Ventriculoatrial Shunt Placement

Ventriculoatrial shunt (VAS) placement enables cerebrospinal fluid (CSF) to flow from the cerebral ventricular system to the atrium of the heart. This procedure is indicated for hydrocephalus, which is among the most common conditions encountered in neurosurgical practice.

There are multiple potential anatomic configurations for CSF shunts. Most commonly, a catheter is positioned within the cerebral ventricle (the proximal site) and connected to a unidirectional valve that limits flow based on pressure or flow rate. The outflow port of the valve is connected to another length of catheter, which is passed subcutaneously to a space within which CSF can be absorbed (the distal site).

The preferred distal site is the peritoneal space (ventriculoperitoneal shunt [VPS]), in that this space is generally safely accessible and possesses more than adequate absorptive capacity for the CSF volume produced by a given person. In rare cases, the peritoneal space is inadequate, necessitating the use of an alternative distal site. The most common of these alternative sites are the cardiac atrium (VAS[1] ) and the pleural space (ventriculopleural shunt).

The primary consideration in any patient who may undergo VAS placement is whether the patient has symptomatic hydrocephalus that necessitates CSF diversion. If so, the surgeon must rule out the peritoneal space as an acceptable location before selecting the cardiac atrium as the site for the distal catheter. Common reasons for this include previous intra-abdominal infection and scarring due to prior procedures or intra-abdominal pathology. Once the peritoneum is ruled out, the surgeon must verify that the atrium is an acceptable target.

Thrombosis of the feeding jugular or subclavian veins may preclude successful access to the cardiac atrium. Long-term thrombosis or previous instrumentation of these vessels can lead to scarring and permanent occlusion. Doppler ultrasonography (US) can be used to effectively evaluate the patency of these large veins if this is of concern. The presence of other intravascular devices (eg, a peripherally inserted central catheter [PICC] line or indwelling central venous catheter) may complicate placement of the shunt catheter or removal of either catheter.

Other considerations include whether the pleural space might represent an adequate alternative. In children younger than 4 years and in patients with poor pulmonary reserve, the pleura may not provide the necessary CSF-absorbing capacity.

VAS placement is indicated for patients with shunt-dependent hydrocephalus in whom the peritoneum is not an acceptable site for distal catheter placement; it may also be considered for patients who do not improve with VPS placement.[2] Some surgeons may prefer to position the distal portion of the shunt into the pleural space rather than the cardiac atrium.

A retrospective analysis by McGovern et al found that for normal-pressure hydrocephalus, VAS appeared to be at least as safe as the more commonly used VPS.[3]

Contraindications for VAS placement include the following:

• Bacteremia
• CSF infection
• Endocarditis
• History of immune-complex glomerulonephritis (eg, shunt nephritis)
• Prothrombotic state
• Pulmonary hypertension
• Congestive heart failure

Relative contraindications may include a history of pulmonary embolism and systemic anticoagulation.

A retrospective analysis by Rymarczuk et al compared the long-term outcomes and adverse occurrences associated with both VPS and VAS in 544 pediatric patients followed for at least 90 days (VPS, 5.9 y; VAS, 5.3 y).[4] Overall survival was superior for VPSs, but if electively scheduled VAS-lengthening procedures were not considered true failures, no significant survival difference was noted. VPSs demonstrated significantly greater survival in patients younger than 7 years but not in older children. VASs had significantly lower rates of infection and proximal failure.

Gmeiner et al retrospectively analyzed long-term outcomes of VAS surgery in 61 pediatric patients with hydrocephalus.[5] Primary VAS was the first shunt type in 68.85% of patients; conversions to second-line VAS were carried out the remaining 31.15%. The rates of VAS revision for dysfunction or elective lengthening of a short atrial catheter were 52.2% and 22.9%, respectively. There was no difference in the number of revisions between patients with primary VASs and those with second-line VASs. Specific VAS complications were rarely observed and completely reversible after treatment.

A C-arm fluoroscopy unit is required to verify atrial catheter position.

If the Seldinger technique is used for vascular access, an appropriately sized vascular access kit is needed. The standard 2.4-mm shunt catheter will fit down a 10-French peelaway sheath.

