Overcoming the black box of aspiration thrombectomy in acute ischemic stroke: An early clinical experience of using contrast injections to understand clot-catheter interactions

Abstract

Background

Factors responsible for failure of aspiration thrombectomy in patients with acute ischemic stroke are poorly understood. In order to examine catheter–clot interactions, we modified our current aspiration thrombectomy technique by performing contrast injections near the tip of the aspiration catheter prior to the initiation of aspiration thrombectomy.

Methods

Small volume injections of contrast were performed using a microcatheter positioned inside the aspiration catheter immediately proximal to the occlusion site. Continuous fluoroscopy during the entire duration of each aspiration pass was recorded. We report our initial results with this new technique and examine potential associations of patterns of contrast behavior with procedural success of each thrombectomy pass.

Results

Seventeen patients were included in final analysis, consisting of 24 total aspiration thrombectomy passes. Microcatheter injections showed no safety concerns. Three angiographic patterns of contrast behavior near the aspiration catheter tip were observed: “occlusive” with no forward contrast flow, “side branch opacification” and “anterograde opacification” with anterograde flow. Movement of the contrast column during aspiration thrombectomy depended on the degree of aspiration catheter redundancy. Manual reduction of excessive catheter turns and higher position of long guide sheath at the petrous or cavernous segments seemed to improve contrast clearance and aspiration force.

Conclusions

This initial experience indicates that multiple complex factors may affect success rates of aspiration thrombectomy. The technique of microcatheter injection near the occlusion site may prove helpful in optimizing the existing aspiration thrombectomy techniques.

Introduction

Aspiration thrombectomy is a well-established first-line treatment of patients with acute ischemic stroke from emergent large vessel occlusion (LVO). Despite continuous advances in aspiration technologies, rates of first pass effect (FPE) remain relatively low, in the order of 26% to 57%.^1–3 This suggests a further need in technical innovation, either by improving characteristics of existing catheters or introducing new devices such as super-bore catheters or pulsatile aspiration. Alternatively, changing the technique of aspiration thrombectomy with existing tools may improve procedural success rates. An obvious limitation of aspiration for LVO is lack of visual feedback to inform the neurointerventionalists about the interactions between thrombus and aspiration catheter prior to or during a thrombectomy pass.

Attempts to study the thrombus-aspiration catheter milieu using various in vitro models of aspiration thrombectomy so far have provided conflicting data with limited influence on everyday clinical practice. Whether positioning the catheter tip proximally to or in direct contact with the thrombus,^4,5 maximizing catheter-to-vessel size to achieve flow arrest ⁶ versus under-sizing the catheter to prevent vessel collapse, ⁷ or even switching from static to cyclic aspiration^8,9 for optimal aspiration results all remain highly controversial. These and other technical and anatomical factors likely greatly influence the performance of aspiration technique, yet we often are not able to predict or directly estimate the success rate of an individual thrombectomy pass intraprocedurally until a control angiographic injection is performed. This results in a “black box” problem where the input is known (aspiration is applied), the unknown events happen, and then the outcome (sometimes, disappointing) is seen after a control injection.

The goal of this study was to evaluate the feasibility of using local contrast injections at the tip of the aspiration catheter immediately prior to the initiation of thrombectomy to determine its diagnostic value and possible association with aspiration success or failure. Here, we summarize our early experience and discuss the possible implications, limitations and future application of this relatively simple technique which attempts to open the “black box” of aspiration thrombectomy.

Methods

This study was approved by the local institutional review boards with a waiver for informed consent. We retrospectively reviewed cases of acute stroke with LVO treated with direct aspiration to identify cases where local injections of contrast at the tip of the aspiration catheter were performed during the thrombectomy procedure. Such injections were performed at the discretion of operator to visualize catheter tip position prior to aspiration attempt.

