The NxStage System One continuous renal replacement therapy (CRRT) system provides convective and diffusive clearance through a simplified cartridge system. It has an interface centered on three large, color-coded numeric fields that offer the most essential information for therapy. When I’m rounding in the CVICU, I often look at these numbers to determine whether they need adjustment based on the situation at hand.
The green field shows dialysate flow, replacement flow, or a combined therapy fluid rate depending on the setup. Dialysate flow primarily drives diffusion, so increasing it improves clearance of small solutes such as urea, creatinine, electrolytes, and lactate until the gradient reaches its limit. Larger molecules do not clear well by diffusion, so higher dialysate flows add little. Lactate behaves like a small solute, but in shock, its production often exceeds what CRRT can remove, so dialysate adjustments are supportive rather than corrective.
The yellow field in the middle represents the ultrafiltration rate. This is the balance point between how much fluid the machine removes and how much is returned to the patient through therapy fluids. When set to zero, the therapy provides clearance only (e.g., in unstable patients who cannot tolerate volume removal). When set to a positive value, it becomes a precise way to achieve gentle fluid removal. For example, in most of my patients who are volume overloaded, I want a net negative fluid loss. Therefore, hour-to-hour, the yellow number must be adjusted to account for ongoing infusions, electrolyte repletion, antibiotics, etc. For example, if all infusions for a given hour total 150 mL, and I want my patient to be “net negative” by 100 mL per hour, the yellow value will need to be 0.25 L/hr. Fluid removal can gradually reduce edema, improve oxygenation, and offload the right ventricle without provoking hypotension.
The red field is the blood pump flow through the extracorporeal circuit. Adequate flow reduces the risk of filter clotting and maintains effective clearance. If the access line is positional or if there is suction on the venous side, the machine may alarm at lower flows. When a patient has a small catheter or difficult access, I decrease this speed to maintain stability while accepting slightly lower clearance. When access is strong, the ability to run higher blood flow improves filter longevity and therapy efficiency.
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