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The different forms of the left brachiocephalic vein visualized during cardiovascular implantable electronic device implantation procedures

Background: Among the factors ensuring successful completion of such minimally invasive procedures as cardiac implantable electronic device (CIED) implantation and central venous catheter (CVC) placement are the morphometry and topography of the vessels used for cardiac lead or catheter advancement. Venous access through the left clavipectoral triangle makes use of the left brachiocephalic vein (BCV). The purpose of this study was to present the radiology images of various individual forms of this vessel observed during CIED implantation procedures. Methods: Our analysis included 100 venography recordings illustrating the left BCV, obtained during de novo CIED implantation procedures. We assessed the mediastinal course of the left BCV, with its natural angles, including angle α (in the middle section of the vessel) and the two angles created by the left BCV and the left subclavian vein (angle β) and the left BCV and the superior vena cava (angle γ). Results: The mean values of angle α tended to be higher (approximately 141°) than those of the two remaining angles (γ and β), which were comparable at 123° and 127°, respectively. An increase in mean angle α values were accompanied by increased mean angle γ and β values (p = 0.05), with only 5% of β and γ angles, in total, having values close to those of a right angle (90 ± 10º). Conclusions: Individual variability of left BCV topography and morphology comes from developmental formation of the physiological angle in the middle section of this mediastinal vessel’s course. The presence of near-right angles along the course of left BCV may potentially result in injuries to the vessel during intravascular procedures.


INTRODUCTION
Performing minimally invasive transvenous procedures, such as cardiac implantable electronic device (CIED) implantation, facilitates making observations as to the individual differences in vessel layout. These differences can be due to either normal anatomical variations or developmental anomalies [1,2,3,4].
Minimally invasive CIED implantation or central venous catheter (CVC) placement procedures, which make use of mediastinal systemic veins, depend to a large extent on favorable morphometry and topography of the brachiocephalic veins (BCVs), also known as the innominate veins. When the venous system is accessed via the left clavipectoral triangle, the vein used for catheter or cardiac lead advancement within the middle mediastinum is the left BCV-formed by the left jugular vein and the left subclavian vein (SV) merging-which subsequently drains to the superior vena cava (SVC) (Fig. 1A).
Left BCV topography, which depends on the nature and size of the natural anatomical angulations along the course of the vessel, may affect the course of CIED implantation or CVC placement procedures [5,6]. These natural angulations include: the angle at the middle segment of the left BCV, where its wall is anterior and adjacent to the arteries branching off the aortic arch (angle α, Fig. 1), and two other evaluated angles: one located at the site where the left SV continues into the left BCV (angle β, Fig. 1B, C) and the other located at the site of drainage of the left BCV into the SVC (angle γ, Fig. 1 B, C). 2 The extent of individual variations and the spatial relations of the left BCV to adjacent anatomical structures may contribute to vessel injury during CIED implantation or CVC placement procedures even in the case of a physiological course of the left BCV [ 7,8,9,10]. There may be direct mechanical injury to some or all the layers of the venous wall, with additional damage to the adjacent arteries in more severe cases; there may also be long-term complications in the form of post-thrombotic lesions at the site of vascular injury [11,12,13,14,15].
The risk of vascular injury is particularly high in the case of anatomical variations of considerably unusual morphometry and topography. With the angulations along the left BCV course seemingly constituting the most vulnerable sites [5,6,16].
The fact that left BCV variations mentioned above are typically asymptomatic means that they are usually detected incidentally during an intravascular procedure [17].
The purpose of this study was to present radiographic data on different forms of the left BCV and the analysis of the types and rates of the individual forms of the vein encountered during CIED implantation procedures over a six-year period.

METHODS
We retrospectively analyzed the venography images obtained during de novo CIED implantation procedures performed during the period of 2014-2019. Only the images that definitively and clearly showed the left BCV course, including the entire length of its lumen and the junctions with the left SV and the SVC visualized via a contrast medium, were considered in the analysis ( Fig. 1) The cases where left BCV lumen morphometry had been difficult to interpret or the angles to measure due to such factors as developmental anomalies of the vessel, poorly contrasted images, past cardiothoracic surgeries, or an atypical left BCV course, were excluded from analysis ( Fig. 2 A, B). The evaluated angles were measured at the points where the straight lines drawn along the middle of the contrasted lumina of adjacent venous segments intersected ( Fig. 1 B, C). In order to ensure data comparability, all venous angle measurements were made on the images obtained during the expiratory phase. We believe that the changes to the left BCV course, including the angles, taking place during that phase may increase the risk of a potential local injury to the vessel during CIED implantation or CVC placement. This study had been approved by the Institutional Review Board.