Some surgeons recommend using an open-ended distal shunt catheter to allow for the positioning of a guide wire through the catheter during future replacement or revision. It is also recommended that the distal catheter have a radiopaque tip to aid visualization.

Portable ultrasonography (US) may be useful in identifying access vessel location.

Some authors promote the use of intraoperative transesophageal echocardiography (TEE).[6]

Anesthesia

As with other cerebrospinal fluid (CSF) shunt operations, general anesthesia is administered. The anesthetist must assume that intracranial pressure (ICP) may be elevated. Rapid-sequence intubation should be considered in patients with a recent history of nausea or vomiting. Situations that might increase the risk of elevated ICP, such as premedication with anxiolytics that may result in hypoventilation, should prompt caution.

Anesthetic considerations specific to ventriculoatrial shunts (VASs) include the increased risk of cardiac arrhythmias associated with placement of a wire or catheter into the cardiac atrium and the risk of venous air embolism that may occur if spontaneous ventilation is used.

All shunt operations carry with them a significant risk of surgical infection and the subsequent need for complicated treatment of such infections. Preoperative antibiotics, diligent adherence to aseptic technique, and other measures to minimize the risk of infection are essential.

Positioning

VASs are not commonly chosen as an initial CSF-shunting procedure. Accordingly, patient positioning is usually influenced by the location (side and cranial entry site) of the existing shunt system. In the case of initial shunt placement, the surgeon’s selection of the most favorable site for ventricular catheter placement will dictate the patient’s position.

In all cases, the patient is placed supine on an operating table that will allow anteroposterior (AP) fluoroscopy with minimal artifact. The shoulders should be appropriately bolstered to facilitate the subcutaneous passage of the shunt catheter and access to the prospective feeding vein. The surgeon should be prepared to access a second vein in the event that difficulty is encountered with the initial vessel. For example, the ipsilateral subclavicular region would be prepared in addition to the neck.

The authors’ practice is to administer 24 hours of intravenous (IV) vancomycin to all patients who undergo surgery for a VAS. Pain is controlled with alternating acetaminophen and ibuprofen, ketorolac, or low doses of opiates for breakthrough pain.

A radiographic shunt series consisting of AP and lateral skull radiographs and an AP view of the chest is obtained in the recovery room. (See the images below.) Postoperative axial imaging, usually rapid T2-weighted magnetic resonance imaging (MRI) of the brain, is obtained between 1 and 4 weeks after surgery, depending on the surgeon's preference.

Ventriculoatrial shunt placement: Anteroposterior radiograph.

Ventriculoatrial shunt placement: Lateral radiograph.

In growing children, annual AP chest radiography is recommended to assess the level of the atrial catheter tip. If the tip is positioned above T4, revision or conversion to another distal site is recommended.

VASs can become secondarily infected from transient bacteremia. Antibiotic prophylaxis should be considered for dental and other invasive procedures.

Before preparing the field, perform a trial fluoroscopy to be certain that the appropriate anatomic landmarks are easily visualized.

In order to prevent backflow of blood and subsequent thrombosis and catheter occlusion, flush the distal catheter with heparinized saline immediately before connecting it to the proximal aspect of the system, and occlude the catheter until it is connected. Place the distal tip of the atrial catheter in the middle to lower atrium, approximately at the level of T6.

Administer antibiotics preoperatively, and adhere to sterile technique to minimize the risk of infection.[7]

Placement of the ventricular catheter does not differ significantly from that in other cerebrospinal fluid (CSF) shunt operations. In other shunt cases, the authors choose to place the ventricular catheter as one of the final steps in the procedure in order to minimize the risk of dislodgment. However, in the case of a ventriculoatrial shunt (VAS), the authors place the ventricular catheter and valve before accessing the distal vessel in order to minimize the risk of distal catheter malposition or backflow.

If the ventricular catheter is placed frontally, a length of distal shunt tubing is secured to the outflow port of the unidirectional shunt valve. A 1-cm vertical retroauricular incision is made, and the tubing is pulled subcutaneously out through this incision. A hemostat or temporary suture is placed to prevent CSF egress. If the ventricular catheter is placed through an occipital burr hole, the distal shunt tubing is tunneled directly to the proposed site of entry for vascular access.

