Advantages
The rapid use of echocardiography as a clinical decision-making tool in the setting of acute coronary syndrome carries a number of potential advantages. These include early detection of regional wall motion abnormalities confirming a likely ischemic etiology and potentially for anatomical localization, a rapid estimation of left ventricular function, early recognition of mechanical complications, and perhaps most importantly recognition of alternate etiologies with the potential to decrease unnecessary invasive procedures. We will discuss each of these components in further detail.
Diagnostic utility and clarification
Echocardiography stands as a fairly rapid, noninvasive, and accurate test to detect early myocardial dysfunction. [1] Given that wall motion abnormalities may manifest prior to the development of EKG changes or a troponin elevation, it also has the potential for ruling in ischemia early on. [2] Perhaps one of the best uses of echocardiography is in the setting of mixed or atypical presentations of chest discomfort. In this setting, it plays a crucial rule in ruling out other known culprits such as tamponade, endocarditis, or pericarditis where cardiac catheterization would commit the patient to additional risks without the potential for benefit.
The Focused Cardiac Ultrasound (FoCUS) exam has been described in the literature as an adjunct to physical examination to emergently recognize structural causes of cardiac dysfunction. This exam focuses on the systolic function, chamber sizes, valvular abnormalities, the presence of pericardial effusion or tamponade, and likelihood of volume responsiveness. [3] The use of echocardiography in this application can both guide and expedite treatment. For cardiogenic shock patients, the FoCUS exam may help to guide mechanical circulatory device therapy.
One recent study investigating the use of echocardiography in the evaluation of ACS in the pre-hospital setting demonstrated the sensitivity and specificity of pre-hospital transthoracic echocardiography for non-ST segment elevation myocardial infarction at 90.9% and 100% respectively. [5] This demonstrates clear potential for the use of echo as a standardized initial evaluation strategy.
In addition, the FoCUS exam can detect contraindications to certain cardiac mechanical support devices that may have resulted from ACS. Several examples include the Impella that has a contraindication in patients with LV thrombus, the Intra-aortic balloon pump that has an absolute contraindication to moderate to severe aortic regurgitation, and the Tandem Heart Devices that are contraindicated with left atrial thrombus. Any suspicion of these findings before or during cardiac catheterization by physical examination or by catheterization findings should lead to further evaluation by FoCUS exam before proceeding with mechanical circulatory support.
Early identification of mechanical complications of myocardial infarction
Echocardiography remains the most accessible and reliable initial imaging modality to evaluate for structural cardiac disease. The management of life-threatening complications of ACS including ventricular free wall rupture, ventricular septal defect, acute papillary muscle rupture, ventricular aneurysm, cardiac tamponade, and ventricular thrombus formation all benefit from early recognition. While a thorough physical exam can frequently raise suspicion for one of these complications, it invariably requires imaging confirmation.
A patient presenting with cardiogenic shock and a newly auscultated murmur raises suspicion for a mechanical complication of an MI. An urgent, focused echocardiogram offers the possibility of immediate visualization of the VSD, imaging and quantification of new valvular regurgitation and also allows the rapid identification of a pericardial effusion associated with free wall rupture. [6] Mechanical complications of myocardial infarction carry a poor prognosis without immediate surgical intervention; coronary angiography in such situations may delay such life-saving surgery.
It is also important to mention that such complications can also present as late complications of ACS, where the door to balloon time will not be considered or in patient with STEMI who have been treated with thrombolytics and transferred to PCI capable facilities. The use of echocardiography may be prudent and a rapid assessment by an experienced ultrasonographer can help evaluate for these complications before proceeding with cardiac catheterization.
Reduction in unnecessary cardiac catheterizations
Prompt echocardiography in ACS has the potential to reduce unnecessary cardiac catheterizations as well as associated morbidity and mortality. Emergent coronary angiography is indicated when a thrombotic or embolic cause of ACS is suspected. In the absence of a strong indication, it may delay other more appropriate therapies and expose the patient to an invasive procedure with its concomitant risks
In cases such as left ventricular thrombus, echocardiography may both confirm the need for catheterization (i.e. LV thrombus with associated wall motion abnormality) and decrease complication rates by identifying the need to avoid left ventricular catheterization.
