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Interact CardioVasc Thorac Surg 2008;7:244-248. doi:10.1510/icvts.2007.163154 © 2008 European Association of Cardio-Thoracic Surgery
A study of aortic dimension in type B aortic dissection
a Cardiac Surgery Division, Beijing Institute of Heart, Lung and Vascular Diseases, Beijing Anzhen Hospital, Capital Medical University, Beijing 100029, China Received 13 July 2007; received in revised form 8 November 2007; accepted 9 November 2007
*Corresponding author. Tel.: +86-10-64456776; fax: +86-10-64443324.
Difference between arch diameter and true lumen diameter in the descending aorta was studied in patients with type B aortic dissection. The diameters of the aortic arch (Proximal  ) and mid-descending aorta (Distal ) were measured on computer tomography angiography (CTA) in 20 healthy adults. Forty-two patients with type B aortic dissection who underwent endovascular repair were divided into two groups: an acute group (23 patients) and a chronic group (19 patients). The diameters of the arch (Proximal ) and the true lumen of the mid-descending aorta (Distal ) were measured on digital subtraction angiography (DSA) and CTA. The taper ratio was defined as (Proximal –Distal )/(Proximal )x100%. In the control group, the taper ratio was 13.0±4.7% on CTA. In the acute patients group, the taper ratio was 23.6±11.3% on DSA and 21.9±12.1% on CTA. In the chronic patients group, the taper ratio was 31.5±13.6% on DSA and 30.1±11.4% on CTA. In both acute and chronic type B aortic dissection, the aorta tapers significantly from arch to true lumen in the descending aorta. Stent-graft with tapered design may be a viable treatment option for endovascular repair of type B aortic dissection.
Key Words: Aortic dissection; Stent-graft; Endovascular repair
During endovascular repair of type B aortic dissection, the proximal end of the stent-graft is positioned in the aortic arch and the distal end in the middle portion of the descending aorta. As the aorta tapers from the proximal to the distal end, the diameter of the descending aorta is smaller than that of the arch. But nowadays, during clinical practice of endovascular repair of type B dissection, only the proximal landing zone (the aortic arch) is used for sizing. Most of the commercial stent-grafts used do not have a tapered design. In this paper, tapering feature of the aorta was studied both in healthy adults and patients with type B aortic dissection. We want to know through this study if there is a significant difference between arch diameter and true lumen diameter in the descending aorta.
2.1. Control group Twenty healthy adults underwent computer tomography angiography (CTA) of the thoracic aorta. There were 12 males and 8 females with a mean age of 52.7 years (S.D. 13.8, range 32–72 years). The diameters of the aortic arch (Proximal ) and the mid-descending aorta (Distal ) were measured.
From January 2003 to June 2005, forty-two patients with type B aortic dissection underwent endovascular repair. They were divided into two groups: an acute group (23 patients) and a chronic group (19 patients). In the acute group, the time from onset of dissection to stent-graft implantation was less than one month. In the chronic group, the time from onset of dissection to stent-graft implantation was more than two years (range 2–10 years). Indications for endovascular repair of acute aortic dissection were: 1) confirmation of type B aortic dissection on CTA or magnetic resonance angiography (MRA); 2) adequate access route for stent-graft placement; 3) landing zone diameter <38 mm; 4) no aberrant right subclavian artery. In patients in any of the following situations, stent-graft implantation was performed emergently: contained rupture, organ or extremity ischemia, signs of impending rupture (pleural fluid), and persistent pain or refractory blood pressure. In the chronic group, descending aorta was significantly enlarged in each patient (total diameter of true lumen and false lumen >40 mm). Five patients had contained rupture. The study was approved by the Institutional Review Board. Clinical characteristics of the two groups are listed in Table 1. The primary tear was just distal to the opening of left subclavian artery (LSCA) in all patients, and the arch at the origin of the LSCA was used as the proximal landing zone.
