A 45-year-old African-American man presenting with chest pain radiating to his back was found to have a Stanford type B dissection on computed tomography (CT) scan, with the flap originating distal to the left subclavian artery (LSCA) and not involving the arch. The scan also revealed a “bovine arch” pattern of vessel branching (Fig. 1). The patient was initially treated medically with antihypertensive drugs but lost left lower extremity pulses and became obtunded on hospital day 2, requiring intubation. A head CT demonstrated an old lacunar infarct but no acute process, and a repeat chest CT confirmed the LSCA origin of the dissection without significant change from the original scan. He was taken emergently to the operating room for a femoral-femoral arterial bypass and, because of his declining mental status in the context of an aortic dissection, possible chest exploration for retrograde extension of the dissection.
Intraoperative transesophageal echocardiography (TEE) was performed that demonstrated the dissection flap in the LSCA and an entry point just distal to it (Video 1, see Supplemental Digital Content 1, http://links.lww.com/AA/A685), without any flap noted in the ascending aorta. By turning the probe to the right from the LSCA, the bovine trunk, measuring 24 mm in diameter, was clearly seen (Fig. 2). Compared with the LSCA, there was qualitatively less color flow Doppler signal obtained in the trunk, and the velocity-time integral (VTI) by pulse wave Doppler was significantly lower (Video 2, see Supplemental Digital Content 2, http://links.lww.com/AA/A686). Given these data suggesting reduced flow in the bovine trunk, along with the patient’s clinical picture, sternotomy was performed. Surgical inspection of the aorta revealed a blue discoloration of the arch and, after aortotomy, extension of the dissection into the bovine trunk that contained mural clot. Circulatory arrest with selective antegrade cerebral perfusion was initiated, and a total arch replacement was performed with reimplantation of the LSCA and bovine trunk into a Hemashield® graft. Total body perfusion was reestablished, and the patient was weaned from cardiopulmonary bypass after 257 minutes. The patient was taken to the intensive care unit in stable condition, although his postoperative course was complicated by multiple cerebral infarctions and eventual transfer to a long-term acute care facility on postoperative day 19. Consent for publication of this case was obtained from the patient’s family.
A normal aortic arch branching pattern consists of 3 major vessels: the innominate artery (IA), left common carotid artery (LCCA), and LSCA. The most common variant has only 2 branches with the IA and LCCA sharing a single trunk, as illustrated in Figure 1. The common trunk is often called a “bovine trunk” because the normal arch in cattle has 1 common trunk from which all arch vessels then branch. The “bovine arch,” although not strictly resembling that of cattle, refers to patients with this IA/LCCA common takeoff and has a reported incidence of approximately 10% to 20%.1
Initially thought of as a benign normal variant, patients with a bovine arch may be predisposed to aneurysms and dissections. One study showed patients with thoracic aortic aneurysms are more prevalent with bovine arch anatomy (20.7% incidence vs 6.7% in 3-vessel arch).1 They may also have a propensity for more rapid growth and aortic dissection, particularly Stanford type B, and require increased surveillance.2–4 Since the bovine arch has 1 less vessel to act as an anchor point, it may also be more vulnerable to torsion and rapid deceleration in traumatic injury.
Imaging the aortic arch using transthoracic echocardiography is relatively straightforward using the suprasternal notch window. The arch vessels can typically be imaged in their longitudinal orientation, often simultaneously.5 In contrast, imaging the arch with TEE can be more challenging due to limited windows from bronchial interposition. However, the LSCA is identifiable in most patients from the upper esophageal aortic arch short-axis view and is an important landmark.6 Turning the probe to the left in this view images more distally along the aorta, while turning to the right images more proximally. To properly identify each branch vessel, we recommend beginning from the upper esophageal aortic arch short-axis view, with the probe turned to the extreme left and slowly turning back to the right, identifying each branch in sequence (Video 3, see Supplemental Digital Content 3, http://links.lww.com/AA/A687). Slight advancement or withdrawal of the probe may be necessary during this process. The average size of branch vessels is 5 to 9 mm,7 but anatomic variants with common trunks may be larger and thus easier to identify.
Spectral Doppler analysis of the arch vessels is problematic using TEE due the axis of flow along them being almost perpendicular to the imaging plane. Nevertheless, color flow Doppler provides a useful qualitative measure of flow and should raise some concern if no signal is present. In our particular patient, we used pulse wave Doppler to obtain a VTI in both the LSCA and bovine trunk. While the absolute values are of questionable significance, the fact that the bovine trunk had a demonstrably lower VTI than the branch vessel next to it helped to prompt further surgical investigation.
In conclusion, intraoperative echocardiographers should be familiar with imaging the aortic arch to the extent that TEE allows. Carefully going from the distal to proximal arch can help ensure the important LSCA landmark is properly identified and not confused with other branch vessels. It may also help identify common anatomical variants, such as the bovine trunk, that may have implications for aortic disease.
Clinician’s Key Teaching Points
By Nikolaos J. Skubas, MD, FASE, Donald Oxorn, MD, and Martin J. London, MD
- The normal aortic arch (AA) has 3 branches, the left subclavian artery (LSCA), the left common carotid artery (LCCA), and the innominate artery (IA) arising from the distal to the proximal AA. A “bovine arch” is the most common congenital anomaly of the AA, wherein the LCCA and IA arise from the same enlarged (“bovine”) trunk. Since a bovine AA is particularly vulnerable to torsion and rapid deceleration in traumatic injury, transesophageal echocardiography (TEE) can be useful in diagnosing complications, such as dilatation, rapid growth, and descending aorta dissection.
- These branches can be imaged in their long-axis orientations using transthoracic echocardiography in the suprasternal notch window also allowing measurement of blood velocity spectral Doppler. The AA branches can be interrogated with TEE by locating the LSCA first, in the upper esophageal AA short-axis view. The LCCA and IA are then imaged with gradual turn of the probe to the right and slight withdrawal or advancement. However, with TEE, only qualitative flow information can be derived using color flow Doppler, as it is not possible to measure the respective blood velocities because of the near perpendicular angle between the Doppler beam and the various branches.
- In this case, a patient with bovine AA and type B aortic dissection developed loss of lower extremity pulses. A computed tomography scan showed proximal progression of the dissection distal to the LSCA, and the patient was initially scheduled for femoral-femoral arterial bypass. Intraoperatively, TEE imaging with color flow Doppler showed decreased flow inside the proximal end of the bovine trunk, which was enlarged (24 mm), as compared with 5 to 9 cm average diameter of the arch branches. This confirmed the proximal progression of dissection and prompted surgical replacement of the AA.
- A wide takeoff of the AA branches should prompt the consideration of a bovine trunk and a reduced color or spectral Doppler velocity should be considered suspicious of compromised flow, as in dissection.
Name: Logan Kosarek, MD.
Contribution: This author helped analyze the data and write the manuscript.
Attestation: Logan Kosarek approved the final manuscript.
Name: Sofia Fischer, MD.
Contribution: This author helped analyze the data.
Attestation: Sofia Fischer approved the final manuscript.
Name: Roman Sniecinski, MD.
Contribution: This author helped analyze the data and write the manuscript.
Attestation: Roman Sniecinski approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
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