Myocardial Infarction

Chapter 41 Myocardial Infarction



C. Kate Meier, DVM, DACVM (Cardiology), Mark A. Oyama, DVM, DACVIM (Cardiology)



KEY POINTS



• Myocardial infarction is relatively uncommon in small animal patients.

• Myocardial infarction can be associated with primary cardiovascular disease or can be secondary to other systemic disease processes.

• Antemortem diagnosis is challenging and is based on certain clinical pathologic derangements and electrocardiographic (ECG) and echocardiographic changes.

• Management is largely supportive to help maintain cardiac perfusion, suppress arrhythmias, and manage the underlying causes of the infarction or ischemia.


INTRODUCTION


Myocardial infarction is the end result of either acute or chronic myocardial ischemia. Myocardial ischemia differs slightly from myocardial hypoxia in that ischemia results in a stasis of waste products of cellular metabolism in addition to a lack of oxygen delivery, leading to cellular damage above and beyond that from hypoxemia.1 Myocardial infarction is a pathologic diagnosis and, depending on whether it is acute or chronic, is characterized by loss of normal cardiac myocyte structure (i.e., myocytolysis, coagulative necrosis, inflammatory cell infiltration, and fibrosis). Myocardial infarction has a host of causes and is a leading cause of cardiovascular disease and death in humans.2 The vast majority of myocardial infarction in people stems from coronary artery disease and atherosclerosis, both of which are relatively uncommon in the veterinary patient population. In dogs, collateral circulation of the coronary arterial supply is relatively extensive,1 so the probability of irreversible myocardial ischemia from any one coronary arterial occlusion may be lower in this species.


The clinical manifestations of myocardial ischemia and infarction in veterinary patients can range from nonspecific symptoms to severe life-threatening arrhythmias and congestive heart failure. Thus, clinical suspicion of myocardial infarction should lead to aggressive management and supportive care. Prognosis depends on the severity of clinical signs, presence of concurrent disease, and response to initial therapy.



ASSOCIATED DISEASE PROCESSES


Multiple extracardiac disease processes have been associated with myocardial infarction in dogs and cats. In theory, any disease that induces a hypercoagulable state or thromboembolic complications can increase the risk of myocardial infarction. These include, but are not limited to, neoplasia, immune-mediated hemolytic anemia, trauma, heatstroke, loss of antithrombin III through renal or gastrointestinal disease, excess circulating corticosteroids (exogenous or endogenous), systemic inflammatory response syndrome, and sepsis. Atherosclerosis, a primary risk factor for infarction in humans, has been documented in dogs with hypothyroidism and diabetes mellitus.3,4 In one study of 21 dogs with pathologically confirmed atherosclerosis, all had disease of the coronary arteries as well as fibrotic changes in the myocardium.3 In another study, 20 of 30 dogs with atherosclerosis had coronary artery involvement.4 Not all dogs with coronary atherosclerosis had clinical signs suggestive of cardiovascular disease, and of seven dogs that had ECG monitoring, only four had abnormalities.3 This suggests that not all myocardial infarctions are clinically apparent.


Myocardial infarction can also be associated with primary cardiovascular disease. Bacterial endocarditis has been associated with myocardial infarction because emboli are showered from an infected valve and may lodge in the coronary arterial circulation.5,6 Feline hypertrophic cardiomyopathy can lead to myocardial infarction through several different mechanisms. First, an increased oxygen demand of the hypertrophied ventricle can lead to chronic myocardial ischemia, especially in the endocardium. It has been hypothesized that this myocardial ischemia or infarction may lead to fatal ventricular arrhythmias and sudden deathin some cats with hypertrophic cardiomyopathy. Second, decreased oxygen delivery to the hypertrophied ventricle results in proliferative remodeling of the coronary arterioles and obliteration of the vessel lumen.7,8 Furthermore, cats with hypertrophic cardiomyopathy develop diastolic dysfunction, which leads to elevated left ventricular filling pressures and eventual left atrial dilation. It is thought that left atrial dilation, via altered hemodynamics and blood pooling, is a predisposing factor for thrombus formation, and this may lead to microthrombi that embolize to the coronary circulation, and result in myocardial infarction. Lastly, cats with significant hypertrophic cardiomyopathy may be persistently tachycardic, and as the ratio of time spent in systole versus diastole increases, diastolic flow into the coronary circulation is impaired, leading to myocardial hypoxia. Dogs with subaortic and pulmonic stenosis are thought to be at higher risk for myocardial infarction for reasons similar to those in cats with hypertrophic cardiomyopathy.9-11 Dogs with cardiac neoplasia and chronic valvular disease also can have myocardial infarction.11,12

Related posts:

  1. Deteriorating Mental Status
  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Pulmonary Contusions and Hemorrhage

Stay updated, free articles. Join our Telegram channel
Tags:
Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Myocardial Infarction

Full access? Get Clinical Tree
Get Clinical Tree app for offline access