Arteriovenous malformation draining vein physiology and determinants of transnidal pressure gradients. The Columbia University AVM Study Project

Cerebral arteriovenous malformation feeding artery aneurysms: a theoretical model of intravascular pressure changes after treatment

OBJECTIVE

A quantitative model may be used to estimate the magnitude of expected pressure changes along the vascular tree with shunt ablation and may provide information to assess the hemodynamic risk of arteriovenous malformation (AVM) treatment.

METHODS

A computer model of the cerebral circulation was applied to estimate the changes in intravascular pressure, velocity, biomechanical stress, and shear stress that might be expected from either endovascular or surgical ablation of AVMs. Two AVM sizes and two feeding artery constellations were simulated. The effect of different shunt flows on vascular pressure was modeled. In each simulation, AVMs were occluded in a stepwise fashion. The effects of systemic hypertension and hypotension in various vascular zones were also simulated.

RESULTS

As large (1000 ml/min) AVMs were occluded, the mean feeding arterial pressure increased from 18 to 68 mm Hg; the percent-occlusion at half-maximal pressure increase was 92%. For medium (500 ml/min) AVMs, feeding arterial pressure increased from 37 to 66 mm Hg; the percent-occlusion at half-maximal pressure increase was 71%. During manipulation of systemic pressure, hemodynamic changes in the circulation close to the nidus were proportionally less than changes in systemic pressure; the degree of proportionality depended on the magnitude of AVM shunt flow.

CONCLUSION

In this simulation, shunt obliteration increased pressure in the nidus and feeding arteries with little effect on the proximal circulation. The shunt provided a "buffering" effect, i.e., higher flow fistulas were exposed to smaller variations in intravascular pressure in feeding artery and nidal pressures during manipulation of systemic pressure.

A theoretical model of cerebral hemodynamics: application to the study of arteriovenous malformations

A comprehensive computer model of the cerebral circulation, based on both hydrodynamics and electrical network analysis, was used to investigate the influences of arteriovenous malformations (AVM) on regional cerebral hemodynamics. The basic model contained 114 normal compartments: 55 arteries, 37 veins, 20 microvessel groups (MVG), one compartment representing systemic and extracranial vascular resistance, and one representing the heart. Each microvessel group, which represented the arteriolar bed, consisted of 5000 microvessels. Cerebral blood flow autoregulation was simulated by a formula that determined the resistance and therefore the flow rate of the microvessel groups (arterioles) as a function of perfusion pressure. Elasticity was introduced to describe the compliance of each vessel. Flow rate was made a controlling factor for the positive regulation of the diameters of conductance vessels by calculation of shear stress on the vessel wall (vessel dilation). Models containing an AVM were constructed by adding an AVM compartment and its feeding arteries and draining veins. In addition to the basic model, AVM models were simulated with and without autoregulation and flow-induced conductance vessel dilation to evaluate the contributions of these factors on cerebral hemodynamics. Results for the model with vessel dilation were more similar to clinical observations than those without vessel dilation. Even in the presence of total vasoparalysis of the arteriolar bed equivalent, obliteration of a large (1000 mL/min) shunt flow AVM resulted in a near-field CBF increase from a baseline of 21 to a post-occlusion value of no more than 74 mL/100 g/min, casting doubt on a purely hemodynamic basis for severe hyperemia after treatment. The results of the simulations suggest that our model may be a useful tool to study hemodynamic problems of the cerebral circulation.

Transvenous hemodynamic assessment of arteriovenous malformations and fistulas. Preliminary clinical experience in Doppler guidewire monitoring of embolotherapy

BACKGROUND AND PURPOSE

Transvenous monitoring of blood flow through intracranial vascular malformations was performed with an intravascular Doppler guidewire to assess hemodynamic changes during endovascular embolotherapy.

METHODS

Flow velocity was assessed in the intracranial venous sinuses of two patients with arteriovenous malformations and seven patients with dural arteriovenous fistulas. In all cases, the Doppler guidewire was positioned in the dural sinuses coaxially through a 2.1F microcatheter. The Doppler guidewire was then advanced to the site of arteriovenous shunting for sampling of venous average peak velocity (APV) and pulsatility index. In two cases, simultaneous feeding artery flow velocity was monitored by transcranial color-coded duplex sonography.

