Continuous and Prolonged Intermittent Renal Replacement Therapies

Fahad Edrees

Anitha Vijayan

Modalities of Renal Replacement Therapy

The available modalities for renal replacement therapy (RRT) in the hospital are:
- Intermittent hemodialysis (IHD)
- Continuous RRT (CRRT)
- Prolonged intermittent renal replacement therapy (PIRRT) or sustained low-efficiency dialysis (SLED)
- Peritoneal dialysis (PD)
The choice of modality depends on the availability of therapies at the institution, physician preference, the patient’s hemodynamic status, and the presence of comorbid conditions. Table 22-1 illustrates the advantages and disadvantages of different modalities.
Patients with sepsis or hepatic failure may have potential benefits with continuous therapies.
The Kidney Disease Improving Global Outcomes (KDIGO) clinical practice guidelines for acute kidney injury (AKI) in 2012 suggested that CRRT is preferred in hemodynamically unstable patients and in patients with acute brain injury or other causes of increased intracranial pressure or brain edema.¹
Intermittent modalities are generally accepted to cause greater fluctuations in blood pressure and produce greater fluid shifts in a short amount of time.
Continuous modalities allow for the same solute clearance and fluid removal, but spread out over a 24-hour period, and thus are favored in hemodynamically unstable patients.
In the United States, CRRT is performed in approximately 30% of patients with AKI and has almost completely replaced PD in the treatment of AKI.
CRRT has not shown improved survival over IHD in critically ill patients.
Likewise, randomized trials have not shown a difference in time to renal recovery or length of intensive care unit (ICU) or hospital stay between groups treated with IHD versus CRRT.
The principles of hemodialysis and PD are discussed in other chapters. This section will focus primarily on CRRT and PIRRT, and how these modalities compare with IHD in AKI.

Dosing of RRT

Evidence from end-stage renal disease patients suggests that a thrice-weekly regimen for IHD, a urea reduction ratio (URR) of approximately 65% to 68% per session, is considered adequate dialysis. This correlates to a fractional urea clearance (Kt/V_urea) of 1.2, where K is the dialyzer efficiency, t is the time of treatment, and V is the volume of distribution of urea.

Given the acuity of the AKI population, urea clearances are notoriously unreliable, with frequent volume shifts, sepsis, high catabolic state, and so forth.

The acute renal failure trial network (ATN) study published in 2008, compared intensive RRT (IHD/SLED six times per week with single-pool Kt/V_urea of 1.3 per session or
continuous venovenous hemodiafiltration [CVVHDF] at 35 mL/kg/hr) to less intensive RRT (IHD/SLED three times per week or CVVHDF at 20 mL/kg/hr). There was no difference in 60-day mortality or renal recovery between the two groups.²

TABLE 22-1 ADVANTAGES AND DISADVANTAGES OF RENAL REPLACEMENT THERAPIES

Modality	Advantages	Disadvantages
IHD	High-efficiency transport of solutes when rapid clearance of toxins or electrolytes is required Allows time for off-unit testing	Hemodynamic intolerance secondary to fluid shifts “Saw-tooth” pattern of metabolic control between sessions
CRRT	Gentler hemodynamic shifts than IHD Steady solute control	Continuous need for specialized nursing Requires continuous anticoagulation (heparin vs. citrate)
PIRRT	Fewer hemodynamic shifts compared to IHD Less work for intensive care nursing staff compared to CRRT Can be performed at night, avoiding cessation of therapy for procedures No need for expensive dialysate and replacement fluids	Needs to be performed 5–6 days per week to achieve adequate clearance Insufficient data on drug dosing
PD	Gentler hemodynamic shifts than IHD	Requires invasion of peritoneal cavity, which may not be possible in postoperative patients Less predictable fluid removal rates Efficiency of urea removal low compared with other therapies
CRRT, continuous renal replacement therapy; IHD, intermittent hemodialysis; PD, peritoneal dialysis.

The randomized evaluation of normal versus augmented level replacement therapy (RENAL) study evaluated CVVHDF with effluent flow rates of 40 mL/kg/hr in the high-dose group versus 25 mL/kg/hr in the low-dose group. There was no difference in 90-day mortality between the two arms.³

Based on current data, our recommendation is to prescribe CRRT at 20 to 25 mL/kg/hr of effluent flow rate.
If prescribed therapy is not being delivered at least 80% of the time, because of interruptions (machine malfunction, catheter malfunction, patient interruption for procedures), then the prescription dose can be increased to ensure delivery of effluent flow rate of at least 20 mL/kg/hr per day.
IHD should be prescribed to achieve a URR of 70% (Kt/V_urea of 1.3) per treatment, three times per week.
URR should be performed during each treatment, and subsequent dialysis treatment should be adjusted (change of duration, dialysis filter, blood flow, and so forth, to improve adequacy) accordingly.⁴

Continuous RRT

Principles of CRRT

CRRT utilizes the principles of diffusion, convection, or both, depending on the modality.⁵
The nomenclature of CRRT is outlined in Table 22-2.
Diffusion:
- This involves the same principles as dialysis and drives solutes such as urea across the dialysis membrane from the blood (higher concentration) to the dialysate (lower concentration), which is running countercurrent to the blood.
- The dialysate flow rate is approximately 15 to 40 mL/min compared with dialysate flow rate in IHD of 400 to 800 mL/min.
- This process is called continuous venovenous hemodialysis (CVVHD).