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Continuous renal replacement therapy: Dialysis for critically ill patients


Critically ill patients have much higher metabolic rates than normal as their bodies attempt to recover from disease. Many are septic and hypotensive. They require vasoactive drugs and continuous waste clearance while simultaneously receiving large volumes of fluid, such as drug infusions and nutritional and inotropic agents to treat hypotension. Quick removal of large amounts of plasma water, as occurs during intermittent hemodialysis (IHD), isn’t indicated because it worsens hypotension.

Instead, hypotensive critically ill patients need a renal replacement therapy that removes wastes and water gently while causing little or no hypotension. The therapy most commonly used is continuous renal replacement therapy (CRRT). In this slow form of hemodialysis, the patient’s blood is removed and pumped through a hemofilter, which resembles a dialyzer. CRRT helps prevent the hemodynamic fluctuations common with the more rapid IHD. Also, the CRRT hemofilter contains numerous hollow fibers through which blood is pumped; these pores allow removal of both smaller and larger waste products, electrolytes, and other substances (such as pro- and anti-inflammatory cytokines), depending on the therapy prescribed and molecular size of the substance. Although the hemo­filter membrane can remove these substances, its membrane pores are too small to remove the larger molecules of blood cells and proteins.

CRRT is an effective alternative renal replacement therapy for removal of wastes and excess plasma water in critically ill patients—especially those who are hypotensive and can’t tolerate the rapid fluid and electrolyte shifts of hemodialysis. It provides continuous control of fluid status, maintains hemodynamic stability, provides protein-rich nutrition while achieving uremic control, maintains electrolyte balance, and helps prevent intracerebral water changes. It slowly removes plasma water (ultrafiltrate) through ultrafiltration, achieved by generating a transmembrane pressure in the hemofilter that exceeds plasma oncotic pressure. The pressure differential leads to water removal from the blood. (See Indications for CRRT by clicking the PDF icon above.)

Types of CRRT

The four types of CRRT therapy are continuous venovenous hemodialysis, continuous venovenous hemo­filtration, continuous venovenous hemo­diafiltration, and slow continuous ultrafiltration. (For information on these modalities, click the PDF icon above)

Obtaining access

A temporary dialysis catheter is inserted for removal and return of the patient’s blood. The catheter is placed in a large vein—usually the subclavian, femoral, or internal jugular vein. The internal jugular vein is preferred because it permits access to blood near the right atrium and is least likely to cause infection. (With a subclavian catheter, subclavian stenosis may occur.) A temporary dialysis catheter is considered a central line, so strict sterile technique must be used during catheter access or manipulation. (See CRRT color-coding by clicking the PDF icon above.)

A properly functioning catheter is essential for efficient CRRT. If blood can’t be removed at an adequate flow rate, therapy won’t be effective. Typical blood flow rates in CRRT range from 150 to 250 mL/min—much slower than the typical IHD rates of 300 to 400 mL/min. The, lower rates are adequate because CRRT runs 24 hours/day for several days instead of just a few hours, as with IHD.

Maintaining fluid balance during CRRT

The nephrologist specifies the ultrafiltrate volume to be removed hourly, based on the patient’s blood pressure, daily fluid intake, fluid output (if any), and additional fluid the patient has gained. The ordered removal volume, which may range from none to 100 mL or more each hour, is programmed into the CRRT machine.

Clinicians should assess the effects of fluid removal every hour. The total daily amount of fluid removed may exceed 75 mL/hour to bring the patient closer to his or her normal (“dry”) weight. How­ever, clinical status must be considered when determining hourly fluid volume removal. A markedly hypo­tensive patient may not tolerate removal of 75 mL/hour. So even if the order calls for removal of 75 mL/hour, plasma water may need to be removed in smaller amounts initially, such as 25 to 50 mL/, followed by assessment of patient tolerance. This amount, along with the previous hour’s intake, is programmed into the CRRT machine as a net loss after calculation of the amount based on fluid balance. (See Calculating fluid removal during CRRT by clicking the PDF icon above.)

Most patients have some degree of fluid overload during CRRT; many are significantly above their “dry” weight due to inability to handle the large daily fluid volumes infused (such as parenteral nutrition, blood products, antibiotics, and maintenance fluids). Also, many critically ill patients can’t mobilize fluids effectively because of intravascular fluid shifts and inadequate renal function.

Urine output during CRRT

In a normal healthy person, urine output ranges from about 0.5 to 2.0 mL/kg/hour, depending on fluid intake; this includes insensible losses. But critically ill patients may be receiving larger fluid amounts. So the amount of fluid to be removed over 24 hours could be much greater than in a healthy person.