Aspiration thrombectomy procedure

All neurointerventionalists performing acute stroke interventions at our center use a similar set up for thrombectomy. Direct aspiration is uniformly used as a first line approach to the treatment of stroke with identifiable arterial occlusion. In brief, typically, after placement of a short 8F femoral sheath, an 0.088-inch long sheath (Zoom 88 [Imperative Care]; Millipede 088 [Perfuse]) or 0.087-inch balloon guide (Walrus [Q’Apel]) was delivered over a 5F support catheter into the target carotid artery. Next, over a 0.025-inch microcatheter (Velocity [Penumbra]) and 0.018- to 0.024-inch guidewire (Aristotle [Scientia Vascular], an aspiration catheter was delivered to the occlusion site. The size and brand of the aspiration catheter were selected by the operator based on the patient's anatomical characteristics. Aspiration using −29.5 inHg pump (Imperative Care) was performed by directly connecting the catheter to aspiration tubing. Next, we would allow approximately 30 seconds of aspiration before slowly withdrawing the aspiration catheter to reduce any kinking and removing the catheter from the body. The entire aspiration pass thus would take approximately 60 to 90 seconds on average before the catheter was fully withdrawn from the body.

Contrast injections and data interpretation

Prior to removing the 0.025-inch microcatheter and connecting the aspiration catheter to the pump, a gentle injection of approximately 0.1 to 0.2 ml was performed through the microcatheter positioned just proximally (approximately 10–15 mm) from the aspiration catheter tip under a blank roadmap (Supplemental Figure S1). The microcatheter was then slowly removed not to disturb the column of contrast and the aspiration catheter was then connected to the aspiration pump. Throughout aspiration, contrast stasis behavior could be observed and recorded as fluoroscopy loops (Supplemental Figure S1). If repeat aspiration attempts were required, a new contrast injection at the catheter tip was performed and a new recording was obtained. If the operator eventually chose to use a stent retriever as a bail out technique, no local contrast injections at the catheter tip were performed in such passes.

For analysis, all angiographic data were retrospectively adjudicated by our neurointerventionalists via group consensus. Procedural data including target occlusion site, device selection, radiographic outcome after each pass were extracted from operative reports and review of procedural images. Operators graded reperfusion using the mTICI scale. ¹⁰ Contrast injections at the aspiration catheter and characteristics of contrast movement during each aspiration thrombectomy pass were analyzed from intraprocedural recordings. Since this was an exploratory proof of concept study with a small sample size, formal statistical analysis was not feasible.

Results

Seventeen patients were included in final analysis, yielding a total of 24 aspiration thrombectomy passes. Procedural and radiographic data of individual cases including description of contrast characteristics during each aspiration pass are summarized in Table 1 and Supplemental Figures S3–S19. The majority were middle cerebral artery M1 and M2 occlusion passes (11/24 [46%] and 10/24 [42%], respectively). Mean and median vessel size at the site of occlusion was 2.2 and 2.1 mm, respectively. Aspiration was performed with a variety of catheters, ranging from .038 to .071 inches in inner diameter. FPE (TICI 2c/3 on first pass) was achieved in six cases (35%). Rescue use of a stent retriever was required in 18% (3/17) of cases.

Table 1.

Procedural characteristics for each aspiration thrombectomy pass.

