Page 1360 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 98: Renal Replacement Therapy in the Intensive Care Unit 933

diffusion; the movement of solutes will depend not only concentration
• Retrospective and observational studies suggest that the early ini- gradient, but also on the size of the solute. Smaller solutes are more dif-
tiation of RRT may improve patient outcomes; however, definitive fusible, because they randomly move more in solution than large solutes.
randomized, controlled trials have yet to be performed. Convective clearance, used in hemofiltration, is an alternative means
• In the setting of acute kidney injury (AKI), no specific RRT modality of achieving solute clearance that generally provides better clearance
(intermittent, continuous, or peritoneal) provides a mortality benefit of larger size solutes (see below, “Modality of RRT: Convective Versus
over another. However, certain clinical scenarios (eg, hepatic failure, Diffusive”). Ultrafiltration operates on the principle that water will move
increased intracranial pressure) may mandate a specific modality. across a semipermeable membrane from a higher-pressure system to a
• In the setting AKI, randomized controlled trials have demon- lower-pressure system. Solutes dissolved in ultrafiltered water also move
across the membrane via “solvent drag,” or convection. Importantly,
strated that a minimum dose of 25 mL/kg/h of continuous renal solute clearance by ultrafiltration requires high volumes of water move-
replacement therapy (CRRT) be delivered in order to improve ment. Thus, low-volume hemofiltration, as utilized in slow continuous
patient survival. Data on dosing of intermittent dialysis suggest ultrafiltration (SCUF), is effective at removing water, with limited solute
prescription of a minimum of three treatments per week. removal (or “clearance”). Conversely, high-volume hemofiltration, such
• No singular method of systemic or regional anticoagulation, in the as used in CVVH, is effective at removing solutes, but large amounts of
setting of AKI requiring renal replacement therapy, has demon- water are removed and fluid must be returned to maintain blood volume
strated superiority. Several options including heparin, citrate, and in the form of a replacement solution. The replacement solution also
no anticoagulation remain extremely common and each has their contains supplemental electrolytes (eg, potassium, phosphorus, calcium)
own risks and benefits. and a buffer (lactate or bicarbonate) to prevent iatrogenic depletion of
• In the setting of AKI requiring RRT, nutritional support consistent these solutes, in addition to treating metabolic acidosis, and both dilut-
with the current ESPEN guidelines and monitoring of parameters ing and removing circulating uremic solutes. The use of acute peritoneal
of nutritional status in critically ill patients are appropriate. dialysis in the setting of AKI has largely fallen out of favor in many
• Depending on the modality of RRT (intermittent, continuous, or countries, although still commonly used in critically ill children and for
peritoneal), dosing strategies for medications (including antimi- adult acute RRT in developing countries. In the large, multicenter inter-
crobials) differ significantly. national observational study of the epidemiology of AKI in the setting
• Adherence to dosing guidelines is critical to ensure that the tar- of critical illness conducted by the Beginning and Ending Supportive
Therapy for the Kidney (BEST for the Kidney) investigators, only 40 of
geted therapeutic dose is delivered in the setting of AKI and RRT, 1
as inappropriate dosing has a significant impact on patient out- 1258 (3.2%) individuals requiring RRT underwent PD or SCUF. Thus,
the focus of this chapter will be on the use of blood-based extracorpo-
comes and increases the risk of mortality.
real therapies to achieve small solute clearance, including IHD and the
forms of CRRT outlined above. The choice of modality (intermittent vs
continuous therapy and diffusive vs convective therapy) remains contro-
Despite advances in medicine and critical care, the nephrology commu- versial and the data supporting the different modalities will be outlined
nity has yet to develop a consistent, proven intervention to predictably in more detail in the sections below. Nevertheless, the goal of all RRT
prevent or hasten the recovery of all forms of acute kidney injury (AKI), therapy remains the same; ameliorate the severe metabolic and volume
including its most severe form, acute tubular necrosis (ATN). Thus, derangements that contribute to the poor prognosis of AKI in the setting
care for the patient with AKI is focused on supportive measures includ- of critical illness. Combinations of these complementary therapies are
ing treatment of the underlying disease state and, when needed, renal commonly used to support such patients at various stages of their acute
replacement therapy (RRT). While advances in nephrology have not illness and recovery.
identified a consistent therapy for the prevention or improved recovery
for AKI, there have been considerable advances in the field of RRT.
INDICATIONS FOR RENAL REPLACEMENT THERAPY
RRT: AN INTRODUCTION AND TIMING OF INITIATION
RRT, in this setting, refers to the use of extracorporeal support to The indications for renal replacement therapy vary between the clear
remove solutes and water. The current available modalities of RRT and the obscure. Medical students and physicians-in-training are rou-
are intermittent hemodialysis (IHD), peritoneal dialysis (PD), and the tinely instructed that there are some uncontroversial, standard “acute
various blood-based modalities of continuous renal replacement therapy indications for hemodialysis” (see Table 98-1).
(CRRT). CRRT modalities include continuous venovenous hemodialysis These established “indications,” however, are severely limited. Firstly,
(CVVHD), continuous venovenous hemofiltration (CVVH), and com- they are reactive in nature, as they aim to avert potentially life-
bination therapies, that is continuous hemodiafiltration (CVVHDF). threatening complications of renal dysfunction as they become clinically
The advances in technology with readily available large bore temporary problematic. Secondly, while some of the indications are objective and
and tunneled venous catheters and blood pumps have made the use readily apparent (ie, hyperkalemia with ECG changes or pulmonary
of arteriovenous circuits, in the form of continuous arteriovenous edema requiring mechanical ventilatory support), others are potentially
hemofiltration/hemodialysis (CAVH/CAVHD) essentially obsolete. subjective and nonspecific (the clinical diagnosis of uremia).
The general principles underlying these various modalities remain In recent years, many clinicians have opted to initiate RRT earlier
the same: Solutes and water move across a semipermeable membrane in the evolving course of AKI, attempting to be more proactive and
and are ultimately removed from the body. The process by which solute
and water transfer occur differs based on the modality of RRT. Dialysis
operates on the principle of diffusion, that is solutes move across a semi- TABLE 98-1 Common Indications for RRT initiation
permeable membrane down their concentration gradient (moving from Severe acidemia (pH <7.1) secondary to metabolic acidosis refractory to medical care
higher concentration to lower concentration). This is utilized in modern
hemodialysis techniques with blood flowing adjacent to a dialysate Severe hyperkalemia (K >6.5 mmol/L) or rapidly rising K refractory to medical care
solution separated by a biocompatible filtering membrane. To maximize Ingestion of dialyzable toxins
the concentration gradient between the blood and dialysate space, the Volume overload with pulmonary edema refractory to medical care
dialysate flow is countercurrent to the flow of blood. Diffusion-based
clearance of solutes remains limited by the principles governing all Uremic complications of renal dysfunction

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