Special Diagnostic Techniques for Evaluation of Cardiac Disease

Chapter 5 Special Diagnostic Techniques for Evaluation of Cardiac Disease*



Meg M. Sleeper,



INTRODUCTION


Many special diagnostic techniques are available for evaluation of animals with cardiovascular disease. Ambulatory electrocardiographic equipment is available through various services, continuous in-hospital electrocardiographic monitoring equipment is widely available, and several large diagnostic laboratories provide specialized clinical pathology services. Additionally, veterinary cardiology referral centers are increasingly available for special diagnostic techniques such as cardiac catheterization.



CONTINUOUS IN-HOSPITAL ELECTROCARDIOGRAPHIC MONITORING


Continuous electrocardiographic monitoring is recommended for hospitalized patients at risk of heart rate or rhythm disturbances. These patients include:


• Patients with congestive heart failure

• Patients hospitalized with clinical signs (e.g., syncope) secondary to arrhythmia

• Patients with systemic disease that puts them at risk for arrhythmias (e.g., shock, sepsis, gastric dilation-volvulus, etc.)


Technique


• Continuous electrocardiographic monitoring requires an ECG unit with an oscilloscope or light-emitting diode (LED) display.

• Use of a chest lead configuration with adhesive electrode patches will minimize artifacts on the tracing while allowing the patient the most freedom for mobility.

• Clip two 2- to 3-cm square areas at the left apex and the heart base (where the apex beat is palpable on the left chest and at the heart base caudal to the right or left scapula).

• Clean and de-fat the area with 70% isopropyl alcohol. Allow to dry.

• Place the positive electrode patch at the left apical site and the negative electrode at the heart base site. A ground electrode may be placed at either site (Figure 5-1).

• Apply a light chest wrap to secure the electrodes and wires.

• If a multiple-lead electrocardiographic (ECG) unit is being used, then use the left arm electrode for the positive electrode, the right arm electrode for the negative electrode lead, and lead I for display/recording.

• If the patient is recumbent and unlikely to move, leads attached directly to the patient limbs can be used.

• Depending on the unit available, the signal is transmitted to the machine by cables or by radiotelemetry.

• Many ECG units will print out an ECG strip, which can be added to the permanent patient record.

image

Figure 5-1 Chest lead preparation in a Boxer. The negative lead should be placed in the upper patch behind the scapula and the positive lead should be placed at the left apex (behind the elbow). This configuration can be used for in hospital continuous ECG monitoring or ambulatory monitoring.



Limitations


• Excessive patient motion may result in displacement of electrodes or motion artifact in the ECG.

• Adhesive patches may occasionally result in contact dermatitis.


PROVOCATIVE ELECTROCARDIOGRAPHIC TECHNIQUES


In some patients, the history or baseline ECG is suggestive of pathologic arrhythmias, but a definitive diagnosis is elusive. In these cases, vagal stimulation or abolition may be informative. A provocative vagal maneuver will transiently elevate parasympathetic tone. In normal animals, the technique typically slows the heart rate or has no effect. However, when there is sinoatrial or atrioventricular (AV) nodal dysfunction, or an abnormal sensitivity to parasympathetic tone, transient sinus arrest or significant AV block may occur. Similarly, a vagal maneuver may be diagnostic and/or therapeutic for an ectopic supraventricular tachycardia. A sinus tachycardia typically slows over several seconds while an ectopic supraventricular tachycardia may terminate abruptly (see Frequently Asked Questions).


An ECG recorded immediately post exercise may demonstrate cardiac arrhythmias associated with increased sympathetic tone. Likewise, an atropine response test can be used to determine if a slow heart rate is associated with elevated vagal tone, as can occur with respiratory disease, gastrointestinal disease or neurologic disease, or is associated with primary cardiac conduction system disease. If the bradycardia is due to elevated vagal influence, then atropine administration or exercise will result in its abolition.



Vagal Maneuver


• The techniques listed below can be used for elevating vagal tone. The patient should be restrained and calm so that a good quality lead II ECG is obtained prior to and during the technique.


Ocular Pressure


• Moderately firm digital pressure is applied over the closed eyelid to one or both eyes for a period of 5 to 10 seconds or until significant slowing of the heart rate occurs.


Carotid Body Massage


• The carotid bodies are located in the area behind the larynx. Apply moderate digital pressure around the larynx while monitoring the ECG. Initiation of a gag response or a cough yields similar results and suggests adequate pressure has been applied.