Potential disadvantages
The additional step of performing echocardiography in cases of suspected acute coronary syndrome also carries potential drawbacks. In many institutions, the ability to obtain an echocardiogram may be limited. If the etiology of ischemia is otherwise clear, any additional imaging reduces time to potential revascularization. Therefore, such utilization of echocardiography relies upon a rapidly available, appropriately trained technician and physician.
Delaying time to the catheterization lab
Current guidelines for the treatment of ST-segment elevation myocardial infarction recommend a 90-minute door-to-balloon time at a catheterization capable facility. Any delay for additional imaging should be avoided unless mechanical complications are suspected. A delay in care has to be carefully balanced relative to its potential benefit. Patients to be targeted are those with troponin elevation without an obvious type 1 MI and patients in whom the diagnosis of a mechanical complication will significantly affect their outcomes.
The purpose of the previously established FoCUS exam is to perform a more limited, high-yield examination by a skilled provider. Loss of time remains an unavoidable drawback of adding additional diagnostic testing. A useful test must be incorporated into a system of implementation that allows for timely performance and interpretation. This will allow for meaningful clinical impact.
Literature review
There have been prior studies investigating the utility of echocardiography as a preliminary diagnostic modality in evaluating patient with acute chest pain and suspected acute coronary syndrome (ACS). The results of these studies have been promising and its implementation in emergent ACS assessment seems beneficial. [9-17]
Table 1 reviews nine studies that measured the accuracy of echocardiography as a predictor of cardiac events by assessing the presence and absence of wall motion abnormality. The primary endpoints for these studies range from major endpoints such as myocardial infarction, to revascularization, angiographic findings of coronary artery disease, and abnormal study findings. [11,17]
The positive predictive value (PPV) of the various studies is variable ranging from 31-100 % but it is important to note that the PPV correlates with low-risk and high-risk patients. Conducting echocardiography for assessment of ACS in the high-risk population, such as the study by Mohler and colleagues [17], reveals a high PPV in high-risk patients. This study identified all patients with new wall motion abnormality; of those, myocardial infarction was identified in 43% giving a PPV of 100% and a negative predictive value (NPV) of 57%.
However, in the low-risk population, echocardiogram seems to be less predictive. In the study with Sabha and colleagues [11], who had a lower-risk population, the overall prevalence of a cardiac events was 17 % giving echocardiography a PPV of 31% and NPV of 98%. In addition, Krontos and colleagues [9] found that the PPV and NPV between echocardiography and electrocardiography in the low-risk population were similar with a PPV of 44% and 60% and a NPV of 98% and 44% respectively.
Overall, echocardiogram is more useful in evaluating for acute coronary syndrome in moderate to high risk populations. Its value in low-risk populations is non-superior to a standard electrocardiogram.
Prior case series regarding the use of echocardiography in ACS
A case series investigating the use of echocardiography by ER physicians to identify clinically significant acute occlusive coronary artery lesions concluded that ultrasound-trained Emergency Physicians can identify significant wall motion abnormalities. These cases involved high-risk patients in whom echocardiography would not have changed management. The strengths of this study included real-life cardiac function assessment and quicker assessment than laboratory data could allow. Some of the drawbacks include body habitus which may make it difficult to obtain certain views, interpretation of images that varied from provider to provider, and the ability to identify new versus old wall motion abnormalities. [19]
Another more recent study investigated the use of echocardiography in the evaluation of ACS in the pre-hospital setting. The study demonstrated very high sensitivity (90%) and specificity (100) for the early detection of NSTEMI. [5]
Suggested implementation strategy
Wall motional abnormalities in acute coronary syndrome may occur before electrocardiographic changes and before chest pain. [18] Suggested implementation of echocardiography in acute coronary syndromes, is in a moderate-to-high risk patient presenting with chest pain in the emergency department after an initial electrocardiogram is negative for STEMI. . This may aid in identifying new wall motion abnormalities for early identification of a probable ischemic etiology while blood work is pending.
A moderate-to-high risk patient who presents with an ST elevation myocardial infarction (STEMI), may benefit from emergent echocardiography just prior to cardiac catheterization if this does not delay door-to-balloon time. This may be especially useful in patients with hemodynamic compromise to identify possible mechanical complications of the MI in whom further interventions prior to coronary angiography (e.g circulatory support, surgical consultation) may be critical. The assessment should be performed by a trained ultrasonographer in the emergency department or on the cath table immediately preceding the procedure. (Table 2)