Category Archives: Mechanism

Mechanism

Closing Velocity And Injury Severity

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Trauma professionals, both prehospital and in trauma centers, make a big deal about “closing velocity” when describing motor vehicle crashes.  How important is this?

So let me give you a little quiz to illustrate the concept:

Two cars, of the same make and model, are both traveling on a two lane highway at 60 mph in opposite directions. Car A crosses the midline and strikes Car B head-on. This is the same as:

  1. Car A striking a wall at 120 mph
  2. Car B striking a wall at 60 mph
  3. Car A striking a wall at 30 mph

2010-saab-9-5-head-on-crash-test_100313384_m1

The closing velocity is calculated by adding the head-on components of both vehicles. Since the cars struck each other exactly head-on, this would be 60+60 = 120 mph. If the impact is angled there is a little trigonometry involved, which I will avoid in this example. And if there is a large difference in mass between the vehicles, there are some other calculation nuances as well.

So a closing velocity of 120 mph means that the injuries are worse than what you would expect from a car traveling at 60 mph, right?

Wrong!

In this example, since the masses are the same, each vehicle would come to a stop on impact because the masses are equal. This is equivalent to each vehicle striking a solid wall and decelerating from 60 mph to zero immediately. Hence, answer #2 is correct. If you remember your physics, momentum must be conserved, so both of these cars can’t have struck each other at the equivalent of 120 mph. The injuries sustained by any passengers will be those expected in a 60 mph crash.

If you change the scenario a little so that a car and a freight train are traveling toward each other at 60 mph each, the closing velocity is still 120 mph. However, due the the fact that the car’s mass is negligible compared to the train, it will strike the train, decelerate to 0, then accelerate to -60 mph in mere moments. The train will not slow down a bit. For occupants of the car, this would be equivalent to striking an immovable wall at 120 mph. The injuries will probably be immediately fatal for all.

Bottom line: Closing velocity has little relationship to the injuries sustained for most passenger vehicle crashes. The sum of the decelerations of the two vehicles will always equal the closing velocity. Those injuries will be consistent with the change in speed of the vehicle the occupants were riding, and not the sum of the velocities of the vehicles. 

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General, Mechanism

Trauma Activation For Hanging: Yes or No?

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In my last post, I discussed a little-reviewed topic, that of strangulation. I recommended activating your trauma team only for patients who met the physiologic criteria for it.

But now, what about hangings? There are basically two types. The judicial hanging is something most of you will never see. This is a precisely carried out technique for execution and involves falling a certain height while a professionally fashioned noose arrests the fall. This results in a fairly predictable set of cervical spine/cord, airway, and vascular injuries. Death is rapid.

Suicidal hangings are far different. They involve some type of ligature around the neck, but rarely and fall. This causes slow asphyxiation and death, sometimes. The literature dealing with near hangings is a potpourri of case reports, speculation, and very few actual studies. So once again, we are left with little guidance.

What type of workup should occur? Does the trauma team need to be called? A very busy Level I trauma center reviewed their registry for adult near-hangings over a 19 year period. Hanging was strictly defined as a ligature around the neck with only the body weight for suspension. A total of 125 patients were analyzed, and were grouped into patients presenting with a normal GCS (15), and those who were abnormal (<15).

Here are the factoids:

  • Two thirds of patients presented with normal GCS, and one third were impaired
  • Most occurred at home (64%), and jail hangings occurred in 6%
  • Only 13% actually fell some distance before the ligature tightened
  • If there was no fall, 32% had full weight on the ligature, 28% had no weight on it,  and 40% had partial weight
  • Patients with decreased GCS tended to have full weight on suspension (76%), were much more likely to be intubated prior to arrival (83% vs 0% for GCS 15), had loss of consciousness (77% vs 35%) and had dysphonia and/or dysphagia (30% vs 8%)
  • Other than a ligature mark, physical findings were rare, especially in the normal GCS group. Subq air was found in only 12% and stridor in 18%.
  • No patients had physical findings associated with vascular injury (thrill, bruit)
  • Injuries were only found in 4 patients: 1 cervical spine fracture, 2 vascular injuries, and 1 pneumothorax
  • 10 patients died and 8 suffered permanent disability, all in the low GCS group

