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Part 5: PD in Pediatric, Elderly, and Diabetic Patients |
Nephrology Dialysis and Transplantation Children's Unit, University Hospital, University Louis Pasteur, Strasbourg, France
Correspondence to: M. Fischbach, Pédiatrie 1, University Hospital, Avenue Molière, Strasbourg Cedex 67098 France. Michel.Fischbach{at}chru-strasbourg.fr
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ABSTRACT |
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 TOP ABSTRACT DISCUSSIONREFERENCES |
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The peritoneal dialysis (PD) prescription should be adequate before
being optimal. The peritoneal membrane is a dynamic dialyzer: the surface area
and the vascular area both have recruitment capacity.
At bedside, prescription is based mainly on tolerance of the prescribed
fill volume, and therefore a too-small fill volume is often prescribed. A
too-small fill volume may lead to a hyperpermeable exchange, with potentially
enhanced morbidity—or even mortality—risks. Better understanding
of the peritoneal membrane as a dynamic dialysis surface area allows for an
individually adapted prescription, which is especially suitable for children
on automated PD.
Fill volume should be scaled for body surface area (mL/m2)
and, to avoid a hyperpermeable exchange, for a not-too-small amount. Fill
volume enhancement should be conducted under clinical control and is best
determined by intraperitoneal pressure measurement in centimeters of
H2O. In children 2 years of age and older, a peak fill volume of
1400 – 1500 mL/m2 can be prescribed in terms of tolerance,
efficiency, and peritoneal membrane recruitment.
Dwell times should be determined individually with respect to two
opposing parameters:
The new PD fluids (that is, those free of glucose degradation products,
with a neutral pH, and not exclusively lactate-buffered) appear to be the best
choice both in terms of membrane recruitment and of preservation of peritoneal
vascular hyperperfusion.
KEY WORDS: Children; fill volume; dwell time; peritoneal membrane; hyperpermeable exchange.
For a long time, peritoneal dialysis (PD) prescription was based mostly on clinical experience—very empirically—especially for continuous ambulatory PD (CAPD) patients. Better understanding of the peritoneal membrane as a dynamic dialyzer leads to individual prescription (1), especially for children on automated PD (APD). The PD prescription should be adequate before it is optimal.
The bedside PD prescription (1) is based primarily on
Nevertheless, considering the "dialyzer"—the peritoneal membrane—in terms of both surface area and vascular area is also of importance (6). In fact, the peritoneal exchange area directly affects adequacy (6). Knowing more about the possibility of both the recruitment capacity of the contact surface area (the "wetted membrane") and the changes in the vascular surface area with the hope of preserving hyperperfusion (7–11), the PD prescription should progress from adequate to optimal considering the "dialyzer."
The PD prescription should be individualized and adapted to achieve at least two main targets:
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DISCUSSION |
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TOPABSTRACTDISCUSSIONREFERENCES |
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The main clinical question related to fill volume prescription is the child's tolerance of the filled abdominal cavity: What is the fill volume that can be well tolerated? The main adequacy question arising from fill volume prescription is how to avoid a hyperpermeable exchange (12–16), potentially resulting from a too-small fill volume (15–17).
In the mid-1980s, the fill volume was prescribed per kilogram of body weight, with 30 – 50 mL/kg less fluid being used in infants than in older children, and less at the beginning of dialysis than thereafter, leading to relatively small fill volume prescriptions. This limited fill volume concept also led to a false perception of differences in peritoneal permeability between children and adults, with a hyperpermeable peritoneal state being presumed especially in infants (12,13). Scaling of the fill volume by BSA (mL/m2), particularly in infants and small children, avoided hyperpermeable exchanges as defined in a peritoneal equilibration test (12–14).
Fill volume prescription in children should therefore be scaled to BSA and prescribed in a not-too-small amount (1,12–15). A too-small fill volume (1,15–17) is a factor in "functional hyperpermeability," this state presumably being related to the ratio between the peritoneal exchange surface area and the small amount of dialysate with which it is in contact (6,17). A too-small fill volume prescription can be corrected by a change in the dialysis prescription (1,6,15–17).
The hyperpermeable state is a major risk factor for ultrafiltration failure because of a too-rapid loss of the glucose-related osmotic crystalloid gradient (1–4), an impaired statural growth rate (18), and a discrepancy between urea in a normal high range and creatinine in a normal low range for the parameters of adequacy (1,5,19), all leading to increased morbidity and possibly even mortality (2,15–17).
