The 12Lead ECG

The 12-lead ECG is usually the earliest available objective test for the presence or absence of cardiac ischemia and can provide important diagnostic and prognostic information in patients with chest pain. In the presence of ST-segment elevation on the 12-lead ECG, the diagnosis of acute MI is confirmed in over 90% of cases by serial CK-MB testing (11,12). Unfortunately, only about 10% of all acute MIs present with ST-segment elevation on the initial ECG; most are confirmed only in retrospect, by serial tracings showing the development of new Q waves or by serial CK-MB testing (11,13,14). The initial 12-lead ECG is further limited in that it provides only a static image of what is usually a dynamic ischemic process and that it has limited ability to evaluate for

Table 1

Likelihood of Significant CAD in Patients with Symptoms Suggesting Unstable Angina

Intermediate likelihood (e.g., 0.15-0.84)

Any of the following features:

History of prior MI or sudden death or other known history of CAD. Definite angina: males > 60 or females > 70 yr of age. Transient hemodynamic or ECG changes during pain.

Variant angina (pain with reversible ST-segment elevation).

ST-segment elevation or depression > 1 mm. Marked symmetrical T-wave inversion > 1 mm in multiple precordial leads.

Absence of high likelihood features and any of the following:

• Definite angina: males

< 60 or females

• Probable angina: males > 60 or females > 70 years of age.

• Chest pain probably not angina and 2 or 3 risk factors other than diabetes.a

• Extracardiac vascular disease.

ST-segment depression 0.05-1 mm.

T-wave inversion > 1 mm in leads with dominant R-waves.

Absence of high or intermediate likelihood features, but may have:

• Chest pain classifies as probably not angina.

• One risk factor other than diabetes.

• T-wave flattening or inversion < 1mm in leads with dominant R-waves.

aCoronary artery disease risk factors include diabetes, smoking, hypertension, and elevated cholesterol.

ischemia in the posterior basal and lateral walls. Despite the lack of diagnostic sensitivity of the initial ECG for acute MI, it can support the overall clinical impression of underlying coronary artery disease (for example, the presence of Q waves) and can provide prognostic information. Dynamic ST-segment elevation or depression and T-wave changes predict a higher short-term risk of death or MI and can be used along with the clinical evaluation to risk-stratify patients presenting with chest pain into high-, moderate-, and low-risk categories for initial triage (1,15-19) (Tables 1 and 2).

Analysis of presenting ECGs from patients in the Global Use of Strategies To Open Occluded Arteries in Acute Coronary Syndromes (GUSTO-IIa) trial showed that the presenting ECG category (ST-segment elevation, ST-segment depression, T-wave inversion/normal, or confounding ECG factors) was an important predictor of short-term mortality in a logistic regression model (20). The highest-risk group included patients with ECG confounders that obscured interpretation of the ST-segment (left bundlebranch block, paced rhythm, or left ventricular hypertrophy), who had a 30-d mortality of 11.6%, followed by ST-segment depression (8.0%), ST-segment elevation (7.4%), and finally, the very-low-risk T-wave inversion/normal group, with a 30-d mortality of only 1.2%. The relationship of the baseline ECG findings with mortality and nonfatal cardiac events in the GUSTO-IIa population is shown in Table 3. A similar gradation

Table 2

Short-Term Risk of Death or Nonfatal MI in Patients with Unstable Angina

High risk

Intermediate risk

Low risk

At least one of the following features must be present:

Prolonged ongoing (>20 min) rest pain.

Pulmonary edema, most likely related to ischemia.

Angina at rest with dynamic ST-segment changes > 1 mm.

Angina with new or worsening MR murmur. Angina with S3 or new and/or worsening rales.

Angina with hypotension.

No high risk feature, but must have any of the following:

Prolonged (>20 min) rest angina, now resolved, with moderate or high likelihood of CAD.

Rest angina (>20 min or relieved with rest or sublingual nitroglycerin). Nocturnal angina.

Angina with dynamic T-wave changes.

New onset CCSC III or IV angina in the past 2 wk with moderate or high likelihood of CAD.

Pathologic Q waves or resting ST-segment depression < 1 mm in multiple lead groups (anterior, inferior, lateral). Age > 65 yr.

No high or intermediate risk feature, but may have any of the following features:

• Increased angina frequency, severity, or duration.

• Angina provoked at a lower threshold.

• New onset angina with onset 2 wk to 2 mo prior to presentation.

• Normal or unchanged ECG.

Abbreviations: MR, mitral regurgitation; CAD, coronary artery disease; CCSC, Canadian Cardiovascular Society Class.

of risk by initial ECG characteristics occurred in studies of chest pain patients by Brush and colleagues (21) and Villanueva et al. (8). In 12,124 acute coronary syndrome patients enrolled in the GUSTO-IIb trial, Savonitto and colleagues showed that as for 30-d events, the baseline ECG correlated with mortality at 6 mo. In their analysis, patients with T-wave inversion had the lowest mortality (3.4%), followed by ST-segment elevation (6.8%), ST-segment depression (8.9%), and combination ST-segment elevation and depression (9.1%) (22).

In summary, using the conventional tools of the history, the physical examination, and the initial ECG evaluation, the sensitivities of ED physicians for admitting acute MI and unstable angina patients are 92-98% and 90%, respectively (23-27). Specificity is low, however, as only about 30-40% of admitted patients ultimately are found to have an acute coronary syndrome as the etiology for their symptoms (28,29). In the Thrombol-ysis in Myocardial Infarction (TIMI) IIIb study of conservative vs early interventional care in patients meeting clinical and ECG criteria for unstable angina, 18% of patients were found to have no significant CAD at cardiac catheterization (30) (Fig. 1).

Table 3

Characteristics and Outcomes by Admission Electrocardiographic Category"

Table 3

Characteristics and Outcomes by Admission Electrocardiographic Category"

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