Pre-hospital ultrasound

Prehospital ultrasound is the specialized application of ultrasound by paramedics,^[1] to guide immediate care and treatment procedures. Like conventional ultrasound, it is a device that produces cyclic sound pressure to penetrate a medium (flesh) and reveal details about the inner structure of the medium.^[2]

Differences from conventional ultrasound

Many emergency physicians now view screening ultrasound as a tool, and not a procedure or study. It is primarily used to quickly and correctly ascertain a limited set of internal injuries, specifically those injuries where conventional methods of determining them, such as trauma to the torso or heart, would either take too long, require too much time to prepare, or introduce greater risk to the patient.^[3]

While conventional ultrasound can be a lengthy process, and is usually conducted with non-mobile units and advanced image filtering and manipulation built into the unit, emergency ultrasound is as simple and quick to operate as possible, and narrowly focused on a small set of criteria.^[4]

Indications

Indications for the use of prehospital ultrasound are determining the severity of trauma to the midsection, determining immediate trauma or penetration of the heart, and to process sources and extent of internal bleeding. Specific implementations vary, some areas use the German originated FAST scan system which focuses on fluids in the paracolic gutter and the Douglas, Koller's and Morison's pouch. Others focus more on cardiac ultrasounds. As a part of the FAST exam, it is common for the examiner to quickly image the sliding lung against the chest wall to rule out pneumothorax. If the visceral and parietal lung have separated, the lung sliding will disappear indicating a probable pneumothorax. If lung sliding is seen, there is no significant pneumothorax. This simple addition to the FAST exam can accurately identify lung collapse in only 1 additional minute. This Enhanced FAST exam is often called EFAST. To be able to exclude a pneumothorax fast and accurate can prevent trauma victims from receiving an unnecessary chest tube.^[5] Another useful indication is assessing intravascular volume by looking directly at the inferior vena cava (IVC). While studies have largely not supported its use as a singular indicator of intravascular volume, in the correct clinical picture the diameter of the IVC can help a practitioner determine a patient's overall fluid status. Cardiac imaging can give the provider a sense of ejection fraction and heart function but in the setting of prehospital evaluation is primarily used to discover life-threatening pericardial tamponade caused by rapidly accumulating blood around the heart. Patients with pulmonary embolism can have a markedly enlarged right ventricle during the event, giving additional clues to the provider for correct diagnosis and treatment. As time goes on, it is likely that there will be dozens of additional indications for field use.

Procedures

In most cases, prehospital providers will employ the use of a portable ultrasound unit. In every instance, an attempt is made to find the area best suited to an ultrasound and utilize bare skin if possible. Resolution is vastly decreased if soundings must be made through any form of clothing.^[6]

There are two main areas of emergency ultrasound. The Focused Assessment with Sonography for Trauma (or FAST) focuses on trying to ascertain if there is internal bodily fluid in blunt abdominal trauma, in the areas between organs, which is a sign of severe internal injury.^[7] Echocardiography is used to attempt to find valvular disease, clots, cardiomyopathies or penetrations of the heart. Both systems are scanning methodologies, they use identical equipment.^[8]

Ultrasound visualization of the optic nerve sheath has been shown to be useful as a surrogate for more invasive intracranial pressure monitoring, allowing for more advanced monitoring of brain injuries in the field.^[9]

Strengths

1. It images muscle, soft tissue, and bone surfaces very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces, unlike most other methods of trauma diagnosis, which are little more than educated guesses.
2. It renders "live" images, where the operator can dynamically select the most useful section for review, and narrows down the problem area, rather than having to wait until the patient is at the hospital.
3. It has no known long-term side effects and rarely causes any discomfort to the patient.

Weaknesses

1. Sonographic devices have trouble penetrating bone. For example, sonography of the adult brain is very limited. This means that in terms of trauma diagnosis involving brain injury, sonography will be difficult and requires high-end ultrasound machines.
2. The depth penetration of ultrasound is limited, making it difficult to image structures deep in the body, especially in obese patients.
3. The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses, which is one more skill that a limited EMS team must develop. Since most EMS teams are small and suffer high turnover, retaining qualified personnel can be difficult.

See also

• Duplex ultrasonography

References