Case	Pass	Occlusion and catheter characteristics	Delivery system	Guide type and location during aspiration	Radiographic findings	Result, TICI	Additional info
1	1	ICA ophthalmic 2.8 mm; catheter OD (Cereglide 71) 2.1 mm; catheter tip 15 mm into clot	025 microcatheter, 024 guidewire	Walrus BGC; cervical ICA → petrous ICA	Puff: occlusive Aspiration: no contrast movement until aspiration catheter loops reduced	1	Supplemental Figure S3 (case 1)
	2	ICA-T 3.8 mm; catheter OD (Cereglide 71) 2.1 mm; catheter tip at clot interface	Same	Walrus BGC; petrous ICA	Puff: occlusive Aspiration: no contrast movement until aspiration catheter loops reduced	2a
	3	M1 2.7 mm; catheter OD (Cereglide 71) 2.1 mm; catheter tip at clot interface	same	Walrus BGC; petrous ICA	Puff: occlusive (Figure 1A) Aspiration: no contrast movement until aspiration catheter loops reduced	2c
2	1	M1 2.5 mm; catheter OD (Zoom 71) 2.1 mm; catheter tip 3 mm into clot	Carrier 68, 018 guidewire	Walrus BGC; cervical ICA	Puff: side branch opacification (Figure 1B) Aspiration: no contrast movement until catheter loops reduced (Figure 3A, Supplemental video 2)	2a	Supplemental Figure S4 (case 2)
	2	M1 2.4 mml; catheter OD 2.1 mm; catheter tip 12 mm into clot	Same	Walrus BGC; cervical ICA	Puff: not recorded Aspiration: no contrast movement until catheter loops reduced	2a
	3	SR used for rescue
3	1	M1 2.3; catheter OD (Zoom 71) 2.1 mm; catheter at clot interface → clot movement → catheter advanced further	Carrier 68, 018 guidewire	Zoom 88; ophthalmic ICA	Puff: anterograde flow and evidence of clot migration → aspiration catheter advanced further into M2, puff repeated with less anterograde flow (Figure 2) Aspiration: continuous contrast movement (Figure 3C, Supplemental video 2)	3	Supplemental Figure S5 (case 3)
4	1	M1 2.8 mm; catheter OD (6F Sofia Plus) 2.1 mm; catheter 12 mm into clot	025 microcatheter, 024 guidewire	Zoom 88; cavernous ICA	Puff: occlusive Aspiration: contrast cleared early	2c	Supplemental Figure S6 (case 4)
5	1	M1 2.7 mm; catheter OD (millipede 070) 2.1 mm; catheter 4 mm into clot	025 microcatheter, 024 guidewire	Millipede 88; ophthalmic ICA	Puff: side branch flow Aspiration: contrast cleared early	2b	Supplemental Figure S7 (case 5)
6	1	M1 3.1 mm; catheter OD (6F Sofia plus) 2.1 mm; catheter at clot interface	035 guidewire	Zoom 88; cavernous ICA	Puff: occlusive first, then contrast washout with continuous saline flush Aspiration: no residual contrast seen in catheter at beginning of aspiration	0	Supplemental Figure S8 (case 6)
	2	SR used for rescue
7	1	M1 2.2 mm; catheter OD (5F Sofia) 1.7 mm; catheter at clot interface	025 microcatheter, 014 guidewire	Benchmark 71; petrous ICA	Puff: occlusive. Suspected kinking/ovalization of aspiration catheter at petrous ICA segment Aspiration: not recorded	0	Supplemental Figure S9 (case 7)
	2	SR used for rescue
8	1	M1 2.6 mm, catheter OD (6F Sofia) 2.1 mm; catheter 6 mm into clot	025 microcatheter, 018 guidewire	Zoom 88; cavernous	Puff: occlusive Aspiration: slow contrast movement first, faster clearance with catheter loops reduced (Figure 3B, Supplemental video 2)	3	Supplemental Figure S10 (case 8)
	2	A2 1.5 mm; catheter OD (Zoom 55) 1.75 mm; catheter 14 mm into clot	025 microcatheter, 014 guidewire	Zoom 88; cervical	Puff: side branch opacification Aspiration: not recorded	3
9	1	M1 1.9 mm; catheter OD (Hippo) 1.8 mm; catheter tip 3 mm into the clot	Cheetah 026	Zoom 88; petrous ICA	Puff: side branch opacification Aspiration: contrast cleared instantly	0	Supplemental Figure S11 (case 9)
	2	M1 1.9 mm; catheter OD (Hippo) 1.83 mm; catheter tip 3 mm into the clot	Cheetah 026	Zoom 88; cavernous ICA	Puff: minimal anterograde flow Aspiration: low movement of contrast back	3
10	1	M2 1.8 mm; catheter OD (Zoom 71) 2.1 mm; catheter tip 9 mm into clot	Snaking	Zoom 88; cavernous ICA	Puff: occlusive Aspiration: contrast cleared early	3	Supplemental Figure S12 (case 10)
11	1	M2, 2.1 mm; catheter OD (Millipede 70) 2.1 mm; catheter tip at clot interface.	Carrier 68, 018 guidewire	Millipede 88, MCA M1	Puff: anterograde + side branch flow (Figure 1D). Carrier removal caused partial retrograde contrast clearance. Aspiration: rapid clearance of residual contrast	0	Supplemental Figure S13 (case 11)
	2	M2, 2.1 mml catheter OD; catheter tip 5 mm into the clot	Carrier 68, 018 guidewire	Millipede 88; petrous ICA	Puff: anterograde flow Aspiration: not recorded	2b
	3	SR used for rescue for residual clot
12	1	M2, 2.3 mm; catheter OD (Zoom 55) 1.75 mm; catheter tip 5 mm into the clot	025 microcatheter, 014 guidewire	Zoom 88; cervical ICA	Puff: anterograde flow (Figure 1C) Aspiration: no residual contrast seen in catheter at beginning of aspiration	0	Supplemental Figure S14 (case 12)
	2	SR used for rescue
13	1	M2, 1.4 mm; catheter OD (Zoom 45) 1.5 mm; catheter tip 10 mm into clot	025 microcatheter, 018 guidewire	Zoom 88; petrous ICA	Puff: anterograde flow Aspiration: no residual contrast seen in catheter at beginning of aspiration	2a	Supplemental Figure S15 (case 13)
	2	M2, 1.4 mm; catheter OD (Zoom 45)×mm; catheter tip 12 mm into clot	025 microcatheter, 018 guidewire	Zoom 88; petrous ICA	Puff: anterograde flow Aspiration: not recorded	2b
14	1	M2, 2.0 mm; catheter OD (5F Sofia) 1.7 mm; 3 mm into clot	035 guidewire	Zoom 88; petrous ICA	Puff: side branch opacification Aspiration: no contrast movement until catheter was removed (clot was corked into catheter tip)	3	Supplemental Figure S16 (case 14)
15	1	M2, 1.6 mm; catheter OD (Socrates 38) 1.35 mm; ∼ 5 mm proximal to the clot.	018 guidewire	Zoom 88; cavernous ICA	Puff: anterograde + side branch flow and clot migration (vs better visualization of true occlusion site) → aspiration catheter advanced further into M3 Aspiration: not recorded	2a	Supplemental Figure S17 (case 15)
	2	IA tPA given for rescue
16	1	M2 1.8 mm; catheter OD (zoom 55) 1.75 mm; catheter tip 2 mm into the clot	Zoom 55, marksman microcatheter, Aristotle 14 guidewire	Zoom 88; petrous ICA	Puff: anterograde slow Aspiration: Slow clearance of contrast	3	Supplemental Figure S18 (case 16)
17	1	M2 1.9 mm; catheter OD (Zoom 55) 1.75 mm; catheter tip 1.5 mm into the clot	Zoom 55, marksman microcatheter, Aristotle 14 guidewire	Zoom 88; petrous ICA	Puff: robust anterograde flow and evidence of two distinct clots (one distant from the Zoom 55 catheter tip) Aspiration: retrograde contrast clearance while removing aspiration catheter	2A	Supplemental Figure S19 (case 17)
	2	SR used for rescue

