Renal Replacement Therapy	45
Sunil Prakash and Arghya Majumdar

A 50-year-old diabetic and hypertensive male patient was admitted with acute pancreatitis requiring ventilatory support. Despite aggressive volume resuscitation, he had a mean arterial pressure (MAP) of 60 mmHg on multiple vasopressors. An echo showed global hypokinesia. His intra-abdominal pressure was 20 mmHg. He was catheterized but had 100 mL of urine output in the past 12 h. Serum urea was 150 mg/dL, creatinine was 3.5 mg/dL, and potassium was 6.5 mEq/L.

Acute kidney injury is a common occurrence in the ICU and often requires renal replacement therapy (RRT). ICU physicians should be aware of the different modalities of renal replacement therapy (RRT) with their advantages and disadvantages.

Step 1: Initiate resuscitation and decide on RRT

•Along with resuscitation measures with ventilatory and hemodynamic support, early RRT should be considered in patients with acute kidney injury.

•Optimal timing of starting RRT remains controversial, and a joint decision between the nephrologist and the intensivist should be taken.

•The usual indications of commencing RRT are the following:

–Volume overload/pulmonary edema

–Refractory hyperkalemia (>6.5 mEq/L)

–Severe metabolic acidosis (pH <7.1)

–Anuria

S. Prakash, M.D., D.M. (*)

Department of Nephrology, Artemis Health Institute, Gurgaon, India e-mail: [e-mail]

A. Majumdar, M.D., M.R.C.P.

Department of Nephrology, AMRI Hospitals, Kolkata, India

R. Chawla and S. Todi (eds.), ICU Protocols: A stepwise approach,	361
DOI 10.1007/978-81-322-0535-7_45, © Springer India 2012

362	S. Prakash and A. Majumdar

–Uremic encephalopathy

–Uremic pericarditis

Step 2: Decide on appropriate modality of RRT (Table 45.1)

•Continuous RRT (CRRT)—this modality of RRT may be preferable in the following situations:

–Hemodynamically instability patients on multiple vasopressor therapy

–Unable to maintain MAP of more than 70 mmHg

–Need of large volume infusions (e.g., total parenteral nutrition [TPN])

–Raised intracranial pressure (ICP)

•Sustained low efficiency dialysis (SLED) may be preferable in some situations:

–If the patient is able to maintain MAP of more than 70 mmHg on low-dose vasopressors, this may be a reasonable option.

•Intermittent hemodialysis (IHD):

–If the patient is hemodynamically stable

–No significant volume overload

Table 45.1 Different modalities of RRT
Modality	Mechanism	Methodology
Hemodialysis (HD)	Diffusion	Here, the solute passively diffuses down its
		concentration gradient from one fluid
		compartment (either blood or dialysate) into
		the other. The dialysate is made to flow in a
		direction which is opposite to blood flow
		(countercurrent flow) through the hollow
		fiber dialyzer, to maintain a continuous
		concentration gradient between the two
		compartments and therefore maximize solute
		removal. Diffusion-based dialysis mostly
		removes small molecular weight solutes of
		less than 1 kD (kilodalton)
Hemofiltration (HF)	Convection	Hydrostatic pressure gradient is used to induce
		the filtration (or convection) of plasma water
		across the membrane of the hemofilter. The
		frictional forces between water and solutes
		(called “solvent drag”) result in the convective
		transport of small and middle molecular weight
		solutes (less than 5,000 D) in the same
		direction as water
Hemodiafiltration	Diffusion and	This modality offers the maximum solute
(HDF)	convection	removal as it combines convection with
		diffusion for achieving this
Ultrafiltration (SCUF)	Hydrostatic pressure	Slow continuous removal of fluid alone, by
		steady hydrostatic pressure
Peritoneal dialysis	Diffusion, convection,	Solute removal is accomplished by diffusion,
(PD)	and osmosis	and most of the ultrafiltration is by osmosis

The extracorporeal blood purification procedures are usually performed by a veno-venous circuit, and the modalities are accordingly referred to as CVVHD, CVVH, or CVVHDF

45 Renal Replacement Therapy	363

Fig. 45.1 Algorithm for choosing modalities of RRT

•Besides the medical indications, the selection of a particular modality of RRT is based on infrastructure, available resources, affordability, availability of appropriate fluids, hemofilters, and preference of the physician.

