A Novel Device for Additional Guide Wire Support to Cross Tortuous Anatomy and Tight Lesions - The Micro Rx Rapid Exchange Microcatheter

Successful crossing of the target coronary lesion with a guidewire is an essential step in percutaneous coronary intervention. Guidewire advancement can be challenging, especially in tortuous, severely stenosed, and heavily calcified lesions. The use of a microcatheter significantly improves the guidewire steerability and penetration force, but it requires specific training and is associated with increased procedural costs. We present the first in vivo experience with a new type of rapid exchange microcatheter (Micro Rx, Interventional Medical Device Solutions), describe bench testing of combinations of guidewires and microcatheters, and suggest potential applications.

Successful crossing of the target coronary lesion with a 0.014-in guidewire is an essential step in percutaneous coronary intervention (PCI).^1–3 Guidewire advancement can be challenging, especially in tortuous, severely stenosed, and heavily calcified lesions.⁴ Various specialty guidewires, such as polymer-jacketed guidewires, can facilitate advancement across challenging lesions, but crossing may still fail.⁵ The use of a microcatheter significantly improves the guidewire steerability and penetration force, but it requires specific training and is associated with increased procedural costs.⁶ We present the first in vivo experience with a new type of rapid exchange microcatheter, provide bench testing results of combinations of guidewires and microcatheters (Micro Rx, Interventional Medical Device Solutions), and suggest potential applications.

Micro Rx device description and applications. The Micro Rx (Figure 1) consists of a 15-cm braided polymer distal shaft and a proximal metal push rod. The distal tip has embedded tungsten to provide X-ray visibility. The tip has a leading-edge entry profile of 0.44 mm. Exit markers are present on the proximal shaft to indicate the tip exit into the coronary artery and the proximal end of the distal shaft. The distal shaft has a taper starting at the tip resulting in a 0.85-mm shaft profile. The Micro Rx can provide additional guidewire support but is not intended to replace regular over-the-wire (OTW) microcatheters, as it does not allow guidewire exchange without losing the distal guidewire position. Typical applications include wiring through tortuous anatomy and through severely stenosed vessels (but not chronic total occlusions) (Figure 2).⁷

Application 1. Subtotal occlusion. To overcome the resistance of a subtotal occlusion, higher penetration force is often needed. Advancement of the Micro Rx close to the guidewire tip may overcome this resistance with a regular workhorse wire and may increase the precise control of the guidewire, facilitating crossing. Furthermore, the advancement of the Micro Rx through the tight lesion creates a microchannel that facilitates subsequent equipment delivery, such as balloon delivery. Figure 2, Panel 1 illustrates these technical challenges.

Application 2. High tortuosity. In tortuous anatomy, guidewires exhibit increased friction that can result in the inability to further advance the guidewire. Additional support by the Micro Rx facilitates crossing of the tortuous lesion. The insertion of a guide catheter extension over the end stop without removing the Micro Rx provides additional support and facilitates successful crossing of further tortuosity. The support and tip load of regular workhorse guidewires can be gradually modified with these two rapid exchange devices.

In vitro evaluation of the Micro Rx. The amount of backup support provided by the Micro Rx has been quantified in a simulated tortuous vascular in vitro model where a guidewire is positioned against a load cell (Figure 3). The penetration force was calculated by dividing the measured force by the lesion contact surface of the guidewire tip. First, the guidewire is advanced without the Micro Rx inserted to have a baseline guidewire force transmission. Then, the Micro Rx was advanced over the guidewire with its rapid exchange port and positioned at different distances from the guidewire tip whereafter the guidewire was pushed as in the baseline measurement. Tip load measurements are made with the tip of the Micro Rx positioned between 0 to 20 mm from the distal end of the guidewire.

In vivo evaluation of the Micro Rx. Most guidewire crossing difficulties are related to tortuosity, vessel occlusion, or a combination of these two factors. To objectively evaluate the ability of the Micro Rx to support guidewire crossing under these circumstances, we created the following in vivo test setup. The right subclavian artery of a female light weighted ( ̴ 40 kg) pig was used, as it has a similar diameter compared with the human coronary artery (3-5 mm). Furthermore, the pig remained more hemodynamically stable when the artificial stenosis was created on the subclavian artery instead of a coronary artery.

Reference devices

The Micro Rx is a rapid exchange microcatheter. Its performance was compared in vivo to 3 current state-of-the-art OTW microcatheters, ie, Finecross MG (Terumo)⁸, Corsair Pro (Asahi Intecc),⁹ and Turnpike Spiral (Teleflex).¹⁰ Tip entry profiles and the outer diameter of the distal shaft are presented in Table 1 for direct comparison.

The artificial occlusion model. The artificial occlusion model (AOM) (Figure 4, Panel A) consists of 4 ridged pins where the distance between the upper row pins and lower row pins can be adjusted. The pins are placed directly opposite of each other and, by decreasing the distance between the pins, the occlusion can be adjusted to find the point of failure to cross. Guidewire support and device crossability were rated.

