Mediastinum
∑ Aortic arch and the three supra-aortic trunks via the suprasternal route (Fig. 19.3)
∑ Descending thoracic aorta, over several cen- timeters, behind the heart, via the cardiac apical route (Fig. 19.4)
The abdominal aorta is then followed via the ab- dominal route up to its bifurcation (Fig. 19.5). It is thus possible to reconstitute a puzzle. The aortic isthmus is, however, generally missing from this puzzle.
A left pleural effusion (for instance, a hemotho- rax in the case of aneurysm leakage) provides an acoustic window that makes the analysis of the descending aorta possible, via the posterior route (see Fig. 15.14, p 101).
A thoracic aortic aneurysm gives a large medi- astinal mass at the aorta. The walls of the aorta generally have a sacciform pattern. The content can show massive thrombosis and then appear as a tissular mass (Fig. 19.6). However, this mass will contain a central lumen, with a stratified periph- ery. Often, the most central layers of the thrombo- sis are still mobile, and one can see them driven Can the mediastinum be analyzed within a gener-
al ultrasound approach, i.e., using a route other than the transesophageal route? Certainly yes, with an effort to sort out perspective, and if one accepts a low feasibility rate. A small probe will be a pre- cious tool here, as elsewhere. A suprasternal approach has been described [1]. A parasternal approach is contributive, when the mediastinum is shifted to one side. Sometimes, through a not perfectly closed sternotomy, it is again possible to have a modest route for ultrasound.
Thoracic Aorta
In good conditions, which depend to a large extent on the patient’s morphotype, it is possible to ana- lyze:
∑ Initial aorta via the left parasternal route (Fig. 19.1)
∑ Ascending aorta via the supraclavicular route (Fig. 19.2)
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Fig. 19.1. Initial aorta (A) visible in a parasternal long- axis scan, between left auricle (LA) and right ventricle (RV). LV, left ventricle. Note that in this scan, the right of the image corresponds to the head of the patient
Fig. 19.2. Ascending aorta (A), inside the superior vena cava (V). Right supraclavicular approach. The origin of the brachiocephalic artery can be seen
back to the periphery a few millimeters in each systole.
In the case of thoracic aortic dissection (Fig. 19.7), an enlarged lumen of the aorta can be observed, and in some cases the intimal flap. This flap has an anatomical shape, i.e., never complete- ly regular, and in our opinion is easily distin- guished from the numerous artifacts that are always too regular and generally located in a strict- ly parallel or meridian plane. However, the search can be difficult, depending on the morphotype, the
situation of the flap with respect to the probe axis, and probably also the operator’s experience here.
The supra-aortic vessels can be followed to var- ious lengths, but the application seems rare, at least in medical ICU use (see Chap. 21).
Acute Mediastinitis
Studying the mediastinal content after cardiac surgery can be delicate. However, the smallest ster- Fig. 19.3. Rare observation of the aortic arch in a young
woman with a favorable morphotype, suprasternal approach. The origin of the supra-aortic trunks (ar- rows) and the right pulmonary artery (PA) in transverse section are exposed in detail
Fig. 19.4. The descending thoracic aorta is exposed over 12 cm in this scan that exploits the cardiac window (api- cal scan of the heart)
Fig. 19.5. Terminal aorta, sequel of Figs. 19.4 and 4.1.
Arrows, origin of the iliac arteries. This type of image can replace more invasive modalities such as CT or angiography in emergency situations
Fig. 19.6. Thoracic aorta aneurysm. Suprasternal scan in a patient in shock with thoracic pain. Note the substan- tial thrombosis, with regular layers. A, circulating lumen of the aorta
nal disunity can offer a large route for the ultra- sound beam. Acute mediastinitis can then be diag- nosed. In a patient who had sepsis 1 month after aortic dissection cure, the transsternal route showed a large, echoic mass of the retrosternal space (Fig. 19.8). An ultrasound-guided puncture of this mass immediately withdrew frank pus.
Staphylococcus was isolated in a few minutes by the laboratory, and adapted antibiotic therapy was begun before prompt surgery.
Acute mediastinitis can often be diagnosed by the anterior parasternal route, if the collection is anterior and voluminous, and extends beyond the sternum.
