The vessel being occluded must be at the proper site and level to simulate the anticipated permanent occlusion. It is important to test-occlude beyond any potential collateral vessels that may still provide flow to the brain during the test, yet may be lost after more distal permanent occlusion. In the carotid circulation, more than half the population has angiographically apparent branches of the proximal intracranial carotid that can be a pathway for collateral flow to the brain, during a test occlusion in the cervical carotid.3 The ophthalmic artery is a significant collateral pathway in many patients and some patients who pass a test occlusion with a balloon proximal to the ophthalmic, may fail when the balloon is placed at the level of the ophthalmic, occluding the collateral flow via that vessel.4 A simple rule of thumb is to perform a test occlusion of a vessel with balloon inflation at the same level as the anticipated permanent occlusion.

The test should reliably predict neurological consequences of the vascular occlusion. Temporary occlusion of a vessel could sufficiently lower the blood flow to an eloquent neuroanatomical region, so that a demonstrable neurological deficit occurs. The situation is simple if the test result is abnormal, and the patient exhibits a neurological deficit during the test: it is very likely that the patient would suffer some haemodynamic ischaemic injury due to permanent occlusion of that vessel. When a neurological change occurs during a test occlusion, it may not always be true that a permanent deficit would occur with permanent occlusion of the artery, thanks to the potential for collateral enlargement (arteriogenesis) after occlusion. However, it is never wise to ignore a test occlusion that produces a deficit. Somewhat more problematic is the situation in which no deficit occurs during the test occlusion. Does this imply that the patient will never have an ischaemic problem from permanent occlusion of the vessel, or is there a potential for false negative test occlusions? Neurological examination during arterial test occlusion is usually combined with additional manoeuvers, when possible, to corroborate the clinical findings. These additional manoeuvers include cerebral blood flow imaging, acetazolamide administration, and pharmacological lowering of blood pressure.

Hypotensive challenge.6–10 Lowering the blood pressure magnifies the haemodynamic effect of vascular occlusion, making it more likely that a neurological deficit will occur in the case of limited collateral flow. When the carotid artery is occluded and no deficit occurs in a normotensive patient, the blood pressure is pharmacologically lowered to a target pressure (e.g. 66% of mean baseline pressure6), or until the patient develops a focal neurological deficit or becomes too nauseated and uncomfortable to allow adequate clinical assessment. Agents that can be used for lowering blood pressure should be fast-acting and quickly reversible, such as nitroprusside or esmolol.

Neuropsychological testing. 9, 11, 12 In addition to simple neurological testing during temporary vessel occlusion, a battery of standardized neuropsychological tests are given to test higher cortical functions.

Electroencephalography (EEG). 13, 14 Continuous EEG monitoring is done throughout the procedure. Slowing or other deviations from baseline conditions can be secondary signs of developing ischaemia.

Somatosensory evoked potentials (SSEP) 15, 16 EEG electrodes are attached and electrical stimulation of a peripheral nerve (usually the median nerve) contralateral to the hemisphere being tested, is performed. Cortical responses are recorded and the timing and amplitude of the response indicates cortical function. Testing is done prior to and following balloon inflation.

Cerebral Oximetry. 17, 18 Commercially available cerebral oximeter, such as INVOS^® (Somanetics, Troy, MI) can be applied to the forehead and allows measurement of frontal lobe oxygenation.

Angiography. 19, 20 Cerebral angiography before and during balloon test occlusion allows qualitative, semiquantitative assessment of brain blood flow and potential collateral circulation to the occluded vascular territory. A posterior communicating artery diameter <1 mm is a risk factor for subsequent stroke with carotid occlusion.21 Similarly, the absence of a functional anterior communicating artery is a risk factor for haemodynamic stroke after carotid occlusion.22 Semi-quantitative assessment consists of looking for synchronous filling of cortical veins with angiography of the contralateral carotid or vertebral during trial occlusion of a carotid, and measuring the difference between hemispheres in the time it takes to achieve venous filling.19, 20 This provides a rough approximation of differences in mean transit time between the hemispheres.

Back-pressure (“stump-pressure”) measurement. 23,24 Blood pressure is measured through the end-hole of the catheter distal to the site of balloon occlusion. The absolute value of the back-pressure or better yet, a ratio of minimum mean back-pressure to mean systemic blood pressure can be recorded. A ratio of 60% or greater is indicates good collateral flow and predicts tolerance to occlusion.25

Transcranial Doppler (TCD) 26–28 Sonographic evaluation of the middle cerebral artery is obtained before and after balloon inflation. Mean blood flow velocity and pulsatility index that do not decrease more than 30% are highly predictive of tolerance to carotid occlusion.27

¹³³ Xenon imaging 29 Radioactive Xenon is administered while the carotid is occluded and the patient’s brain is imaged with a detector, and blood flow data calculated.

