Electrocardiogram Standard 12Lead ECG

A complete summary of evidence related to the diagnostic utility of the standard ECG was recently published (19,21), and this background will not be repeated here. However, the NHAAP's Working Group on Evaluation of Technologies for Identifying ACI (21) found that most studies evaluated the accuracy of the technologies and only a few evaluated the clinical impact of routine use. Furthermore, they concluded that although the standard ECG is a safe, readily available, and inexpensive technology with a relatively high sensitivity for AMI, it is not highly sensitive or specific for ACI. However, the ECG remains an integral part of the evaluation of patients with chest pain and the Working Group recommended that it remain the standard of care for evaluating patients with chest pain in the ED.

The ECG provides essential information when the diagnosis is not obvious by symptoms alone (53), despite one study noting that the results of the ECG infrequently changed triage decision based on initial clinical impressions (54). The generally dominant weights given to ECG variables in mathematical models for predicting ACI substantiate this impression (6,7,10,15,16). Moreover, the initial ECG is increasingly important in intrahospital triage, because of its value in predicting complications ofAMI (55-57).

Despite its central role in the evaluation of patients with suspected ACI, there are fundamental limitations in the standard ECG (Table 7). First, it is a single brief sample of the whole picture of the changing supply and demand characteristics of unstable ischemic syndromes. If a patient with UAP is temporarily pain free when the ECG is obtained, the resulting tracing may poorly represent the patient's ischemic myocardium.

Second, 12-lead electrocardiography is limited by its lack of perfect detection (58). Small areas of ischemia or infarction may not be detected; conventional leads do not examine satisfactorily the right ventricle (59) or posterior basal or lateral walls well (i.e., AMIs in the distribution of the circumflex artery) (60,61).

Third, some ECG baseline patterns make interpretation difficult or impossible including prior Q waves, early repolarization variant, left ventricular hypertrophy, bundlebranch block, and dysrhythmias (62). Lee et al. (9) demonstrated that when the current ECG shows ischemic findings, availability of a prior comparison ECG improved triage.

Fourth, ECG waveforms are frequently difficult to interpret causing disagreement among readers, so-called missed ischemia. In a study of AMI patients sent home, ECGs tended to show ischemia or infarction not known to be old, with 23% of the missed diagnoses owing to misread ECGs (8). Jayes et al. (63) compared ED physician readings of ECGs with formal interpretations by expert electrocardiographers and calculated sensitivities of 0.59 and 0.64 and specificities of 0.86 and 0.83 for ST-segment and T-wave abnormalities, respectively. Both McCarthy et al. (18) and a review of litigation in missed AMI cases (64) emphasized this factor of incorrect ECG interpretation. In the largest study to date of ACI in the ED, Pope et al. (3) found that although the rate of missed diagnoses of ACI (2.1% AMI, 2.3% UAP) was low, there was a small but important incidence of failure by the ED clinician to detect ST-segment elevations of 1 to 2 mm in the ECGs of patients with myocardial infarction (11%). Correct ECG interpretation by ED physicians is doubly important today because of the need to use interventions such as thrombolytic agents and percutaneous coronary angioplasty appropriately in ACI.

Fifth, the implications of the ECG findings must be interpreted in their clinical context, a process done intuitively by clinicians and formally stated in Bayesian analysis. When symptoms alone strongly suggest ischemia, a normal or minimally abnormal ECG will not substantially decrease the probability of ischemia. Conversely, when the presentation is inconsistent with acute ischemia, an abnormal ECG, unless diagnostic abnormalities are present, will only modestly increase the likelihood of ischemia. Bayes' rule tells us that the ECG will have the greatest impact when symptoms are equivocal (65). This is illustrated by Table 8, which shows the probability of acute ischemia for combinations of history and ECG findings among 2801 emergency patients (66); this formed the basis for the Acute Ischemic Heart Disease Predictive Instrument (6).

Finally, the ECG suffers from imperfect sensitivity and specificity for ACI. When interpreted according to liberal criteria for MI (i.e., ECGs that show any of the following as positive for AMI: nonspecific ST-segment or T-wave changes abnormal but not diagnostic of ischemia; ischemia, strain, or infarction, but changes known to be old; ischemia or strain not known to be old; and probable AMI), the ECG operates with relatively high (but not perfect) sensitivity (99%) for AMI, at the cost of low specificity (23%; positive predictive value 21%; negative predictive value 99%). Conversely, when interpreted according to stringent criteria for AMI (only ECGs that show probable AMI), sensitivity (61%) drops and specificity equals 95% (positive predictive value 73%; negative predictive value, 92%) (67).

Despite its usefulness, the ECG is insufficiently sensitive to make the diagnosis of ACI consistently. The ECG should not be relied on to make the diagnosis, but rather should be included with history and physical examination characteristics to identify patients who appear to have a high risk for ACI (i.e., a supplement to, rather than a substitute for, physician judgment). In "rule out AMI" patients, a negative ECG carries an improved short-term prognosis (55,68-71). Providing the interpreter with old tracings would intuitively seem to be of value because baseline abnormalities make current evaluation difficult, yet, Rubenstein and Greenfield (72), in a study of 236 patients presenting to EDs with the complaint of chest pain, found that only a small proportion might have benefited from having a previous baseline ECG available (5% might have avoided unnecessary admission). Furthermore, there was no patient for whom a baseline ECG would have aided in avoiding an inappropriate discharge. ECG sampling

Table 8

The Original ACI Predictive Intrument's Probabilities of Acute Ischemia for ED Patients

ECG Abnormalities (%)

Table 8

The Original ACI Predictive Intrument's Probabilities of Acute Ischemia for ED Patients


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