Tag Archives: aorta

What Is A Wide Mediastinum Anyway?

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.

Related post:

Blunt Aortic Injury And New Cars

Car crashes are a significant cause of trauma death worldwide. Aortic injury is the cause of death in somewhere between 16% and 35% of these crashes (in the US). Over the years, automobile safety through engineering improvements has been rising. A recent poster presented at EAST 2012 looked at the effect of these improvements on mortality from aortic injury.

The authors analyzed the National Automotive Sampling System – Crashworthiness Data System database (NASS-CDS) for car model years dating from 1994 to 2010. They included any front seat occupants age 16 or more. Over 70,000 cases were reviewed.

Interesting findings:

  • Overall mortality from aortic injury was 89%
  • 75% of deaths occurred prior to arrival at a hospital
  • Risk for suffering an aortic injury was statistically associated with age >=60, being male, being the front seat passenger, position further back from the steering wheel, and ejection from the vehicle
  • The injury was more likely to occur when speed was >= 60mph, impact occurred with a fixed object, and in SUV vs pickup truck crashes
  • Newer cars protected occupants from aortic injury in side-impact crashes, but the incidence actually increased in frontal-impact crashes

Bottom line: Aortic injury will remain a problem as long as we find ways to move faster than we can walk. Engineers will continue to make cars safer, but the increase in aortic injury in frontal impact in late model cars is puzzling. This phenomenon needs further analysis so that safety can be improved further. Trauma professionals need to keep this injury in mind in any high energy mechanism and order a screening chest CT appropriately.

Related posts:

Reference: Aortic injuries in new vehicles. Ryb et al, University of Maryland and Johns Hopkins. Poster presented at EAST Annual Meeting, January 2012.

When To Image The Aorta In Blunt Trauma

Blunt injury to the thoracic aorta is one of those potentially devastating ones that you (and your patient) can’t afford to miss. Quite a bit has been written about the findings and mechanisms. But how do you put it all together and decide when to order a screening CT?

There are a number of high risk findings associated with blunt aortic injury. Recognize that they are associated with the injury, but are still not very common. They are:

  • Fractures of the sternum or first rib
  • Wide mediastinum
  • Displacements of mediastinal structures (left mainstem down, trachea right, esophagus right)
  • Loss of the aortopulmonary window
  • Apical cap over the left lung

Here’s a sensible method for screening for blunt aortic injury, using CT scan:

  • Reasonable mechanism (fall from greater than 20 feet, pedestrian struck, motorcycle crash, car crash at “highway speed”) PLUS any one of the high risk findings above.
  • Extreme mechanism alone (e.g. car crash with closing velocity at greater than highway speed, torso crush)

Note on torso crush: I have seen three aortic injuries from torso crush in my career, one from a load of plywood falling onto the patient’s chest, one from dirt crushing someone when the trench they were digging collapsed, and one whose chest was run over by a car.

Related post:

Trauma 20 Years Ago: CT Imaging of the Aorta

CT scan is now the standard screening test for injury to the thoracic aorta. But 20 years ago, we were still gnashing our teeth about how to detect this injury.

An interesting paper was published in the Journal of Trauma 20 years ago this month on this topic. Over a 2 year period, the Medical College of Wisconsin at Milwaukee looked at all patients who underwent imaging for aortic injury. At the time the gold standard was aortogram. They looked at patients who underwent this study and CT, which was not very common at the time.

They had 50 patients who underwent aortography alone and 17 who underwent both tests. Of the 17, 5 had the injury, but only three were seen on CT. There were also two false positives. Sensitivity was 83%, specificity was 23%, with 53% accuracy. The authors concluded that any patients with strong clinical suspicion of aortic injury should proceed directly to aortogram.

Why the difference today? Scan technology and resolution has increased immensely. Also, the timing of IV contrast administration has been refined so that even subtle intimal injuries can be detected. CT scan is now so good that we have progressed from the CV surgeon requiring an aortogram before they would even consider going to the OR, to the vascular surgeon / interventional radiologist proceeding directly to the interventional suite for endograft insertion.

Trauma 20 Years Ago: Blunt Aortic Injury in Children

We always worry about the aorta after high-energy blunt trauma in adults. Should we be doing the same in kids? After all, they are very elastic and for the most part they are tough to break.

A 13 year review was undertaken by the CV surgeons at Harborview twenty years ago which tried to answer this question. They looked at medical examiner records of all pediatric deaths (16 or younger) and identified the ones with traumatic aortic injury. They found only 12 deaths (2.1%), and somehow they also tracked one survivor (from ME data???). The age range was 3-15, with a mean of 12 (which means that the majority were in the older age group).

Six children were pedestrian struck, 5 were involved in car crashes, and two were on motorized bikes or ATVs. None of the children in car crashes were restrained and two were ejected. Four of the five were traveling > 55mph. All had other serious injuries, including abdominal and orthopedic.

It’s tough to draw any meaningful conclusions from this paper due to the small numbers, the retrospective design, and the lack of a denominator. The only thing it does tell us is that aortic injury is bad, and that kids should not get hit by cars and should wear their seat belts. The mean age suggests that it involves primarily older children. But we kind of knew all that already.

What it does not help with is figuring out at what age we need to start thinking about imaging the aorta with CT scan. I’ll be digging into that a little more this week.

Reference: Eddy et al. The epidemiology of traumatic rupture of the aorta in children: a 13 year review. J Trauma 30(8): 989-992, 1990.