A percutaneous (Seldinger) or open vascular cutdown technique may be used to access the appropriate vessel for atrial catheter positioning. The authors prefer a percutaneous technique, which is similar to that used for the placement of central venous catheters. Ultrasonographic (US) guidance may be used to facilitate access.[8]

The internal jugular vein (IJV) is the most commonly selected access vessel. An appropriately sized (often 20- or 22-gauge) needle is used to puncture the skin one to three fingerbreadths (depending on the size of the patient) above the clavicle, between the heads of the sternocleidomastoid muscle. If subclavian vein access is chosen, the entry site is inferior to the clavicle, at the junction of the middle and lateral third of the bone.

After successful cannulation of the vessel, a flexible guide wire is passed through the needle, and the tip is positioned in the superior vena cava (SVC) or the cardiac atrium under fluoroscopic guidance. Instability on electrocardiography (ECG) may indicate that the guide wire is positioned within the heart.

The needle is then removed, and a nick incision is made to facilitate dilator entry. The authors prefer to load the peelaway sheath on the dilator and pass them together into the vessel. Occasional advancing and retracting of the guide wire ensures that the dilator is following the wire’s subcutaneous course.

The dilator and guide wire are then removed, and the shunt tubing is passed down the peelaway sheath. Brisk back-bleeding is often encountered upon removal of the dilator. The authors find it most effective to pass the distal shunt well beyond the lower atrium before splitting and removing the peelaway sheath; it is not always possible to advance the shunt tubing once the sheath is removed. The catheter is then pulled back to an appropriate final position (as described in more detail below), flushed with heparinized saline, and connected to the proximal shunt system.

An open approach can also be used to gain access to the vascular system. A cutdown is made to the IJV, where a purse-string suture is placed on the anterior wall of the vein. A stab incision is made inside the purse-string, and the catheter is placed directly into the vein. The suture is then tied down around the catheter to stop any back-bleeding.

A third technique is to isolate the transverse facial vein as it enters the IJV. This vein can then be divided and the shunt passed down into the IJV. A tie is used to secure the catheter to the transverse facial vein and to prevent back-bleeding.

The position of the distal catheter tip is verified by means of fluoroscopy. The tip of the distal catheter is optimally positioned in the middle to lower atrium, at approximately the T6 vertebral body level. If the tip is not clearly visualized, injection of a small volume of radiopaque contrast material may facilitate fluoroscopic visualization. Other techniques, including recording pressure waves through the catheter[9] or using the catheter as an ECG lead,[10] may also be used.

Once satisfactory positioning of the atrial catheter has been achieved, the proximal and distal catheters are trimmed in such a way as to allow a small amount of redundancy. Excellent flow of CSF through the proximal catheter system is verified, and the distal catheter is flushed with heparinized saline and connected to the proximal catheter via a straight connector. The incisions are thoroughly irrigated with antibiotic-impregnated saline and closed in layers.

Complications of VAS placement may include the following:

• Shunt-induced immune-complex glomerulonephritis - Chronic shunt infection with Staphylococcus epidermidis may result in immune complex deposition at the renal glomerulus, causing severe glomerulonephritis; this injury generally responds immediately to externalization of the shunt and appropriate antibiotic therapy
• Pulmonary hypertension due to microemboli - A catheter within the cardiac atrium may casue chronic microembolization and thus result in pulmonary hypertension
• Endocarditis
• Wound/CSF shunt infection ^[7] - CSF shunt infection rates vary but are commonly reported to be in the approximate range of 5-8%; a CSF shunt infection warrants removal of the shunt and placement of an external ventricular drain during antibiotic treatment of the infection
• Outgrowing the shunt - As a child with a VAS grows, the shunt will slowly back out of the atrium, a process that potentially leads to shunt malfunction
• ECG instability
• Shunt malfunction - All shunts are prone to malfunction, at a rate of 40% in the first 2 years after insertion or revision; patients and their families should be aware of this and take appropriate action should it occur
• Delayed intracranial hemorrhage ^[11]

SVC thrombosis is a rare complication of VAS placement.[12]  Intramuscular migration of the venous catheter is also quite rare but has been reported.[13]  Migration of the distal catheter into the pulmonary arteries has been observed.[14]  A migrated catheter may be managed via an open or an endovascular approach.[15]