Page 47 - Small Animal Internal Medicine, 6th Edition

CHAPTER 2 Diagnostic Tests for the Cardiovascular System 19

motion are readily detectable and can guide initial manage- blocked by a strongly hyperechoic object (such as a rib), and
ment. Nevertheless, follow-up evaluation by a veterinary an acoustic shadow (where no image appears) is cast behind
VetBooks.ir cardiologist or other individual with advanced echocardio- the object.
For most echocardiographic examinations, the animal is
graphic training usually is advisable. In addition, a lung
ultrasound examination (see p. 33) could rapidly narrow the
images usually are obtained when the heart is imaged from
differential diagnosis list and help guide initial therapy in gently restrained in lateral recumbency; better-quality
patients with respiratory signs. the recumbent side. For this the animal is placed on a table
or platform with an edge cut out, which allows the echocar-
BASIC PRINCIPLES diographer to position and manipulate the transducer from
Echocardiography uses pulsed, high-frequency sound waves the animal’s dependent side. Some animals can be adequately
that are reflected, refracted, and absorbed by body tissue imaged while standing; however, patient movement often is
interfaces. Only the reflected portion can be received and challenging. Shaving a small area of hair over the transducer
processed for display. Transducer frequency, power output, placement site improves skin contact and usually image
and various processing controls influence the intensity and clarity. Coupling gel is applied to produce air-free contact
clarity of the displayed echo images. Individual patient char- between skin and transducer. The transducer is placed over
acteristics also affect the quality of images obtained. Sound the area of the precordial impulse (or other appropriate site),
waves do not travel well through bone (ribs) and air (lungs); and its position is adjusted to find a good “acoustic window”
these structures may preclude good visualization of the that allows clear visualization of the heart. The right and left
entire heart. Several echo modalities commonly are used for parasternal transducer positions are used most often. Minor
clinical examinations: two-dimensional (2-D, real-time), adjustment of the animal’s forelimb or torso position may be
M-mode, and Doppler modalities. Each has important appli- required to obtain a good acoustic window. Once the heart
cations (described later). is located, the transducer is angled or rotated to obtain the
Sound waves are propagated through soft tissue at a char- desired views. Controls for factors such as beam strength,
acteristic speed (≈1540 m/sec), which allows the location focus, and postprocessing parameters are adjusted as needed
and size of various structures to be determined in relation to to optimize the image. For 2-D and M-mode studies, better
the origin of the ultrasound beam at any point in time. With image definition is achieved when the ultrasound beam is
2-D and M-mode echocardiography, stronger echoes are oriented perpendicular to the cardiac structures. Image arti-
returned when the ultrasound beam is oriented perpendicu- facts are common and can mimic a cardiac abnormality.
lar to the imaged structure. Stronger echoes also result when Sometimes a lesion is suspected that is not truly present;
there is greater mismatch in acoustic impedance (related to other times an actual abnormality is obscured. If a suspected
tissue density) between two adjacent tissues, because this lesion can be visualized in more than one imaging plane, this
produces a more reflective boundary. Very reflective inter- provides greater assurance that it is real.
faces such as bone/tissue or air/tissue interfere with imaging A basic echocardiographic examination is obtained from
of weaker echoes from deeper tissue interfaces. The ultra- the right parasternal position and includes standard 2-D
sound beam decreases in intensity as it penetrates through imaging planes and carefully obtained M-mode views. A
the body’s tissues (because of beam divergence, absorption, more complete examination includes standard left paraster-
scatter, and reflection of wave energy at tissue interfaces); nal views as well as any other modified views needed to
echoes returning from deeper structures tend to be weaker. further evaluate specific lesions. Doppler evaluation pro-
In general, higher frequency ultrasound energy permits vides important additional information. A complete exami-
better resolution of small structures because of the beam’s nation may be quite time consuming in some patients.
characteristics (longer near field and lesser far field diver- Echocardiography usually can be performed with minimal
gence). However, higher frequencies have less penetrating or no chemical restraint. For animals that will not lie quietly
ability as more energy is absorbed and scattered by the soft with gentle manual restraint, light sedation is helpful.
tissues. Conversely, a transducer that produces lower fre- Sedation protocols for dogs include either butorphanol
quencies provides greater penetration depth but less well- (0.2-0.3 mg/kg, IV or IM), or butorphanol (same dose)
defined images. Frequencies generally used for small animal mixed with acepromazine (0.02-0.03 mg/kg, IV or IM), or
echocardiography range from about 3.5 megahertz (MHz) buprenorphine (0.005-0.01 mg/kg, IV or IM) combined with
for large dogs to greater than 10 MHz for cats and small acepromazine (0.02-0.03 mg/kg, IV or IM). For cats, butor-
dogs. However, image optimization also involves many other phanol (0.2-0.25 mg/kg IM) mixed with acepromazine
technical factors and settings that can vary among manufac- (0.05-0.1 mg/kg IM) or midazolam (0.2 mg/kg IM) often is
turers and are beyond the scope of this chapter. adequate after a 20 to 30 minute rest period in a quiet room.
Strongly reflective tissues are referred to as being hyper- However, some cats require more intense sedation. A com-
echoic or of increased echogenicity. Poorly reflecting tissues bination of butorphanol (0.2-0.4 mg/kg IM) and alfaxalone
are hypoechoic; fluid, which does not reflect sound, is anechoic (1-2 mg/kg IM) can be effective and does not raise heart rate
or sonolucent. Tissue behind an area of sonolucency appears like ketamine does. Predictably fractious cats also can be
hyperechoic because of acoustic enhancement. On the other pretreated at home (~2-3 hours before the echo appoint-
hand, through-transmission of the ultrasound beam is ment) with gabapentin at 50 mg (for small cats) to 150 mg