MYOCARDIAL INFARCTION Echocardiography is more sensitive and specific than history and ECG findings in diagnosing myocardial infarction in patients presenting with chest pain. In several clinical series, left ventricular wall motion abnormalities were observed in 89 to 100 percent of patients with transmural infarction.89 The sensitivity of echocardiography in detecting a nontransmural infarction is somewhat less and depends on the transmural thickness of the infarcted myocardial segment. In addition, patients who had no echocardiographically detected wall motion abnormalities had smaller infarctions and fewer complications. 8,9
Echocardiography is also useful in the diagnosis and management of chest pain due to causes other than myocardial infarction. Patients with chest pain due to myocardial ischemia usually have echocardiographically detectable wall motion abnormalities with pain, even in the absence of infarction. Other causes of chest pain, including aortic dissection, hypertrophic cardiomyopathy, valvular heart disease, pericarditis, and pulmonary embolism, can frequently be identified or suspected based on transthoracic echocardiography.
In addition to facilitating diagnosis, echocardiography early in the course of acute myocardial infarction provides information important in guiding early management and determining short-term prognosis. The location and size of wall motion abnormalities correlate well with the coronary artery involved in an infarction. Infarction resulting from obstruction of the left anterior descending coronary artery usually causes abnormal function in the anterior, septal, and apical segments of the left ventricle. Akinesis of the basal anterior segment predicts occlusion of the left anterior descending coronary artery, proximal to the first septal perforator, a finding of prognostic significance. There is overlap in the areas of abnormal wall motion in myocardial infarcts involving the left circumflex and right coronary arteries. Occlusion of either vessel can cause abnormal motion in the middle and basilar segments of the posterior and inferior walls. Wall motion abnormalities confined to the posterior and lateral walls are usually caused by obstruction of the circumflex, whereas abnormalities confined to the inferior basal segment are characteristic of obstruction of the right coronary artery.
A number of factors, including the presence of ischemia, stunning, or overload, confound the relationship between the size of regional wall motion abnormalities and the amount of infarcted myocardium. When compared with pathologic examination, echocardiography tends to overestimate the amount of necrosed myocardium early in the course of myocardial infarction. Despite this limitation, echocardiographic estimation of the extent of myocardial infarction correlates well with infarct size as determined by peak creatine kinase, radionuclide imaging, and contrast ventriculography. Echocardiographic evaluation of left ventricular function early in the course of myocardial infarction has been found to be a better predictor of in-hospital mortality than the prognostically strongest clinical parameters. An echocardiographically determined ejection fraction of greater than 40 percent has correlated with a low short-term mortality and could potentially be used to select low-risk patients for early discharge.10 By contrast, a tall peaked E wave on Doppler evaluation of left ventricular filling has been correlated with elevated left ventricular end-diastolic pressure and worse prognosis.
Echocardiography has been used to assess the efficacy of thrombolytic therapy. The time required for improvement of left ventricular function after reperfusion is related to the duration of ischemia and size of the ischemic zone. In patients with successful reperfusion induced by thrombolytic agents or direct angioplasty, echocardiographically detectable improvement in contractility can occur in the first 24 h. The delay in recovery of function, even after successful reperfusion, limits the utility of echocardiography in acutely assessing the success of thrombolytic therapy.
The widespread use of echocardiography early in the course of myocardial infarction reflects the clinical utility, availability, and safety of this technique. Echocardiography does have a number of potential shortcomings in the early diagnosis and management of acute myocardial infarction. The quality of echocardiographic images depends on the skill of the operator and the habitus of the patient. Images adequate for evaluation of wall motion can not be obtained in 5 to 15 percent of patients in clinical studies of myocardial infarction. 8,9 Abnormal wall motion can be present in patients with Wolfe-Parkinson-White syndrome, bundle branch block, right ventricular overload, and after cardiac surgery. However, in contrast to patients with myocardial infarction, in these patients, wall thickening is usually preserved. On the other hand, myocarditis may cause regional loss of wall thickening and be indistinguishable from infarction by echocardiography. In addition, it is sometimes difficult to differentiate new wall motion abnormalities from preexisting ones by echocardiography.
ACUTE MITRAL REGURGITATION This requires emergent surgery, and has a higher mortality rate than that of chronic mitral valve disease. Etiologies include endocarditis, acute ischemia or myocardial infarction, prosthetic valve failure, and chordal rupture due to myxomatous mitral disease or trauma.
Echocardiographic signs in acute mitral regurgitation include the "snake" sign, corresponding to high-frequency fluttering of a ruptured chord in the left atrium. Rupture of the papillary muscle may cause disconnection of the involved tip from the body of the muscle. The direction of the mitral regurgitant distinguishes anterior from posterior flail leaflet: anterior leaflet failure is associated with posteriorly directed jet, and posterior leaflet failure shows anteriorly directed jet. Transesophageal echocardiography appears to be superior to transthoracic imaging for determining the size and etiology of the valvular disorder.
ACUTE AORTIC REGURGITATION Acute aortic regurgitation results from trauma, endocarditis, or aortic dissection. This condition is rapidly fatal and requires immediate surgical repair. Transesophageal echocardiography may be necessary to delineate disruption of valvular or annular architecture in this condition.
For chronic aortic regurgitation, color Doppler demonstrates a regurgitant jet in the left ventricle. In the acute case, however, rapid equilibration of pressures in the aorta and the normal-sized left ventricle causes a small, or no, Doppler jet. The spectral pattern of the continuous wave Doppler will have a low peak velocity and an extremely rapid deceleration slope. These findings correspond to the inaudibility of the murmur of acute regurgitation. In the most severe case, there is little diastolic flow into the ventricle, since the aorta and left ventricle become a continuous chamber. Early closure of the mitral valve or diastolic mitral regurgitation on M-mode or
2D echocardiography are ominous prognostic signs. Early mitral valve closure indicates severe and acute decompensated aortic insufficiency. MAGNETIC RESONANCE IMAGING
MRI is rapidly becoming the noninvasive gold standard for the assessment of cardiac structure and function. The high contrast between moving blood and the myocardium by MRI, as well as high spatial resolution (Fig 57-10) and lack of ionizing radiation, results in a superb technique for myocardial assessment. Although availability remains a concern, it is anticipated that major medical centers will increasing have MRI services available for emergency applications in myocardial imaging. Previously, MR machines had been designed to image static areas of the body, specifically, the brain and spine. Currently, all major equipment manufacturers have devoted substantial resources to developing MR equipment specifically for the diagnosis of cardiovascular disorders. This section reviews principles of MRI to provide a basis for emergency medicine professionals in evaluating mR studies. Additionally, the range of emergency medicine applications for cardiac imaging is presented.
FIG. 57-10. MRI of normal short-axis cardiac anatomy. Short-axis images from the base of the heart (A) through the cardiac apex (B). Successive images are displayed left to right, top to bottom. Abbreviations: LAD, left anterior descending coronary artery; LV, left ventricle; PM, papillary muscles; RVOT, right ventricular outflow tract; RV, right ventricle; S, septum; T, trabeculations in right ventricle. (Reprinted from Bluemke and Boxerman, 19 with permission.)
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