Continuous-Flow Peritoneal Dialysis as Acute Therapy

Rationale and Physiology

Peritoneal dialysis (PD) has been used in the treatment of acute renal failure (ARF) for a generation, and its details described elsewhere. Use of PD in ARF has declined largely because the slow solute clearance it achieves renders it inadequate to deal with the modern hypercatabolic patient with ARF who has multiorgan system failure (MOSF). Urea clearance in acute PD is limited, even under optimal circumstances, to 10 to 15 mL/min. This limitation is not due to membrane surface area, permeability, or blood flow, which should be more than capable of delivering clearances of 40 to 80 mL/min. Solute clearance is limited by the fill-dwell-drain cycle of standard PD.

Fig. 187.1 depicts an idealized PD exchange: dialysis, or solute flux across the membrane, requires dialysate contact with the membrane, which makes the fill and drain segments extremely inefficient. Flux (J) is defined mathematically as the permeability coefficient of the membrane or mass-transfer area coefficient (MTAC) multiplied by the difference between the solute concentration in the blood (C _B ) and that in the dialysate (C _D ), or concentration gradient, as shown in the following equation:

In standard PD, the concentration gradient decreases continuously as solute transport occurs during a dwell, steadily reducing the flux, or rate of transport (see Fig. 187.1 ).

CFPD works by constantly replenishing the dialysate, either with fresh, sterile dialysate in single-pass mode, or by externally purifying the dialysate with a hemodialysis (or sorbent) system in recirculation mode. In either case the net effect is to lower C _D and to keep it low throughout the treatment. This greatly enhances clearance and allows the system to perform up to the level of its inherent permeability/blood flow limitations ( Fig. 187.2 ). CFPD has been modeled mathematically in vitro and in vivo. Clearance approaches the MTAC as intraperitoneal solute concentration approaches zero. Clearance varies with intraperitoneal volume, rate of dialysate flow through the peritoneum (Q _P ), and efficiency of the external regenerating circuit, which in turn depends on external dialysate flow rate or Q _D ( Fig. 187.3 ).

Historical Perspective

Our work in CFPD is based largely on the pioneering observations of James A. Shinaberger et al., who in 1965 reported on the first successful series of patients treated with this technique. They compared intermittent PD (IPD), then the standard of care, with CFPD in five patients. They used two catheters, one placed deep within the pelvis and the other near the diaphragm. A 2-L to 3-L intraperitoneal reservoir was drained via the pelvic catheter, recirculated through a primitive extracorporeal circuit consisting of a twin-coil dialyzer sitting in a 50-L vat of dialysate, and returned to the patient through the subdiaphragmatic catheter ( Fig. 187.4 ). Dialysate flow was varied between 20 and 300 mL/min. At flows of 200 to 300 mL/min, urea clearances ranging from 46 to 125 mL/min were obtained.

Other researchers reported experiences over the next 15 years using different setups and home-built dual catheters. Most of these investigators reported urea clearances of around 30 mL/min. The 1980s were dominated by continuous ambulatory PD (CAPD) and continuing cycling PD (CCPD), and little work was done on CFPD until the mid-1990s, which coincided with the end of the honeymoon with CAPD. At that time, CFPD was rediscovered.