BGC, balloon guide catheter; IA, intra-arterial; ICA, internal carotid artery; M1, middle cerebral artery M1 segment; M2, middle cerebral artery M2 segment; MCA: middle cerebral artery; OD, outer diameter; SR, stent retriever; tPA, tissue plasminogen activator; TICI, thrombolysis in cerebral infarction.

There were no associated safety concerns with performing microcatheter injections prior to aspiration. Microcatheter injections demonstrated three distinct patterns of contrast behavior: (1) “occlusive” with no visible contrast flow outside of the catheter; (2) “side branch opacification” when the catheter is only partially occlusive and retrograde flow of contrast around the catheter would opacify adjacent branches, and (3) “anterograde opacification” when anterograde contrast flow is seen, sometimes in combination with side branch opacification or alone. Examples of these patterns are illustrated in Figure 1.

Figure 1.

Patterns of microcatheter injections in different situations and cases. (A) Anteroposterior projection of left internal carotid artery (ICA) injection demonstrating M1 occlusion. (B) Anteroposterior projection of selective microcatheter injection under blank roadmap, showing occlusive pattern. Continuous live fluoroscopy (subsequent panes in B) shows no anterograde flow of contrast indicating that the aspiration catheter (Cereglide 071) is completely occlusive. The contrast only fills the aspiration catheter instead. (C) Anteroposterior projection of right ICA injection demonstrating M1 occlusion. (D) Anteroposterior projection of selective microcatheter injection demonstrating side branch opacification. Not that this side branch was not visualized on baseline ICA injection. (E) Roadmap view of anteroposterior projection of a left ICA injection demonstrating M2 occlusion. (F) Anteroposterior projection of selective microcatheter injection. Here, robust anterograde flow of contrast is present. (G) Anteroposterior projection of left ICA injection demonstrating M2 occlusion. (H) Anteroposterior projection of selective microcatheter injection. Both side branch contrast flow and robust anterograde flow can be seen.

