What kinds of mechanisms can actually cause a thoracic aortic injury? Most physicians are aware that it involves sudden deceleration. This includes falls from a height and head-on car crashes. However, other mechanisms are associated with this injury as well.
Sudden acceleration can also tear the aorta. This can occur from a rear-end type car crash where one car is stopped and the other is traveling at a high rate of speed. It can also occur when pedestrians are struck by a car.
T-bone crashes also have a significant association with aortic injury. Twenty years ago, this was not really recognized, but now we know better.
One very interesting mechanism that I’ve seen about 5 times is the torso crush. This can occur when heavy objects tip over onto someone’s chest. I’ve seen this injury when multiple sheets of plywood have fallen on someone, and when a ditch caved in and the patient was crushed by dirt.
So when should you be concerned about the aorta enough to image it? In all cases, there must be a significant mechanism (see above). Falling over or being bumped at low speed just can’t do it. It’s also very rare in children under 10. I use the following guidelines:
- Significant mechanism plus any one chest x-ray finding (see last 2 days of discussion)
- Extreme mechanism alone. I define this as a closing velocity > 60mph, although you probably won’t know exactly how fast they were really going. You’ll need to estimate based on the usual speed on that particular road in the case of a car crash. Err on the side of predicting a higher speed. Extreme mechanism also includes pedestrian struck at moderate speed or better and torso crush.
- Physical signs or symptoms consistent with aortic injury. These include tearing chest pain, especially between the shoulder blades, and pulse discrepancy (right radial pressure higher than left radial)
The gold standard screening test is now the helical chest CT. If the results are indeterminate, then a good old-fashioned aortogram may be needed.
Yesterday I talked about the most common chest xray finding in patients with a thoracic aortic injury, the wide mediastinum. There are several lesser known (and less common) findings that may also occur. These can be divided into three broad categories: associated fractures, displacements, and other weird findings.
The associated fractures indicate that a lot of energy has passed into the chest and generally involve bones that are difficult to break. They consist of:
- First rib (or second). These are flattened with a pronounced curve and are very difficult to break.
- Scapula. Also irregular, and thick in some areas.
The displacements are shifts in other mediastinal structures causes by a hematoma near the aortic arch. They generally involve the bronchial tree and esophagus.
- Left mainstem bronchus is pushed down, since it is nestled under the arch.
- Trachea is angled to the right as the whole bronchial tree on the left side is pushed down
- Esophagus is shifted to the right. This can only be seen if an NG is in place.
Weird stuff are just miscellaneous things that people have found to be associated with this injury:
- Apical cap. This is blood that has dissected away from the aorta and is extrapleural. Think of it as an epidural hematoma of the pleura, so it pushes in from the outside making it somewhat lenticular (lens-shaped). It is only significant if seen on the left, since the hematoma can’t dissect all the way over to the right. (see image below)
- Loss of the aortopulmonary window. This is a small space seen between the shadows of the aorta and pulmonary artery on chest x-ray. It is best seen on the lateral view, which we don’t get in trauma patients.
Tomorrow, I’ll talk about what kind of mechanism is needed to tear the aorta, and finish up with some guidelines on when to image people for this injury.
A. The apical cap. Note how it bows inward
B. Blood along the spine dissecting up from the arch.
Trauma professionals are always on the lookout for injuries that can kill you. Thoracic aortic injury from blunt trauma is one of those injuries. Thankfully, it is uncommon, but it can certainly be deadly.
One of the screening tests used to detect aortic injury is the old-fashioned chest xray. This test is said to be about 50% sensitive, with a negative predictive value of about 80%. However, the sensitivity is probably decreasing and the negative predictive value increasing due to the rapidly increasing number of obese patients that we see.
A wide mediastinum is defined as being > 8cm in width. In this day and age of digital imaging, you will need to use the measurement tool on your workstation to figure this out.
Unfortunately, it seems like most chest xrays show wide mediastinum these days. What are the most common causes for this?
- Technique. The standard xray technique used to reduce magnification of the anterior mediastinum (where the aortic arch lives) is a tube distance of 72 inches from the patient, shot back to front. We can’t do this for trauma patients because we can’t stand them up and are reluctant to prone them. The standard trauma room technique is 36 inches from the patient shot front to back. This serves to magnify the mediastinal image and make it look wide.
- Obesity. The more fat in the mediastinum, the wider it looks. The more fat on the back, the further the mediastinum is from the xray plate and the greater the magnification.
- Other mediastinal blood. Major blunt trauma to the chest can cause bleeding from small veins in the mediastinum, making it look wide.
- Thymus. Only in kids, though.
- Aortic injury. Last but not least. Only a few percent of people with wide mediastinum will actually have the injury.
If you encounter a wide mediastinum on chest xray in a patient with a significant mechanism for aortic injury, then they should be screened using helical CT.
Tomorrow, I’ll talk about other xray findings that can clue you in to the presence of a thoracic aortic injury. Friday, I’ll finish by discussing what the significant mechanisms are for this injury.
Where Are All The Pediatric Trauma Centers?
I have constructed this map from available resources from the American College of Surgeons and numerous state agencies. ACS verified pediatric centers have a diamond in their icon; Level I is red and Level II is yellow. Non-ACS centers are pink (Level I) or blue (Level II). The Level I pediatric center at Regions Hospital is the green star.
I have made every attempt at accuracy, but things do change. If I have omitted any centers or misclassified them, please leave a comment or email me!