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.
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!
Members of the trauma team must frequently protect the cervical spine when moving the patient or performing certain procedures. In most cases, a cervical collar is placed which does a fine job of this. Occasionally, though, the collar must be removed to provide access to areas near or under the collar.
When the collar is off, someone must be charged with immobilizing the cervical spine. Sometimes this is incorrectly referred to as providing inline traction and not inline stabilization.There is a big difference!
Inline traction is used to try to realign cervical vertebra that are malpositioned due to fracture or ligamentous injury. This should only be performed under the guidance of a neurosurgeon!
Inline stabilization merely means that the patient (or trauma professional) is restrained from moving the cervical spine. This is commonly needed while intubating the patient, so that the intubator does not extend the neck when trying to visualize the cords.
Why is this important? Check out the images below. If a severe injury has already occurred, traction on the neck may have devastating consequences! Inline stabilization is the only way to go.
Okay, so you’ve seen “other people” wearing perfectly good lead aprons lifting them up to their chin during portable xrays in the trauma bay. Is that really necessary, or is it just an urban legend?
After hitting the medical radiation physics books (really light reading, I must say), I’ve finally got an answer. Let’s say that the xray is taken in the “usual fashion”:
Tube is approximately 5 feet above the xray plate
Typical chest settings of 85kVp, 2mAs, 3mm Al filtration
Xray plate is 35x43cm
The calculated exposure to the patient is 52 microGrays. Most of the radiation goes through the patient onto the plate. A very small amount reflects off their bones and the table itself. This is the scatter we worry about.
So let’s assume that the closest person to the patient is 3 feet away. Remember that radiation intensity diminishes as the square of the distance. So if the distance doubles, the intensity decreases to one fourth. By calculating the intensity of the small amount of scatter at 3 feet from the patient, we come up with a whopping 0.2 microGrays. Since most people are even further away, the dose is much, much less for them.
Let’s put it perspective now. The background radiation we are exposed to every day (from cosmic rays, brick buildings, etc) amounts to about 2400 microGrays per year. So 0.2 microGrays from chest xray scatter is less than the radiation we are exposed to naturally every hour!
The bottom line: unless you need to work out you shoulders and pecs, don’t bother to lift your lead apron every time the portable xray unit beeps. It’s a waste of time and effort!
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