patient upright, and allow patient ambulation if able. 6. Obtain a serum sample at 120 min. 7. At the end of study (240 min), drain the dialysate with the patient in the upright position (drain time not to exceed 20 min). 8. Measure the drained volume, and take a 10-mL sample from the drained volume after a good mixing. 9. Analyze the blood and dialysate samples for creatinine and glucose concentrations. 10. Correct the serum and dialysate creatinine concentrations for high glucose level (correction factor 0.000531415). 11. Calculate the dialysate to plasma ratios for creatinine, and so on, and calculate the Dt/D0 glucose. FIGURE 4-9 Correction of creatinine levels Equation to correct the creatinine levels in dialysate and serum. Corrected creatinine (mg/dL) The creatinine levels in dialysate and serum need to be corrected = Observed creatinine (mg/dL) – (glucose [mg/dL] x 0.000531415) for high glucose levels, which contribute to formation of noncreatinine chromogens during the creatinine assay. The correction factor may vary from one laboratory to another. In our laboratory at the University of Missouri–Columbia, the correction factor is 0.000531415. Accordingly, the corrected creatinine is calculated as in the equation. The correction in the serum is minimal due to low blood sugar levels; however, it is significant in dialysate, especially during the early phase of dwell (0- and 2-hour dialysate samples).
  7. 7. Principles of Peritoneal Dialysis 4.7 FIGURE 4-10 Intraperitoneal residual volume Equation to calculate the intraperitoneal residual volume. Residual volume is the volume Vin(S3 – S2) of dialysate remaining in the peritoneal cavity after drainage over 20 minutes. The residual R= (S1 – S3) volume can be determined by knowing the dilution factor for solutes such as potassium, urea, and creatinine during the next instillation. The calculation of residual volumes is based on the assumption that the mixing of fluid in the peritoneal cavity is instantaneous and com- plete. This equation is used for the calculation, where Vin is instillation volume; S1 is solute concentration in pretest exchange dialysate; S2 is solute concentration in instilled dialysis solution; and S3 is solute concentration immediately after instillation (0 dwell time). The residual volumes by urea, creatinine, glucose, potassium, and protein are calculated and averaged for accuracy. The measurement of residual volumes is of limited clinical useful- ness; however, it is of great value in a research setting in which accurate determination of intraperitoneal volume is required. FIGURE 4-11 1.1 1.1 Urea Creatinine Classification of peritoneal transport func- tion. Based on the peritoneal equilibrium 0.9 0.9 test results, peritoneal transport function Dialysis to plasma ratio Dialysis to plasma ratio may be classified into average, high (H), 0.7 and low (L) transport types. The average 0.7 transport group is further subdivided into high-average (HA) and low-average (LA) 0.5 0.5 types. For the population studied by Twardowski and coworkers [6], the trans- 0.3 port classification is based on means; stan- 0.3 dard deviations (SDs); and minimum and maximum dialysate to plasma ratio (D/P) 0.1 0.1 values over 4 hours for urea, creatinine, glucose, protein, potassium, sodium, and 0 1/ 1 2 3 4 0 1/ 2 1 2 3 4 corrected creatinine (panels A–G). 2 A Hours B Hours (Continued on next page) 1.1 35 Glucose Protein Final to initial dialysate glucose ratio 30 Dialysate to plasma ratio × 1000 0.9 25 0.7 20 0.5 15 0.3 10 5 0.1 0 0 1/ 1 2 3 4 0 1/ 2 1 2 3 4 2 C Hours D Hours
  8. 8. 4.8 Dialysis as Treatment of End-Stage Renal Disease FIGURE 4-11 (Continued) Potassium Sodium 1.1 1.00 The volume of drainage correlates positively with dialysate glucose and negatively with 0.9 D/P creatinine values at 4-hour dwell times (panel H). (From Twardowski and cowork- Dialysate to plasma ratio Dialysate to plasma ratio 0.90 ers [6]; with permission.) 