E
Echocardiography
INDICATIONS
• Congenital or acquired cardiac disease (valvular, myocardial, pericardial), cardiac neoplasia, pulmonary hypertension, systemic hypertension, and pleural effusion or respiratory distress of uncertain etiology
EQUIPMENT, ANESTHESIA
• Scanning table with holes cut out because examination from beneath the animal improves image quality
• Ultrasound equipment; sector-scanning transducers preferred:
○ For cats and small dogs (usually 7.0-8.0-MHz transducer), medium-sized dogs (5.0-MHz transducer), large dogs (2.5-3.5-MHz transducer)
• The echocardiography machine should have M-mode, two-dimensional (2D), Doppler (pulsed-wave, continuous-wave, and color-coded), and electrocardiogram (ECG) capabilities.
PREPARATION: IMPORTANT CHECKPOINTS
• Hair might be clipped over the right third to the sixth intercostal spaces (ICS) and left at the fourth to the seventh ICS (precordial transducer locations); however, in most dogs and cats, satisfactory images can be obtained by parting the haircoat, applying isopropyl alcohol liberally over area of interest, and finally applying transmission gel.
POSSIBLE COMPLICATIONS AND COMMON ERRORS TO AVOID
• Although the technique has no known physical hazard, there are risks associated with the improper interpretation or use of the results of the ultrasound.
• Adjust gain to avoid producing a white, distorted image due to a high setting; too low a setting will produce a weak signal.
PROCEDURE
• Image quality is improved in lateral recumbency; however, dogs and cats may be examined in a standing, sitting, or sternal position.
• The ultrasound machine’s ECG clips are attached to the legs as recommended by the manufacturer (ECG is used for measurements and timing within the cardiac cycle).
• Starting in right lateral recumbency (animal’s legs toward the examiner), the right parasternal location (window) is between the right third and sixth ICS, between the sternum and costochondral junctions (landmark: palpate right precordial heartbeat and place transducer at this location to start). Attention is paid to having the assistant restrain the animal, with the forelegs drawn cranially to open the axilla/acoustic window.
• Long-axis views: The beam plane is oriented slightly clockwise from perpendicular to long axis of the body, parallel to the long axis of the heart, and with the transducer index mark pointing toward the heart base (craniodorsal, approximately toward the animal’s shoulder). The following two views are obtained: first, a four-chamber view with the ventricles displayed to the left and the atria to the right; second, a view obtained by slight clockwise rotation of the transducer from the four-chamber view into a slightly more craniodorsal orientation revealing the left ventricular outflow tract, aortic valve, and aortic root.
• Short-axis views: Rotate the transducer about 90○ toward the sternum from the four-chamber view (keep probe in same position except for the rotation) such that the beam plane is oriented perpendicular to the long axis of the heart, with the transducer index mark now pointing cranially toward the animal’s elbow (proper orientation identified by circular symmetry of the left ventricle or aortic root). There are five standard transverse images (left ventricle with papillary muscles, left ventricle at mitral chordae tendineae level, left ventricle at mitral valve level, heart base–aorta/left atrium level, and heart base–pulmonary artery) obtained from this position by pivoting the transducer from the apex to the base of the heart (caudal/ventral to cranial/dorsal).
• Turn animal over into left lateral recumbency, with the animal’s legs still toward the sonographer.
• Left caudal (apical) parasternal location: The location is between the left fifth and seventh ICS, as close to the sternum as possible (landmark: palpable left apical heartbeat):
○ Left apical two-chamber views: The transducer index mark is pointing toward the heart base (dorsal), and the beam plane parallel to the long axis of the heart—the left side of the heart—is visualized (left atrium, left ventricle, and mitral valve). Slight rotation of the transducer into a craniodorsal to caudoventral orientation reveals left ventricle, outflow tract, aortic valve, and aortic root in a long-axis view.