In mediastinitis with the thorax opened, we have not yet seen an advantage to in situ ultra- sound analysis. If indicated, the probe can be inserted in a sterile sheath.
It is assumed that the possibility of early diag- nosis of acute mediastinitis by transesophageal echography is promising.
Thoracic Esophagus
Thoracic esophagus cannot be explored by a retro- tracheal approach. It can be approached below the carina as a tubular flattened structure that passes in the dihedral angle between the heart and descending aorta (Fig. 19.9). Its analysis is uncer- tain but should always be tried.
Esophageal rupture is an emergency whose infrequency makes it all the more severe, since this diagnosis is rarely evoked immediately. Our obser- vations show that a routine ultrasound examina- tion of any thoracic drama will promptly recognize these disorders: partial pneumothorax, pleural effusion (with alimentary particles yielding a com- plex echostructure), and frank pus withdrawn from the ultrasound-guided thoracentesis.
In the critically ill patient, the gastric tube and above all its frank acoustic shadow make a good 136 Chapter 19 Mediastinum
Fig. 19.7. An 80-year-old female with violent chest pain.
Suprasternal scan demonstrating an enlarged aortic lumen with an internal image that is irregular, nonarti- factual, and mobile indicating intimal flap (arrow).
Dissection of the thoracic aorta
Fig. 19.8. Substantial collection (M) visible by the trans- sternal route, in a recently operated patient. The collec- tion is echoic and tissue-like. The tap withdrew frank pus. Note the heart (LV) located more deeply
Fig. 19.9. Location of the thoracic esophagus (O) in a transverse, pseudo-apical scan of the heart. The eso- phagus is surrounded by the rachis (R), the right auricle (RA), the left ventricle (LV) and the descending aorta (A)
landmark that facilitates the location of the esoph- agus. The esophageal balloon of a Blakemore tube can be visualized posterior to the left auricle (Fig. 19.10). Ultrasound help in this situation is discussed in Chap. 6.
Pulmonary Artery
In patients with a favorable morphotype, the aortic arch can be exposed by suprasternal route. In the concavity of the aorta, a transverse scan can more or less easily bring the right pulmonary artery into
view (Fig. 19.11). Detection of a frank blood clot using this route is rare, but can provide immediate diagnosis of severe pulmonary embolism.
Internal Mammary Artery
The internal mammary artery crosses just outside the sternal border. Locating it can be useful before certain punctures.
An internal mammary artery false aneurysm once had this very suggestive pattern: an egg- shaped, vertical, long-axis mass. Ultrasound analy- sis of its content showed a blatant whirling flow (Fig. 19.12). The vascular origin of this mass was proven, once again without Doppler. It goes with- out saying that this pattern seriously contraindi- cates diagnostic puncture.
Other Mediastinal Structures
The recognition of the following elements, even if they are responsible for disorders such as tracheal compression, rarely leads to therapeutic decisions in the emergency room. Diving goiter, adenomegaly or mediastinal tumors can be quietly diagnosed when not compressive (see Fig. 12.8, p 73).An ante- rior mediastinal mass in a clinical context of myas- thenia gravis will be suggestive of thymoma. A pneumomediastinum yields, in our observations, a complete acoustic barrier, of value if (1) the heart was previously located in this area and (2) lung Fig. 19.10. Inflated esophageal balloon of a Blakemore
probe (asterisk), driving the posterior aspect of the left auricle (LA) away
Fig. 19.11. Another transverse scan of the right pulmo- nary artery (PA), surrounded by the aortic arch (A).
Suprasternal scan. A pulmonary embolism could thus be proven in extreme emergency
Fig. 19.12. False aneurysm of the left internal mammary artery. Transverse scan of a parasternal intercostal spa- ce. Egg-shaped mass with vertical long axis. In real- time, an echoic whirling flow indicated the arterial na- ture of this mass. H, heart
sliding is recognized outside this area, which rules out pneumothorax.
Let us remind the reader here that complete atelectasis can considerably favor the ultrasound analysis of the mediastinum by the external approach (see Fig. 12.20, p 80, and Fig. 17.11, p 124).
References
1. Matter D, Sick H, Koritke JG,Warter P (1987) A supra- sternal approach to the mediastinum using real-time ultrasonography, echoanatomic correlations. Eur J Radiol 7:11–17
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