Step 3: Understand different modalities of RRT (Table 45.1)

1.CRRT (Figs. 45.1, 45.2, and 45.3 and Tables 45.2 and 45.3)

•CRRT more closely mimics normal kidney function, by gradually processing the blood and slowly removing excess fluid, uremic toxins, and electrolytes, 24 h a day and thereby improving hemodynamic stability.

•CRRT can provide up to 24–30 L of fluid exchange each day compared to 3–6 L per dialysis session for IHD. This greater fluid elimination can prevent fluid overload.

•CRRT improves the nutritional status of critically ill patients by allowing infusion of necessary volume of parenteral nutrition (2–3 L).

•It is the preferred therapy in septic shock.

•CRRT is gentler than IHD as electrolyte concentrations are slowly and continuously corrected, thereby preventing osmotic shift and variations in intracranial pressure.

•However, CRRT has failed to show unequivocal survival advantage though it may portend a better renal recovery.

•Maintain adequate anticoagulation during CRRT:

–Normally, 1,000–2,000 units of heparin are given as a bolus followed by a continuous infusion of 300–500 units per hour. Therapy is monitored every 6 h with the aim of maintaining the APTT 1.5–2 times control.

–Saline infusions sometimes suffice if the patient has already a bleeding diathesis.

364	S. Prakash and A. Majumdar

Fig. 45.2 CVVHDF

Indications of RRT

	Volume			Rising urea,	Uremic
		Acidosis	Hyperkalemia		symptoms
				creatinine
	overload				(encephalopathy)

Hemodynamically	Mildhemodynamic	Hemodynamically
Unstable	instability	Stable

CRRT	SLED	IHD

Fig. 45.3 The dialysis circuit

45 Renal Replacement Therapy		365
Table 45.2 CRRT advantages
Advantage	Methodology
Hemodynamic stability	Avoids hypotension, which is seen in ultrafiltration
	Avoids major swings in intravascular volume
	Allows slow and continuous tissue refilling
	Maintains steady cardiac filling pressures
	Avoids swings in intracranial pressures (beneficial in patients
	with raised ICP)
Easy to replenish and	Ultrafiltration is continuous and gentle
regulate fluid volume	Can adjust ultrafiltration rate according to hourly MAP status
	Can vary ultrafiltrate according to hourly variation in rate of
	infusates
	Can accurately adjust it to the intravascular blood volume or
	stroke volume variation when such monitors are being used
Customize replacement	According to the metabolic parameters, replacement fluid
solutions	composition may be altered like high lactate, calcium, high/low
	potassium, high/low sodium

CVVHD will probably be more effective than CVVH in the highly catabolic patient with a large solute load. CVVHDF with its convective removal of larger solutes is preferred in the patient with septic shock in whom the removal of inflammatory mediators is desirable. CVVHDF combines the convective solute removal of CVVH with the diffusive solute removal of CVVHD

Table 45.3 CRRT disadvantages

Lack of rapid solute and fluid removal

Glomerular filtration rate equivalent of 15–20 mL/min

Limited role in drug overdose setting

During filter clotting, entire system shuts down and the patient loses a lot of blood

Necessitates continuous anticoagulation

Limits mobility for various investigations

Requirement of ultrapure fluids and high-flux dialyzers

–Citrate anticoagulation may be used with custom-made, calcium-free dialysate. Frequent calcium monitoring and calcium infusion may be required.

–Bivalirudin and argatroban may be considered as an anticoagulant in cases of heparin-induced thrombocytopenia requiring RRT.

–Regional anticoagulation can be achieved with heparin and protamine.

–Prostacyclin infusions are an option, but may cause hypotension.

•Continuous RRT must be provided with an effluent flow rate (the sum of hemofiltration rate and dialysate flow rate) of at least 20 mL/kg/h.

•There is improved survival at effluent flow rates of 35 mL/kg/h but not much with 45 mL/kg/h (57% and 58%, respectively) as compared to an effluent flow rate of 20 mL/kg/h (41%) in patients with septic shock.

2.SLED

•SLED is not a continuous therapy and achieves lower solute clearances that are maintained for longer period.