○ Left apical four-chamber views: This is the only view in which the transducer index mark is pointing to the left and caudally, opposite of all other views. Note: The transducer should be as far back toward the apex of the heart as possible, often around the seventh ICS, and tilted to point cranially. The beam plane is in a left-caudal to right-cranial orientation and then directed dorsally toward the heart base, revealing the ventricles in the near field closest to the transducer and the atria in the far field (heart is oriented vertically; left ventricle, mitral valve, and left atrium should appear to the right). Modest cranial tilting of the beam from the above view will bring the left ventricular outflow region into view (five-chamber view).
• Left cranial parasternal location: The location is between the left third and fourth ICS, between the sternum and costochondral junctions.
○ Long-axis views: With the transducer index mark pointing cranially and the beam plane oriented parallel to the long axis of the body and heart, a view of the left ventricular outflow tract, aortic valve, and ascending aorta is obtained (left ventricle will be displayed to the left and aorta to the right). From this position, angling of the beam ventral (toward the sternum) to the aorta brings out the right atrium/right auricle, tricuspid valve, and inflow region of the right ventricle (displayed to the right, while the left ventricle will be noted to the left). Finally, angling the transducer dorsally (transducer will almost be horizontal and parallel with the table) in relation to the ascending aorta produces a view of the main pulmonary artery, pulmonary valve, and right ventricular outflow tract.
○ Short-axis views: Remain in the same location as for the cranial long axis, and rotate the transducer until the transducer’s index mark is toward the thoracic spine (dorsally, about 90○ from the location of long-axis view; aorta should appear circular in the center of the image). The right ventricular inflow tract should be to the left, and outflow tract and pulmonary artery to the right.
Bonagura JD. Echocardiography. J Am Vet Med Assoc. 1994;204:516-522.
Oyama MA. Advances in echocardiography. Vet Clin North Am Small Anim Pract. 2001;34(5):1083-1104.
Thomas WP, Gaber CE, Jacobs GJ, et al. Recommendations for standards in transthoracic two-dimensional echocardiography in the dog and cat. J Vet Intern Med. 1993;7:247-252.
Electrocardiography
EQUIPMENT, ANESTHESIA
• Electrocardiograph (ECG machine). Each ECG wire should end in an atraumatic clip that connects the machine to the animal’s skin.
• A table with a waterproof foam-core pad on which the animal can lie comfortably (to reduce electrical interference emerging through the table)
POSSIBLE COMPLICATIONS AND COMMON ERRORS TO AVOID
• Animals that are in respiratory distress should not be restrained for electrocardiography; these animals should be assessed while they are standing (or in any other position in which they are not distressed), or the ECG should be postponed until dyspnea has improved or resolved.
• Artifact can be minimized by:
○ Avoiding metal-on-metal contact between clips; often, with smaller dogs and cats, the wires need to be held apart by the person performing the ECG to avoid wire-to-wire or clip-to-clip contact during the procedure.
○ Choosing a quiet, comfortable environment to perform the tracing; a cold or stressful environment may trigger shivering (and motion artifact), whereas an excessively warm or stressful environment can induce panting in dogs (and motion artifact).
○ Replacing the ECG wire set every 5 years under all circumstances or annually with heavy use (e.g., used multiple times weekly) because the wires tend to fracture internally with age
• Overinterpretation of the dimensions of P waves and T waves as indicating cardiac chamber enlargement is a common pitfall; echocardiography is much more sensitive and specific than electrocardiography for assessing cardiac structure.
• In-hospital monitoring that lasts for hours or more should involve the use of atraumatic or minimally traumatic skin clips: either (1) a small patch of skin can be shaved on each side of the thorax and stick-on, human ECG patches can be used or (2) loops of small-gauge steel suture may be passed through the skin in the usual locations on the four limbs, and the ECG wires are clamped to these loops instead of directly to the skin.
