The Physics of Guiding Catheters for the Left Coronary Artery
in Transfemoral and Transradial Interventions

Although the backup force of a guiding catheter is important for successful percutaneous coronary intervention (PCI), no theory has been proposed thus far regarding the factors involved in its generation. Thus far, our understanding stems only from the opinions of experienced individuals in this field. In this study, we constructed an arterial tree model and measured the backup force of guiding catheters in simulations of transfemoral interventions (TFI) and transradial interventions (TRI). In this paper, we discuss our observations on the important factors that determine the backup force of guiding catheters in the left coronary artery. Materials and Methods Aorta model. An arterial tree was constructed of polyvinyl chloride tubes. The outer and the inner diameters of the tubes (outer/inner) representing the vessels were: 36 mm/30 mm for the aorta, 10 mm/8 mm from the start of the radial artery to the aorta, and 6 mm/4 mm at the coronary artery. The structure of the arterial tree is three-dimensional, as shown in Figures 1A–C. Measurement of backup support. The aortic model was filled with water at 37°C. A guiding catheter was inserted into the model of the coronary artery via the right radial or the femoral artery approach. A 0.014 inch Runthrough floppy guidewire (Terumo, Tokyo, Japan) was passed through the coronary artery to the second curve. A 1.5 x 20 mm Arashi balloon catheter (Terumo) was automatically pushed at 5 mm/sec by a motor drive with a push-pull gauge (Figure 1D) until the guiding catheter dislodged from the coronary ostium. The maximum resistance was considered as the maximum backup force at which the guiding catheter could remain in the working position. Guiding catheters. All guiding catheters in this study consisted of 6 French (Fr) Heartrail shafts (Terumo), except for the size variation study. Judkins Left (Figure 2A),¹ Ikari Left (Figure 2C),^2,3 and Backup Left (EBU/XB) (Figure 2D) catheters were used. To study the factors generating backup force, we constructed several guiding catheters with special shapes that were not commercially available; for example, we constructed a Judkins catheter with a reverse bend at the brachiocephalic artery (Figure 2B), and an Ikari catheter with a longer flat portion (Figures 2E–2H). Reproducibility of data in this model. First, we tested the reproducibility of this experimental model. Backup support was measured 10 times in the left coronary artery model in TFI modeling experiments using a 6 Fr Judkins Left 4 (JL4). The average maximum resistance was 63.1 ± 2.1 gram force (gf), and the coefficient of variance was 3.29%. By these criteria, we considered that the reproducibility of this model was within acceptable limits. Statistical analysis. Data were expressed as mean ± SD. Analysis of variance (ANOVA) was used for comparison of means of the three groups. A p-value (Cary, North Carolina). Results Catheter size and backup force. First, in this model, we compared the backup force of different-sized guiding catheters. The maximum resistance was considered to be the maximum backup force, as noted in the methods. When a JL4 was applied via the transfemoral approach, the maximum resistance was 63.1 ± 3.1 gf with a 6 Fr catheter, 96.7 ± 2.7 gf with a 7 Fr catheter, and 139.0 ± 4.3 gf with an 8 Fr catheter. The larger French size guiding catheter produced a significantly greater backup force (Figure 3). Approach site and backup force. Second, we compared the backup force between right TRI and TFI. The backup force measured in the in vitro model was 1.6 times larger when a JL4 was used in TFI (Figure 4). When a JL4 was applied in TRI, the angle theta-r was smaller than theta-f in TFI (Figures 4 A and B). When we used a backup (EBU/XB) type catheter in TRI and TFI, the backup force was larger in TFI than in TRI (Figure 5). There was a small (8%) but statistically significant difference between the TRI and TFI (p Techniques to increase backup force in TRI using Judkins. Many experienced operators prefer the JL3.5 to JL4 in TRI. When the JL4 is applied in TRI, the angle theta-4 is smaller than the theta-3.5 of the JL3.5 (Figures 7A and B). The backup force measured in the in vitro model is significantly larger when the JL3.5 is used (Figure 7D). Furthermore, experienced operators use a deep engagement technique with the JL to generate greater backup force. Deep engagement results in an angle theta-4 (deep), which is larger than theta-4 in the usual position (Figure 7C). The change of angle may be a factor that generates greater backup force in addition to friction between the guide and the coronary artery (Figure 7D). Comparison of backup force in TRI including power position of Ikari guide. Backup (EBU/XB) type catheters have greater backup force than a JL4 with deep engagement. The IL4 had a small (8%), but significantly greater, backup force than the backup-type catheters. Operators experienced with the Ikari-type catheter use a power position technique. When an IL guide catheter is pushed along the guidewire, the catheter shape changes. This is called the power position of the IL (Figure 8D). Although it is a simple technique, it generates greater backup force than the original position (Figure 8E). The backup force may be determined by the angle q (Figures 8A–D). As the angle became larger, the backup force was greater in this model (Figure 8E). Contact