RESULTS

Before embolotherapy, the flow pattern in the venous sinuses was pulsatile, with a mean (+/-SD) APV of 39.0 +/- 22.5 cm/s. Total or near-total embolization was achieved in six of the nine cases. After embolization, the flow pattern became less pulsatile and the APV was reduced to a mean of 21.2 +/- 14.6 cm/s (P = .0123, one-tailed paired t test). The pulsatility index was used to calculate the maximum minus the minimum peak velocity (MxPV-MnPV). This was reduced from an average of 27.0 +/- 8.7 cm/s to 13.5 +/- 8.3 cm/s after treatment (P = .0456). A parallel reduction in APV of the feeding arteries was observed with embolization.

CONCLUSIONS

Preliminary clinical experience indicates that transvenous assessment of two parameters, APV and MxPV-MnPV, is useful in the hemodynamic evaluation of intracranial arteriovenous shunts. This valuable hemodynamic information may be used for objective and quantitative monitoring during embolotherapy of intracranial vascular malformations.

Transvenous hemodynamic assessment of experimental arteriovenous malformations. Doppler guidewire monitoring of embolotherapy in a swine model

BACKGROUND AND PURPOSE

A Doppler guidewire was used to monitor progressive changes in draining vein flow parameters during experimental embolotherapy in a swine arteriovenous malformation (AVM) model.

METHODS

A microcatheter was positioned superselectively in the main arterial feeder and main draining vein in each of 10 AVM models in swine. With use of the Doppler guidewire, preembolization arterial and venous average peak velocities (APVs) and pulsatility indices were recorded. The device was left in the draining vein during transarterial particulate (in 8 swine) or liquid adhesive (in 2 swine) embolization, and continuous transvenous flow during and after treatment was monitored. Periembolization Doppler flow parameters were correlated qualitatively with angiographic changes in the nidus.

RESULTS

Preembolization draining vein flow was pulsatile, with a mean APV of 38.9 +/- 13.7 cm/s. After embolization, this changed significantly to a less pulsatile or nonpulsatile pattern, with a lower mean APV of 9.2 +/- 4.9 cm/s (P = .0001). A novel expression, the maximum minus the minimum peak velocity (MxPV-MnPV), was used in evaluating the transvenous Doppler spectra. This was reduced significantly after embolization from a mean of 11.1 +/- 3.5 cm/s to 6.7 +/- 2.5 cm/s (P = .0025). Objective periembolization hemodynamic changes were detected in the draining veins earlier than the visually subjective angiographic changes within the nidus.

CONCLUSIONS

Transvenous Doppler guidewire assessment of two parameters, APV and MxPV-MnPV, is useful in the hemodynamic evaluation of experimental arteriovenous shunting and may be used for future objective and quantitative monitoring during endovascular AVM embolotherapy.

Risk of intracranial arteriovenous malformation rupture due to venous drainage impairment. A theoretical analysis

BACKGROUND AND PURPOSE

Increased resistance in the venous drainage of intracranial arteriovenous malformations (AVMs) may contribute to their increased risk of hemorrhage. Venous drainage impairment may result from naturally occurring stenoses/occlusions, or if draining veins (DVs) undergo occlusion before feeding arteries during surgical removal, or after surgery in the presence of "occlusive hyperemia." We employed a detailed biomathematical AVM model using electrical network analysis to investigate theoretically the hemodynamic consequences and the risk of AVM rupture due to venous drainage impairment.

METHODS

The AVM model consisted of a noncompartmentalized nidus with 28 vessels (24 plexiform components and 4 fistulous components), 4 arterial feeders, and 2 DVs. An expression for the risk of AVM nidus rupture was derived on the basis of functional distribution of the critical radii of component vessels. Risk was calculated from biomathematical simulations of volumetric flow rate with both DVs patent and for four stages of venous drainage obstruction: (1) 25%, (2) 50%, (3) 75%, and (4) 100%. Each stage of occlusion was applied to each DV while the other DV was patent and then to the patent DV while the other DV was totally occluded.

RESULTS

For flow through the AVM when both DVs were unobstructed, the baseline risk of AVM nidus rupture ranged from 4.4% to 91.2%. Theoretical rupture occurred in nidus components proximal to the DVs when the risk exceeded 100%, as was observed with the obstruction of DV1 and a patent DV2. The ranges for risk of rupture across the nidus for the four stages were (1) 4.7% to 90.5%, (2) 5.9% to 86.9%, (3) 0% to 98.4%, and (4) 0% to 106.3%, respectively. Rupture was observed for an 86% occlusion of DV1 (ie, the DV fed by the intranidal fistula) and DV2 patent, primarily because of the dramatic shift in the hemodynamic burden toward the weaker plexiform nidus vessels.