ECG Clip/Electrode Placement for Standard Limb Leads (I, II, III, aVR, aVL, aVF)
| RA, white | Right forelimb; clip to skin just proximal to the olecranon (caudal triceps region). |
| LA, black | Left forelimb; clip to skin just proximal to the olecranon (caudal triceps region). |
| RL, green | Right hind limb; clip to skin just proximal to the stifle (cranial thigh); ground wire. |
| LL, red | Left hind limb; clip to skin just proximal to the stifle (cranial thigh). |
PROCEDURE
• Performing an ECG requires two people (clinician and assistant), unless animal restraint is not necessary (anesthetized or unconscious animal).
• The assistant restrains the animal in right lateral recumbency. Sitting, sternal, and dorsal positions are equally acceptable; determination of the mean electrical axis from these other positions is inaccurate, but this has no effect on assessing the cardiac rhythm.
• The assistant stands or sits behind (dorsal to) the right-laterally recumbent animal and holds the animal’s limbs perpendicular to the long axis of the animal’s body. Note: For the humeri and femurs to be truly perpendicular to the long axis of the body, the assistant needs to extend the limbs sometimes quite substantially. Fore-limbs (at the level of the carpi) are held in the assistant’s right hand and hind limbs (at the level of the tarsi) in the left.
• The clinician attaches the ECG clips (electrodes) to the appropriate points on the animal for the limb leads as well as the precordial leads.
• A standard ECG includes representative recordings from all six limb leads, usually 3-15 seconds each for I, II, III, aVR, aVL, and aVF, and will provide further information if precordial leads are also recorded.
• A standard ECG also includes a “rhythm strip,” which is a continuous tracing of the same lead for 12 seconds to 1 minute or more:
○ The most commonly used lead for the rhythm strip is lead II, but the lead that is chosen should be the one that shows the clearest P waves, QRS complexes, and T waves.
• Generally, if an abnormality has not been detected over a period of 1-4 minutes of ECG recoding on paper, then the procedure is terminated. The tracing obtained thus far is reviewed meticulously, and if further ECG evaluation is needed, ongoing ECG monitoring either with an in-hospital display monitor (e.g., oscilloscope) or portable telemetry (see p. 1287) can be used.
POSTPROCEDURE
The clips are carefully detached from the skin prior to releasing the animal’s restraint.
ALTERNATIVES AND THEIR RELATIVE MERITS
• Thoracic radiographs: provide some information on cardiac structure (e.g., chamber enlargement), but this information is limited by substantial overlap between normal and mild to moderate abnormalities. Also indicated when ST-segment elevation or depression is present (assess for pulmonary or airway lesions).
• Echocardiography: the clinical gold standard for assessing cardiac chamber size; indicated if cardiac enlargement is suspected based on ECG and/or radiographs
• Pulse oximetry/arterial blood gas measurement: if ST segment elevation or depression is noted in the absence of structural heart disease (e.g., cardiomyopathy, advanced valvular disease, congenital heart disease)
• Serum potassium measurement: if lack of P waves is noted, especially in an animal with a history and/or physical findings suggesting a reason for hyperkalemia (urgent)
Electromyography (EMG) and Motor Nerve Conduction Velocity (NCV)
OVERVIEW AND GOALS
• Integral component of diagnostic evaluation of animals suspected of having neuromuscular disorders (neuropathy, myopathy, neuromuscular junctional disease).
• May confirm and characterize the presence of nerve, neuromuscular junction, or muscle disease in these animals.
• If the EMG is abnormal, a neuromuscular disease is present but a normal EMG does not rule out the possibility of a neuromuscular disease.
INDICATIONS
• Animals with clinical signs and neurologic examination suggestive of neuromuscular disease (diffuse or focal).
EQUIPMENT, ANESTHESIA
• General anesthesia and tracheal intubation are required. For sensory nerve conduction studies in which paralysis with atracurium is required, a ventilator is needed.
• Differential amplifier, computer with appropriate software for labeling and calculations, needle electrodes for stimulating and recording impulses (concentric needle electrode preferred for electromyography [EMG]), isopropyl alcohol, and measuring tape.
• Surgical pack: often, muscle and nerve biopsies (see p. 1305) are done following electrodiagnostics.
