Why are Continuous Ambulatory Peritoneal Dialysis (CAPD) manual Peritoneal Dialysis (PD) bags more effective in treating fluid overload patients than using the Automated Peritoneal Dialysis (APD) machine?

Why Manual CAPD Bags Are More Effective Than APD for Fluid Overload

Manual CAPD bags are superior to APD machines for treating fluid overload because they prevent the net peritoneal fluid reabsorption that occurs during the long daytime dwells characteristic of APD, while providing more frequent, shorter exchanges that optimize cumulative ultrafiltration. 1

The Fundamental Problem: Long Dwells Cause Fluid Reabsorption

• Net peritoneal fluid absorption commonly occurs with long-duration dwells, such as the diurnal (daytime) dwell in APD, which directly counteracts efforts to remove excess fluid 2, 1
• Long dwells (8-16 hours) allow significant fluid reabsorption, especially in high and high-average transporters who equilibrate rapidly 1
• In volume-overloaded patients, ultrafiltration should never be negative for any exchanges, making standard APD with long day dwells problematic 1
• The American Journal of Kidney Diseases guidelines explicitly state that particular attention should be given to avoiding net peritoneal fluid absorption during long dwells because this can be prevented by altering the PD prescription 2

Why CAPD Provides Superior Ultrafiltration

• CAPD allows 4-5 exchanges per day with shorter dwell times (4-6 hours each) that prevent the fluid reabsorption seen with prolonged dwells 2, 1
• Shorter dwell times optimize ultrafiltration by preventing the equilibration that leads to fluid reabsorption 1
• CAPD provides greater ultrafiltration than APD cyclers: median 650 mL/day for CAPD versus 337 mL/day for APD without a day exchange 3
• CAPD removes significantly more sodium: median 57 mmol/day (after adjusting for flush-before-fill) versus 23 mmol/day for APD, which is critical for volume control 3
• Manual exchanges provide immediate flexibility to adjust dwell times and glucose concentrations based on real-time volume status without waiting for programmed cycles 1

Specific APD Limitations in Fluid Overload

• APD patients often prefer "dry day" prescriptions (no daytime dwell), which severely compromises ultrafiltration capacity when volume removal is most needed 2, 1
• Even when APD includes a day dwell, the long duration allows significant fluid reabsorption that negates the overnight ultrafiltration gains 1
• APD requires greater use of hypertonic glucose solutions to compensate for poor ultrafiltration, which damages the peritoneal membrane and causes adverse metabolic effects 2, 1, 3
• The intermittent nature of APD with less stable fluid and osmotic loads may contribute to worse outcomes 4

Evidence-Based Strategies for Volume Overload

• Avoiding long-duration dwells that are associated with ineffective fluid removal or net fluid resorption is the preferred approach 2, 1
• If APD must be used, shorten or eliminate the day dwell, leaving the patient "dry" for a portion of the day 2, 1
• Alternatively, drain and replace the day dwell partway through with fresh dialysis solution to prevent reabsorption 2, 1
• Use icodextrin solution exclusively for any necessary long dwells, as RCTs demonstrate it increases peritoneal ultrafiltration and decreases extracellular fluid volume 2, 1
• Increasing dwell volumes in CAPD (from 2-2.5 L to 3 L) is preferred over increasing exchange frequency, as it enhances ultrafiltration without compromising adherence 2

Critical Pitfalls to Avoid

• Do not assume APD provides superior volume control simply because it is automated—CAPD demonstrably provides better ultrafiltration and sodium removal 3
• Avoid relying on hypertonic 4.25% glucose solutions to compensate for poor APD ultrafiltration, as frequent use damages the peritoneal membrane 2, 1
• Never ignore negative ultrafiltration (fluid absorption) in any exchange when managing volume overload, as this directly worsens the clinical condition 1
• Do not prescribe APD with long day dwells for volume-overloaded patients without using icodextrin for that dwell 2, 1

When APD Can Be Modified (But CAPD Remains Superior)

• If patient preference strongly favors APD despite volume overload, use icodextrin for the entire day dwell to maintain ultrafiltration 2, 1
• Consider nocturnal APD with complete "dry day" only if combined with aggressive diuretic therapy in patients with preserved residual kidney function 2
• Monitor drain volumes from the daytime dwell monthly to detect fluid reabsorption early 2
• Ultrafiltration volume and sodium removal become similar between CAPD and APD only when APD includes a day exchange, essentially mimicking CAPD's pattern 3