Best Of: IV Contrast
We use CT scanning in trauma care so much that we tend to take it (and its safety) for granted. I’ve written quite a bit about thoughtful use of radiographic studies to achieve a reasonable patient exposure to xrays. But another thing to think about is the use of IV contrast.
IV contrast is a hyperosmolar solution that contains some substance (usually an iodine compound) that is radiopaque to some degree. It has been shown to have a significant impact on short-term kidney function and in some cases can cause renal failure.
Here are some facts you need to know:
- Contrast nephrotoxicity is defined as a 25% increase in serum creatinine, usually within the first 3 days after administration
- There is usually normal urine output and minimal to no proteinuria
- In most cases, renal function returns to normal after 3-4 days
- Nephrotoxicity almost never occurs in people with normal baseline kidney function
- Large or repeated doses given within 72 hours greatly increase risk for toxicity
- Old age and pre-existing diabetic renal impairment also greatly increase risk
If you must give contrast to a patient who is at risk, make sure they are volume expanded (tough in trauma patients), or consider giving acetylcysteine or using isosmolar contrast (controversial, may still cause toxicity).
Bottom line: If you are considering contrast CT, try to get a history to see if the patient is at risk for nephrotoxicity. Also consider all of the studies that will be needed and try to consolidate your contrast dosing. For example, you can get CT chest/abdomen/pelvis and CT angio of the neck with one contrast bolus. Consider low dose contrast injection if the patient needs formal angiographic studies in the IR suite. Always think about the global needs of your patient and plan accordingly (and safely).
Reference: Contrast media and the kidney. British J Radiol 76:513-518, 2003.
The Pan-Scan For Trauma
Diagnostic imaging is a mainstay in diagnosing injuries in major trauma patients. But the big questions are, how much is enough and how much is too much? X-radiation is invisible but not inocuous. Trauma professionals tend to pay little attention to radiation that they can’t see in order to diagnose things they can’t otherwise see. And which may not even be there.
There are two major camps working in emergency departments: scan selectively and scan everything. It all boils down to a balance between irradiating enough to be satisfied that nothing has been missed, and irradiating too much and causing harm later.
A very enlightening study was published last year from the group at the University of New South Wales. They prospectively looked at their experience while moving from selective scanning to pan-scanning.They studied over 600 patients in each cohort, looking at radiation exposure, missed injuries, and patient injury and discharge disposition variables.
Here are the interesting findings:
- Absolute risk of receiving a higher radiation dose increased from 12% to 20%. This translates to 1 extra person of every 13 evaluated receiving a higher dose.
- The incidence of receiving >20 mSv radiation dose nearly doubled after pan-scanning. This is the threshold at which we believe that cancer risk changes from low (<1:1000) to moderate (>1:1000).
- The risk of receiving >20 mSv was lower in less severely injured patients (sigh of relief)
- There were 6 missed injuries with selective scanning and 4 with pan-scanning (not significant). All were relatively minor.
Bottom line: Granted, the study groups are relatively small, and the science behind radiation risk is not very exact. But this study is very provocative because it shows that radiation dose increases significantly when pan-scan is used, but there was no benefit in terms of decreased missed injury. If we look at the likelihood of being helped vs harmed, patients are 26 times more likely to be harmed in the long term as they are to be helped in the short term. The defensive medicine naysayers will always argue about “that one catastrophic case” that will be missed, but I’m concerned that we’re creating some problems for our patients in the distant future that we are not worrying enough about right now.
Reference: Comparison of radiation exposure of trauma patients from diagnostic radiology procedures before and after the introduction of a panscan protocol. Emerg Med Australasia 24(1):43-51, 2012.
Repeat Imaging: What Good Is It?
I’ve written previously about how often imaging gets repeated once a trauma patient gets transferred to a trauma center (click here). There are many reasons, including clinical indications, need for advanced imaging (reconstructions), or lack of contrast. But at least 20% have to be repeated because the media is incompatible or not sent with the patient. Sounds like a problem, but is it a significant one?
A recent retrospective analysis of about 2,000 transfers to a Level I center looked at the reasons for repeat imaging and changes in outcome due to it. The paper found several interesting things:
- Repeat imaging was more likely in more severely injured patients
- Hospitals that transferred more patients to the trauma center tended to do more scans before transfer
- Patients who had repeat imaging stayed in the ED longer waiting for definitive disposition
- Repeat images did not improve outcomes (LOS, DC home, mortality)
- A rough estimate of $354 more in charges was attributed to repeat imaging
Bottom line: Repeat imaging is wasteful, expensive and increases time in the ED. And don’t forget about the radiation exposure. With all the emphasis on pushing hospitals to use an electronic medical record, there needs to be a similar push to standardize methods for transferring radiographic images between hospitals to address the problem of repeat imaging.
Reference: Repeat imaging in trauma transfers: A retrospective analysis of computed tomography scans repeated upon arrival to a Level I trauma center. J Trauma 72(5):1255-1262, 2012.
Identifying Bowel and Mesenteric Injury by CT
CT scan is an invaluable tool for evaluating blunt abdominal trauma. Although it is very good at detecting solid organ injury, it is not so great with intestinal and mesenteric injuries. Older studies have suggested that CT can detect mesenteric injuries if done right, but a newly published study has shown good accuracy with a few imaging tweaks.
A Taiwanese study looked at a series of prospectively studied victims of blunt abdominal trauma. Patients with abdominal pain or a positive FAST were entrolled (total 106). IV contrast was given, and scans during the arterial, portal, and equilibrium contrast phases were performed using a multidetector scanner. Images were read in a blinded fashion.
A total of 13 of 23 patients who underwent laparotomy were found to have a bowel or mesenteric injury. Five had bowel injury, 4 had mesenteric hemorrhage, and 4 had both. Mesenteric contrast extravasation was seen in 7 patients, and this correlated with mesenteric bleeding at laparotomy.
The authors found that the following signs on CT scan indicated injury:
- Full or partial thickness change in bowel wall appearance
- Increased mesenteric density
- Free fluid without solid organ injury
Bottom line: This study shows that CT scan can detect bowel and mesenteric injury reliably if you scan the patient 3 times! This seems like over-radiation and overkill. A more intelligent way to approach this would be to perform a normal trauma abdominal scan. If a suspicious area of mesenteric or bowel thickening is seen, then a limited rescan through the affected area only for equilibrium phase images may be warranted. If actual contrast extrvasation is seen, no further scanning is needed. A quick trip to the OR is in order.
Reference: Contrast-enhanced multiphasic computed tomography for identifying life-threatening mesenteric hemorrhage and transmural bowel injuries. J Trauma 71(3):543-548, 2011.