Approximately 6 million patients are evaluated in the ED setting
for chest pain or symptoms suggestive of an acute coronary syndrome (ACS)
each year.1 A small number of these patients will
present with ST-segment elevations on their initial 12-lead ECG
and go on to immediate thrombolysis or percutaneous coronary intervention.
A much larger subset of these patients with chest pain will ultimately
be found to have an alternative diagnosis other than ACS. Unfortunately,
these patients with nonischemic chest pain often exhibit symptoms
that are no different than patients with true myocardial ischemia.
Indeed, no symptom alone or in combination is sufficiently discriminatory
to preclude further diagnostic workup.2 Initial
serologic markers of cardiac injury (total creatine phosphokinase
and its MB subfraction, troponin) and ECG changes are also nondiagnostic
in this subset of patients, with very poor sensitivities of 20% and
50%, respectively.3,4 Thus, emergency
physicians routinely face the question of how to manage patients
who are at a non-negligible risk for ischemic chest pain, but have
no certain diagnostic indications of ACS on presentation to the
In practice, the determination of ischemic chest pain in the
ED is difficult, and errors occur. Approximately 2% to
4% of patients presenting to the ED with symptoms suggestive
of myocardial ischemia are mistakenly discharged.5 The
consequences of premature discharge of patients at high risk for
adverse coronary events can be catastrophic, with a mortality risk
nearly twice as high as those who are admitted.5 As
such, physicians manage this complaint quite conservatively. This
approach yields many unnecessary admissions, however, and only approximately
30% of patients admitted for a suspected myocardial infarction are
ultimately diagnosed with ischemic heart disease.6 With
this practice, national expenditures for admissions to rule out
ACSs approach $10 billion each year.7 Many
centers have adopted short-stay “chest pain units” to
work toward expeditious and cost-effective management of this issue,
and emergency physicians are increasingly responsible for the downstream management
of these chest pain workups.
This chapter addresses the robust field of noninvasive myocardial
imaging as it relates to the evaluation of patients with symptoms
concerning for acute myocardial ischemia. These imaging modalities
are used as a means of cardiac risk stratification, aiming to increase
the diagnostic sensitivity and specificity for ACS. Furthermore,
these tests help to guide patients’ management and prognosis.
Each modality has its strengths and limitations, and a thorough
understanding of these characteristics will guide the clinician
to choose the most useful study for each patient.
Multiple imaging modalities employ the concept of stress imaging.
This technique seeks to unmask coronary flow limitations that are
otherwise silent at rest. Under increased metabolic demand, limited
perfusion can alter the metabolic activity of the myocardium or
affect regional myocardial function. The myocardial ischemic cascade
(Figure 299.1-1) provides the theoretical
framework for these changes. Patient symptoms and ECG changes typically
occur after perfusion and regional function have been impaired.