Cardiovascular Dysfunctions Sometimes Reflect Primary Cardiovascular Disturbances or Diseases, but More Often They Are Secondary Consequences of Noncardiovascular Disturbances or Diseases

Some of the cardiovascular dysfunctions encountered in vet­erinary medicine are primary in that the fundamental dis­turbance or disease process affects the cardiovascular system directly.

Cardiovascular dysfunction and disease can be either congenital (present al birth) or acquired (developing after birth). Myocarditis is an example of an acquired cardio­vascular disease, and there are many others. Congenital cardio­vascular diseases frequently involve defective heart valves, which either cannot open fully or cannot close completely. Congenital cardiac defects are common in certain breeds of dogs and horses. Although a heart that has a congenital defect or an acquired disease may be able to pump an adequate amount of blood when the animal is at rest, it usually cannot deliver the increased blood flow required by the body during exercise. When a dysfunction in the heart impairs its ability to pump the amount of blood flow normally needed by the body, the condition is called heart failure (“pump failure”). The patient with heart tailure classically exhibits exercise intolerance.

Parasites are another common cause of acquired cardio­vascular dysfunction. In dogs, for example, adult heartworms (Dirofilaria immitis) lodge in the right ventricle and pul­monary artery, where they impede the flow of blood. These worms also release substances into the circulation that disrupt the body’s ability to control blood pressure and blood flow. In horses, bloodworms (Strongylus vulgaris) lodge in the mesen­teric arteries and decrease the blood flow to the intestine.

In many other disease states, cardiovascular complications develop even though the cardiovascular system is not the pri­mary target of the disease. 'Γhese secondary cardiovascular dys­functions often become the most serious and life-threatening aspects of the disease. For example, severe burns or persistent vomiting or diarrhea leads to substantial losses of water and electrolytes (e.g., Na⁺, CΓ, K^,, Ca²*) from the bloodstream. Even if the blood volume is not depleted to dangerously low levels in these conditions, the alteration in electrolyte con­centrations can lead to abnormal heart rhythms (cardiac arrhythmias) and ineffective pumping of blood by the heart (heart failure). The electrolyte abnormalities in such a patient can be made even worse if incorrect fluid therapy is given. Incorrect fluid therapy can also lead to an accumulation of excess fluid in the tissues of the body; this “waterlogging” of tissues is called edema. If the excess fluid gathers in the lung tissue, the condition is called pulmonary edema. Pulmonary edema is life threatening because it slows the flow of oxygen from the pulmonary air sacs (alveoli) into the bloodstream.

Pulmonary edema is a secondary complication in many disease states. A further example is shock-lung syndrome, which results when toxic substances in the body trigger an increase in the permeability of the lung blood vessels. These ^uIeakyⁿ vessels allow water, electrolytes, plasma proteins, and white blood cells to leave the bloodstream and accumulate in the lung tissue and airways. The resulting pulmonary edema can lead to death.

Whereas the effects of shock-Iung syndrome are most serious in the pulmonary circulation, other types of shock depress the cardiovascular system in general.

Anesthetic overdose is another common clinical problem in which the most serious and life-threatening symptoms are the secondary cardiovascular complications. Most anesthetics depress the CNS, and the resulting abnormal neural signals to the heart and the blood vessels can depress cardiac output and lower blood pressure. Some anesthetics, particularly the bar­biturates, also depress the pumping ability of the heart directly.

There are many other examples of primary and secondary cardiovascular dysfunction, but those just mentioned illus­trate the importance and variety of cardiovascular dysfunctions encountered in veterinary medicine. The distinction between primary and secondary cardiovascular dysfunction is some­times unclear, but this difficulty simply emphasizes how intimately the cardiovascular system is interconnected with all the other body systems and how dependent all the other systems are on the normal functioning of the cardiovascular system.

The remainder of this chapter reviews the general features of the cardiovascular system. Chapters 19 to 25 discuss the various elements of the cardiovascular system in detail. Chapter 26 summarizes cardiovascular function and dys­function by describing the overall effects of heart failure, hemorrhage, and exercise.