Anterograde flow of contrast from microcatheter injection was observed in 42% (10/24) of aspiration passes. In three cases, a microcatheter injection demonstrated robust anterograde flow of contrast and clot positioned distal to its expected initial location. In two cases, the operator further adjusted position of the aspiration catheter before attempting aspiration (cases 3 [Figure 2] and 15).

Figure 2.

Example of microcatheter injection leading to adjustment of aspiration catheter position. (A) Baseline anteroposterior projection of right internal carotid artery (ICA) injection, roadmap, showing distal middle cerebral artery M1 occlusion. (B) Under roadmap guidance, Zoom 71 aspiration catheter was brought over Carrier 68 delivery catheter to the occlusion site. (C) Fluoroscopy recording showing robust anterograde flow of contrast injection indicating clot location more distally. (D) Zoom 71 catheter was advanced further into M2 occlusion before aspiration was applied. (E) First pass effect with TICI 3 reperfusion was achieved as demonstrated on repeat anteroposterior projection of ICA injection.

In cases with “occlusive” and “side branch opacification” microcatheter injection, and some cases of “anterograde flow” (when such flow was sluggish), the residual column of contrast within the distal end of the aspiration catheter could be continuously visualization throughout aspiration, yielding several observations. We often witnessed improved movement of contrast as the aspiration catheter was slowly pulled back and its loops were reduced as the tip of the catheter remained in contact with the clot (Figure 3). Higher location of guide catheter (long sheath) seemed to be associated with improved chances of FPE. In all 6 cases when FPE was achieved, long sheath was placed at petrous (n = 2) or cavernous (n = 4) internal carotid artery (ICA) segment prior to clot aspiration.

Figure 3.

Patterns of contrast movement during aspiration. (A) Lateral projection fluoroscopy and (B) roadmap in a case with a tandem internal carotid artery (ICA) occlusion treated with stenting and middle cerebral artery (MCA1) M1 occlusion treated with aspiration. The guide catheter (8F Cook shuttle) could not be advanced distal to the stent. As Zoom 71 aspiration catheter is slowly pulled back under roadmap guidance, catheter turns (indicated by the yellow dashed line) are reduced and improved flow of contrast is seen. (C) Lateral projection of fluoroscopy and (D) roadmap demonstrating an aspiration pass for MCA M1 occlusion. Here, Zoom 88 guide is advanced from the petrous to proximal cavernous segment as 6F Sofia Plus catheter is slowly withdrawn. Improved flow of contrast is seen. (E) Lateral projection of fluoroscopy and (F) roadmap demonstrating an aspiration pass for MCA M1 occlusion is shown. In this case, Zoom 88 guide was brought into the ophthalmic ICA segment prior to beginning of aspiration. Minimal deformation of Zoom 71 aspiration catheter is seen with rapid clearance of contrast once aspiration was applied.

Discussion

Accumulating evidence suggests that increasing FPE rate is associated with improved clinical outcomes,^11,12 thus it is paramount to identify factors responsible for failure of achieving FPE and identify strategies to correct it. While one clear solution is inventing more effective thrombectomy tools, we also argue that even with existing tools, one can achieve a higher rate of FPE by optimizing the technical nuances of aspiration.

Aspiration thrombectomy is attractive to many operators given its simplicity and versatility; one can track an aspiration catheter over a microcatheter/guidewire,^13,14 a guidewire alone, ¹⁵ using a designated delivery assist catheter ¹⁶ or simply “snaking” the aspiration catheter alone. ¹⁷ All of these variations of aspiration technique, however, provide limited ability for the operator to visualize complex clot-aspiration catheter interactions. With stent retriever thrombectomy, a microcatheter injection is often performed, offering additional information about clot burden, distal anatomy, and collateral flow. ¹⁸ Our proposed use of a microcatheter injection prior to aspiration is somewhat analogous to microcatheter contrast injection in stent retriever thrombectomy, along with a benefit of an improved safety profile (with our technique, there is no need to cross the occlusion site thus a risk of vessel perforation or clot disturbance is eliminated). “Dead space” of an average 130 cm long 0.070-inch aspiration catheter is around 3.3 ml and a slow injection of 0.1–0.2 ml of contrast (often, even a smaller injection would suffice) is a very safe maneuver.

