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Focused Cardiac Ultrasound in Emergency Clinical Settings
Focused cardiac ultrasound (FoCUS) refers to a point-of-care ultrasound examination that is goal oriented in a specific clinical setting. , These “focused” bedside echocardiographic examinations differ from diagnostic echocardiography in that they are performed to answer specific clinical questions that are based on a clinically derived differential diagnosis, and they are increasingly performed by clinicians who have not necessarily achieved competency in comprehensive cardiovascular ultrasound. These brief echocardiographic studies are increasingly being performed with small handheld devices and are playing a growing role in the evaluation and management of patients in emergency situations.
In many ways, echocardiography is ideally suited for use in urgent clinical settings. It is the only modality that can be performed portably at the bedside, allowing examination of patients who are too unstable for transport. The acquisition time is short (usually <10 minutes) and provides immediate information regarding life-threatening conditions. The technology does not expose patients to iodinating contrast or ionizing radiation and can be performed safely and serially on virtually any patient. Echocardiography technology, particularly portable devices, is relatively inexpensive to purchase and easy to maintain and store. Finally, echocardiography is well suited to imaging many of the cardiovascular conditions that can threaten life. This chapter reviews the major applications of FoCUS in patients presenting with medical emergencies. These applications are broadly discussed as both common clinical scenarios in which FoCUS is useful and in the evaluation of specific life-threatening conditions.
Chest Pain
Diagnostic Algorithms for Acute Coronary Syndrome
More than 8 million patients in the United States present annually to emergency departments (EDs) with symptoms concerning for acute coronary syndrome (ACS). A minority of these patients (10%–30%) are ultimately diagnosed with either acute myocardial infarction (MI) or other forms of ACS. In those with ACS, the correct diagnosis is often delayed or even missed in up to 5% of cases because of limitations in the accuracy and time required for standard diagnostic algorithms that include patient history, physical examination, 12-lead electrocardiogram (ECG), and circulating biomarkers. Noninvasive echocardiography is now routinely used in many centers to rapidly diagnose ACS, to identify those who have high-risk features of their ACS, and to reduce health care costs by confidently excluding ischemia in those whose symptoms are not from ACS. The use of comprehensive echocardiography in patients with chest pain (CP) caused by suspected acute myocardial ischemia is a recommended when the baseline ECG is nondiagnostic. Many patients presenting with ACS are hemodynamically stable, and urgent bedside echocardiography is not indicated. Conversely, unstable patients can benefit from FoCUS to evaluate CP caused by life-threatening conditions ( Fig. 17.1 ), including complications of acute myocardial infarction, pulmonary embolus, and aortic dissection (discussed later).
Evaluation for Wall Motion Abnormalities
Transthoracic echocardiography (TTE) has a high sensitivity but moderate specificity for the diagnosis of acute MI. When patients are evaluated during symptoms or within a short period of resolution, the sensitivity of echocardiography for detecting regional wall motion abnormalities is as high as 90% in those with nondiagnostic findings on ECG. Echocardiographic evidence of wall motion has also been shown to provide earlier diagnosis of ACS than conventional (non–high-sensitivity) troponin assays. , In particular, echocardiography can be useful for rapid diagnosis of ACS caused by left circumflex disease, which is often “silent” on the standard 12-lead ECG. Assessment of regional wall motion by echocardiography in the acute setting can also provide useful prognostic information in patients with CP. The presence of a definitive wall motion abnormality in those with ACS increases likelihood for related late adverse events by approximately fourfold.
Perhaps more important is the high negative predictive value of echocardiography. The presence of completely normal wall motion is able to exclude ischemia with a greater than 95% negative predictive value provided that imaging is performed during symptoms or soon after its resolution. It should be cautioned that echocardiography may be falsely negative in situations when imaging is performed late after resolution of symptoms or if there is incomplete or inadequate visualization of all myocardial segments. Given that segmental wall motion and wall thickening analysis are some of the most technically demanding aspects of echocardiographic interpretation, FoCUS should be used to exclude ACS only in clinical settings when the value of a negative diagnostic test is high and by those who are competent in the interpretation of wall motion and use of ultrasound contrast agents when necessary.
Contrast-Enhanced Echocardiography
Encapsulated microbubble ultrasound contrast agents that are stable after intravenous injection improve endocardial border delineation of the left ventricle (LV) in otherwise technically suboptimal studies. This issue is of particular importance in the evaluation of patients in whom ACS is suspected because (1) every segment needs to be visualized, (2) a high level of reader confidence is needed, and (3) the imaging environment in the ED is often suboptimal. Clinical studies have demonstrated that ultrasound contrast agents substantially increase the number of interpretable segments, decrease interobserver variability, and increase reader confidence. The evaluation of regional wall motion with contrast echocardiography specifically for ED patients with chest pain has been demonstrated to provide incremental value to clinical information, the ECG, and initial troponin. LV opacification with contrast echocardiography in those with already recognized ACS can also reveal the presence of ventricular pseudoaneurysms or ventricular thrombus. The use of ultrasound-enhancing agents (UAEs) with handheld ultrasound devices, however, is constrained by limited contrast-specific imaging technologies on the current generations of these devices. Although UEAs can be visualized using two-dimensional harmonic imaging available on handheld devices, administration of the agents is best matched to ultrasound systems that optimize contrast signal to noise via adjustable acoustic output controls and contrast-specific imaging sequences.
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