Page 446 - Clinical Small Animal Internal Medicine

414 Section 5 Critical Care Medicine

While it is convenient to consider the heart as consist- burdens that mature rapidly. The severity of disease is
VetBooks.ir ing of two separate pumps (the left and right), it must be directly related to worm burden and patient size and
caval syndrome occurs most often when worm burdens
remembered that the two operate in a dependent man-
ner. The right heart is obligated to pump the same vol-
worm burdens as small as 12.
ume of blood that is returned to it from the venous approach 60–100 although it has been reported with
circulation in the same fashion that the left heart is obli- Tachyarrhythmias can result from primary conduc-
gated to pump the volume of blood that is returned from tional abnormalities such as arrythmogenic right ven-
the pulmonary circulation. This interdependence means tricular cardiomyopathy (ARVC) or may be secondary
that dysfunction of either the left or right heart results in to structural changes within the heart, as is the case
a decrease in total cardiac output. Therefore, sudden with dilated cardiomyopathy (DCM). In fact, sudden
failure of one side of the heart will result in dysfunction death due to ventricular tachycardia occurs in up
of the entire heart and can lead to cardiogenic shock. to 50% of Doberman pinschers affected with DCM.
Once global oxygen delivery is insufficient to main- A rare but severe form of ventricular tachycardia
tain aerobic metabolism, the pathophysiology of shock called Wolff–Parkinson–White syndrome can be seen
at the cellular level is identical regardless of the under- in dogs or cats with ventricular preexcitation. These
lying etiology. patients are at risk for development of paroxysmal
ventricular tachycardia due to the establishment of a
reentry circuit that allows for the establishment of a
Epidemiology reciprocating tachycardia and are at increased risk of
developing cardiogenic shock.
Many of the conditions that cause cardiogenic shock in Third‐degree atrioventricular (AV) block can result
dogs are chronic, progressive diseases that have an acute from progression of a chronic infiltrative cardiac disease,
worsening. Myxomatous mitral valve disease can predis- infectious disease (e.g., Chagas disease), myocarditis or
pose dogs to rupturing one or more chordae tendinae, ingestion of cardiotoxins, including prescription medi-
leading to cardiogenic shock. Unfortunately, the severity cations (e.g., beta blockers, calcium channel blockers) or
of valvular lesions is not predictive of risk of chordae plant alkaloids (e.g., digitalis glycosides). If acute in onset
tendinae rupture in dogs so little can be done to predict (not preceded by second‐degree AV block), third‐degree
which patients will suffer from this condition. When AV block can result in development of cardiogenic shock
rupture of a first‐order chorda occurs, patients will often although this is uncommon and usually coincident with
exhibit acute, severe clinical signs. Rupture of second or significant structural cardiac disease. Sick sinus syn-
third‐order chordae may result in minimal to no clinical drome is a disease that occurs primarily in miniature
signs and may only be detected during routine echocar- schnauzers and infrequently in other breeds. It is charac-
diography or necropsy. terized by periods of severe bradycardia or sinus arrest
Pericardial effusion is arguably the most common with occasional paroxysmal tachycardia resulting in
cause of cardiogenic shock in dogs and can occur more potentially significant perfusion abnormalities.
or less acutely. Chronic pericardial effusion is less likely Myocarditis is an extremely rare cause of cardiogenic
to result in cardiogenic shock while acute pericardial shock that can be caused by infectious organisms, trauma,
effusion almost uniformly causes tamponade and subse- immune‐mediated diseases, and toxins. Infectious causes
quent cardiogenic shock. Acute pericardial effusion can of myocarditis include viral (parvo, distemper), bacterial
occur secondary to a rapid bleeding event from a neo- (various), rickettsial (Bartonella), and protozoal organisms
plastic lesion associated with the heart (hemangiosar- (Trypanosoma, Toxoplasma), among others. Myocarditis
coma of the right atrium), or from rupture of one of the can induce myocardial failure through a cycle of inflamma-
vascular structures within the pericardial sac (atrial rup- tion, myofiber dropout and fibrosis, the cumulative effect
ture in dogs or cats with severe left atrial enlargement of which is a clinical syndrome that mimics dilated cardio-
secondary to chronic cardiac disease). Idiopathic peri- myopathy. In general, myocarditis results in a subacute to
carditis is an uncommon cause of acute pericardial effu- chronically progressive form of cardiac dysfunction, but
sion but is a frequent cause of cardiac tamponade in patients with undiagnosed myocarditis can present with
dogs. Pericardial effusion in cats is rare and infrequently end‐stage disease in cardiogenic shock.
results in tamponade. Rarely, intracardiac neoplasia can result in develop-
Canine heartworm disease can occur anywhere in the ment of cardiogenic shock by directly affecting blood
continental United States, but the incidence reaches 45% flow within the heart itself. Neoplasia associated with
along the Atlantic and Gulf Coast regions and the the atrioventricular valves, semilunar valves, right ven-
Mississippi River region. Caval syndrome can manifest tricular outflow tract, and right and left atria has been
as cardiogenic shock when dogs have large heartworm described. Tumor types that have been reported include