CONCLUSIONS

On theoretical grounds, venous drainage impairment was predictive of AVM nidus rupture and was strongly dependent on AVM morphology (presence of intranidal fistulas and their spatial relation to DVs) and hemodynamics. Specifically, stenosis/occlusion of a high-flow DV induces a rapid redistribution of blood into the weak plexiform vessels of the opposing region of the nidus, causing a hemodynamic overload and an increased risk of rupture. These findings should be carefully considered among all factors affecting the natural history of intracranial AVMs and the mechanisms implicated in their spontaneous rupture. They may also provide a theoretical rationale for some of the hemorrhagic complications that occur during and after surgical treatment.

A biomathematical model of intracranial arteriovenous malformations based on electrical network analysis: theory and hemodynamics

Hemodynamics play a significant role in the propensity of intracranial arteriovenous malformations (AVMs) to hemorrhage and in influencing both therapeutic strategies and their complications. AVM hemodynamics are difficult to quantitate, particularly within or in close proximity to the nidus. Biomathematical models represent a theoretical method of investigating AVM hemodynamics but currently provide limited information because of the simplicity of simulated anatomic and physiological characteristics in available models. Our purpose was to develop a new detailed biomathematical model in which the morphological, biophysical, and hemodynamic characteristics of an intracranial AVM are replicated more faithfully. The technique of electrical network analysis was used to construct the biomathematical AVM model to provide an accurate rendering of transnidal and intranidal hemodynamics. The model represented a complex, noncompartmentalized AVM with 4 arterial feeders (with simulated pial and transdural supply), 2 draining veins, and a nidus consisting of 28 interconnecting plexiform and fistulous components. Simulated vessel radii were defined as observed in human AVMs. Common values were assigned for normal systemic arterial pressure, arterial feeder pressures, draining vein pressures, and central venous pressure. Using an electrical analogy of Ohm's law, flow was determined based on Poiseuille's law given the aforementioned pressures and resistances of each nidus vessel. Circuit analysis of the AVM vasculature based on the conservation of flow and voltage revealed the flow rate through each vessel in the AVM network. Once the flow rate was established, the velocity, the intravascular pressure gradient, and the wall shear stress were determined. Total volumetric flow through the AVM was 814 ml/min. Hemodynamic analysis of the AVM showed increased flow rate, flow velocity, and wall shear stress through the fistulous component. The intranidal flow rate varied from 5.5 to 57.0 ml/min with and average of 31.3 ml/min for the plexiform vessels and from 595.1 to 640.1 ml/min with an average of 617.6 ml/min for the fistulous component. The blood flow velocity through the AVM nidus ranged from 11.7 to 121.1 cm/s with an average of 66.4 cm/s for the plexiform vessels and from 446.9 to 480 dyne/cm2 with an average of 463.5 dyne/cm2 for the fistulous component. The wall shear stress ranged in magnitude from 33.2 to 342.1 dyne/cm2 with an average of 187.7 dyne/cm2 for the plexiform vessels and from 315.9 to 339.7 cm/s with an average of 327.8 cm/s for the fistulous component. The described novel biomathematical model characterizes the transnidal and intranidal hemodynamics of an intracranial AVM more accurately than was possible previously. This model should serve as a useful research tool for further theoretical investigations of intracranial AVMs and their hemodynamic sequelae.

Factors that predict the bleeding risk of cerebral arteriovenous malformations

BACKGROUND AND PURPOSE

Arteriovenous malformations (AVMs) have an overall 2% to 4% annual risk of hemorrhage. The purpose of this study was to determine whether specific clinical and radiographic factors predispose AVMs to bleed and to predict the bleeding risk for individual AVM patients.

METHODS

We reviewed the clinical histories and cerebral angiograms of 315 AVM patients who underwent stereotactic radiosurgery at our center. One half of the patient data (analysis cohort) was used to determine risk factors for bleeding and to construct AVM hemorrhage risk groups. These risk groups were then tested with the second half of the patient data (test cohort).

