Continuous Veno-Venous Hemofiltration, or CVVH, is a specialized form of kidney support used for critically ill patients. It is a type of continuous renal replacement therapy (CRRT) that is performed at the patient’s bedside within an Intensive Care Unit (ICU). This treatment acts as a temporary, artificial kidney, gently and continuously cleaning the blood of waste products and excess fluid when a person’s own kidneys are unable to do so. It is designed as a supportive measure, providing the body with the balance it needs while the kidneys are given time to heal.
Why CVVH Is Used
The primary reason a patient requires CVVH is the development of Acute Kidney Injury (AKI), a condition where the kidneys suddenly lose their ability to function. This can happen in up to 70% of patients admitted to an ICU. AKI is a complication of other severe medical issues, such as major trauma, heart surgery, or widespread infection (sepsis). These conditions can compromise the body’s stability and reduce blood flow to the kidneys, causing them to fail.
Another reason for using CVVH is to manage severe fluid overload. Critically ill patients may receive large volumes of intravenous fluids and medications, but if their kidneys cannot produce urine, this fluid accumulates in the body. This excess fluid can strain the heart and lungs. CVVH is employed for patients who are hemodynamically unstable, meaning their blood pressure and heart rate are fragile and cannot withstand more conventional treatments.
The gentle nature of CVVH makes it the preferred choice for these unstable patients. Conditions like sepsis can lead to a state of shock where blood pressure is dangerously low and the kidneys are not receiving enough blood. The continuous process of CVVH helps maintain stability, removing fluid and waste without causing abrupt shifts that could compromise the patient’s condition. This makes it a suitable intervention when other forms of renal support are too aggressive.
The CVVH Process Explained
The process begins with the placement of a specialized intravenous line called a central venous catheter. This catheter, often referred to as a vascath, is inserted into a large vein to access the patient’s circulatory system. The catheter has two separate channels: one to draw blood out of the body and another to return the cleaned blood. This setup creates a closed loop known as an extracorporeal circuit, managed by the CVVH machine.
Blood is slowly pumped from the patient to a filter called a hemofilter, which acts as the artificial kidney. Inside the filter, a semipermeable membrane separates waste products and excess water from blood cells and proteins. This removal occurs through a process called convection, where the movement of water across the membrane pulls dissolved waste solutes with it.
To maintain the body’s electrolyte balance, a sterile solution known as replacement fluid is administered into the circuit before the blood is returned to the patient. The composition and rate of this fluid are precisely controlled by the CVVH machine based on the nephrologist’s prescription. To prevent the blood from clotting as it passes through the tubing and filter, an anticoagulant medication is infused into the circuit.
CVVH Versus Traditional Hemodialysis
The main difference between CVVH and traditional hemodialysis is the timing and intensity. Traditional hemodialysis is an intermittent therapy, performed over three to four hours, a few times per week. During these sessions, it removes a large volume of fluid and waste products at a high speed. This rapid removal can cause complications, such as a sudden drop in blood pressure, which is risky for critically ill individuals.
CVVH, in contrast, is a continuous therapy that operates 24 hours a day. Its rate of fluid and solute removal is much slower, mimicking the constant function of healthy kidneys. This steady process avoids the abrupt fluid shifts associated with intermittent hemodialysis.
This continuous nature makes CVVH the preferred method for managing AKI in the ICU, especially for patients with low blood pressure. While traditional hemodialysis is effective for stable patients with chronic kidney failure, the gentle approach of CVVH provides a supportive bridge for critically ill patients. It allows for careful management of their fluid and electrolyte balance until their kidneys recover or they become stable.
What to Expect During Treatment
For family members, the ICU environment can seem overwhelming. The patient will be connected to the CVVH machine, which has a screen displaying pressures, flow rates, and other data. Multiple plastic tubes will be visible, connecting the patient’s catheter to the machine and carrying blood to and from the hemofilter.
A dedicated ICU nurse, who has received special training in the therapy, will closely monitor both the patient and the machine. This nurse is responsible for managing the system, troubleshooting any issues, and ensuring the treatment is running safely. Patients are often sedated to ensure their comfort and to prevent movement that could dislodge the central venous catheter.
The CVVH machine is equipped with alarms designed to alert the nursing staff to any changes or potential problems within the circuit. These alarms are a normal part of the monitoring process. They might sound due to a change in pressure, a potential clot forming in the filter, or an issue with blood flow. The nurse will promptly address these alarms to maintain the system’s integrity and ensure patient safety.