Over a 24-hour period, CRRT should remove the total amount of fluid the patient receives over the course of a day. So for a patient receiving a total of 1,500 mL of fluid per day, fluid should be removed at a rate of about 62 mL/hour. Keep in mind that to prevent fluid overload during CRRT, the extra fluid administered must be considered additional fluid intake. To calculate hourly urine output, multiply the patient’s weight by 0.5 mL/kg.

Preventing clotting in the circuit

Clotting in the circuit is a common complication of CRRT. Several methods can be used to keep the circuit patent. Heparin infusion is inexpensive and simple, although it’s not always as efficient as other anticoagulation methods. (With any type of CRRT, the goal isn’t to anticoagulate the patient but to prevent bleeding and avoid clotting of the circuit.) Heparin is infused on a continuous basis into the arterial side of the CRRT circuit (called the prefilter) immediately before blood enters the hemofilter. Target coagulation studies should be done to help maintain activated clotting time at 140 to 180 sec and activated partial thromboplastin time (aPTT) at 55 to 100 sec, or about twice the normal value.

Argatroban, a thrombin inhibitor, may be used in patients who develop heparin-induced thrombocytopenia. Although highly effective in CRRT anticoagulation, it’s more expensive than heparin. Like heparin, it’s infused into the prefilter. During argatroban administration, monitor coagulation factors regularly; as ordered, keep aPTT at about twice the normal value.

Citrate anticoagulation

Citrate is becoming the preferred CRRT anticoagulant even though its use for this purpose is off-label. It’s infused either prefilter or as a replacement solution. Blood requires calcium to complete the clotting process; citrate binds with calcium in the blood and prevents clotting. Citrate is then metabolized to bicarbonate and water in the liver and skeletal muscle.

Because citrate decreases blood ionized calcium levels, calcium must be given I.V. continuously to replace bound calcium. Citrate administration necessitates careful monitoring of ionized calcium both from the CRRT circuit and the patient. Ionized calcium in the circuit typically is kept between 0.20 and 0.30 mmol/L, which interrupts the coagulation process while blood travels through the circuit. As ordered, increase or decrease the citrate and calcium drips according to the patient’s calcium level, which should be drawn at regular intervals. When CRRT is suspended (as when the patient needs to travel), stop the citrate and calcium infusions to prevent adverse events.

Know that infusing citrate alone without calcium may cause significant bleeding and arrhythmias, and infusing calcium without citrate can cause arrhythmias or cardiac arrest. Also be aware that as each citrate molecule rapidly metabolizes to three molecules of bicarbonate, continuous citrate infusion puts the patient at risk for metabolic alkalosis. Be sure to monitor laboratory values regularly for alkalosis. Using a protocol for citrate administration and titration is helpful. Despite the drawbacks of citrate anticoagulation, studies show it keeps the CRRT circuit free from clotting and reduces bleeding risk.

Nursing management

Provide the following care for a patient receiving CRRT:

  • Weigh the patient daily to assess fluid removal.
  • Remember that anticoagulation during CRRT can lead to bleeding. Monitor for signs and symp­toms of bleeding in the oral mucosa, gastric aspirate, stool, and injection sites. Check coagulation studies regularly.
  • Take steps to prevent hypothermia. During CRRT, about 150 to 250 mL of blood volume remains outside the body in the CRRT circuit. Therefore, blood may become cool and cause the patient’s temperature to drop; monitor temperature regularly. To help prevent hypothermia, manufacturers provide warmers for CRRT solutions or blood warmers to warm the blood as it returns to the patient. Using a warming blanket also helps prevent hypo­thermia.
  • Check blood urea nitrogen and creatinine levels at least daily to assess CRRT efficacy.
  • Routinely monitor the patient’s complete blood counts to check for unintended blood loss in case the CRRT circuit suddenly clots.
  • Check electrolyte levels. (Electrolytes are filtered out in the ultrafiltrate removed by CRRT.) Hypo­phosphatemia is common in patients on CRRT and can be treated with supplements or by adding phosphate to CRRT fluids. (See Nursing assessment during CRRT by clicking the PDF icon above.)

Nurse education

To ensure effective CRRT, nurses who care for patients receiving this therapy should attend classes on this specialized therapy and demonstrate clinical competency on a regular basis. Classes typically include education on how to use the CRRT machine, identify machine alarms, perform troubleshooting, and calculate CRRT fluid balance. Other ways to ensure staff competency include use of preceptors to monitor nurses initially and to serve as mentors.

Click here for a list of selected references.

Susan M. Dirkes is president of Nursing Resource Consultants LLC, in Newport, Michigan.

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