Page 1904 - Cote clinical veterinary advisor dogs and cats 4th

954 Syncope

activity (i.e., failure of subsidiary pacemaker blood flow, hemoglobin concentration, and • Echocardiogram (p. 1094): ± congenital
oxygen tension; seizures > syncope
in third-degree atrioventricular [AV] block, • Tussive syncope: proposed mechanisms or acquired heart disease, estimation of
pulmonary arterial pressure (p. 838)
VetBooks.ir drome [SSS]) or physical exertion that is not ○ Increased intrathoracic pressure transiently • Resting ECG (p. 1096): ± rhythm
sinus arrest associated with sick sinus syn-
abnormalities
matched by an increase in cardiac output
increases intracranial pressure and dimin-
in a patient with inappropriate bradycardia
(e.g., third-degree AV block). ishes cardiac venous return and resultant Advanced or Confirmatory
cardiac output; cerebral blood flow is
• Tachyarrhythmias: rates > 300 beats/min for decreased during paroxysms of coughing. Testing
> 6 seconds (i.e., supraventricular tachycardia ○ Coughing stimulates vagal afferent • Holter monitor (p. 1120): 24-hour ECG;
[SVT] or ventricular tachycardia) in dogs; transmission to the vasomotor center in helpful in establishing a diagnosis in 42%
cats uncommonly experience syncope with the medulla, with subsequent stimulation of patients
tachyarrhythmias but more so with brady- of vagal efferents to the heart and blood • Cardiac event recorder (p. 1120): digital loop
cardias. Heart rate and time necessary for vessels (bradycardia, hypotension). recorder that is patient or owner activated,
syncope can vary, depending on underlying • Autonomic dysfunction: excessive barore- programmed to capture the heart rate
cardiac structure (i.e., a slower heart rate flex resulting in cardiac inhibition and/or and rhythm before and after the syncopal
for a shorter duration may cause syncope peripheral vasodilation (i.e., neurocardiogenic episode(s); diagnostic yield is 85%
in an animal with underlying myocardial syncope, carotid sinus sensitivity, carotid • Cardiac troponin I level is increased in
or valvular heart disease). Various outcomes: body tumor) dogs with cardiac syncope as compared
nonsustained tachyarrhythmia results in a • Peripheral vasomotor dysfunction: abnormal- with seizure, but overlap in values occurs so
return to sinus rhythm; overdrive suppression ity of peripheral vasoconstriction, muscle that troponin cannot be used to distinguish
leads to sinus arrest or possibly ventricular tone, heart rate, and/or respiration, resulting between the disorders.
fibrillation. in syncope (postural hypotension) • Assessment of oxygenation: pulse oximetry,
• Outflow obstruction (i.e., pulmonic stenosis, • Undetermined: ≈40% of human syncope arterial blood gas analysis (p. 1058)
subaortic stenosis, cor pulmonale due to cases despite extensive testing; likely similar • CT/MRI/cerebrospinal fluid tap: neurologic
heartworm disease with secondary pulmonary percentage in veterinary medicine disease (pp. 1080 and 1132)
hypertension) • Other tests may be useful, as indicated by
○ Classic theory: exercise results in DIAGNOSIS prior findings (e.g., abdominal ultrasound,
vasodilation of systemic arterioles. Fixed bronchoalveolar lavage, fine needle aspirate
obstruction to flow results in inadequate Diagnostic Overview or lung biopsy)
increase in cardiac output, leading to Diagnosis is largely based on patient signal-
hypotension and syncope. ment; historical events before, during, and TREATMENT
○ Alternate theory: systolic function, cardiac after the episode; and complete cardiovascular
output, and flow increase through stenosis and neurologic exams (p. 1136). Because the Treatment Overview
during exercise. Increased left ventricular episode often is not witnessed directly by the Treatment involves addressing the primary
systolic pressure causes overstimulation of veterinarian (to confirm or refute syncope), cause to alleviate syncopal episodes and prevent
left ventricular mechanoreceptors. Reflex request video recording by the owner when an sudden death.
activation of cardiac afferent vagal fibers event occurs at home. Confirmatory testing may
results in increased parasympathetic involve ambulatory electrocardiogram (ECG) Acute General Treatment
tone to heart and systemic blood vessels, monitoring and/or central nervous system Acute treatment is aimed at the underlying
resulting in bradycardia, vasodilation, and (CNS) imaging, depending on the suspected cause or stabilization procedures:
subsequent syncope. cause. Consider consultation with and/or • Blood loss: fluids, blood products, others
• Cyanotic heart disease (e.g., right-to-left referral to a veterinary cardiologist ± veterinary • Pericardial effusion with cardiac tamponade:
patent ductus arteriosus, tetralogy of neurologist. pericardiocentesis
Fallot): syncope is due to hypoxemia and/or • Bradyarrhythmias
hyperviscosity of blood from erythrocytosis, Differential Diagnosis ○ SSS, third-degree AV block: medical
or else is caused by arrhythmias. • Seizures (p. 1280) management may be effective if arrhyth-
• Masses obstructing inflow or outflow of • Narcolepsy/cataplexy mia is due to excessive vagal tone and
blood (rare): intracardiac mass lesions reduce • Weakness/collapse (e.g., metabolic, atropine responsive (i.e., anticholinergics
cardiac output by restricting blood flow neuromuscular) [propantheline] and beta-agonists [terbu-
through atrioventricular valves or obstructing taline]; see individual diseases for specific
a ventricular outflow tract. Initial Database treatment recommendations). Temporary
• Pericardial disease: compromises cardiac • CBC: often unremarkable; ± anemia, pacemaker if bradyarrhythmia is refractory
output by interfering with systemic venous thrombocytopenia to medical management and animal is
return (i.e., cardiac tamponade) • Serum biochemistry panel: often unremark- showing persistent clinical signs.
• Congestive heart failure: syncope mechanism able; ± electrolyte abnormalities, metabolic • Tachyarrhythmias (p. 1457)
presumed to be similar to autonomic dys- derangements ○ Ventricular tachycardia: oral or intrave-
function (see below) • Urinalysis: often unremarkable; ± proteinuria nous medical management, depending
Noncardiac: (antithrombin loss and subsequent hyperco- on severity of the arrhythmia (i.e., oral
• Neurologic syncope: increased intracranial agulable state leading to thromboembolism) options: mexiletine, sotalol, amiodarone,
pressure with resultant decrease in cerebral • Blood pressure (p. 1065): normal result does atenolol; intravenous options: lidocaine,
perfusion (e.g., cerebral edema, brain not rule out transient hypotension. procainamide, amiodarone, esmolol. Cau-
tumors, meningitis, encephalitis, cerebral • Heartworm testing: antigen test, microfilaria tious use or avoidance of beta-blockers
vascular obstructions, acute bleed). Seizures test in cases with systolic dysfunction/dilated
are far more likely than syncope with these • Thoracic radiographs: to identify structural cardiomyopathy
diseases. heart disease, pulmonary parenchymal
• Metabolic syncope: abrupt decrease in oxygen abnormalities (e.g., pulmonary artery abnor- Chronic Treatment
or nutrient delivery (i.e., glucose) to the malities with heartworm disease, cardiogenic • Treatment of underlying systemic disorder,
brain. Oxygen concentration is affected by pulmonary edema) if identified

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