RESULTS

The mean AVM volume was 4.0 +/- 3.4 mL (approximate maximum diameter of 2 cm). One hundred ninety-six initial hemorrhages occurred in 10,348 patient-years for an annual initial bleed rate of 1.89%; 44 of these 196 patients had a repeat bleed in 591 patient-years for an annual rebleed rate of 7.45%. The overall crude annual hemorrhage rate was 2.40%. Multivariate analysis revealed three factors associated with hemorrhage: history of a prior bleed (relative risk [RR], 9.09; 95% confidence interval [CI], 5.44 to 15.19; P < .001), a single draining vein (RR, 1.66; 95% CI, 1.13 to 2.38; P < .01), and a diffuse AVM morphology (RR, 1.64; 95% CI, 1.12 to 2.46; P < .01). Four AVM hemorrhage risk groups were constructed on the basis of the significant factors. The annual rate of bleeding was 0.99% for low-risk AVMs, 2.22% for intermediate-low-risk AVMs, 3.72% for intermediate-high-risk AVMs, and 8.94% for high-risk AVMs.

CONCLUSIONS

Analysis of a large group of AVM patients who underwent stereotactic radiosurgery demonstrated that small AVMs have an annual hemorrhage risk similar to that of the general AVM population. AVM patients have a wide variability of bleeding risk that can be predicted from their clinical presentation and the angiographic characteristics of the AVM. The management of AVM patients should be based not only on the morbidity of the proposed treatment but also those factors that predispose individual patients to either a low or high hemorrhage risk.

Blood pressure monitoring in feeding arteries of cerebral arteriovenous malformations during embolization: a preventive role in hemodynamic complications

To study the hemodynamics of arteriovenous malformations and to avoid hemodynamic complications during and after artificial embolization, we measured arterial blood pressures in 21 feeders in 14 patients through a microcatheter system. Before embolization, the pressures were significantly low in feeders with branches terminating in the malformation (terminal divided branches) and comparatively low in arteriovenous malformations with rapid blood flow through the malformation. The pressures in feeders with brain-nutrifying branches distal to the nidus (transient branches) were significantly high. Therefore, transient branches might be distinguishable from terminal divided branches with the use of feeder pressure monitoring. A hemorrhagic complication occurred in one patient. The feeder pressure in this patient was low before embolization and showed the maximum change among the patients after embolization. It seems that the lower the feeder pressure, the more likely complications are to occur, owing to remarkable hemodynamic alterations. Feeder pressure monitoring may be useful for preventing hemodynamic complications, especially when angiographic findings show feeding arteries giving off terminal divided or transient branches or rapid blood flow through the malformation.

'Steal' is an unestablished mechanism for the clinical presentation of cerebral arteriovenous malformations

BACKGROUND AND PURPOSE

Focal neurological deficits (FNDs) in patients with arteriovenous malformations (AVMs) have been widely attributed to the phenomenon of "cerebral steal." The incidence of focal deficits was investigated in a large prospective sample.

METHODS

Using data from patient history and examination, CT or MRI, and transcranial Doppler sonography, we studied 152 consecutive, prospective AVM patients for evidence of FNDs unrelated to a hemorrhagic event. Feeding mean arterial pressure was measured during superselective angiography.

RESULTS

Two (1.3%) of 152 patients met the criteria for a progressive FND. Nonprogressive FNDs were seen in 11 (7.2%) patients (stable in 4.6%, reversible in 2.6%). The median observation time period was 17 months (range, 1 to 60 months). There were no differences in transcranial Doppler mean velocities in feeding arteries in FND versus non-FND groups (118 +/- 44 versus 112 +/- 37 cm/s, P > .05) or pulsatility indexes (0.53 +/- 0.20 versus 0.55 +/- 0.15, P > .05). Feeding artery pressure was similar in FND (n = 10) and non-FND (n = 96) groups (39 +/- 16 versus 39 +/- 16 mm Hg at a systemic pressure of 82 +/- 18 versus 75 +/- 14 mm Hg, NS).

CONCLUSIONS

Nonhemorrhagic focal neurological syndromes in AVM patients are infrequent. Progressive deficits are especially rare. There was no relation between feeding artery pressure or flow velocities and FND. There does not appear to be sufficient evidence to assign steal as an operative pathophysiological mechanism in the vast majority of AVM patients.