Bottom line: It is obvious that patients with normal GCS after attempted hanging are very different from those who are impaired. The authors developed an algorithm based on the initial GCS, which I agree with. Here is what I recommend:

  • Do not activate the trauma team, even for low GCS. This mechanism seldom produces injuries that require any surgical specialist. This is an exception to the usual GCS criterion.
  • The emergency physician should direct the initial diagnosis and management. This includes airway, selection of imaging, and directing disposition. A good physical exam, including auscultation (remember that?) is essential.
  • Patients with normal GCS and minimal neck tenderness or other symptoms do not need imaging of any kind.
  • Patients with abnormal GCS should undergo CT scanning, consisting of a CT angiogram of the neck and brain with soft tissue images of the neck and cervical spine recons.
  • Based on final diagnoses, the patient can be admitted to an appropriate medical service or mental health. In the very rare case of a spine, airway, or vascular injury, the appropriate service can be consulted.

Reference: A case for less workup in near hanging. J Trauma 81(5):925-930, 2016.

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Mechanism, Trauma Center

Trauma Activation For Strangulation: Yes or No?

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I received a request to discuss this topic from a reader in Salina, KS. Thanks!

Trauma activation criteria generally fall into four broad categories: physiology, anatomy, mechanism of injury, and co-factors. Of these, the first two are the best predictors of patients who actually need to be assessed by the full trauma team. Many trauma centers include a number of mechanistic criteria, usually much to their chagrin. They typically end up with frequent team activations and the patient usually ends up have trivial injuries.

However, there are some mechanisms that just seem like they demand additional attention. Death of another occupant in the vehicle. Fall from a significant height. But what about a patient who has been strangled?

Unfortunately, the published literature gives us little guidance. This usually means that trauma centers will then just do what seems to “make sense.” And unfortunately, this frequently results in significant overtriage, with many patients going home from the emergency department.

Since there is little to know research to show us the way, I’d like to share my thoughts:

  • As a guiding principle, the trauma  team should be activated when the patient will derive significant benefit from it. And the benefit that the team really provides is speed. The team approach results in quicker diagnosis from physical exam and FAST. It gets patients to diagnostic imaging quicker, if appropriate. And gets them to the OR faster when it’s not appropriate to go to CT.
  • Activating for a strangulation mechanism alone is probably a waste of time.
  • Look at the patient’s physiology first. Are the vital signs normal? What is the GCS? If either are abnormal, activate.
  • Then check out the anatomy. If the patient has any voice changes, or has obvious discoloration from bruising, crepitus, or subcutaneous emphysema, call the team. They may suffer a deteriorating airway at any moment.

If physiologic and anatomic findings don’t trigger an activation, then standard evaluation is in order. Here are some things to think about:

  • A complete physical exam is mandatory. This not only includes the neck, but the rest of the body. Strangulation is a common injury from domestic violence, and other injuries are frequently present.
  • If there are any marks on the neck, CT evaluation is required. This includes soft tissue, CT angiography, and cervical spine evaluation. All three can be done with a single contrast-enhanced scan. The incidence of spine injury is extremely low with strangulation, but the spine images are part of the set anyway.
  • CT of the chest is never indicated. There is no possibility of aortic injury with this mechanism, and all the other stuff will show up on the chest x-ray, if significant enough for treatment.
  • Even if there are no abnormalities, your patient may need admission while social services arranges a safe place for their discharge. Don’t forget the social and forensic aspects of this injury. Law enforcement may need photographic evidence or statements from the patient so this event can’t happen again.

Next post: Trauma Activation for Hanging: Yes or No?