Our first key observation is the significant variability in patterns of blood flow near the tip of the aspiration catheter. Microcatheter injections can provide accurate information by opening and illuminating the “black box” in each aspiration pass by providing direct visualization via contrast. For example, they can show if the catheter is completely or partially occlusive. These injections may also help bridge the gap between bench research and actual patient cases in other ways such as in offering insights into the value of flow arrest or risk of potential vessel collapse with excessive aspiration.

A second important observation is real-time intraprocedural ability to recognize a phenomenon of clot migration and instances of suboptimal catheter position before aspiration is initiated. Indeed, in two of our cases, microcatheter injections informed the operator of the need to optimize catheter placement, potentially preventing a futile aspiration attempt and greater time before recanalization. Novel delivery catheters designed to assist in aspiration catheter navigation are increasingly used, ¹⁹ with some operators routinely advancing such devices deep into the clot. This maneuver may cause clot fragmentation, migration, and decrease subsequent aspiration success which we observed in some of our cases. Perhaps microcatheter injections can be used to optimize delivery techniques for this type of assist devices. When aspiration thrombectomy is tested in vitro using three-dimensional models where operators are able to directly visualize clot location and accurately navigate the catheter toward target occlusion, successful recanalization rates substantially exceed the numbers that are observed in clinical practice. ²⁰ We posited and then observed in real-time that the microcatheter injection technique can impart some of this advantage into clinical scenarios.

This observation seems to be especially important for cases of medium or distal vessel occlusions. Most of the cases in our series with anterograde contrast flow from microcatheter injections were in occlusions beyond the M1 segment. It appears that at least in some cases, a baseline injection from the ICA does not accurately pinpoint the exact location of clot and the true occlusion is in fact more distant. This may explain in part why thrombectomy for medium and especially distal vessels remains a challenge. ²¹

The third observation comes from the real-time visualization of contrast flow inside the aspiration catheter during the pass itself, which may represent the nature of aspiration power inside the catheter, and especially the strength of aspiration at the tip of the catheter. We frequently observed the effect of relaxing the aspiration catheter and reducing excessive sharp turns on increased speed of contrast movement. This may indicate improved aspiration power and the negative effect of catheter deformations such as kinking or ovalization—the phenomena that are poorly understood and likely under-recognized in real world cases. ²² Careful relaxation and straightening of aspiration catheter under fluoroscopy to ensure the tip remains engaged into the clot has become our standard technique.

Finally, the observation of the beneficial effect of “high” guide catheter position (i.e. at the petrous ICA segment or higher) is another relatively simple maneuver that may improve FPE rate. Various highly navigable 0.088-inch and higher diameter catheters are now available in any interventionalist's armamentarium.^23,24 This supports recent data published that large bore guide catheter positioning into the petrous ICA or more distally results in a higher rate of FPE.^25,26 Potential future directions of applying the microcatheter injection technique may include studying aspiration with 088-inch super large bore aspiration catheters and comparing the efficacy of different aspiration cycles including cyclical aspiration.

Our study has some limitations. The number of cases and individual passes was relatively small, precluding a formal statistical analysis. Some of the data on microcatheter injection or aspiration recordings were missing from individual cases. It remains to be determined if digital subtraction angiography, the road mapping technique, or live fluoroscopy is optimal for microcatheter injections and contrast flow analysis, which in part may depend on the type of angiography unit one uses. There was a great variability in types of aspiration catheter and delivery systems used in our cases, making formal statistical analysis even more challenging. Characterization of contrast injection patterns was subjective, qualitative, and analysis of intra- and extracranial vessel tortuosity was not performed.

Conclusions

This initial experience indicates that multiple complex factors may affect success rates of aspiration thrombectomy. The technique of microcatheter injection near the occlusion site may prove helpful in optimizing the existing aspiration thrombectomy techniques by opening the “black box” of aspiration thrombectomy with direct visualization. We encourage neurointerventionalists to adopt and modify this technique and share their results by means of additional research.

Supplemental Material

Video 1.

Video 2.