Practical emergency ultrasound (Proceedings)
The practice of emergency ultrasound is based on two paradigms: immediate results and focused examinations. The benefit of emergency ultrasound to patient care is realized when a specific clinical question is raised in the history or physical examination findings or when specialty services are delayed. A focused examination does not replace a comprehensive abdominal ultrasound examination or echocardiogram. Instead, the emergency ultrasound provides timely diagnostic information and guidance for high-risk procedures.
The purpose of this presentation is to introduce basic ultrasound techniques and interpretations for focused examinations in veterinary emergencies. This can serve as an initial approach to training emergency medicine veterinarians. Trainees can then build on these topics with direct supervision and didactic training from experienced veterinarians and ultrasound technicians. Following a review of the basics of ultrasound physics, some general indications for focused ultrasound examinations in veterinary practice will be discussed. The indications are based on peer reviewed articles and common procedures performed at the Animal Medical Center.
The perspective in human patientsOver the past 20 years since initial investigations, the bedside ultrasound exam has evolved into its own discipline. The American Medical Association and the American College of Emergency Physicians recognize emergency physician-performed ultrasound examinations. Acceptance of the focused ultrasound examination by other specialty organizations has perhaps been an uphill battle. Some providers of consultative ultrasound services recommend training standards for practitioners outside their own specialties that far exceed the standards accepted by emergency medicine authorities. Despite differences in standards of training, the emergency ultrasound examinations worth is proven in peer-reviewed journals for numerous clinical scenarios including trauma, abdominal aortic aneurysms, ectopic pregnancy, pericardial effusion, cardiac activity, and procedure guidance.
A veterinary perspective
An initial review article on emergency veterinary ultrasound examinations dates back to 1988. Since then, the use of focused ultrasound examinations in critical veterinary patients has been touched upon in other reviews and studies. Standards for training of these exams have not been established by any specialty organization. The American Association of Veterinary Radiologists (AAVR) is currently developing standards for training both technicians and veterinarians. More information on this can be found at http://www.aavr.org/.
Indications for focused emergency ultrasound examinations are generally based on prior suspicion of a particular disease. The most well established indication in both human and veterinary patients is for the identification of fluid in cases of trauma. This is probably because fluid identification (whether it be in an infected uterus, around the heart, or within a body cavity) is a specific and relatively simple positive finding to interpret. Other specific ultrasound diagnoses in veterinary medicine could include identification of a living fetus, an intussusception, gallbladder mucocele, splenic torsion, or high-grade hydronephrosis. However, one can see that as we add to this list, the room for error in interpretation can increase. Furthermore, some of these diagnoses may not require immediate therapeutic intervention. Emergency clinicians must weigh the confidence of their interpretation with the urgency of the diagnosis (can a suspected hydronephrosis wait 8 hours for a more complete abdominal scan?).
Ultrasound wave characteristics
Sound energy is periodic changes in pressure within a medium, causing the molecules within that medium to compress and relax as an advancing pressure wave. Therefore, sound requires a medium for travel. People hear frequencies between 15 and 20,000 cycles per second. Contrast this with the inaudible sonic beams of diagnostic ultrasounds that have a frequency of 1,000,000 to 20,000,000 cycles per second (1 to 20 MHz).
Velocity: Since sound requires a medium for travel, we must discuss this medium of travel when discussing the characteristics of ultrasound. Ultrasound travels slowest in gases, fastest in solid tissue, and intermediate velocities in fluids/soft tissues. Velocity of an ultrasound in tissue is based on the tissue's compressibility and density. Gases are less dense (molecules less closely packed together) and therefore will be less effective in compressing and relaxing to allow for sound travel.
Frequency: When sound passes from one medium to another, its frequency remains constant but wavelength changes to accommodate the new velocity in the second medium. The frequency is chosen by the operator and will be discussed under 'technical parameters'.
Intensity: The intensity of sound is the power of the sound wave's compression of the medium as it travels. The intensity of the sound wave diminishes as it travels deep into tissue. Other factors that diminish a sound wave's intensity include the number of tissue interfaces, the type of tissue, and the frequency of the sound wave. Operator controls for both frequency and intensity are discussed under 'technical parameters'.