Inhibition of Vagal Tone


• Two techniques are possible for abolishing vagal tone and are useful if the patient is suspected to have a vagally mediated bradycardia. With either technique, it is important to first obtain a baseline ECG.


Postexercise Electrocardiography


• Typically, strenuous leash running is used; however, the duration of exercise is not standardized. Evidence of exertion, such as panting, is sufficient to stop exercise and an immediate post-exercise ECG is obtained. A delay of as little as 30 seconds may alter results, and heavy panting can result in artifacts making the ECG difficult to interpret.


Atropine Response Test


• Atropine (0.04 mg/kg IV or IM) will result in abolition of vagal influence. An ECG should be obtained 10 to 15 minutes after administration of the drug for comparison with the baseline ECG.


Clinical Utility



Vagal Maneuver


Clinically useful in two scenarios:


• In the evaluation of dogs with a history of syncope, a vagal maneuver may demonstrate sinus arrest or AV block, suggestive of parasympathetic hypersensitivity or primary nodal disease.

• In dogs with tachycardia, abrupt termination of the arrhythmia with a vagal maneuver suggests an ectopic supraventricular origin because ventricular tachycardias are not usually sensitive to vagal tone; however, lack of response is not helpful in differentiating the origin of the tachycardia. The maneuver can also be useful to differentiate sinus tachycardia (no response or gradual slowing of heart rate) from pathologic ectopic supraventricular tachycardia (no responce or abrupt termination of tachycardia).


Postexercise Electrocardiography


• This test can be performed in the evaluation of dogs with subaortic stenosis or occult cardiomyopathy. In affected dogs, the combination of exercise, myocardial disease and left ventricular hypertrophy may result in electrocardiographic indicators of myocardial ischemia (i.e., ST segment depression or ventricular ectopy). Lack of response does not necessarily rule out heart disease. In dogs with bradycardia, abolition of the arrhythmia after exercise suggests a vagally mediated etiology. The increased availability of ambulatory ECG recordings has markedly reduced the use of post exercise ECGs in clinical practice.


Atropine Response Test


• Sinus tachycardia with a heart rate greater than 135 beats per minute suggests normal sinus node function in the dog. Additionally, this response suggests that medical management with vagolytic therapy may be effective if the bradycardia is associated with clinical signs such as syncope or collapse and implantation of a pacemaker is not possible.


Key Point


Provocative ECG techniques are easily performed, relatively inexpensive, and can be very helpful for diagnostic and therapeutic purposes in animals with bradycardias and/or tachycardias.



AMBULATORY ELECTROCARDIOGRAPHY (HOLTER MONITORING AND CARDIAC EVENT RECORDING)



Indications


Routine in-hospital electrocardiography only provides a glimpse of the daily electrocardiographic activity. Moreover, arrhythmia detection may be confounded by iatrogenic changes in the autonomic nervous system activity. A 24-hour ambulatory (Holter) recording of the ECG increases the sensitivity of arrhythmia detection. Major indications include:


• Detection of transient arrhythmias associated with syncope or periodic weakness.

• Screening of high-risk breeds for cardiomyopathy (e.g., Boxers, Doberman Pinschers).

• Evaluation of frequency, severity, and significance of arrhythmias detected on in-hospital ECG.

• Monitoring efficacy of antiarrhythmic therapy (e.g., control of heart rate in patients with chronic atrial fibrillation).

• Determining the true incidence and type of arrhythmia in heart disease patients.


Technique


Modern Holter monitors use a high-fidelity digital recorder to capture and store the cardiac electrical activity for 24 hours. Some digital recorders are now capable of monitoring the patient for up to 7 days. Two to three simultaneous ECG chest leads are typically recorded.


• Electrode sites are prepared by clipping, shaving, cleaning and drying the chest.

• Adhesive patches are firmly adhered to the skin (see Figure 5-1).

• A chest wrap is essential for securing electrodes, wires and the recorder. A vest or harness can also be used (Figure 5-2).

• Once the monitor has been applied, the animal returns home to resume normal activity. Very small dogs and cats that find it cumbersome to ambulate with the monitor in place may respond better to hospitalization with cage restraint during the 24-hour recording session.

Related posts:

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  2. Systemic Hypertension
  3. Heartworm Disease
  4. Radiology of the Heart

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Special Diagnostic Techniques for Evaluation of Cardiac Disease

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