EMS ULTRASOUND
Dave Spear, M.D., F.A.C.E.P

 

The first practical use of sound wave reflection was in world war II in searching for German submarines. If you were scuba diving near an underwater wall and began striking your air tank with a wrench, the sound would travel to the wall and bounce back as an echo. The time it took would tell you the distance it took to travel to the wall and back. If you knew the speed of sound in water you could determine the distance to the underwater wall. This is the same principle used by bats and dolphins.
                                                                   
Back to the scuba diver … if a dolphin appears it would also be sending echoes to the wall. Because dolphins rely on this method as another sense, they send many more waves and analyze the reflections with their brain. By analyzing this data the dolphin can “see” the wall.

Ultrasound machines have a "brain" like the dolphin and translate sound wave into images we can see on a screen. Whether the machine is called an ultrasound machine, a sonogram machine or and echocardiography machine, the same process is going on.

Some important characteristics of the waves we use to image humans:

• Air Scatters these US Waves and you lose the image

• Water & Fluid transmit waves - appear black

• Interfaces between two high fluid content structures gives sharp delineation – for example liver next to kidney gives a "line" between the two.


Characteristics of Thorax vs Abdomen


The thorax is mostly air. There are fewer places to place a sonogram probe on the chest in which air is not getting in the way. When US waves hit air they scatter and the image is lost. 
The abdomen has many more areas to approach which don’t have air.
 

TRAUMA ULTRASOUND
Abdominal Imaging with US in Trauma Patients
 

The liver happens to be an excellent structure to sonogram. There is no overlying air and its high water contenttransmits ultrasound waves very well. If the ultrasound probe is placed on the patient's side "laterally" you can see liver and kidney next to each other.

 


We don’t desire to do an official abdominal ultrasound like a radiologist would want done. Our use is to confirm what we already may suspect. If a blunt trauma patient is hypotensive, we want to do a US screening exam just to look for suspected blood in the abdomen.



Note that water, blood, urine and other fluids look identical with ultrasound. Therefore, it is the fact that we suspect blood in the belly of a trauma patient that leads us to equate fluid with blood in these cases.
As stated previously, if the ultrasound probe is placed on the right lateral side of the abdomen, the liver and kidney are easy to visualize. This happens to also be the area where blood would collect in the supine abdomen.

        


Hepatorenal (Morison’s) Pouch


 If a surgeon were to open you mid abdomen and pour in fluid into the peritoneal space it would collect or “puddle” in the deepest (most dependent) part of the supine abdomen. This happens to be between the liver and the kidney and is called Morison’s Pouch.



 Blood (fluid) in the abdomen of a trauma patient:

 

                   


Notice that the liver in this US is "floating" or surrounded by blood!
 

Blood (fluid) in the abdomen of a trauma patient:


Can you point out the blood? (more subtle on this one)


•  True Positive (Blood truly is in the peritoneum)
– Needs exploratory surgery or CT abdomen
•  True Negative (No blood in the abdomen)
– Can place patient in Trendelenberg or repeat Study q 30 min to 60 min
•  False Positive (See fluid – but it is not blood)
– Bladder Rupture, Ascities
•  False Negative (There is blood – but not enough to detect)
– Takes 500cc-1000cc of blood to detect
 

The Role of the Ultrasound Machine in a Trauma Patient

•  Where?: the field ultrasound should be done en route
•  When?: should be part of the secondary survey
•  Why?: will let you know the patient has a serious problem if there is blood in the abdomen
 

The Shortcomings of Ultrasound in Trauma

  • Air is the enemy and scatters waves losing the image

  • Needs to be 500cc - 1000cc to be able to image

  • Does not evaluate for a retro peritoneal hematoma

 

Summary of Diagnostic Studies in Blunt Abdominal Trauma

Hemoperitoneum (blood in the abdomen) can be diagnosed by any of the following ways:

• Diagnostic Peritoneal Lavage
• CT Scan of Abdomen & Pelvis
• Ultrasound
 

Why EMS Ultrasound now?
 

  • Portability

  • Cost

  • Ease of Use

  • Proven efficacy
     

"The sooner EMS Ultrasound use becomes widespread, the more lives that will be saved."