2.3. Stent-grafts Four commercially available stent-grafts were used: Talent (Medtronic, Inc., Minneapolis, MN, US), Aegis (Microport, Shanghai, China), Griking (Grikin, Beijing, China) and Endofit (Endomed, Phoenix, AZ, US). All stent-grafts had a cylindrical nitinol skeleton covered with polytetrafluoroethylene (PTFE) or polyester graft fabric. All had a bare stent at the proximal end and a covered length of 100–120 mm. Different sizes were available, ranging in diameter from 26 to 42 mm. The delivery system consisted mainly of an outer sheath and a pushing rod. Stent-graft implantation was performed in the catheter lab by a team of cardiovascular surgeons, interventional cardiologists, radiologists and anesthesiologists. A 5F sheath was inserted into the left radial artery or surgically exposed left brachial artery. A calibrated 5F pigtail catheter was introduced into the ascending aorta through the LSCA. One femoral artery was surgically exposed and used to introduce a 6F pigtail catheter into the ascending aorta. Digital subtraction angiography (DSA) was performed using a left anterior oblique projection (usually 60°). The precise location of the primary tear was identified. The diameter of the landing zone was measured using the calibrated pigtail catheter and compared with the measurement obtained from MRA or CTA before the procedure. A stent-graft with a larger diameter (usually 10–15%) was chosen. Prior to the deployment of the stent-graft, 1 mg/kg or 125 IU/kg heparin was given intravenously. An extra-stiff guide wire was threaded into the ascending aorta through the pigtail catheter. A transverse or longitudinal arteriotomy was made in the femoral artery. The delivery system was introduced over the super-stiff guide wire to the target position. After the systolic blood pressure had been lowered to <100 mmHg, and the heart rate to <90 beats/min, the stent-graft was deployed under fluoroscopic guidance. Repeat DSA was performed to confirm that the primary entry tear had been covered. Each patient had one stent-graft placed. In all patients, the proximal bare stent surpassed the opening of the LSCA. 2.5. Measurements and statistics Three-dimensional CTA was used to measure the diameters of the aortic arch at the origin of the LSCA (Proximal) and the mid-descending aorta (Distal ) in the control group (Fig. 1).
After deployment, the stent-grafts extended from the aortic arch to the mid-descending aorta. DSA during the procedure (Fig. 2) and CTA one week after the procedure (Fig. 3) were used to measure the diameters of the arch (Proximal ) and the true lumen in the mid-descending aorta (Distal ). The calibrated catheter was used as a scale for measurements on DSA.
All measurements were made by an independent radiologist. The difference in diameter between the arch and the mid-descending aorta was defined as (Proximal – Distal ). The taper ratio was defined as (Difference/Proximal )x100%. All data were expressed as mean±S.D. All statistical analysis was undertaken by a specialist.
Comparison of clinical characteristics between the two patient groups shows that patients in the chronic group were older than patients in the acute group. The diameter of the descending aorta was significantly larger in the chronic group than the acute group (Table 1).
In all three groups, Distal was significantly smaller than Proximal (Table 2). In the control group, the taper ratio was 13.0±4.6%. In the acute group, the taper ratio was 23.6±11.3% on DSA and 21.9±12.1% on CTA. In the chronic group, the taper ratio was 31.5±13.6% on DSA and 30.1±11.4% on CTA. The taper ratio was significantly higher in the acute group than the control group (P=0.002). The taper ratio was nearly 10% higher in the chronic group than the acute group, but the difference was not significant (Table 3).
The aorta tapers from the proximal to the distal end. In most cases of endovascular repair of aortic diseases such as thoracic aortic aneurysm, penetrating aortic ulcer or pseudoaneurysm, the whole length of the stent-graft is deployed in the descending aorta. The tapering of the descending aorta is not obvious and is not given much attention. Stent-grafts used in the treatment of the above conditions usually do not have a tapered design. Endovascular repair of aortic dissection is a different situation. In most cases, the aortic arch is the proximal landing zone and the diameter of the stent-graft is decided only according to the diameter of the aortic arch. After deployment, the proximal portion of the stent-graft anchors in the intact aortic arch and the distal portion anchors in the true lumen of the dissected descending aorta. Most currently available commercial stent-grafts used in the treatment of aortic dissection do not have a tapered design. When we had a patient whose descending aorta showed excessive dilatation six months after stent-graft implantation (Fig. 4), we realized the importance of the problem and decided to study the taper ratio between the arch and the true lumen of the descending aorta.