Xenon CT 5, 30 Dynamic CT imaging is done as the patient inhales non-radioactive xenon gas. Scans are obtained prior to balloon inflation to determine baseline flow, and the study is repeated during balloon occlusion to determine the effect of occlusion on blood flow. Can also be done with acetazolamide injection during balloon inflation to evaluate for the presence of vascular reserve.

CT perfusion 32 Dynamic CT imaging with a bolus of intravenous contrast, and post-processing can provide blood flow, blood volume and mean-transit time. Scans are obtained without balloon inflation to determine baseline flow, and the study is repeated with balloon inflation to determine the effect of occlusion on blood flow. CT perfusion can also be done with acetazolamide injection during balloon inflation to evaluate cerebrovascular reserve. See the Primer on Imaging in Stroke in Chap.9 for more detail.

Positron emission tomography (PET) 34 Short acting, radioactive tracers such as.¹⁵O H₂O are administered and PET scanning done. Postprocessing allows blood flow calculation. Scans are obtained without balloon inflation to determine baseline flow, and the study is repeated with balloon inflation to determine the effect of occlusion on blood flow. Can also be done with acetazolamide injection during balloon inflation, to evaluate for the presence of vascular reserve.

Single-photon emission computed tomography (SPECT) 36–39 ^99mTechnicium-HMPAO is injected intravenously within 5 min after the balloon is inflated, and the radioactive tracer deposits in the brain in quantities proportional to the regional blood flow. After the test occlusion is completed, SPECT scanning shows activity from the tracer, and asymmetry is detected qualitatively by visual inspection of the scan and by measuring the number of radioactive counts in each region of interest.

Magnetic resonance (MR) perfusion 43,44 Diffusion-weighted scans, perfusion imaging with a bolus of intravenous gadolinium contrast, and post-contrast T1 weighted and FLAIR imaging are performed prior to and following the balloon inflation. The diffusion and post-contrast scans are observed for signs of ischaemia with the balloon inflated. Calculation of cerebral blood volume, mean transit time and blood flow can be done on a computer workstation using the MR perfusion data. See the Primer on Imaging in Stroke in Chap. 9 for more detail.

Computer simulation 45, 46 Proprietary software allows computer modeling of blood flow in the intracranial circulation using data from MR and digital subtraction angiography imaging.

Intracranial haemorrhage

Aneurysm

Arteriovenous malformation

Arteriovenous fistula

Tumours involving a vascular structure

Double lumen balloon catheter. This has a central lumen for the microwire and for pressure measurements, and another lumen for inflating and deflating the balloon. These devices are similar to most angioplasty balloons, but high-­pressure, low-compliance angioplasty balloons are not recommended for test occlusions, as they can traumatize the vessel. Soft balloons such as those on standard occlusion balloons or even Swan-Ganz balloon catheters can be used. The advantage of these balloons is the ability to measure pressures through the distal lumen, and also these balloons are relatively inexpensive. The disadvantage of these balloons is that they do not manoeuver well, are somewhat traumatic to vessels and consequently should not be used in very small vessels or intracranial vessels, although The Ascent™ (Codman Neurovascular, San Jose, CA) is designed to be used in intracranial vessels.

Over-the-wire microballoon. The prototypical balloon in this category is the Hyperform™ (ev3 Neurovascular, Irvine, CA) (see Fig. 11.1). The balloon catheter has single lumen and when the appropriately sized wire is advanced beyond the catheter tip, an O-ring type valve in the balloon catheter seals the tip of the catheter around the wire and allows inflation and deflation of the balloon. These balloons have the advantage that they are soft, atraumatic, and very manoeuverable to almost any destination. The downside is that they have single lumen to inflate the balloon and no way to measure backpressure when the vessel is occluded. These small balloons are advanced through a guide catheter placed in the proximal carotid or vertebral artery, depending on the vessel being tested. Measurement of pressure through the guide catheter will show dampening of the pressure waveform, if the balloon is inflated in the vessel a short distance beyond the guidecatheter tip. The microwire must be advanced through the balloon catheter for at least a short segment distal to the balloon. Thus, it requires a straight segment to place the distal wire and care should be taken to keep the wire tip out of small side-branches or acute bifurcations, to prevent perforations or dissections. The risk of injury caused by the microwire can be minimized by creating a tight J-shaped curve on the guidewire tip. One advantage of the microwire in these single lumen balloons is that, if necessary, the balloon can be rapidly deflated by withdrawing the wire. Other balloon types do not have this option for rapid deflation. These microballoons are the most common balloons currently used for cerebrovascular test occlusions.

