BACKGROUND: The aortic knob is thought to be the most useful radiographic landmark for the proper positioning of the intraaortic balloon pump (IABP) tip. However, this has not been studied formally. In this study we assessed whether the aortic knob is a reliable landmark for positioning the IABP and compared it with another potential landmark, the carina.
METHODS: We measured the distance from the top of the distal aortic arch (aortic knob) to the left subclavian artery (LSCA) on three-dimensional computed tomography angiography in 100 patients. The distance from the level of the LSCA origin to the level of the carina was also measured using three-dimensional computed tomography in 150 additional patients.
RESULTS: In 16% of the aortic knob study population, the LSCA to aortic knob distance was <0 cm or 0 cm. The median distance from the LSCA to the carina was 42 mm (range: 30–63 mm). In the carina study population, the origin of the LSCA was 35–55 mm above the carina in 95.3% of patients.
CONCLUSION: In 16% of patients, the IABP was too close to the LSCA origin when it was placed at the aortic knob, whereas positioning the IABP at 2 cm above the carina provided an adequate position for the IABP tip (1.5–3.5 cm distal to the origin of the LSCA) in 95.3% of patients. The carina may be a more reliable landmark for positioning the IABP than the aortic knob.
IMPLICATIONS: The carina in a chest radiograph can be used as a guide for positioning the intraaortic balloon pump.
From the Department of *Anesthesiology, †Thoracic and Cardiovascular Surgery, College of Medicine, Seoul National University, Seoul, Korea.
Accepted for publication May 31, 2007.
Jin-Tae Kim and Jeong-Rim Lee have contributed equally to the project.
Presented in part at the annual scientific meeting of the Korean Society of Anesthesiologists, Seoul, Korea, November, 2006.
Address correspondence and reprint requests to Seung Zhoo Yoon, MD, PhD, Department of Anesthesiology, College of Medicine, Seoul National University, 28 Yeongeon-dong, Jongno-gu, Seoul 110-744, Korea. Address e-mail to firstname.lastname@example.org.
The intraaortic balloon pump (IABP) has been widely used to support patients with cardiogenic shock, myocardial ischemia, or postsurgical myocardial dysfunction (1). The appropriate performance of the IABP is dependent on proper position (2,3). Ideally, the tip of the balloon should be positioned 2–3 cm distal to the origin of the left subclavian artery (LSCA) (2,4,5). This position results in maximum augmentation of coronary artery flow although minimizing the risk of embolization to the cerebral vessels and occlusion of the LSCA (4,6).
A chest radiograph is recommended to confirm IABP tip positioning in the intensive care unit. The aortic knob is thought to be the radiographic landmark of choice for proper positioning (4,7). The recommended position for the tip of the balloon is just distal to the aortic knob. The aortic knob on chest radiography is the shadow of three anatomical structures: the LSCA, the distal aortic arch, and the proximal descending aorta. However, individual variation in the relationship between the aortic knob and the origin of the LSCA has not been studied formally. Furthermore, using the aortic knob on the chest radiograph to confirm the position of the IABP may be erroneous because the aortic knob is a broad shadow on the chest radiograph.
On the contrary, seen on a chest radiograph, the carina is a clear anatomical structure. Moreover, the aortic arch always crosses over the right pulmonary artery and the main bronchus. Therefore, the position of the carina is consistent with respect to the aortic arch. We hypothesized that the carina can be a useful landmark for appropriately positioning the IABP on chest radiography.
The aim of this study was 1) to evaluate whether the aortic knob can be a reliable radiographic landmark for positioning the IABP tip and 2) to assess the carina as a clinically practical landmark for IABP placement.
The study protocol was reviewed by the Institutional Review Board and approved as a minimal risk retrospective study that did not require individual consent based on the institutional guidelines for waiving consent. The three-dimensional computed tomography (3D CT) angiography was performed as a preoperative diagnostic work-up in patients undergoing coronary artery bypass graft surgery (CABG), except for those who had impaired renal function and severe left ventricular dysfunction, or who underwent urgent or emergent operation.
To determine the relationship between the aortic knob and the LSCA, the 3D CT reconstruction angiography films of 100 adult patients undergoing CABG were reviewed retrospectively. The authors interpreted these tomograms using the Picture Archiving and Communication System (PACS, Maroview ver. 5.3, MAROTECH Inc., Seoul, Korea). From among the reconstruction images of the aorta and its branches we selected the image that shows the aortic arch and the origin of the LSCA. The top of the distal aortic arch on the image was marked, and then the distance from that point to the origin of the LSCA (LSCA-Ao distance) was measured (Fig. 1). If the origin of the LSCA was located proximal to the top of the distal aortic arch, the distance was represented as a positive value; if it was located distal to or at the top of the distal aortic arch, the distance was represented as zero.
To determine the relationship between the carina and the LSCA, the 3D CTs of another 150 adult patients with a history of CABG were reviewed retrospectively. Because the carina is difficult to determine on a 3D CT reconstruction angiography image, we selected two transverse plane CT images, one showing the carina and the other showing the origin of the LSCA. The total number of 2 mm sliced images between the selected two images were added and this was considered as the distance from the carina to the LSCA (LSCA-Ca distance).
Patients' ages, genders, weights, and heights were recorded. The relationships between the LSCA-Ao distance or LSCA-Ca distance and the other variables were analyzed.
Data are presented as mean ± sd or median (range) or frequencies (percent). Patients' characteristics of the two study populations were compared with Student's t-test. The relationships between the LSCA-Ao distance or the LSCA-Ca distance and age, weight, or height were analyzed by Pearson correlation analysis. Statistical significance was accepted for P values <0.05. Statistical analyses were performed using SPSS version 10.0 (SPSS, Chicago, IL).