Reference: Strangulation forensic examination: best practice for health care providers. Adv Emerg Nurs J 35(4):314-327, 2013.

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Mechanism

Closing Velocity And Injury Severity

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Trauma professionals, both prehospital and in trauma centers, make a big deal about “closing velocity” when describing motor vehicle crashes.  How important is this?

So let me give you a little quiz to illustrate the concept:

Two cars, of the same make and model, are both traveling on a two lane highway at 60 mph in opposite directions. Car A crosses the midline and strikes Car B head-on. This is the same as:

  1. Car A striking a wall at 120 mph
  2. Car B striking a wall at 60 mph
  3. Car A striking a wall at 30 mph

2010-saab-9-5-head-on-crash-test_100313384_m1

The closing velocity is calculated by adding the head-on components of both vehicles. Since the cars struck each other exactly head-on, this would be 60+60 = 120 mph. If the impact is angled there is a little trigonometry involved, which I will avoid in this example. And if there is a large difference in mass between the vehicles, there are some other calculation nuances as well.

So a closing velocity of 120 mph means that the injuries are worse than what you would expect from a car traveling at 60 mph, right?

Wrong!

In this example, since the masses are the same, each vehicle would come to a stop on impact because the masses are equal. This is equivalent to each vehicle striking a solid wall and decelerating from 60 mph to zero immediately. Hence, answer #2 is correct. If you remember your physics, momentum must be conserved, so both of these cars can’t have struck each other at the equivalent of 120 mph. The injuries sustained by any passengers will be those expected in a 60 mph crash.

If you change the scenario a little so that a car and a freight train are traveling toward each other at 60 mph each, the closing velocity is still 120 mph. However, due the the fact that the car’s mass is negligible compared to the train, it will strike the train, decelerate to 0, then accelerate to -60 mph in mere moments. The train will not slow down a bit. For occupants of the car, this would be equivalent to striking an immovable wall at 120 mph. The injuries will probably be immediately fatal for all.

Bottom line: Closing velocity has little relationship to the injuries sustained for most passenger vehicle crashes. Those injuries will be consistent with the speed of the vehicle the occupants were riding, and not the sum of the velocities of the vehicles. 

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Anatomy, Mechanism

Bucket Handle Injury – Part 1

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Bucket Handle Injury

A bucket handle injury is a type of mesenteric injury of the intestine. The intestine itself separates from the mesentery, leaving a devascularized segment of bowel that looks like the handle on a bucket (get it?).

These injuries can occur after blunt trauma to the abdomen. The force required is rather extreme, so the usual mechanism is motor vehicle crash. In theory, it could occur after a fall from a significant height, and I have seen once case where a wood fragment was hurled at the abdomen by a malfunctioning lathe.

The mechanics of this injury are related to fixed vs mobile structures in the abdomen. Injuries tend to occur adjacent to areas of the intestine that are fixed, such as the cecum, ligament of Treitz, colonic flexures and rectum. During sudden deceleration, portions of the intestine adjacent to these areas continue to move, pulling on the nearby attachments. This causes the intestine itself to pull off of its mesentery.

The terminal ileum is the most common site for bucket handle tears. Proximal jejunum, transverse colon, and sigmoid colon are other possible areas. The picture above shows multiple bucket handle injuries in one patient. There are 3 injuries in the small bowel, and one involving the entire transverse colon. Note the obviously devascularized segment at the bottom center of the photo.

Always think about the possibility of this injury in patients with very high speed decelerations. Seat belt marks have a particularly high association with this injury. If your patient has an abnormal exam in the right lower quadrant, or if the CT shows unusual changes there (“dirty” mesenteric fat, thickened bowel wall, extravasation), I recommend a trip to the OR. In these cases, an injury will nearly always be present.

Tomorrow: These injuries can be subtle in an awake patient with a reliable exam. On Friday I’ll write about how you can detect it in unconscious patients.

Source: personal archive. Not treated at Regions Hospital

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