EARLY DIAGNOSIS OF AMI In those patients whose initial ECG is diagnostic for MI, no further testing is required and appropriate interventions can be initiated. As stated previously, approximately 50 percent of MI patients will have an initially nondiagnostic ECG. The release kinetics of currently available markers generally require 6 h or more after coronary occlusion to exclude infarction; therefore MI cannot be definitively ruled out within the first few hours of the ED visit. However, some MI patients with nondiagnostic initial ECGs will have positive marker tests upon ED arrival, and many more will develop positive tests soon after presentation. Therefore, early, rapid serial sampling of myocardial markers may identify many MI patients with nondiagnostic ECGs (rule in MI) and thus allow earlier utilization of time-dependent treatments.
Studies of single CK-MB measurement upon ED presentation have demonstrated a sensitivity for MI of less than 60 percent. However, by using serial CK-MB
measurements, several authors have demonstrated sensitivities ranging from 80 to 96 percent. —1 and 16
Owing to its earlier release into serum after coronary artery occlusion, the heme-containing protein myoglobin has a potential advantage over CK-MB for early diagnosis of MI. In one study, samples taken at presentation yielded a sensitivity of 62 percent for myoglobin as compared with 14 percent for CK-MB. At 3 h, the sensitivity for MI increased to 90 percent for CK-MB and to 100 percent for myoglobin.17 A subsequent, similar study reported sensitivities at 2 h of 82.1 percent for CK-MB and 100 percent for myoglobin.18
IDENTIFYING "MISSED MI" PATIENTS Single-sample myocardial marker measurements cannot be used to exclude the diagnosis of MI in the ED. However, results of several investigations suggest that the incorporation of markers into patient care algorithms for low-risk patients prior to discharge may help to identify those with unsuspected MI.
In the first such study (1987), three of 482 chest pain patients discharged from the ED were found to have had a positive CK-MB. In another investigation (1991) among 271 ED patients, 5 discharged patients and 2 admitted to unmonitored beds with noncardiac diagnoses were identified by a single positive CK-MB value and later determined to have had a clinically unsuspected MI. All 7 of these patients had nondiagnostic initial ECGs and 3 had presented with symptoms other than chest pain.19 In a later multicenter study that prospectively assessed the effect on ED physician decision making of two CK-MB measurements drawn 3 h apart, 3 of 265 patients selected for discharge by the ED physician were identified and admitted solely on the basis of a positive CK-MB. These results suggest that ED patients to be sent home or admitted to unmonitored beds after presenting with chest pain as well as with other presentations of possible ischemia may benefit from prerelease "screening" for MI using CK-MB.20 Although these results are promising, there is not yet sufficient data concerning either cost or clinical effectiveness of this practice to advocate it's widespread use.
EARLY RISK STRATIFICATION As use of newer anti-ischemic therapies, such as early angiographic interventions and anti-platelet and antithrombotic drugs, become more common, the morbidity and mortality associated with ACS are likely to decline further. However, the greater risk of therapy related complications and the higher costs associated with these newer interventions necessitate a selective approach to their use. Several investigations suggest that markers of myocardial injury can successfully be used in the ED to rapidly identify those patients most likely to benefit from a more aggressive approach.
Positive CK-MB tests obtained upon presentation and 2 h later have been shown to predict up to a fivefold increased likelihood of subsequent ischemic complication, regardless of the final diagnosis. A subsequent similar study of 5120 patients from 53 EDs confirmed the prognostic significance of a positive CK-MB value, regardless of final diagnosis. Other investigations have shown that early ED testing of either troponin-T or troponin-I also yields clinically useful prognostic data and suggest that simultaneous testing of both troponin and CK-MB may identify additional high-risk patients. "i?1,,2.2
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