Role of Echocardiography in Patients With Acute Coronary Syndrome

Diagnostic Role of Echocardiography

The accuracy of echocardiographic diagnosis of an MI depends on its ability to detect WMAs in the involved segment. The severity of the WMA depends on the transmural extent of the infarction, and the circumferential limits depend on the arterial distribution and collateral blood supply. Most patients with ST-elevation myocardial infarction (STEMI) have some hypokinetic LV segments unless there is very early reperfusion, but it is not unusual for patients who have non-STEMI with a small troponin rise to have no appreciable WMA. The most widely used scoring system for grading the severity of a WMA is shown in Table 21.2 .

Other causes of segmental LV dysfunction must be recognized ( Table 21.3 ). An increasingly recognized mimic of acute MI is apical ballooning syndrome, also known as takotsubo cardiomyopathy. It is estimated that approximately 2% of all patients with suspected ACS are ultimately diagnosed with takotsubo cardiomyopathy. In the International Takotsubo Registry, 90% of registrants were women older than 50 years of age. Women are most commonly affected by an emotional trigger, men by a physical trigger, and no trigger is evident in an estimated 28% of cases.

Earlier criteria for the diagnosis of takotsubo cardiomyopathy excluded CAD, but it is now recognized that this syndrome may occur in patients with CAD. The prevalence of CAD ranges from 10% to 61%, with a 15% prevalence in the International Takotsubo Registry. Specific criteria were proposed by Frangieh et al. to help differentiate takotsubo cardiomyopathy from STEMI and non-STEMI.

TTE is invaluable in the diagnosis of takotsubo cardiomyopathy and reveals characteristic features of LV impairment in a noncoronary distribution and early improvement of LV function consistent with myocardial stunning rather than MI. Although the classic presentation is with midwall and apical LV ballooning and hypercontractile basal LV segments ( Fig. 21.1 ), variant forms such as mid-ventricular ballooning and basal ballooning have also been described.

In a cardiac magnetic resonance (CMR) imaging series of 256 patients with takotsubo cardiomyopathy, 82% were of the apical variety, 17% were mid-ventricular, 1% were basal, and 34% had RV involvement. Up to one third of patients experience early complications, including dynamic LV outflow tract obstruction, acute MR, heart failure, cardiogenic shock, and arrhythmias. Mortality rates vary from 2% to 5%, similar to those for STEMI, and death occurs mainly from cardiogenic shock and ventricular arrhythmias. Natural history studies suggest improvement in LV function in most cases within weeks or months with reported recurrence rates of 5% to 22% at 6 months to 10 years after the index event.

Myocarditis is an important mimic of ACS. Patients typically present with chest pain, a rise in troponin levels, and nonspecific ECG changes. Regional WMAs may exist but with a normal or near-normal coronary angiogram. Shirani et al showed a predominance of subepithelial inflammation in the LV free wall. The posterolateral LV segments are most commonly involved, but other LV segments may be involved, or the LV may be globally dilated and/or impaired. In a study by Mahrholdt et al., patients with biopsy-proven myocarditis due to parvovirus PVB19, the lateral wall of the LV was predominantly affected, whereas in patients affected with herpesvirus HHV6, the anterior septum was predominantly affected.

Clues to the diagnosis of myocarditis rather than an ACS include clinical features such as a viral prodrome, pleuritic chest pain, a more persistent rise in troponin levels (rather than the typical rise and fall of troponin levels in an ACS), and a pericardial effusion. Magnetic resonance imaging (MRI) with gadolinium is a very accurate modality to confirm myocarditis.

Localization of Infarction

In an attempt to establish segmentation standards applicable to all types of imaging, including echocardiography, nuclear perfusion imaging, cardiovascular MRI, and cardiac CT, a 17-segment model of LV segmentation has been proposed. This model includes the apical cap, which is the segment beyond the end of the LV cavity that usually is seen only with some contrast and myocardial perfusion studies. With routine echocardiographic studies, the 16-segment model can be used without the apical cap. The LV segments and the approximate and most common coronary arterial distributions in relation to these segments are depicted in Fig. 21.2 . The extent of the segmental wall motion is related to the exact coronary anatomy in an individual patient but may vary among patients. The presence of collaterals and previous bypass surgery alters the distribution of ischemia and infarction relative to the involved arterial supply.

Direct visualization of the ostia of the left main coronary artery and right coronary artery (RCA) and the proximal left anterior descending coronary artery (LAD) in adults may be achieved with the use of TTE ( Fig. 21.3 ). This may be helpful to exclude anomalous coronary artery origin, especially in younger patients.

RV Infarction

Although RV infarction was first described in the 1930s, it was only after the hemodynamic consequences were recognized in the 1970s that RV infarction was considered a clinical entity.

