Equine Abdominal, Thoracic Ultrasonography in the Field
Many veterinarians carry and use ultrasound machines extensively, primarily for assessing horses’ reproductive and musculoskeletal systems. But they can maximize its use by looking into other body cavities, such as the thorax and abdomen, said Tracy Norman, VMD, Dipl. ACVIM (large animal), owner of Valley Veterinary Ultrasound, in Central Virginia.

During her presentation at the 2021 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Nashville, Tennessee, Norman provided tips and best practices for preparing equine patients for thoracic and abdominal ultrasonography and getting quality images. She also acknowledged that learning new ultrasound tricks can be challenging.

“It’s really frustrating, and because we get frustrated, we stop using it,” she said. “I want to encourage you to use your ultrasound machine more.”

Keys to success, said Norman, revolve around case selection, understanding your equipment, and knowing how to prepare your patient—and yourself—for examinations. She shared these considerations with conference attendees:

1. Case selection

“By far the most common application of abdominal sonography in equine practice is the horse with acute signs of abdominal pain,” or colic, she said.

Fast localized abdominal sonography for horses (FLASH) can provide rapid and efficient examinations to address specific concerns, but veterinarians must use it with caution.

“It only provides a limited exam and is only good for acute abdominal pain,” said Norman. “It is not appropriate to use this technique for horses with fever, chronic colic, diarrhea, anemia, abdominal distention, weight loss, or renal failure cases, for example.” Abdominal ultrasound can’t penetrate deep enough into the abdominal cavity to get a “big picture” of what’s going on with these other conditions.

An abdominal ultrasound will yield valuable information about abnormalities of hollow viscera (large organs), solid organs, and fluid, whereas thoracic ultrasound examinations will show pleural (lung) irregularities, effusion (swelling), and rib fractures.

For ultrasound to be valuable, “clinicians must have a commanding knowledge of anatomy and sufficient experience to accurately interpret subtle alterations of the anatomy,” Norman said.

2. Equipment

“There are lots of options available, including the transducers,” she said. “Typically, we use a large curvilinear, low-frequency probe for the abdomen and thorax, but others can be useful, too. A rectal probe can look at the lung surface, for example.”

Manipulating the equipment is a little more intensive for abdomens and the thorax than for musculoskeletal structures and reproductive tracts because of the different depths and distances of the targets.

Importantly, Norman reminded practitioners, “Label your images! This is especially important once the anatomy gets abnormal.”

3. Patient preparation

Clipping the fur can improve image quality, making interpretation of the ultrasound easier for some veterinarians.

However, “clients have big frowns when you map out the area where you want to clip,” Norman said.

“If you have the latitude to clip, wash the area with soap and water afterward, and use warm ultrasound gel,” she recommended.

Warming the gel isn’t just for patient comfort; it also increases blood flow to the area and improves transmission of the sound waves.

If the owner will not allow clipping, brush the area to remove as much dirt and debris as possible. Wet the hair with alcohol, but only apply the alcohol to a small area at a time.

“If it is cold, and evaporation of the alcohol contributes to piloerection (raised hairs), the ultrasounds will be impeded,” explained Norman.

Owners of horses with pituitary pars intermedia dysfunction (aka Cushing’s disease) are often worried their horses will freeze if they’re clipped in the winter. With these horses, at least clip the target area you want to examine.

In her opinion, Norman recommends clipping whenever possible because it makes a dramatic difference in the quality of the images.

Another preparation tip to keep in mind: When planning an abdominal ultrasound (e.g., for a horse with weight loss)—withhold feed for 12-24 hours, but still offer free-choice water.

4. Veterinarian preparation

Norman recommended veterinarians attend wet labs for hands-on instruction and, importantly, reviewing equine anatomy.

“You need to know where you should expect things,” she said. “For example, the ribs run at about a 30-degree angle in reference to the spine. Knowing this will help you nestle the probe in the rib space, which can be hard in a fat horse.

“You can successfully scan these body cavities,” she continued. “Select cases and prepare your patients appropriately, understand your equipment to use it to the best of its ability, and practice. No matter how busy you are, scan a lot of normal horses first before trying your hand on clinical cases.”