0.7 0.5 0.80 0.3 Max H +SD HA 0.1 –SD LA Min L 0 0.70 1/ 0 1/ 1 2 3 4 0 2 1 2 3 4 2 E Hours F Hours ADK vol05 ch p04 fig11F 1.1 3500 Corrected creatinine Max +SD 3000 x 0.9 –SD Min 2500 Dialysate to plasma ratio 0.7 2000 mL 0.5 1500 1000 0.3 H 500 HA 0.1 LA L 0 0 Drain Residual Volume 0 1/ 2 1 2 3 4 volume pre-eq post-eq G Hours H FIGURE 4-12 CLINICAL APPLICATIONS OF THE In clinical practice it is customary to perform the baseline standard- PERITONEAL EQUILIBRATION TEST ized peritoneal equilibrium test (PET) approximately 3 to 4 weeks after catheter insertion. The PET is repeated when complications occur. The standardized test for clinical use measures dialysate Peritoneal membrane transport classification creatinine and glucose levels at 0, 2, and 4 hours of dwell time 1. Choose peritoneal dialysis regimen. and serum levels of creatinine and glucose at any time during 2. Monitor peritoneal membrane function. the test. The extensive unabridged test, as originally proposed by Twardowski and coworkers [6], has become a very important 3. Diagnose acute membrane injury. research tool. 4. Diagnose causes of inadequate ultrafiltration. 5. Diagnose causes of inadequate solute clearance. 6. Estimate dialysate to plasma ratio of a solute at time t. 7. Diagnose early ultrafiltration failure. 8. Predict dialysis dose. 9. Assess influence of systemic disease on peritoneal membrane function.
  9. 9. Principles of Peritoneal Dialysis 4.9 FIGURE 4-13 Baseline peritoneal equilibrium test Population distribution of peritoneal membrane transport types. Baseline peritoneal equilibrium test results of patients on long-term peritoneal dialysis in the United States suggest that approximately High High average Low average Low transporter transporter transporter transporter 68% have average transport rates, 16% have high transport rates, D/P creatinine D/P creatinine D/P creatinine D/P creatinine and another 16% have low transport rates [6]. Similar distributions of transport types have been documented worldwide [14–16]. D/P—dialysate to plasma ratio. 16% 68% 16% FIGURE 4-14 Baseline peritoneal equilibrium test Using transport type to select a peritoneal dialysis regimen. Because clearance rates continue to increase with time, patients with low High High average Low average Low transport rates are treated with long dwell exchanges, ie, continu- ous cyclic peritoneal dialysis (CCPD). Owing to the low rate of NIPD NIPD High-dose CAPD increase in the dialysate to plasma ratio (D/P), the clearance rate High-dose CCPD DAPD CAPD High-dose CCPD only when significant per unit of time is augmented relatively little by rapid exchange residual renal techniques such as nightly intermittent peritoneal dialysis (NIPD). function is present On the contrary, the clearance per exchange rate over long dwell exchanges would be less in patients with high transport rates. During the short dwell time, patients with high transport rates capture maximum ultrafiltration and small solutes are completely equilibrated. Therefore, these patients are best treated with tech- niques using short dwell exchanges, ie, NIPD or daytime ambulato- ry peritoneal dialysis (DAPD). Patients with average transport rates can be effectively treated with either short or long dwell exchange techniques. CAPD—continuous ambulatory peritoneal dialysis. FIGURE 4-15 1.0 0.97 Diagnosis of early ultrafiltration failure. The dialysate to plasma ratio (D/P) curve of sodi- um, during the unabridged peritoneal equilibrium test (2.5% dextrose dialysis solution), typically shows an initial decrease owing to the high ultrafiltration rate. Because of sodium Dialysate to plasma ratio 0.92 sieving, the ultrafiltrate is low in sodium. Consequently, the dialysate sodium is lowered, 0.9 resulting in a lower D/P ratio of sodium. Later, during the dwell when ultrafiltration ceas- 0.88 es, dialysate sodium tends to equilibrate with that of capillary blood, returning the D/P ratio of sodium to baseline. Absence of the initial decrease of the D/P of sodium is an indi- 0.85 cation of ultrafiltration failure and is typically seen in the early phase of sclerosing encap- 0.8 0.80 sulating peritonitis. (From Dobbie and coworkers [17]; with permission.) High High average Low average Low 0.7 0.0 1.0 2.0 3.0 4.0
