We dive into when to initiate renal replacement therapy, the modalities, settings, and physics involved, troubleshooting problems, and more, with Dr. Paul Adams, a dual-trained nephrologist and intensivist at the University of Kentucky.
- One of the better indications for early dialysis in the ICU patient is to control volume, which in an oliguric patient you know is likely to keep accumulating.
- Help determine who is likely to eventually need dialysis (and hence deciding early vs late, not early vs maybe never) with a furosemide stress test: give 1-1.5 mg/kg of furosemide (160 mg is often about right), then if they don’t make about 1 ml/kg/hr of urine for a few hours, they’re likely to end up needing renal replacement therapy.
- Realistically, most true indications for acute dialysis in the ICU are hyperkalemia, volume overload, or occasional toxicology.
- CRRT is generally more effective at volume management, particularly preemptive volume management, because it continues throughout the day and can more easily keep up with inputs. It is also more hemodynamically stable.
- CRRT can be done via CVVH (using convective flow to drag out fluid and solutes via pressure across a filter), CVVHD (using diffusion gradients to clear solute and fluid), or CVVHDF (using both). Which modality of CRRT is used tends to come down to institution and practitioner practice, although there are some clinical differences in amount of solute clearance and such.
- Effluent is the balanced electrolyte fluid which is used for therapy, and can be run into the blood before reaching the filter (diluting it and improving filter life, but decreasing efficiency), after reaching the filter (purely to replace what was lost), and on the other side of the filter (creating a dialysis effect). Total effluent rate gets divided among these sites as you like.
- UF (ultrafiltration) is essentially whatever fluid is lost that you’re not replacing.
- About 25–30 ml/kg/hr is usually about the right effluent rate. A higher rate helps make up for interruptions during the day.
- 150–250 ml/hr bloodflow is about right; it generally has relatively little effect on clearance in CRRT (unlike in intermittent HD, where it directly impacts clearance).
- Circuit life can be prolonged with anticoagulation. Heparin can be used either systemically or regionally (infused at the start of the circuit, then reversed at the end using protamine), or citrate can be used regionally (replaced with calcium at the end), although it requires close monitoring of ionized calcium levels (really the ratio between total and ionized calcium, since citrate-bound calcium still registers on total calcium assays; a total calcium more than 2-2.5x higher than ionized levels suggests citrate toxicity).
- 16–18 hours of CRRT is usually needed before you start to see an impact on serum solute levels. For critical levels like severe hyperkalemia, start with IHD instead to get a quick correction.
- Pressure problems at the dialysis access are almost always due to anatomic issues like catheter placement. Try adjusting the line, such as placing it deeper. Reducing bloodflow may help, using a different site, or rarely pharmacologically paralyzing the patient.
- Pressure problems at the filter (“transmembrane pressure” or TMP) are usually from clotting. Consider anticoagulation if not already being used, or pre-filter fluid. Inflammatory patients like in sepsis can have very dirty, clotty blood.
- If a patient starts making 600-1000ml of urine daily, consider weaning of renal replacement. That is not common in the critically ill, even if they eventually have later renal recovery; transition to IHD is more common.
- If volume inputs are still ample (many liters a day), it’ll be hard to keep up using IHD, since UF rates top out around a liter per hour. Stick with CRRT in that case.
- Rhabdomyolysis “disproportionately” increases BUN and creatinine, since those are products of muscle breakdown; they may have adequate renal function (demonstrated by robust urine output) despite high numbers.