Chapter 147 Heart Failure in Dogs
Heart failure (HF) is a state wherein the cardiac output is inadequate to meet the perfusion needs of the metabolizing tissues and exercise capacity is limited. The causes of HF in dogs are diverse, but there are stereotypical cardiovascular and systemic responses to impaired cardiac function, regardless of cause. This chapter provides a brief overview of HF; considers the causes and diagnoses of HF in dogs; and reviews treatment plans for management of the cardiac failure patient. The clinical pharmacology and pathophysiologic rationale for drugs used in treatment of HF are detailed in Chapter 146. Management of HF in cats is discussed in the chapter “Cardiomyopathy” (Chapter 150).
Heart failure is a not a specific disease but a pathophysiologic disorder. Some of the key abnormalities of this condition are summarized below, and the pathophysiologic rationale for using particular cardiovascular drugs is summarized in Chapter 146. The reader is directed to other textbooks for a detailed review of pathophysiology of HF.
Next in this chapter we will consider the key causes of heart disease in dogs, as well as an overview of cardiac disease classification and applicable diagnostic studies. The following are salient points regarding cardiac diagnosis and the clinical workup.
Identifying the predominant form and cause of heart disease and classifying the pathophysiologic mechanism of cardiac failure allows the clinician to direct appropriate therapy and render a more accurate prognosis. Cardiac diagnoses can be classified in a number of ways.
Table 147-1 OVERVIEW OF MORPHOLOGIC/ANATOMIC DIAGNOSES
Table 147-2 OVERVIEW OF ETIOLOGIC DIAGNOSES
Cardiac murmur: functional—a prolonged audible vibration associated with blood flow in the heart or great vessels but unrelated to structural heart disease (also termed physiologic or innocent murmur); functional murmurs are often caused by anemia, fever, thyrotoxicosis, elevated sympathetic tone, or protracted bradycardia.
Cardiac murmur: organic—audible vibration associated with abnormal blood flow in the heart or great vessels associated with structure disease; these murmurs typically indicate pathology affecting the heart valves or a congenital shunt.
Congestive heart failure—an advanced pathophysiologic state of heart failure characterized by renal sodium retention, elevated venous pressures, and fluid accumulation in the lung, subcutaneous tissues, or body cavities.
Diastolic heart failure (diastolic dysfunction)—impairment of ventricular diastolic filling or distensibility as with concentric hypertrophy of the ventricle or pericardial disease; the ventricle must be filled by higher than normal filling (venous and atrial) pressures.
Heart failure—a pathophysiologic state caused by systolic or diastolic failure of the heart and characterized by neurohormonal activation and inadequate cardiac output relative to exercise and tissue perfusion demands.
Hemodynamic overload—a condition characterized by increased demand on the ventricle to pump a greater stroke volume or a higher pressure than normal; typically these are subdivided into volume overloads and pressure overloads (see the following).
Pressure overload—condition wherein the ventricular systolic (pumping) pressure must be higher than normal to eject the stroke volume; usually associated with hypertension or obstruction to ejection as with aortic stenosis.
Pulmonary hypertension—elevated pulmonary arterial blood pressure generally caused by left-sided heart failure, pulmonary vascular or parenchymal disease, or marked increase in pulmonary blood flow (without concomitant decline in pulmonary vascular resistance).
Shock—an acute, life-threatening disorder characterized by diminished tissue perfusion, impaired oxygen delivery, altered metabolism, and tissue injury; various causes of shock include cardiogenic shock (from profound heart failure), hypovolemic shock (from fluid loss or hemorrhage), and distributive shock (excessive vasodilation secondary to sepsis, anaphylaxis, or spinal injury).
Systolic heart failure (systolic dysfunction)—impairment of ventricular systolic pumping that limits stroke volume and cardiac output; typical of dilated cardiomyopathy, severe valvular heart diseases, and myocardial failure due to chronic volume or pressure overloads.
Tachyarrhythmia—a cardiac arrhythmia generally characterized by a fast heart rate, as with sinus tachycardia, atrial tachycardia/flutter/fibrillation, supraventricular re-entrant tachycardia, and ventricular tachycardia.
The most important causes of canine heart disease involve a limited number of acquired disorders and a handful of important congenital heart defects. These conditions, as well as usual diagnostic findings, are summarized in Table 147-4 and in other chapters across this section. The most important acquired cardiac disorders responsible for HF in dogs are:
The most important reasons for CHF in dogs are degenerative valvular disease, dilated cardiomyopathy, pericardial diseases, and heartworm heart disease. These conditions can be complicated by systemic hypertension, pulmonary hypertension, and arrhythmias such as atrial fibrillation or ventricular tachycardia.
There are obvious risks for cardiovascular disease relative to these factors. Young animals will be suspected of congenital heart disease; mature animals of acquired, degenerative, and neoplastic diseases. There are dozens of genetic breed predispositions to cardiovascular diseases. These tend to be tabulated in standard reference textbooks and are described in other chapters within this volume.