Ultrasound has become the standard imaging modality in many austere situations. It has multiple clinical applications and can easily be operated by the clinician. In addition, its low cost (in comparison to other imaging hardware) and portability are ideal for remote areas. While clinicians now perform a wide variety of ultrasound exams (the E-FAST and gallbladder being the most common), only the less common, but easily performed exams to diagnose fractures and neonatal intracranial bleeding, are described here. Eye exams are discussed in Chapter 27, Ophthalmology.
In austere situations, "disposable" often means unavailable. That is frequently the case with ultrasound gel. Substitutes include ECG jelly, liquid soap, any water-soluble gel (e.g., K-Y Jelly), and vegetable oil. All provide images comparable to those produced using commercial gels. The problem with vegetable oil, however, is that it is not water-soluble and it stains everything it touches with oil marks. While ordinary soap or water can be used as ultrasound gel, do not use petroleum-based materials, such as Vaseline, since they reportedly damage ultrasound heads.
A nonsterile very thin plastic bag, large condom, or rubber glove can be used to protect the ultrasound head when it is being used in the vagina or mouth. Put contact gel into the bag or glove before inserting the probe. Then put additional gel on the outside of the plastic for a good seal. When a sterile probe cover is needed to help guide procedures done under aseptic conditions (e.g., paracentesis, central line placement, and bladder aspiration), use sterile surgical gloves as a probe cover.
Neonatal Ultrasound of the Head
Use cranial ultrasound in infants to check for bleeding. Cranial ultrasonography can easily identify a subependymal hemorrhage or intraventricular blood. It can also be used to identify posthemorrhagic ventricular dilation reasonably well and midline shifts. Evaluation of arterial flow (especially in the neck) and intraparenchymal lesions is operator dependant.
Use a 7.5 MHz probe for premature infants <32 weeks gestation or <1500 g, and a 5.0 to 3.0 MHz probe for older infants with an open fontanelle. With enough gel on the head and probe and with the child restrained, place the probe over and perpendicular to the anterior fontanelle in the coronal plane (across the head) and slowly angle it forward and backward to see the entire brain. The structures, especially those that are blood-filled, should be easy to see. Repeat the process with the probe oriented in the sagittal plane (front to back) and slowly angle it from side to side to see the lateral ventricles and temporal lobe.
Although bone is a natural obstacle to high-frequency sound transmission, the large difference in acoustic impedance between soft tissue and bone results in a strong acoustic interface. This leads to an almost total reflection of sound energy from the bone, providing an excellent way to examine patients for fractures.13
Selecting the correct patients for examination is the key to using ultrasound for fracture diagnosis (Fig. 18-8). In patients exhibiting signs and symptoms of a fracture, clinicians with minimal training can diagnose many fractures using ultrasound, even those that cannot be seen on radiographs. In patients with a medium-to-low probability of fracture, they are able to successfully rule out the presence of long-bone fractures.14
Algorithm for use of ultrasound in fracture diagnosis. (Abbreviation: Fx, fracture)
The ultrasound exam for fracture, which generally takes less than 5 minutes to complete, is usually performed with a high-frequency linear probe (10 to 5 MHz). Physicians can become proficient in administering the exam with only an hour-long standardized formal training session and a practice session on a live normal model.14 Although, demonstrating on a child with a femur fracture, I taught rural Zambian nurses to successfully recognize fractures with ultrasound in 5 minutes. Some inexperienced examiners use pictorial reference cards to locate both ultrasound equipment controls and where to place the probe for various examinations.15
The most common exam is of a long bone. Especially for those not completely familiar with the technique, a useful way to begin is to examine the unaffected extremity and to adjust the depth and maximize visualization of the cortical interface during this part of the exam. The cortical interface appears as an unbroken, strongly echoic line. Using sufficient gel, place the transducer transversely (across) the extremity just proximal to the patella or elbow. Identify the femur or humerus and rotate the transducer 90 degrees so that it lies along the long axis of the bone. Slide it up the extremity (Fig. 18-9). Either the smooth shaft or an obvious discontinuity will be seen. Approaching the femoral neck, angle and rotate the transducer slightly and continue scanning to the midpoint of the inguinal ligament to visualize the femoral neck, head, and pelvic acetabulum. Near the shoulder, scan just distal to the acromial process of the scapula to visualize the humeral head.14,16 If significant bleeding around the fracture has occurred, there may be an overlying hematoma that appears acutely as a hyperechoic area; the hematoma appears hypoechoic in the subacute stage.13