Conversely, a too-large fill volume may contribute to patient morbidity (1,15,16) by causing complications such as pain, dyspnea, hydrothorax, hernia formation, gastroesophageal reflux with anorexia, and loss of ultrafiltration by enhanced lymphatic drainage. These morbidities can also result in prescription noncompliance. Furthermore, increasing the fill volume over a socalled peak volume will not improve dialysis efficiency, but may even reduce it (1,6).
All in all, fill-volume prescription should be low enough to be clinically well tolerated, but should thereafter consider "dialyzer" (membrane) recruitment. Fill volume can be adjusted under the control of intraperitoneal pressure measurement to achieve adequate ultrafiltration and urea purification, both of which are directly related to the achieved ratio: fill volume to the membrane surface area and vascular area recruited for the exchange (6,17). In children over the age of 2 years, the presumed optimal fill volume should be increased stepwise close to the upper limit of 1400 – 1500 mL/m2 for a night exchange in the prone position, while sleeping.
DWELL TIME DETERMINATION: PRINCIPLES AND PRACTICE
Dwell duration (Table 2)
directly affects dialytic exchange capacity
(1,4):
short dwell times favor urea purification and ultrafiltration; long dwell
times favor creatinine and phosphate purification.
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In CAPD, dwell times are by definition long. Therefore, the major risk is loss of the glucose osmotic gradient, leading to ultrafiltration failure and even more dialysate reabsorption by the child, especially in the case of residual polyuric dysfunctional diuresis. To avoid this condition, hypertonic dialysate is often prescribed, despite the potentially enhanced toxicity to the peritoneal membrane, which is related to the amount of glucose degradation products (GPDs) in the fluid (2,11,20). Icodextrin fluid was proposed to limit time-related dialysate reabsorption, but icodextrin dialysate should be limited to 1 exchange daily (2,20).
Most children are treated with APD because of both the better tolerance of a full peritoneal cavity in the supine position and the need for short dwell times to limit dialysate reabsorption (1,2,20,21). However, shortdwell exchanges are more appropriate for urea clearance (Kt/Vurea) than for phosphate clearance (1,4,19). In fact, during a peritoneal equilibration test, the time needed to achieve a dialysate-to-plasma concentration ratio of 50% is 3 – 4 times longer for phosphate than for urea (1,4). Thus, short-dwell exchanges are more adapted to achieve adequate ultrafiltration and urea purification (21). Long-dwell exchanges favor higher creatinine and phosphate clearance, but at the risk of impaired ultrafiltration (2,20,21).
A dwell time close to 1 hour for APD exchanges appears to be the usual initial prescription in children, but that choice needs to be adapted to the patient's condition, the residual renal function, and the main goal of the treatment (1,21): ultrafiltration using a short dwell time, or phosphate purification needing a longer dwell time. It is also important to note that exchange kinetics are also influenced by the composition of the PD fluid (2,7,9,11,20) and the prescribed fill volume (22).
THE PD MEMBRANE: A DYNAMIC DIALYZER
The peritoneal membrane is a dynamic dialysis membrane
(6,8),
a contact surface area, and a vascular exchange surface area. The surface area
in contact with the dialysate—the wetted membrane—is only a
fraction of the anatomic area, 30% – 60% in humans as measured by
computed tomography (6). The
wetted contact area depends on various factors
(6), among them posture (for
example, positive recruitment in the supine position) and fill volume (that
is, progressive recruitment until, in children, a peak fill volume close to
1400 mL/m2 BSA is reached).
The peritoneal vascular perfusion and the density of the functional pores of the perfused capillaries together determine the vascular exchange surface area. This vascular surface area is dynamically affected by factors such as the PD fluid composition and possible inflammatory agents (8,17). It appears that the new, more physiologic PD fluids—GPD-free, not exclusively buffered with lactate, and at neutral pH—offer the best combination in terms of both membrane recruitment and vascular hyperperfusion preservation (6,7,9,10). But the best choice—either mixed lactate and bicarbonate PD fluid, which preserves peritoneal hyperperfusion and possibly enhances fill volume because of lower induced intraperitoneal pressure (7), or pure bicarbonate PD fluid, which potentially affects surface area recruitment and limits peritoneal hyperperfusion (6,9,10)—is not yet clear.
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