There were no differences in age, height, or weight between the aortic knob and the carina study populations (Table 1).
LSCA-Ao distance was as follows: <0 cm or 0 cm in 16 patients (16%), 0 cm < LSCA-Ao ≤ 1 cm in 14 patients (14%), 1 cm < LSCA-Ao ≤ 2 cm in 53 patients (53%), and >2 cm in 17 patients (17%). When the age increased, LSCA-Ao distance increased (r = 0.425, P < 0.05). LSCA-Ao distance was not correlated with weight or height. There was no statistical difference of the distribution of LSCA-Ao distances between the males and females.
The median LSCA-Ca distance was 42 mm (30–63 mm). The LSCA-Ca distance was in the range from 35 to 55 mm in 95.3% of patients (Fig. 2). Although the LSCA-Ca distance was not correlated with weight or age, it was correlated with height (r = 0.196, P < 0.05). The median LSCA-Ca distance was 45 mm (29–63 mm) in men and 40 mm (30–60 mm) in women (P = 0.053).
Our results show that the aortic knob may not be a proper radiographic landmark for IABP tip positioning because of the variation in the origin of the LSCA on the aortic arch, whereas the carina can be a proper landmark for correct placement of IABP because the LSCA originates 35–55 mm above the carina in 95.3% of the study population.
In about 80% of individuals, three branches of the aorta arise from the aortic arch: the brachiocephalic trunk, the left common carotid artery, and the LSCA. Adachi first classified this branching pattern as Type A. Another 11% of reported cases have a common trunk incorporating the left common carotid artery and the brachiocephalic artery, leaving only two branches originating from the aortic arch; this is referred to as Adachi Type B. The third common pattern, Type C, has the left vertebral artery, a fourth branch of the aortic arch, originating proximal to the LSCA (8). In the aortic knob study population, all patients were Adachi Type A. In 16% of the investigated patients, however, the LSCA was located distal to the aortic knob.
When the IABP tip is placed proximal to the LSCA, the chances of the IABP disturbing the blood flow to the LSCA and left common carotid artery may be increased. Although the major vascular complications related to the IABP occur in the lower extremities after cardiac surgery (9,10), the presence of an IABP is one of the independent predictors for cerebrovascular accident in patients undergoing percutaneous coronary intervention (11,12).
The aortic knob study shows that the longer LSCA-Ao distances are related to old age; the LSCA-Ao distance was longer than 2 cm in all 17 patients whose ages were over 60 yr. The aortic arch transverse diameter increased with age (13) and the aortic knob width was significantly correlated with age (14,15). The elongation of LSCA-Ao distances that occurs with aging can affect the distance from the top of the distal aortic arch, which is the main portion of the aortic knob on the chest radiograph, to the origin of the LSCA. Even though the relation between the aortic knob and the origin of LSCA is initially normal with respect to IABP positioning, the elongation of LSCA-Ao distance with aging may later mean that the aortic knob is not a proper radiographic landmark for IABP positioning. On the other hand, despite low r values, LSCA-Ca distance was correlated with height, not with age. This can be explained by the fact that the LSCA-Ca distance is the vertical distance from the carina to the LSCA.
The carina has advantages over the aortic knob as a radiographic landmark. First, the aortic arch consistently crosses just over the main bronchus. Accordingly, the position of the carina relative to the aortic arch varies little when compared with the position of the aortic knob. In addition, contrary to the aortic knob, the carina is a well-definable anatomic landmark on chest radiography. It is positioned in the center of the body, resulting in an amelioration of the parallax effect (16).
Our results show that the LSCA consistently originates 35–55 mm above the carina in 95.3% of the study population. Considering recommendations that the tip of the IABP be placed 2–3 cm distal to the LSCA, if the IABP tip were positioned 2 cm above the carina, the distance from the IABP tip to the LSCA would be 1.5–3.5 cm (Fig. 3). There are reports that the IABP tip can be displaced 1–4 cm cephalad with a change in the patient's position from recumbent to sitting (17,18). This displacement can result in obstruction of the LSCA blood flow, or aortic tear with subsequent dissection. Positioning of the IABP tip at the level of the carina may be more distal than IABP tip positioning based on the aortic knob. Therefore, using the carina level as a radiographic landmark for IABP tip positioning can be safer than using the aortic knob.
Some limitations of the present study should be mentioned. The main limitation of our study is the ambiguity of the aortic knob. “Aortic knob” is a radiographic term, not an anatomic structure, and, therefore, its definition of is quite vague. We arbitrarily chose the top of the distal aortic arch as a reference point for the study because we think that it is the central portion of the aortic knob in the chest radiograph. Although the correlation between the distal aortic arch on CT and the aortic knob on chest radiography is not confirmed by formal study, our results sufficiently show that the origin of the LSCA is inconsistent among individuals and the aortic knob or distal aortic arch may not be a reliable landmark for IABP tip positioning.
Another limitation is that LSCA-Ca distance is not a real anatomic distance but, rather, the vertical distance on a CT scan. Therefore, our measured LSCA-Ca distance may underestimate the real anatomic distance from the LSCA to the carina. In our opinion, however, this discrepancy would be clinically irrelevant.
In conclusion, the aortic knob may not be a proper radiographic landmark for IABP tip positioning in some patients. The carina, a well-definable anatomic structure on chest radiographs, may be a practical landmark for positioning of the IABP.
The authors thank the Medical Research Collaborating Center at Seoul National University Hospital and College of Medicine for performing the statistical analysis.
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