RV infarction occurs in up to 50% of inferior MIs, but hemodynamic compromise develops in fewer than half of such cases. The RV is predominantly supplied by acute marginal branches of the RCA, and occlusion of the RCA proximal to the origin of these branches results in ischemic dysfunction of the RV. When the occlusion is distal to the right atrial (RA) branches, augmented RA contractility enhances RV function and cardiac output. Conversely, more proximal occlusions result in ischemic depression of RA contractility, which impairs RV filling and function, resulting in more severe hemodynamic compromise.

Less commonly, RV infarction is associated with acute anteroseptal MI. Isolated RV MI is reported to occur in less than 3% of all acute MIs. The clinical importance of RV infarction is emphasized by higher rates of hemodynamic compromise, arrhythmias, and in-hospital mortality compared with MI involving only the LV.

Echocardiography plays a vital role in the evaluation of patients with suspected RV infarction ( Table 21.4 ). In many cases, the LV inferior WMA may be relatively small with preserved LVEF. Assessment of RV systolic function is complex, and commonly used echocardiographic measures, including RV fractional area change and visual assessment, are associated with significant interobserver variability.

Tricuspid annular plane systolic excursion of less than 10 mm is a useful objective measure of RV dysfunction and should be measured in patients with suspected RV infarction. In a study of 60 patients with a first acute inferior MI, a tricuspid valve annulus peak systolic velocity of less than 12 cm/s had a sensitivity of 81%, a specificity of 82%, and a negative predictive value of 92% for RV infarction.

Global longitudinal strain of the RV can provide greater sensitivity and specificity than RV fractional area change or tricuspid annular plane systolic excursion for major adverse cardiovascular events after RV infarction. Reduced RV compliance can be detected by an increased A-wave velocity on the hepatic vein flow signal and by a short pressure half-time of the pulmonary regurgitant jet (<150 ms).

Characteristic echocardiographic features of RV failure with high RA pressures include bowing of the interatrial septum into the left atrium (LA) and dilation of the inferior vena cava with lack of inspiratory collapse. These indicators of RV function correlate with clinical status and prognosis. Hypoxemia resulting from right-to-left shunting across a patent foramen ovale may occur with a large RV infarction associated with elevated RA pressures.

In most cases of RV infarction, RV function improves and returns to near normal within 3 to 12 months, although the improvement may not be complete. Because the acutely ischemic dysfunctional RV represents predominantly viable myocardium, which may spontaneously recover or respond favorably to successful reperfusion even late after the onset of occlusion, it has been suggested that the term RV infarction is largely a misnomer and that the term RV ischemic dysfunction should be used. However, it is important to recognize that RV infarction is associated with relatively high in-hospital mortality rates, and early reperfusion enhances recovery of RV function with an improved clinical course and improved survival. The use of RV speckle tracking–derived longitudinal strain has prognostic value for patients with heart failure and may have a role after MI with RV involvement.

Papillary Muscle Rupture

Acute severe mitral valve regurgitation in the setting of MI is usually a result of necrosis and rupture of papillary muscle tissue. It is a life-threatening complication that almost always requires urgent surgical intervention. MR due to LV remodeling and distortion of ventricular architecture may also occur (discussed later).

2D imaging may detect abnormalities in the mitral valve apparatus, including flail leaflets and PMR. Even if not clearly visualized, PMR should be suspected when there is an eccentric jet of MR with a relatively normal-sized LA. Although color-flow parameters are those most often used, accurate grading of MR severity should also encompass other Doppler-echocardiographic signs. For patients with pulmonary edema, the unexpected findings of a small infarction, a hyperdynamic LV, or increased early mitral inflow velocity should prompt a careful search for MR even when color-flow Doppler is unrevealing.

In a review of PMR, 15 of 17 cases occurred in patients with inferior infarction and rupture of the posteromedial papillary muscle. The more common involvement of the posteromedial papillary muscle occurs because its blood supply is from a single coronary artery (i.e., the posterior descending artery) rather than a dual coronary artery supply, as in the anterolateral papillary muscle, which is supplied by the LAD and the left circumflex coronary artery (LCx) ( Fig. 21.4 ).

Chordae to both leaflets arise from each of the papillary muscles, and in cases of complete rupture of the entire trunk of a papillary muscle, both leaflets are affected. In less severe cases, the rupture is incomplete, and only a single head is torn. The 2D echocardiographic findings may include prolapse of one or both leaflets, a flail leaflet, or liberation of a portion of the papillary muscle. In some patients, the ruptured muscle remains tethered to the chordae, and chaotic motion is present. If only a single head of the papillary muscle is affected (i.e., incomplete rupture), medical stabilization often is possible. When rupture is complete, involving the main trunk of the papillary muscle, the complication is fatal without immediate recognition and prompt surgical repair.