An External Ventricular Drain (EVD) is a temporary neurosurgical device used primarily in intensive care. This system involves a small, flexible tube inserted directly into the brain’s fluid-filled spaces, known as the ventricles. The EVD manages pressure inside the head by controlling the volume of cerebrospinal fluid (CSF), which protects brain tissue from excessive pressure.
Why External Ventricular Drains Are Used
The EVD serves two primary purposes: therapeutic fluid drainage and diagnostic pressure monitoring. The skull is a rigid compartment holding brain tissue, blood, and cerebrospinal fluid (CSF) in balance. If this balance is disrupted by injury or illness, the pressure inside the skull (ICP) can increase rapidly. ICP greater than 15 to 25 mmHg is dangerous because it impedes blood flow and causes tissue damage.
Therapeutically, the EVD treats acute hydrocephalus, which is the buildup of CSF caused by blockages after conditions like hemorrhage or infection. The drain actively diverts excess fluid away from the brain, providing immediate pressure relief.
The second use is continuous ICP measurement, required for patients with severe traumatic brain injury or conditions causing brain swelling. Since the EVD is placed directly into the ventricular system, it provides real-time pressure data, allowing the medical team to adjust the treatment plan quickly.
The Mechanics of EVD Placement and Operation
The EVD system consists of three main components: the ventricular catheter, the external drainage collection system, and a pressure monitoring transducer. The catheter is a thin tube, often treated with antibiotics, inserted into one of the brain’s lateral ventricles. Placement is typically performed by a neurosurgeon or trained specialist, sometimes at the patient’s bedside or in an operating room.
To insert the catheter, a small incision is made in the scalp, and a hole is drilled through the skull, often at Kocher’s point. The catheter is carefully advanced into the ventricle, confirmed by the flow of CSF. Once secured, the catheter connects to a closed, sterile external collection system.
The system drains CSF using gravity, controlled by the collection chamber’s height relative to the patient’s head. This height acts as a pressure threshold. If brain pressure exceeds this set height, CSF flows out; otherwise, it remains in the ventricles. The medical team adjusts this setting to regulate the amount and rate of drainage.
Essential Monitoring and Management
Safe EVD operation requires constant attention from the critical care team. A primary management step is maintaining the correct zero reference point for the drainage height and pressure transducer. This point is often aligned with the patient’s external auditory meatus (tragus of the ear), which approximates the level of the foramen of Monro within the brain.
The EVD system must be re-leveled every time the patient’s head or bed position changes to ensure accurate pressure readings and drainage thresholds. Failure to temporarily clamp and re-level the drain when moving the patient risks over- or under-drainage of CSF.
Medical staff continually monitor the volume of CSF drained, typically aiming for less than 20 mL per hour. ICP readings are continuously observed, and the transducer is often clamped briefly to obtain a true ICP measurement and waveform analysis. This waveform provides insight into the brain’s compliance. Strict adherence to sterile technique is necessary during all interactions to prevent bacteria from entering the brain.
Associated Risks and Complications
Despite being a necessary intervention, EVD placement carries several recognized risks and complications. The most serious risk is ventriculitis, an infection of the ventricles and cerebrospinal fluid. Infection risk increases with the duration the catheter remains in place or if there is a CSF leak around the insertion site.
Another complication is hemorrhage, or bleeding, which can occur during catheter insertion. Although usually minor, severe bleeding may require further neurosurgical intervention. Mechanical malfunctions are also possible, such as occlusion or blockage of the catheter by blood clots or tissue debris.
Blockage prevents the EVD from draining fluid or monitoring pressure, leading to a sudden rise in ICP. Conversely, incorrect height settings can cause overdrainage, potentially collapsing the ventricles and resulting in a subdural hematoma.