In most cases of aortic dissection, the true lumen is compressed by the false lumen and is elliptical, semielliptical or crescentiform in cross section on MRA or CTA, making it difficult to measure its diameter. After deployment of the stent-graft, the true lumen is fully expanded and its diameter is easy to be measured. We, therefore, used DSA and CTA to measure the diameter of the true lumen after implantation of the stent-graft. Taper ratio in normal adults and in patients with aortic dissection was calculated. As the pathologic changes in acute and chronic aortic dissection are different, patients were further divided into an acute group and a chronic group. The results show a taper ratio of 13% in the control group, 20% in the acute group and 30% in the chronic group. The taper ratio is significantly higher in the patient groups than in the control group, which may be related to pathologic changes. The data show a larger arch diameter in patients with either acute or chronic dissection than that in healthy controls. The fact that chronic dissection has the highest taper ratio can be explained by the pathological changes of chronic dissection. The most common changes in chronic dissection are an excessively distended false lumen, thickened dissection membrane and narrowed true lumen. Assuming that acute aortic dissection has a taper ratio of 20% and that a stent-graft without taper design oversizes the aortic arch by 15%, its distal end will oversize the true lumen of the descending aorta by more than 40%. On the other hand, assuming that chronic aortic dissection has a taper ratio of 30% and that a stent-graft without taper design 15% larger than the aortic arch is selected, its distal end will oversize the true lumen of the descending aorta by more than 60%. This may cause problems, the most serious one being rupture of the dissected membrane by the distal end of the stent-graft immediately after deployment because of excessive radial force. This happened in one of our cases, and a cuff was added at the distal end of the stent-graft (the patient was not included in this study). The more rigid the stent-graft, the greater the chance of rupture. Other problems are stenosis of the descending aorta caused by infolding of the stent-graft in a relatively small lumen and slow future dilatation of the descending aorta which may lead to aneurysm formation. Since the introduction of endovascular repair for the treatment of aortic disease by Dotter [1] in 1969, the technique has progressed dramatically. In 1991, Parodi [2] first applied stent-grafts in the treatment of abdominal aortic aneurysm. In 1994, Dake et al. [3] first used stent-grafts in the treatment of descending aortic aneurysm. From the homemade stent-grafts in the early period [4] to commercially available stent-grafts in recent years, their design has continued to improve. Several commercially available stent-grafts designed for thoracic aneurysms such as Excluder, TAG (Gore, US), Talent (Medtronic, US), AneuRx (Medtronic, US), Endofit (Endomed, US) and Zenith TX2 (Cook, US) have been used in the treatment of aortic dissection. Each has its unique features [5–10], but most do not have a tapered design, except for those that are ordered specially. Some stent-grafts are tapered only in large sizes (diameter 36 mm) and have a very limited taper ratio (<6%). The situation is the same in China [11]. At present, there is no stent-graft specifically designed for aortic dissection.
When we realized the problem, we began to use specially ordered stent-grafts with a tapered design [12]. We usually chose a stent-graft with a taper ratio of about 15%. For example, if the arch diameter was 30 mm, a stent-graft with Proximal
Two overlapping stent-grafts, the proximal being larger and the distal being smaller, may be one choice to accommodate the tapering feature of aortic dissection. Surely such a solution will bring more technical challenges to surgeons than single tapered stent-graft. Such challenges would include issues such as determining which one comes first, how much overlapping is sufficient, and the optimal size of each stent-graft. Malina et al. [13] share our opinion. They advocated in their article that stent-grafts used in endovascular repair of type B dissection should be tapered, compliant and soft. They did not mention taper ratio in their article. Our preliminary study is just a beginning of a series of studies on the choice of stent-graft. It is too early to conclude that a tapered stent-graft is better than a non-tapered stent-graft in endovascular repair of type B aortic dissection. A prospective, randomized, control study is needed. However, we still hope that our study may give readers some inspirations and assist surgeons in their choosing of stent-grafts in the treatment of aortic dissection.
In both acute and chronic type B aortic dissection, the aorta tapers significantly from arch to true lumen in the descending aorta. Stent-graft with tapered design may be a viable treatment option for endovascular repair of type B aortic dissection.
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Abstract
1. Introduction
