The Basics of Continuous Kidney Replacement Therapy (CKRT)

The Basics of Continuous Kidney Replacement Therapy (CKRT)

When patients are admitted to the intensive care unit (ICU) with acute kidney injury (AKI), clinicians must decide when and if to start them on kidney replacement therapy (KRT), a form of dialysis. Once it becomes clear that dialysis is necessary, the next decision is which type of KRT is most appropriate, based on the patient’s condition.

For the most critical, hemodynamically unstable patients, it may be important that therapy be gentle and administered slowly—in most cases, continuously.1

The therapies in this category—commonly known as continuous kidney replacement therapy (CKRT)—are explained at length in “Basics of CKRT,” an educational webinar from NxStage®. This webinar is dedicated to helping clinicians manage patients with kidney injury, and delivered by Dr. Casey Gashti,* a board-certified nephrologist and internist at Rush University Medical Center (RUMC) in Chicago.

In this article, we present some of Dr. Gashti’s observations regarding CKRT and provide an overview of the therapy.

What is CKRT?

CKRT is a form of kidney replacement therapy usually delivered to critically ill patients over 24 consecutive hours—in many instances, over several days. Treatments are administered using a CKRT machine such as the NxStage System OneTM, a portable device frequently used at the bedside in the intensive care unit (ICU). CKRT removes fluid and toxic solutes slowly and continuously, helping to stabilize the patient without causing shock to their system.

Mechanisms and Modalities Used in CKRT: Diffusion, Convection, and Combination Therapy

Although all fall into the “continuous” category, CKRT modalities can differ based on the methods for clearance.

“There’s diffusion, which would be hemodialysis, or convection, which would be hemofiltration,” explains Dr. Gashti. “Or if we use a combination of diffusion and convection, that would be hemodiafiltration.”

These three methods of clearance are named and defined as follows:

  • Continuous veno-venous hemofiltration (CVVH)  uses convective clearance. Instead of dialysate, the patient’s blood is run through the dialyzer, and, via ultrafiltration, fluids and solutes are “dragged” out of the blood using pressure. Replacement fluid is introduced pre- or post-filtration to dilute the blood and restore electrolytes and glucose before it returns to the patient.
  • Continuous veno-venous hemodialysis (CVVHD) applies diffusive clearance, which passes blood and dialysate solution through a filter with a semi-permeable membrane (the dialyzer) to remove toxins from the blood based on concentration gradient of molecules on either side of the semi-permeable membrane. When flushed out of the dialyzer, the dialysate carries away toxins and fluid. Diffusion is very efficient at filtering out substances with tiny molecules, such as urea, creatinine, and electrolytes (like potassium).
  • Continuous veno-venous hemodiafiltration (CVVHDF)** combines CVVH and CVVHD, using both replacement fluid and dialysate, and combining solute-removing benefits of both diffusion and convection.

According to Dr. Gashti, “In reality, all diffusive clearances have a convective component, and all convective clearances have a diffusive component, although one component is more prevalent.”

It should also be noted that no single form of CKRT treatment has been found more effective than another,3 and the prescribed therapy is often based on the clinician’s or facility’s familiarity and comfort with the treatment.

CKRT Applications in the ICU

Because of the slow, continuous delivery of CKRT, it is considered most beneficial when administered to ICU patients whose condition may be too unstable for intermittent therapies. KDIGO recommends the use of CKRT in:

  • Hemodynamically unstable patients—those whose circulatory system lacks the pressure required to create adequate blood flow to all the parts of the body where it is needed.1
  • Patients with a brain injury or elevated intracranial pressure—those who need the mildest form of fluid removal to keep cranial pressure stable.1

Dr. Gashti provides a more detailed scenario for continuous therapy, as could be prescribed and administered at his hospital.

“To give you an example, our surgeons are very much believers in aggressive volume management when they come out of the operating room,” he describes. “They want to reduce as much stress off the freshly operated-on heart. They really want these patients dried out as much as possible. And so, they want that ability or the option of hourly ultrafiltration adjustment.”

In these instances, CKRT is administered for several days, and once the patient is stable enough, they may be shifted to less frequent therapy—either prolonged intermittent kidney replacement therapy (PIKRT) or intermittent hemodialysis (IHD), depending on their condition.

 “In our ICUs, most CKRT cases would be continuous ultrafiltration in patients with volume overload. And that patient would need to be on a machine at all times,” Dr. Gashti expounds. “It is rare that we need to keep patients on because their blood urea nitrogen (BUN) level is too high, or their potassium is just persistently elevated. Usually, electrolytes are managed within the first 24-48 hours of being on CKRT. What you struggle with, and one of the reasons patients stay on for days, is really volume management. So, I believe that that is probably the leading indicator of a patient remaining on the machine continuously for multiple days.”

Due to these multi-day, continuous treatments, one of the complications that come with CKRT is coagulation (clotting) inside the patient’s catheter or at the venous access site. According to Dr. Gashti’s experience, alleviating this problem can be accomplished using many approaches, including adjusting blood flow, repositioning, changing the type of catheter, or anticoagulation.

Learn More in the “Basics of CKRT” Webinar

When a case requires the gentlest form of kidney replacement therapy, CKRT is considered a safe and efficacious treatment that may help maintain patient stability in many critical circumstances.

To learn more about this topic, watch the complete webinar, “Basics of CKRT,” an educational resource offered by NxStage.

*Paid speaker for NxStage Medical, Inc.

**The NxStage System One does not perform CVVHDF.2


The NxStage® System One™ is indicated for the treatment of acute and chronic renal failure, or fluid overload using hemofiltration, hemodialysis, and/or ultrafiltration, in an acute or chronic care facility. The NxStage System One is also indicated for Therapeutic Plasma Exchange in a clinical environment. All treatments must be administered under a physician’s prescription and must be observed by a trained and qualified person, considered to be competent in the use of this device by the prescribing physician. NxView™ is a computer-based touch screen user interface that provides online instructions for use, summarized system information, and remote access.

NxView is contraindicated as the sole method of monitoring a patient during treatment.2


Kidney replacement therapy, as with any medical therapy, is not without risks. The decision of which therapy to use should be made by the physician, based on previous experience and on the individual facts and circumstances of the patient. There is no literature demonstrating one therapy is clinically better than another.3

  1. Kidney International. KDIGO Clinical Practice Guideline for Acute Kidney Injury. 8-33, 2012.
  2. NxStage System One User Guide, NC4921 Rev. H 2020-01.
  3. Nash et al. Journal of Critical CareVolume 41, October 2017, Pages 138-144.

CAUTION: Federal law restricts this device to sale by or on the order of a physician.

© 2022 Fresenius Medical Care. All Rights Reserved. Fresenius Medical Care, the triangle logo, NxStage, NxView, and System One are trademarks of Fresenius Medical Care Holdings, Inc. or its affiliated companies. All other trademarks are the property of their respective owners. APM4319 Rev A. P/N 104771-01 Rev A 09/2021.

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