Collapsing dogs: managing common causes of collapse or syncope (Proceedings)
Collapse and shock are a clinical manifestation of many different severe diseases, and requires emergency triage and care. Other dogs may present with a history of episodes of collapse and may be clinically stable on presentation. These two categories of patients are treated very differently, but they have some similar differential diagnoses.
A clinical algorhythm for the collapsed hypotensive dog includes: cardiogenic shock, hypovolemic shock, or distributive shock. Cardiogenic shock is uncommon, and may be seen with acute severe heart disease or end-stage heart disease of many etiologies. Systolic myocardial failure and low output heart failure is seen with severe dilated cardiomyopathy. The most important diastolic disease leading to cardiogenic shock is pericardial effusion and cardiac tamponade. Other diastolic heart diseases that cause cardiogenic shock in cats includes hypertrophic cardiomyopathy and restrictive cardiomyopathy. Heart diseases that cause severe volume overload such as acute severe mitral regurgitation and a major chordae tendinae rupture, or acute infective endocarditis of the mitral or aortic valves may cause cardiogenic shock. Severe sustained tachyarrhythmias or bradyarrhythmias decrease cardiac output and can cause cardiogenic shock. Transient tachyarrhythmias or bradyarrhythmias may cause transient decreased cerebral perfusion and syncope (see below) rather than sustained collapse and shock. Hypovolemia may be secondary to hemorrhage or many diseases that cause severe dehydration and a volume underloaded state. Distributive shock occurs when there is inappropriate vasodilation, and main causes include sepsis and anaphylaxis.
The initial triage of the collapsed patient includes measurement of vital parameters, blood pressure, pulse oximeter, electrocardiogram, and STAT minimum database (CBC, chemistry including electrolytes). If possible urine should be collected for urinalysis. A FAST scan of the abdomen and the thorax is useful to quickly evaluate for free abdominal fluid, pleural effusion, or pericardial effusion. Abdominocentesis should be done to evaluate for hemorrhagic effusion, and the fluid saved to submit for fluid analysis if indicated. In animals with low clinical suspicion of cardiogenic shock and heart failure, placement of a large bore intravenous catheter and shock bolus of intravenous crystalloids (60-90 ml/kg) should be given while additional diagnostics are done. In addition to evaluating for pericardial effusion, a triage echocardiogram is also used to evaluate for severe myocardial failure in dogs with dilated cardiomyopathy and cardiogenic shock. In dyspnic animals, the left atrial size should be assessed either subjectively or quantified by measurement of the ratio of the left atrial diameter to the aortic diameter in the right parasternal short- axis basilar view. Normal LA:Ao is <1.5, and significant left atrial dilation is >1.8. Dogs with severe mitral regurgitation and cardiogenic pulmonary edema typically have severe left atrial dilation (LA:Ao ≥ 2) except in the rare instance of acute major chordae tendinae rupture.