The artificial tortuosity model. The artificial tortuosity model (ATM) (Figure 4, Panel B) consists of 5 stacked pins where the distance between the upper row pins and lower row pins can be adjusted. By decreasing the distance between the pins, the tortuosity can be adjusted until failure to cross. Three physicians who are highly experienced with guidewire and single-lumen microcatheter manipulation worked jointly exchanging “main operator” position to ensure individual opinions on the device performance during the experimental runs. Guidewire support and device crossability was rated on a scale from 1 to 4.

Bench test results. In the in vitro experiment, the following 3 commonly used guidewires were tested: the polymer jacketed Gladius EX (Asahi Intecc) with a tip load of 3.0 g, and 2 softer guidewires, the tapered, polymer-jacketed Fielder XT-A (Asahi Intecc) with a tip load of 1.0 g, and the workhorse Sion (Asahi Intecc) with a tip load of 0.7 g (Figure 5). As expected, the various guidewires exhibited distinct penetration forces. As no significant difference was found between the individual microcatheters including the Micro Rx, data was combined into one curve for improved clarity.

Tip load provided by the manufacturers is derived from a standardized test method where the guidewire is assessed for the distal 10 mm. The proximal part of the guidewire is excluded from this test method. In the in vitro evaluation we tested the performance of the whole guidewire length in a microcatheter within a vessel structure (2.5-mm silicone tube). The combination of the guidewire and microcatheter will bend and settle in the simulated vascular model based on its integral technical design before providing any forward push. Therefore, our results provide different tip penetration forces compared with the standardized tip load test method. However, we assessed a much more clinically relevant situation.

With the addition of a microcatheter (Micro Rx), the guidewire penetration force significantly increased. Even when the tip of the Micro Rx was placed 20 mm behind the tip of the guidewire, the penetration force increased by 2.5-, 2-, and 1.25-fold, respectively, compared with the bare guidewire. The backup support increased gradually as the tip of the Micro Rx moved closer to the tip of the guidewire. With the tip of the Micro Rx at 10 mm behind the tip of the guidewire, the penetration force was approximately 3, 2.5, and, 1.5 times higher than for the bare guidewire, and when the tip of the Micro Rx was 1 mm from the tip, a 5-, 4.25-, and 2-fold increase was found. 

Increase of tip penetration force was distinct for the 3 guidewires, with the Sion having the most modest, and the Fielder XT-A having the most pronounced multiplication of its penetration force. Absolute forces, however, remained within the guidewires' category, with the Gladius in the highest end (70-80 N/mm²), the Fielder XT-A in the mid-range (40-50 N/mm²), and the Sion at the lower end (20-30 N/mm²).

Micro Rx guidewire support and device crossing in an occluded vessel. The AOM was operated in the subclavian artery as presented in Figure 4, Panel C. Multiple 0.014” Sion Blue guidewires (Asahi Intecc) were used in the evaluation of the Micro Rx vs the reference devices.

The Micro Rx and reference devices were inserted multiple times by all operators to prevent experimental bias. The adjustable AOM was closed down to find the point of failure (guidewire crossing and device crossing) and enable comparison of performance between devices. The intervention was fluoroscopy-guided to control the advancements of the Micro Rx and the guidewires (Figure 4, Panel D).

The unanimous consensus among the expert physicians in this in vivo experimental test setup is summarized below:

·     For guidewire support and microcatheter crossing, the Micro Rx was better compared with the Corsair Pro and Turnpike Spiral.

·     For guidewire support, the Micro Rx was equivalent to the Finecross. The Finecross allowed easier crossing compared with the Micro Rx.

·     The ease of use of the Micro Rx was significantly better than all reference devices.

Micro Rx guidewire support and device crossing in a tortuous vessel. The ATM was operated in the subclavian artery as presented in (Figure 4, Panel C). A 0.014” Sion Blue guidewire was used for evaluating the Micro Rx against the reference devices.

The Micro Rx and reference devices were inserted multiple times by all operators to prevent experimental bias. The adjustable ATM was closed down to find the point of failure (guidewire crossing and device crossing) and enable comparison of performance between devices. The intervention was fluoroscopy-guided to control the advancements of the Micro Rx and the guidewires (Figure 4, Panel E).

The unanimous consensus among the expert physicians in this in vivo experimental test setup is summarized below:

·     Guidewire support and microcatheter crossing of the Micro Rx was superior compared with the Turnpike Spiral.

·     Guidewire support and micro catheter crossing of the Micro Rx was better compared with the Corsair Pro.

·     Guidewire support of the Micro Rx was equivalent to the Finecross. The Finecross had better microcatheter crossing compared with the Micro Rx.

·     The Micro Rx was easier to use than all reference devices.

All in vivo tests were performed by each of the 3 operators in both animals. Figure 6 shows all experimental results from the 2 studies.