• In some cases, percutaneous endoscopic gastric (PEG) tube placement (see p. 1270) is beneficial while animal is anesthetized (providing nutritional support, administering medications, etc.).
PREPARATION: IMPORTANT CHECKPOINTS
• Monitor body temperature and prevent hypothermia, which prolong nerve conduction times (1.8 m/s drop for every 1°C drop in limb temperature).
PROCEDURE
• Animal is premedicated, anesthetized, intubated, and then maintained with inhalational anesthetic (isoflurane, sevoflurane).
• EMG:
○ Needle electrodes are used for recording electrical activity in major muscle groups in appendicular muscles and in epaxial and masticatory muscles; rarely esophageal, laryngeal muscles, and tongue.
○ Evaluation is performed both by listening for characteristic sounds of electrical potentials and visually examining the waveforms produced by the potentials on the monitor.
○ Sensitivity is greatly increased by increasing the number of “passes” through the muscle (by continually redirecting the needle within the muscle) and by increasing the number of muscles sampled, as denervation potentials may be present in only a few areas.
○ A short burst of electrical activity occurs during needle placement is normal due to temporary disruption of muscle fibers (insertional activity). Other normal electrical potentials called miniature end plate potentials and endplate spikes may occur as a result of spontaneous depolarization of part or all of a myofiber.
○ Other spontaneous electrical discharges in resting muscle (i.e., anesthetized animal) are abnormal and indicate neuromuscular disease.
○ Abnormal EMG activity includes fibrillation potentials, positive sharp waves, complex repetitive discharges, or myotonic discharges. In general, the type of waveform is not specific for neuroanatomic location (nerve versus muscle) or pathophysiologic mechanism of disease.
○ Abnormal EMG activity confirms the presence of neuromuscular disease and helps localize which muscle groups are affected; however, abnormal EMG activity cannot differentiate between myopathic or neuropathic disease.
• Motor NCV:
○ Using stimulating needle electrodes, a nerve is stimulated at various accessible points along its anatomic pathway.
○ Needle or surface electrodes within or over a muscle innervated by the nerve record a compound muscle action potential (CMAP), which results in a simple biphasic waveform.
○ Measurements made include the nerve conduction velocity (NCV), as well as the CMAP, amplitude, duration and area
○ Nerves commonly tested in domestic animals include the sciatic, peroneal, cranial tibial, radial, musculocutaneous, and median.
○ Nerve conduction velocity along various segments of axon may be determined by measuring the distance between points of stimulation (ideally at least 100 mm to decrease the effect of human measurement error) and difference in latency of waveforms (velocity [meters per second] = Ddistance/D time).
○ The amplitude of the waveform is proportional to the number of available axons. Axonal disease results in a decreased number of axons and decreased CMAP amplitude and may also cause secondary EMG changes due to denervation.
○ Decreases in NCV may be due to demyelination or due to loss of the large, fastest conducting axons.
○ Demyelinating diseases, in addition to causing decreases in NCV, cause a widening and distortion of the CMAP waveform, and temporal dispersion or distortion of the waveform over the time axis.
○ Many peripheral nerve diseases result in both axonal loss and demyelination, causing decreased amplitude, temporal dispersion, and decreased conduction velocity in addition to abnormal EMG activity due to denervation.
○ Measuring nerve conduction velocity is helpful in determining whether disease is demyelinating, axonal, or both, as well as whether the nerves are more affected proximally or distally. It is also helpful in determining which nerves are affected.
(Reprinted with permission from Cuddon PA: Electrophysiology in neuromuscular disease, Vet Clin North Am Small Anim Pract 32:31-62, 2002.)
(Reprinted with permission from Cuddon PA: Electrophysiology in neuromuscular disease, Vet Clin North Am Small Anim Pract 32:31-62, 2002.)
(Reprinted with permission from Cuddon PA: Electrophysiology in neuromuscular disease, Vet Clin North Am Small Anim Pract 32:31-62, 2002.)
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