Continuous kidney replacement therapy
What is continuous renal replacement therapy?
The function of normal kidneys is to filter out waste materials and fluid from our blood. Suppose there is an insult to the kidneys over a long period of time or even an acute injury. In that case, our body is not able to filter our blood, and the wastes accumulate, indicating kidney failure. Continuous renal replacement therapy is a procedure where the function of the normal kidneys is carried out artificially with the help of machines. During this procedure, your blood is drawn from your body through an I/V line, passed through a filter that removes the waste products, and then returned to your body through another I/V line. There are other procedures available to carry out this function, but continuous renal replacement therapy is commonly used if you have had an acute kidney injury and you are hemodynamically unstable. Hemodynamic instability is a condition where your body is unable to maintain normal blood pressure leading to an inadequate blood flow to organs. Continuous renal replacement therapy performs at a slower pace and over a longer time as compared to other procedures, therefore, giving your body enough time to adapt to it and also maintaining the fluid-electrolyte balance of the body. It is the modality of choice in critically ill patients with kidney insult.
When is continuous kidney replacement therapy commonly used?
Following are the conditions where CRRT is preferable:
- Severe uremia (presence of waste material- urea in the blood) or acute kidney injury
- Severe volume overload
- Sepsis or severe infection
- Hemodynamic instability or hypotension
- Cardiac dysfunction
- Inborn error of metabolism
- Intoxication like drug overdose
- Any condition that makes normal hemodialysis very difficult
- If you have an increased intracranial pressure
- Major burns with compromised renal function
Your healthcare team will strictly monitor your vitals and ensure fluid and waste removal during the procedure.
Is continuous renal replacement therapy the same as dialysis?
- CRRT is roughly considered a type of dialysis. It is different from conventional dialysis as dialysis is carried out for 3-4 hours a day intermittently, while CRRT runs 24 hours a day. CRRT is a preferred modality in specific conditions where dialysis may not be safe, as discussed above. However, both dialysis and CRRT are forms of renal replacement therapies.
- Compared with dialysis, continuous renal replacement therapy allows for gradual waste removal that approaches the homeostasis of normal kidney function.
What are the differences between haemodialysis and CRRT?
- CRRT is carried out at a slower speed than hemodialysis. So, it puts less stress on your body.
- CRRT procedures involve more compatible biomembranes compared to haemodialysis. This decreases the chances of infections and inflammatory reactions.
- Because it is carried out at a slower pace, CRRT maintains your haemodynamic stability and also maintains your nutritional status better than haemodialysis
- In haemodialysis, the everyday work of your kidney is carried out artificially over a short period of time. This may lead to decreased blood pressure or hypotension. Chances of hypotension are lower with CRRT.
- In hemodialysis, only the dialysate fluid is used. Both dialysate and replacement fluids are used in CRRT
- The typical net fluid removal rate is 0-1000 ml/hr for haemodialysis, while it is 0-200 ml/hr for CRRT.
Types of CRRT?
CRRT is an umbrella term for four continuous renal modalities — SCUF, CVVH, CVVHD, and CVVHDF.
- SCUF: It stands for Slow Continuous Ultrafiltration. It is used exclusively to remove excess fluids in severely hypovolemic patients. Normally, if you have a fluid overload, your doctor may prescribe you a medicine called a diuretic, whose function is to remove the excess fluid from the body. If, however, your body shows resistance to diuretics and the condition persists, SCUF is initiated to remove the excess fluid. The ultrafiltration rate is 2 to 8 ml/min.
- CVVF (Continuous Veno Venous Hemofiltration): This procedure is used in patients with uremia, severe acid/base or electrolyte imbalance to remove larger molecular weight substances. It is also used in cases of diuretic-resistant pulmonary oedema. The ultrafiltration flow rate is 25ml/kg/hour.
- CVVHD (Continuous Veno-Venous Hemodialysis): In CVVHD, a dialysate solution similar to your blood chemistry is passed countercurrent to the blood to encourage the diffusion of solutes across the membrane. Its advantages include controlled fluid removal and slow acid/base imbalance correction.
- CVVHDF (Continuous Veno Venous Hemofiltration): Its advantages include rapid controlled fluid removal, rapid electrolyte and acid-base imbalance correction, and effective clearance of small molecules. It is the most widely used form of dialysis in ICU patients.
What are the advantages of using continuous kidney replacement therapy?
There are reasons continuous kidney replacement therapy is preferred over other procedures in critically ill patients. It has the following advantages:
- It keeps the fluid-electrolyte balance of your body in control. Derangement of the fluid-electrolyte balance in the body is hazardous as it gives rise to an array of life-threatening complications, including arrhythmias.
- It maintains the acid-base status of your body.
- Continuous renal replacement therapy is initiated with a blood flow of 100 ml/min, which is increased gradually, giving your body enough time to adjust to the procedure.
- The chances of infection with this procedure are minimal.
- It maintains the hemodynamic stability of the patient and helps prevent blood loss. Hemodynamically unstable patients may not be able to tolerate rapid ultrafiltration with intermittent hemodialysis.
- It has an excellent capacity for blood purification for different molecules
- CRRT closely mimics the native kidney in treating AKI and fluid overload.
How long can you stay on CRRT?
The duration of your CRRT will be decided by your doctor, depending on your condition. You may need to stay on CRRT until your kidneys regain their lost function of filtering out the excess waste products and water from the blood. To determine how long you need to stay on CRRT, it is mandatory to consider other parameters other than waste clearance, such as your fluid balance, acid-base and electrolyte homeostasis, and nutritional status.
Complications of CRRT
- CRRT removes excess fluid from your body. If the process is not carefully monitored, it can lead to decreased blood pressure or hypotension.
- The electrolyte levels in your blood must be checked at regular intervals if you are on CRRT. Electrolyte imbalances may lead to an irregular heart rhythm.
- Acid-base disturbances may occur.
- Critically ill patients have a very fast metabolism with increased nutritional needs. Additionally, when you are on CRRT, some key nutrients may be lost through the dialysis machine. This can lead to nutritional imbalance and malnutrition. Therefore strict monitoring of the technique and your physical health is needed by your healthcare provider.
- Water-soluble vitamins and minerals are readily filtered across the dialysis membrane. This can lead to their depletion.
- Anaemia may occur due to loss of volume.
- You may experience hypothermia as a result of RRT. This is because the blood stays outside the body and is returned intermittently; in ICU, the outside temperatures are usually chilly.
- Although RRT is a life-saving modality to compensate for the damaged kidneys in case of an emergency, it may delay the recovery of your kidneys.
- Infections may result if the instruments used during the procedure have not been sterilised properly. However, the risk of infections with CRRT is relatively low.