RESULTS
Out of the 2,112 de novo CIED implantation procedures performed over the evaluated period, we initially selected the 391 that had required the use of venography to 4 visualize the vessels through which the cardiac lead was being advanced. Due to our objectives, which required an unequivocal interpretation of the obtained left BCV images, the number of recordings selected for analysis was ultimately pared down to 100. The selection criteria included good visualization of the entire lumen of the vein's course through the mediastinum, including the SV drainage point and the left BCV drainage point to the SVC, in order to allow the evaluated angles (α, β, and γ) to be measured ( Fig. 1. B, C).
In the cases with no discernible angulation in the left BCV and the vein's oblique course through the mediastinum (Fig. 3B), the vein's slope with respect to the transverse plane ranged from 20° to 45° (Fig. 4 A, B).

Comparisons between angles:
Comments: The scatter plots show a positive correlation between angles α and β and angles α and γ. Moreover, there is a slight positive correlation between angles γ and β; however, it is less pronounced than the other two ( Fig. 3A-C).
There was a significant, though weak, positive correlation between angles α and β and angles α and γ. Higher values of angle α may be expected to be associated with higher mean values of angles γ and β. The positive correlation between angles γ and β was not statistically significant; therefore, no conclusions can be drawn as to a correlation between these two angles.
Another confirmation of significant differences between the angles was provided by the results of analysis of variance and Student's t test for the difference of means (with the Bonferonni correction due to multiple comparisons).
Analysis of variance results demonstrated significant differences between the evaluated angles (p < 0.0001). The results of Student's t test with p = 0.05 showed significant differences between the mean angle values when comparing angles γ and α and angles β and α, with no significant differences between angles γ and β. 6 Therefore, we can conclude that angles α were significantly more obtuse than the other two types of analyzed angles. However, we cannot state that angles γ and β differed significantly in their values. times longer than its right counterpart [18]. Each of the BCVs drains its corresponding side of the chest by collecting blood from the ipsilateral internal jugular vein and SV as they join.
The most typical left BCV topography involves the vein coursing above the aortic arch (AA) and anterior to its branches (the brachiocephalic trunk, left common carotid 7 artery, and left subclavian artery. Once it crosses to the right side of the upper mediastinum, the left BCV joins the right BCV to form the SVC [4].
During transvenous CIED implantation procedures, cardiac leads are typically introduced via the veins of the left clavipectoral triangle. In our center this method is used in approximately 97% of de novo CIED implantation procedures.
Radiographic visualization of the left BCV considerably helps in assessing the vein's morphometry and the nature of its angulations, as well as the position of these angulations with respect to adjacent structures. The nature of the angles along the course of the left BCV through the mediastinum helps observe any relationships between them (Fig.   9, 10). These relationships vary to some extent during chest movements during the expiratory phase of respiration ( Fig. 1 B, C).
The unique character of venous angulations may predispose to venous wall injuries during CIED implantation or CVC placement procedures. In severe cases, the injury may involve damage to adjacent anatomical structures, producing clinical manifestations.
Nonetheless, even an epithelial injury may lead to long-term complications, such as postthrombotic lesions [6,15,16,19,20]. The risk of vascular injury is increased by higher numbers of inserted leads, specific structural characteristics of the vein (diameter, wall stiffness, etc.), and any attempts to forcibly overcome potential difficulties in lead advancement [21,22].
The risk of intraoperative venous injury is also increased in the vascular segments forming right-angle or sharp-angle angulations. In the analyzed materials such vulnerable venous segments were: the site where the left SV and the left BCV meet (angle β) and the site of left BCV drainage to the SVC (angle γ) [15,16,21]. These angles created such unfavorable conditions in 5%-6% of cases, collectively (Fig. 8 A, B).
In most cases angle α was an obtuse angle (Fig. 8 C) Angles whose values were close to 90° were found in 5% of cases and were most