Bedside diagnosis of myocardial ischemia with ST-segment monitoring technology: measurement issues for real-time clinical decision making and trial designs.

Monitoring of the ST segment is a valuable tool for guiding clinical decision making and evaluating anti-ischemia interventions in clinical trials; however, measurement issues hamper its diagnostic accuracy. This study reports the frequency and type of false positives and other measurement issues we have encountered during 12-lead ST-segment monitoring of patients in a cardiac care unit. Of 292 patients, 117 (40%) had one or more false positive events during an average of 41 hours of ST-segment monitoring, for a total of 506 false positive events. The 506 false positive events included 167 (36%) due to body positional change; 132 (26%) due to sudden increase in QRS complex/ST-segment voltage; 96 (19%) due to transient arrhythmia or pacing; 80 (16%) due to heart rate change in steeply sloped ST-segment contours; 26 (5%) due to a noisy signal; and 5 (1%) due to lead misplacement. It is concluded that many conditions in addition to myocardial ischemia can cause transient ST-segment deviation in patients with unstable coronary syndromes. Accurate ST-segment monitoring requires expertise in electrocardiogram interpretation, an understanding of the patient's clinical situation, and knowledge of the functions and limitations of the ST-segment monitoring system.