Inflatable balloon wire. This is typified by the GuardWire^® (Medtronic Vascular, Santa Rosa, CA). This system works well especially for carotid test occlusions. This occlusion balloon is mounted on a 0.014-in hypotube (a small diameter wire with an inner lumen) and has a 0.028-in profile for the 2.5–5-cm balloon, or 0.036 in for the 3–6 mm balloon. The larger balloon can easily be advanced through 6F guide catheter. It is inflated with an inflation device, which can be removed from the wire, leaving the balloon inflated. This allows removal of the guide catheter and permits placement of a diagnostic catheter via the same femoral arterial sheath for performance of control angiography of potential collateral vessels, while the balloon occludes the target vessel. Another advantage of this balloon wire is that, it has such a low profile that it allows safe moving of the patient with the balloon in place, for cerebral blood flow imaging with CT perfusion. The disadvantages of the balloon wire is that the distal stump pressure cannot be measured and the wire tip extends for several centimeters distal to the balloon. The GuardWire^® has other disadvantages, including the fact that it is stiffer than the microballoons, so, as a general rule, should not be used in intracranial vessels or other small vessels. The need for the inflation device also means that there is a bit of a learning curve to be able to use this device efficiently. The most annoying problem with this device is the length of time required for balloon deflation. The balloon should be inflated with a dilute contrast solution (e.g. 30% contrast in saline) to minimize viscosity and decrease the problems associated with inflating and deflating the balloon.

Detachable coils. In extremely small, tortuous distal vessels, it may not be possible to safely advance even the smallest microballoons. However, these vessels may still be accessible using low-profile, ten-system microcatheters. With the microcatheter in the vessel to be tested, a detachable coil can be advanced into the vessel to temporarily occlude it. This method will only work in vessels <3 mm in diameter. Use a 2 or 3 mm diameter coil that is stretch resistant so that it can be easily removed. The coil should also be ultra-soft, to fill the lumen of the vessel without traumatizing the intima. Advance as few loops of coil as necessary to occlude flow. Obviously, the patient must be fully heparized, so that the thrombus does not form in the vessel, and occlusion times must be kept to a minimum. Given the possibilities of thrombus formation and the remote possibility of coil stretching or inadvertent detachment, this method should not be routinely used for test occlusion unless everything else fails.

The ICA requires balloons at least 5 mm in diameter in most cases.

The extracranial ICA can be occluded with Swan-Ganz double-lumen balloon (Edwards Lifesciences, Irvine, CA), 10 mm occlusion balloon catheter (Cook Medical, Bloomington, IN), 7  ×  7 mm Hyperform™ microballoon (ev3 Neurovascular, Irvine, CA), or 6 mm GuardWire^®(Medtronic Vascular, Santa Rosa, CA).

The intracranial ICA is best occluded with a Hyperform™ microballoon (ev3 Neurovascular, Irvine, CA) or the Ascent™ (Codman Neurovascular, San Jose, MA).

The vertebral arteries can usually be occluded with 5 mm diameter or larger balloons.

The straight segment of the cervical vertebral can be occluded with a 10 mm occlusion balloon or a 7  ×  7 mm microballoon listed above.

Above the C2 segment of the vertebral artery, where the artery curves laterally, only flexible microballoons such as the Hyperform™ should be used.

Intracranial vessels such as ICA and vertebral arteries may be as large as 4–5-mm in diameter. The supraclinoid ICA is usually 3.5 mm and basilar artery is usually 3.2 mm in diameter. More distal branches are generally <3 mm in diameter.

Vessels >4 mm can be occluded with the 7  ×  7 mm Hyperform™ microballoon, or 6  ×  9 mm Ascent™ (Codman Neurovascular, Raynham, MA).

Vessels ≤4 mm may be occluded with the 4  ×  7 mm Hyperform™ (ev3, Irvine, CA), a 4  ×  10 mm Hyperglide™ microballoon (ev3, Irvine, CA) or 4  ×  10 mm Ascent™ (Codman Neurovascular, San Jose, CA).

Attach all the catheters to RHVs and attach a continuous infusion of saline containing 10,000 units heparin per liter.

Through the femoral (or brachial) sheath, advance the guide catheter into the ICA or vertebral artery.

Do angiograms to determine the best position for the balloon, and to size the artery for proper balloon selection.

Avoid positioning the balloon in any area containing atherosclerotic plaque.

Obtain a roadmap mask to allow balloon positioning and inflation under roadmap guidance.

Warn patients that the catheter manipulation and balloon inflation may cause a feeling of pressure.

Use extreme caution when advancing or inflating a balloon in intracranial vessels.

Always keep track of where the tip of the guidewire is to avoid vascular perforation or dissection.

When the balloon reaches the desired location, pull back on the catheter to remove any slack. This will prevent the balloon from advancing forward as it is inflated.

Inflate the balloon only just enough to stop the flow. Do not overinflate.

