Approach to Optimizing Endovascular Position in Aortic Dissection Repair Using Intravascular Ultrasound and TEE

Introduction 

Accurate identification of the true vs false lumen is a critical step prior to deploying an endovascular device in patients with aortic dissection. The importance of true lumen deployment cannot be overstated, as inadvertent false lumen deployment may result in significant morbidity and mortality. Intravascular ultrasound (IVUS) and transesophageal echocardiogram (TEE) are valuable tools that help establish true lumen access intraoperatively during both type A and type B aortic dissection repair, building upon the knowledge of dissection anatomy and true lumen shape gathered from preoperative computed tomography angiography (CTA) imaging.  

Specifically, in type B aortic dissection, the use of IVUS has been independently associated with improved long-term survival, with the Society for Vascular Surgery clinical practice guidelines strongly recommending IVUS examination for thoracic aortic aneurysms (1). Similarly, TEE is helpful for visualizing the intimal flap and confirming blood flow within the correct lumen. 

The primary goal of any operation involving endovascular aortic stent placement is to ensure true lumen access in the proximal and distal landing zones of the stent. While both IVUS and TEE can assist in better understanding dissection anatomy and confirming correct placement of the stent, these techniques require significant experience to interpret effectively. Additionally, there are numerous limitations to these techniques that the user must recognize.  

For the cardiothoracic surgical trainee, most familiarity with IVUS and TEE comes through hands-on experience and pattern recognition in the operating room. Given the lack of available educational resources regarding the use of these modalities in aortic dissection, the authors compiled their approach to achieving optimal endovascular positioning of an aortic graft using both IVUS and TEE. 

Type A Dissection

The frozen elephant trunk (FET) is a widely utilized technique for the repair of aortic arch and proximal descending aortic dissections, combining a soft branched graft for replacement of the proximal aorta with a stent graft to treat the distal aorta. For type A dissections requiring a FET repair, TEE was used solely for confirmation of true lumen access. This approach allowed for the use of a 5 French common femoral artery sheath instead of the 9 French required for IVUS. It is important to note that the authors are deliberate about which femoral artery is accessed based on preoperative CT imaging, as one side may lend itself better to cannulation of the true lumen. 
 
Access to the common femoral was gained using a micropuncture needle and wire, enabling the placement of a 5 French sheath. Following this, a stiff J wire was placed through the femoral sheath and advanced until it was seen in the descending aorta on TEE. Unless resistance is met, this move is typically performed blindly without the use of fluoroscopy. If any resistance is met, then this procedure can be done using a hydrophilic-coated guidewire and exchange catheter.  

Once the wire was identified on TEE in the descending aorta, it was followed proximally up to the subclavian artery, ensuring it is within the true lumen for the length of the descending aorta. It is possible the wire may pass through fenestrations between the true and false lumen within the abdominal aorta, however, as long as the wire ends up in the true lumen for the anticipated length of the stent in the descending aorta, this is inconsequential. During circulatory arrest, the aortic arch was resected, and the wire was pulled out from the distal aorta into the operative field. The stent graft device was then loaded onto the wire and advanced down the distal aortic arch into the descending aorta, guaranteeing true lumen placement for the length of the stent.  

Following stent deployment, a final examination with TEE should be performed, demonstrating flow in the true lumen and static flow in the false lumen. This will also provide an assessment of stent expansion within the true lumen, as well as ensure that there is no collapse of the true lumen distal to the stent, which could indicate a new reentry tear. If a reentry tear is identified, it can be repaired with an extension thoracic endovascular aortic repair (TEVAR) via the femoral artery. 
 
Type B Dissection 

TEVAR is the preferred treatment option for complicated type B aortic dissections. With the development of branched designs, branched TEVAR has become a popular modality for addressing aortic pathology extending into the aortic arch. The most common of these interventions involves zone 2 stenting into the left subclavian artery. 
 
For TEVAR cases performed for aortic dissection, both TEE and IVUS are utilized to identify the true lumen, as relying on one modality alone can be misleading. Starting with a micropuncture needle and wire, a 9 French sheath is placed in the common femoral artery. The IVUS catheter is then loaded over the wire through the 9 French sheath, providing a 360-degree, real-time view of the vessel in cross-section and enabling accurate measurement of the vessel as well its relationship and proximity to branch vessels. 

In addition, IVUS provides information on fenestrations within the intimal flap between the true and false lumen, which may add significant complexity to the dissection. Lastly, IVUS provides helpful information on flow characteristics within the false lumen, including the presence or absence of thrombus or static flow. 

To identify the true lumen using TEE, the authors typically utilize several different measurement modalities, which are repeatedly performed at various points along the length of the dissection. These include observing which way the intimal flap moves during systole, using color flow Doppler to determine which lumen fills first, and using continuous wave Doppler to assess the direction of blood flow through a fenestration. However, these methods cannot be utilized in isolation and should be combined with data from IVUS to verify wire placement in the true lumen. Likewise, repeated assessments along the length of the dissection are crucial, as it is easy to be misguided by any one specific view along the aorta. 
 
Limitations 

While TEE and IVUS are helpful adjuncts for better understanding dissection anatomy, they do not always tell the whole story and should only be interpreted in the context of a thorough understanding of a patient’s CT imaging. Information gathered preoperatively from the CTA regarding true lumen shape, extent of the dissection, and location of entry tears or large fenestrations provides valuable insights into what to expect intraoperatively on IVUS and TEE. Ultimately, concerns about creating false lumen flow and ischemic organ beds should overwhelm their confidence in true lumen access.  

If certain views do not make sense, it is imperative to look at the dissection in a different way. Always ask yourself, “Are we missing something because of our modality?”  
 
While various strategies are mentioned to help identify the true lumen from the false lumen on IVUS and TEE, each has specific limitations that are often overlooked. The next section summarizes these techniques and their corresponding potential pitfalls. 
 
Strategies to Identify True Lumen Limitations 

Identifying the lumen that fills first can be misleading, as with an increasing number of fenestrations, the primary flow may occur in the false lumen instead of the true lumen. Visualization of the guide wire can also be challenging; poor acoustic windows or dropout artifacts can inhibit visualization of the wire crossing over into the false lumen. Additionally, the wire may enter and exit the true lumen either distally or proximally to the location measured, as TEE only provides a snapshot at one location, while IVUS can evaluate the length of the aorta. 

The identification of the aortic intima layer can also present difficulties, as the brightness and echogenicity of the intima is not always visible. Furthermore, the systolic motion of the flap can be affected by nearby large fenestrations, which may cause the false lumen to become pressurized first, resulting in paradoxical systolic motion in the false lumen instead of the true lumen. Lastly, the systolic motion of the true lumen becomes less prominent the further away from the primary tear. 

References

1. Upchurch GR, Jr., Escobar GA, Azizzadeh A, et al. Society for Vascular Surgery clinical practice guidelines of thoracic endovascular aortic repair for descending thoracic aortic aneurysms. J Vasc Surg. Jan 2021;73(1S):55S-83S. doi:10.1016/j.jvs.2020.05.076

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