  10. 10. 4.10 Dialysis as Treatment of End-Stage Renal Disease FIGURE 4-16 (DxV) C= Creatinine and urea clearances rates. These rates are estimated by dividing the amount of P where C = clearance in mL/min: solute removed per unit of time by the plasma solute concentration. Alternatively, clearance DxV = dialysate solute removed per minute; also can be estimated by multiplying the solute equilibration rate per unit of time by the D = dialysate solute concentration; volume of dialysate into which equilibration occurred over the same unit of time. By con- V = volume of dialysate in mL/min; and vention, the creatinine clearance rate is normalized to body surface area. P = plasma solute concentration The urea clearance is normalized to total body water (volume of urea distribution in the or body) and is expressed as Kt/V. The Kt/Vvalue is a number without a unit ([mL/min min]/ C=(D/P) x V mL). During intermittent dialysis, with a dialysate flow rate of 30 mL/min, the typical urea where C = clearance in mL/exchange at time t; clearance is about 18 to 20 mL/min [18]. Increasing the dialysate flow rates to 3.5 to 12 D/P = solute equilibrium rate at time t; and V = volume of dialysate at time t L/h by rapid exchanges increases the urea clearance rate to a maximum of 30 to 40 mL/min. A Beyond this maximum rate, the clearance rate begins to decrease owing to the loss of mem- brane-fluid contact time with infusion and drainage; inadequate mixing may also occur Kt/V [19–22]. Clearance could be enhanced by increasing the membrane-solution contact [23]. where K = urea clearance in mL/min; Continuous dialysate flow techniques using either two catheters or double-lumen catheters t = minutes of therapy; and also have enhanced the urea clearance rate to a maximum of 40 mL/min. At these high flow V = volume of urea distribution or total body water rates, poor mixing, channeling, abdominal pain, and poor drainage limit successful applica- B tion. Maintaining a fluid reservoir in the peritoneal cavity (called tidal peritoneal dialysis) and then replacing only a fraction of the intraperitoneal volume rapidly, increases clearance rates by about 30% compared with the standard technique using the same doses owing to maintaining fluid-membrane contact at higher dialysis-solution flow rates [24–29]. During continuous ambulatory peritoneal dialysis (CAPD) in adults, the optimum volume that ensures complete membrane-solution contact is about 2 L [30,32]. Successful use of 2.5- and 3.0-L volumes has been reported in adult patients undergoing CAPD; however, hernial complications are increased [32,33]. FIGURE 4-17 Mass-transfer area coefficient The mass-transfer area coefficient (MTAC). The MTAC represents the clearance rate by The diffusive mass transfer is estimated by diffusion in the absence of ultrafiltration and when the solute accumulation in the dialysis solution is zero [34–39]. MTAC is equal to the product of peritoneal membrane perme- A ability (P) and effective peritoneal membrane surface area (S). Thus, when both capillary M=I (C – C ) R P D blood and dialysate flows are infinite, the clearance rate is directly proportional to the where M = diffusive mass transfer: effective peritoneal surface area and inversely proportional to the overall membrane resis- A = effective membrane surface area; tance. However, infinite blood and dialysate flows cannot be achieved, and the maximum I = coefficient of proportionality; R = sum of all resistances; clearance rate is unattainable. The closest measurable value, the MTAC, was introduced. Cp = solute concentration in the potential The MTAC represents an instantaneous clearance without being influenced by ultrafiltra- capillary blood; and tion and solute accumulation in the dialysate. The MTAC is measured over a test exchange CD = solute concentration in the dialysate during which at least two blood and dialysate samples are obtained at different dwell A times. The precision of the measurement is enhanced with more data points. The MTAC is seldom used clinically; however, it is a very useful research tool. Dividing both sides of the equation by solute concentration in peripheral blood (CB) will yield instantaneous clearance or the MTAC; M A CP CD CB =K=I ( – R CB CB ( B If the peritoneal capillary blood flow is infinite, Cp will equal Cb and A C Ki=I R ( ( 1– D CB If the dialysate flow is also infinite, then Co will equal 0, and A Ki=Kmax=I R C
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