area on the aorta. To study the effect of the catheter’s contact area on the aorta, we constructed modified Ikari guides with varying attachment areas (Figures 9A–D). The measured backup force was greater in catheters with longer attachment (Figure 9E). However, there was a limit in terms of length beyond which there was no further increase in the backup force. The brachiocephalic angle. We attempted to study one additional factor that would account for the backup force of a guiding catheter. The brachiocephalic angle is specific to TRI and does not exist in TFI. Catheters with an angle at the brachiocephalic artery may generate better backup force. To study this hypothesis, we constructed a modified JL with an angle at the point of contact with the brachiocephalic artery (Figures 10A and B). The measured backup force was the same between the original and the modified JL catheter. Thus, the angle of the guiding catheter does not increase the backup force (Figure 10C). Discussion In this study, we quantitatively measured the backup force of guiding catheters for the left coronary artery. Three factors were found to be associated with the backup force: 1) catheter size; 2) angle (theta) of the catheter on the reverse side of the aorta; and 3) contact area. Based on these observations, we derived the power equation at the point of catheter contact on the reverse side of the aorta (Appendix). The angle (theta) determines the vertical vector that can dislodge the guiding catheter. A smaller cos-theta results in a greater backup force. This suggests that the lower position is preferable as the point of contact on the reverse side of the aorta because the angle approaches 90 degrees. Many common observations can be explained by this hypothesis, such as the weak Judkins backup force in TRI and the better performance of the JL3.⁵ in TRI. A strong backup force in deep engagement may be due to the change of the angle. A second factor that determines the backup force is static friction. Newton’s law cannot be applied to predict static friction because the wall of the aorta is elastic. Although the friction mechanism is likely to be complex, the larger contact area may increase friction. Our in vitro model showed that a longer flat portion generated greater backup force; however, if the flat part of the catheter was too long, the backup force was lost, probably because a long flat portion results in a larger angle theta’. Based on the in vitro model, the backup force is greater when the flat portion is 35 mm. However, the length of the flat portion in the actual IL guide is 25 mm. We chose the 25 mm length as optimal due to its ease of manipulation.Furthermore, the power position of the IL can generate a maximum backup force that is greater than the backup force of the catheter with a 35 mm flat portion. Using a catheter designed with an angle to fit the brachiocephalic artery did not increase the backup force. However, in TRI, the modified JL with the brachiocephalic angle was easier to engage than the original JL. Although it was impossible to quantify the ease of catheter insertion into the coronary ostium, we believe that the angle stabilizes the catheter in the ascending aorta. The IL guiding catheter has proven to provide several advantages in TRI. The angle q of the IL is greater than that of the JL. Also, the area of contact on the reverse side of the aorta is greater than the JL. The angle at the brachiocephalic artery does not increase backup force, but makes it easier to engage the left coronary artery. TRI is more convenient for patients than TFI due to lower bleeding risks and shorter hospital stays.^4–7 However, TRI has not been widely performed, probably because of the weak backup support of guiding catheters. An understanding of the mechanism in which the guiding catheter works in TRI and TFI may help guide the choice of TRI as the preferred approach. Finally, the weak backup force of the Judkins in TRI may be the primary reason for TRI’s infrequent use. This study addressed a number of parameters that affect guide catheter performance. TRI is a reasonable and beneficial approach in PCI and should be considered for more patients, although it does have other limitations such as puncture difficulty and the limitation of potential conduit placement in future bypass surgery. Study limitations. The first limitation is that the model may be too simple to explain the actual mechanism. The angle used in the mathematical model is taken as the projection to the sagittal plane of the aorta. Because the human aorta has a three-dimensional curved surface, one should consider the angles in the other planes. If the formula describes the true behavior, the backup force is infinite when the angle q is equal to 90 degrees. However, we know that an infinite backup force is a physical impossibility. The elasticity of the aorta and the strength of the catheter should also be taken into consideration. Too strong a force can break the catheter and/or dissect the aorta. Acknowledgements. We would like to express our gratitude to Mr. Takenari Ito for setting up the experimental model and for his technical assistance, and Dr. Satoshi Takeshita for his professional comments. The Terumo Company supported this study by supplying the guiding catheters and devices for the experiments. Yuji Ikari is the inventor of the Ikari catheter and is publicly associated with the product.

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