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Rocco MV

Body surface area limitations in achieving adequate therapy in peritoneal dialysis patients

Perit Dial Int (Dec) 16:617-622 1996

Recent increases in the recommended clearance targets for patients on peritoneal dialysis have highlighted some of the physical limitations that exist with this dialytic modality. In this paper, Rocco does some simple modelling in order to define the body surface area limitations for achieving these targets with a variety of PD prescriptions. For the purposes of the analysis, all patients are presumed to be functionally anephric. The important variable, therefore, apart from body surface area is the peritoneal transport characteristics for creatinine. A number of very helpful figures are produced looking at two CAPD prescriptions [2L x 4 daily and 2.5L x 4 daily] for each class of transporter. The APD prescriptions studied are five 2-hour dwells nightly, seven 1-hour dwells nightly, and seven 1-hour dwells nightly with a 4-hour day dwell. In each case, the analysis is done for both 2L and 2.5L dwells.

The results are presented in five very helpful figures. In the case of the CAPD prescriptions, there is some effort to validate the calculations by comparing them with 394 dialysis prescriptions that were actually measured. In general, the correlations are quite impressive.

Comment: A number of points are apparent from simply eyeballing the figures. With the 2L q.i.d. prescription the anephric patient cannot reach the 60L target at all if he is a low transporter, and, even if he is a high transporter, he will not reach it if surface area is greater than a rather low 1.45 m2. In other words, this prescription will not be `adequate' in the vast majority of anephric patients. With a q.i.d. 2.5L CAPD prescription, Rocco shows that the low transporter will still not reach the 60L target unless the body surface area is < 1.2 m2. If the patient is a high transporter the target CrCl can be reached as long as BSA is less than 1.75 m2. In other words, this prescription is viable for an anephric patient provided he or she is not above average size and provided the patient is an above average transporter.

In the case of APD, the tables are more complicated. They basically show again that only high transporters have any hope of reaching a 60L a week creatinine clearance target if a day dwell is not used. All the low average and low transporters cannot be adequately dialysed on this regimen. Using 7 x 1-hour dwells per night, only the high transporters and the high average transporters of 2.5L dwells can reach the target if they are any more than average body size. However, if a day dwell is added in there is a significant improvement with the targets becoming achievable for average transporters also. Low transporters remain quite problematic even when 2.5L dwells are used. Two-day-dwell options are not shown in the figure, but clearly these would be required if creatinine clearance targets are to be achieved by low transporters, and even then it would be difficult to get above 60L per week if the patient is significantly bigger than average.

In general, these figures are very helpful and should be kept in mind when prescribing PD. It should be pointed out that creatinine clearance is only one measure of adequacy in PD and that KT/V urea has also been widely used. It is well recognized that it is easier to achieve KT/V urea targets than creatinine clearance targets in the anephric patient. Thus, some of these patients might be adequately dialysed as measured by KT/V even if the creatinine clearance target is not measured. If such patients are doing reasonably well it is probably reasonable to keep them on PD but to follow them closely. It should also be mentioned that all this modelling is an imprecise art and that there is no substitute for frequent 24-hour urine and dialysate collections in patients to see how things are actually going in reality. Nevertheless, this is a useful piece of work and figure 1-5 might be worth keeping handy for any practitioner of PD. (Peter G. Blake, M.D., Victoria Hospital, London, Ontario)