The Micro Rx represents a novel device category that has the potential to enable non-chronic total occlusion (CTO) operators to successfully treat more complex anatomies.² The device features a rapid exchange design with a highly pushable proximal and distal shaft and a high-penetration tip design with the smallest leading-edge profile on the market. The specially designed end stop allows the unique insertion and removal of a 6F guide extension catheter without first removing the Micro Rx. Overall, the unique design of the Micro Rx provides an easy-to-use alternative to OTW microcatheters in tight lesions and tortuous anatomy where additional guidewire support is required without the need for guidewire exchange.

In the experimental in vivo setup, guidewire support of the Micro Rx was at least as good as current state-of-the-art OTW microcatheters commonly used in daily practice. The physicians who evaluated the device had never used a Micro Rx before and found the device easy to handle. The rapid exchange design made it possible to quickly insert and retract the Micro Rx without the need for trapping that is required for OTW microcatheters.

With the exception of Finecross, the Micro Rx demonstrated superior crossing through the AOM. It is expected that the superior crossing of the Finecross is related to its lower crossing profile. The pins of the AOM have no flexibility and remain fully rigid, which results in mechanical limitation to push a catheter through. In the AOM, the backup support provided by all OTW catheters was lower compared with the Micro Rx. This is mainly a consequence of the proximal metal push rod that provides more support compared with a braided or coiled microcatheter.

When investigating guidewire support and microcatheter crossing in ATM, the Micro Rx was again superior compared with the Turnpike Spiral and Corsair Pro, and equivalent to the Finecross. The Finecross had better microcatheter crossing compared with the Micro Rx, which is most likely a consequence of its lower overall profile (0.85 mm vs 0.6 mm, respectively). However, the Micro Rx exhibited an easier overall use than all reference devices.

Modifying guidewire tip penetration force. Contemporary guidewires feature distinct penetration forces for different wire tasks. Our experiments showed that the addition of a microcatheter can modify their tip penetration force and change the wire characteristic. Moving the microcatheter closer to the guidewire tip, especially towards the last few millimeters, increases the ability of the guidewire to penetrate harder tissue or cross a very tight lesion. Using a microcatheter may enable crossing of complex coronary lesions with less aggressive wires. Given its easy handling and improved performance compared with OTW microcatheters, the Micro Rx could facilitate crossing of complex lesions by non-CTO operators.

Unique guide extension feature. The guidewire backup support provided by a microcatheter is sometimes insufficient. A common next step to increase the backup support is the use of a guide catheter extension.¹¹ If short guidewires are used, inserting a guide catheter extension if a microcatheter is already in place requires multiple procedural steps. The microcatheter first requires trapping for removal and to allow loading of the guide extension on the guidewire; after insertion of the guide extension, the microcatheter needs to be loaded back on the guidewire (again with trapping, inside the guide extension or using a Trapliner [Teleflex]).¹ The Micro Rx has a very low-profile proximal end stop (< 1 mm). Due to this design, a guide extension can be simultaneously loaded over the guidewire as well as over the Micro Rx, thereby improving the ease of use and eliminating the extra procedural steps needed with OTW microcatheters.

The ability to dramatically increase tip load and support, combined with its simple application, makes Micro Rx highly attractive for both CTO and non-CTO operators even with workhorse guidewires.

Single-lumen OTW microcatheters allow guidewire exchange, which is especially important when crossing CTOs. The Micro Rx is not recommended as a front-line microcatheter for CTO crossing due to its inability to allow guidewire exchange without losing distal access. However, the very small catheter entry profile of the Micro Rx in combination with the very strong pushability demonstrated in the in vivo experiment make it an attractive option for microcatheter and balloon uncrossable CTOs (and non-CTO lesions) after successful guidewire crossing.¹² In this setting, the Micro Rx might be able to create an initial channel based on its very low entry profile (0.43 mm). Further crossing of the Micro Rx enabled by its great support creates a bigger channel (0.85 mm) through the lesion that may facilitate subsequent balloon crossing or guidewire exchange through a single-lumen OTW microcatheter.

Based on the initial bench tests and in-vivo experience, the Micro Rx is an easy-to-use alternative to OTW single lumen microcatheters in complex PCI where additional guidewire support is required without the need for guidewire exchange. The Micro Rx provides outstanding support, low tip entry profile, and the ability to add a guide extension. Typical applications include tortuous anatomy and severely stenosed (but not CTO) vessels.

Affiliations: ¹University Heart Center, Basel, Switzerland; ²Center for Coronary Artery Disease, Minneapolis Heart Institute and Minneapolis Heart Institute Foundation, Minneapolis, Minnesota, USA; ³Maria Pia Hospital, GVM Care & Research, Turin, Italy.

Data availability statement: The numerical data used to support the findings of this study are included within the article.

Funding statement: Device development and animal testing was financed by the manufacturer (IMDS, The Netherlands).

Disclosure: Gregor Leibundgut, Emmanouil S Brilakis, and Roberto Garbo served on the advisory board for the development of the Micro Rx, and have received consulting fees and honoraria for educational events for IMDS.

Manuscript accepted September 12, 2023.

Address for correspondence: Gregor Leibundgut, MD, Department of Cardiology, University Hospital Basel, Petersgraben 4, 4031 Basel, Switzerland. kardiologie@mac.com.