Measure the volume required to inflate the balloon and occlude the vessel. When doing adjunctive blood flow imaging, requiring patient transfer with the balloon in place, this allows for deflation and inflation of the balloon without fluoroscopic control.

Prepare the balloon by attaching a 10 mL syringe partially filled with contrast to the inflation port of the balloon, aspirate any air, and release suction to allow contrast to enter the inflation port.

Attach a one-way stopcock to the inflation port, and inflate the balloon with 50:50 contrast:saline mixture, then deflate. Angle the balloon to allow aspiration of any residual air as the balloon is deflated.

For those ambitious (or foolish) enough to use a Swan–Ganz (Edwards Lifesciences, Irvine, CA) for test occlusion, expect to struggle getting into the vessel of interest, as these balloons are not designed for arterial catheterization. A 0.010 in. microwire can be used to direct the catheter and to do partial inflations for flow direction, but it is not a pleasant experience in tortuous vessels.

For all other balloons, such as the 10 mm occlusion balloon catheter (Cook Medical, Bloomington, IN), it is usually necessary to use an exchange wire, unless the patient is young or has straight cervical arteries that are easily accessible with a straight catheter.

Using a diagnostic angiographic catheter of desired size and shape, such as a 5F Angled Glidecath^® (Terumo Medical, Somerset, NJ), catheterize the target carotid or vertebral artery that is to be tested.

Using a 300 cm, 0.035 in. diameter exchange wire, exchange the diagnostic catheter for the balloon catheter.

Advance the balloon catheter so that it is just proximal to the site of intended occlusion, and inject the contrast through the central lumen to obtain a roadmap of the vessel.

Advance the balloon to the target site.

Prepare to measure pressures through the end-hole of the balloon catheter, either by attaching a pressure line to the stopcock (or manifold) attached to the central lumen of the balloon catheter, or by using a pressure-sensing guidewire.

Measure a baseline pressure through the central lumen of the balloon catheter.

Gently inflate the balloon just enough to occlude the vessel.

Inject contrast through the end-hole of the balloon catheter; pooling of contrast in the artery will confirm complete occlusion.

Measure the pressures through the central lumen of the balloon catheter again. When the vessel is occluded, there will be dampening of the pressure waveform. A drop in mean pressure by 50% after balloon inflation is suggestive of insufficient collateral flow to the distal territory.

Examine the patient for any neurological deficits and pay particular attention to functions performed by areas of the central nervous system supplied by the vessel being occluded.

At some point during the test occlusion, do a cerebral angiogram using an arterial catheter in a contralateral sheath, and check for collateral flow from other arterial pathways.

If the patient tolerates the balloon inflation clinically and the back pressure in the balloon catheter does not drop <50% post-inflation, keep the vessel occluded for an extended period (∼30 min) to confirm tolerance to occlusion.

Consider using a supplementary test to look for other signs of haemodynamic insufficiency, when the balloon is inflated (see below).

Signs of test occlusion failure:

The procedure is complete when the patient fails the test occlusion, or if they pass for at least 30 min. The balloon should be deflated.

Prior to removing the balloon, ensure that the patient’s symptoms have resolved. If not, a dissection or a thromboembolic complication may have occurred, and leaving the balloon catheter in place provides access for diagnostic angiography and corrective intervention.

Remove the balloon catheter when all testing is complete.

Prepare the Hyperform™ or Hyperglide™ (ev3 Neurovascular, Irvine, CA) by thoroughly flushing the sterile holder housing the balloon to activate the hydrophilic coating.

Attach a one-way stopcock, or Flo-switch (BD Medical, Franklin Lakes, NJ) to a rotating haemostatic valve, and attach this to the balloon catheter.

Fill the balloon catheter with 50% contrast diluted with saline.

Insert the X-pedion™ (ev3 Neurovascular, Irvine, CA) or other 0.010-in. wire through the rotating haemostatic valve and into the balloon catheter.

Make a j-tip curve on the shapeable platinum tip of the wire, to limit the risk of the microwire traumatizing or perforating a vessel.

Place the guide catheter in the carotid or vertebral artery.

Make a roadmap.

Advance the balloon to the target site.

Gently inflate the balloon just enough to occlude the vessel.

Confirm occlusion of the vessel by injecting the contrast through the guide catheter. There will be stasis of the contrast in the vessel proximal to the balloon.

Examine the patient for neurological deficits and pay particular attention to functions performed by areas of the central nervous system supplied by the artery being occluded.

At some point during the test occlusion, do a cerebral angiogram using an arterial catheter in a contralateral sheath. Check for collateral flow from other arterial pathways.

If the patient tolerates the balloon inflation clinically, keep the vessel occluded for an extended period (∼30 min) to confirm tolerance to occlusion.

Use adjunctive tests to assess for haemodynamic insufficiency when the balloon is inflated (see below).

Signs of test occlusion failure: