Dynamic tracking of ischemia in the surface electrocardiogram

Abstract

Background: Accurate detection of the earliest signs of ischemia on the surface electrocardiogram (ECG) is essential for timely diagnosis and management of potentially life-threatening ischemic events. Yet, accuracy of ischemia analysis in ECG monitors remains suboptimal because of a number of confounding factors, including changes in body position and other artifacts. Hence, the goals of this study were (1) to examine the duration and time course of ischemic events and (2) to compare ECG changes caused by "true" ischemic events with those caused by changes in body position. Continuous, 12-lead Holter ECGs obtained from patients who presented to the emergency department with chest pain and enrolled in the Ischemia Monitoring and Mapping in the Emergency Department in Appropriate Triage and Evaluation of Acute Ischemic Myocardium study were analyzed. Holter recordings were initiated within the first 40 minutes after patients' arrival to the emergency department. Here we present preliminary results.

Methods: Twelve patients (age, 59 +/- 16 years; 5 women, 2 with a final diagnosis of non-ST-segment elevation myocardial infarction, 4 with unstable angina, and 6 with other cardiovascular diseases), in whom ischemic ST deviations were identified on Holter data, underwent 4 consecutive, 2-minute recordings in the following body positions: (1) supine, (2) on the left side, (3) on the right side, and (4) sitting (or standing) upright. After baseline correction, beat-to-beat changes in QRS and ST-T segments were examined in all 8 channels and the root-mean-square curve by using an adaptive algorithm that computes the slope, amplitude, duration, area, and the Karhunen-Loève-derived representation of the corresponding segment. To prevent possible biases toward patients with more frequent ischemic events, a single index event was chosen for analysis in each patient. There were 3 ST-elevation events and 9 ST-depression events; these events reached the maximum ST deviation 11 +/- 8 hours (mean +/- SD) after the beginning of the recording.

Results and conclusions: In most patients with transient myocardial ischemia, the microvolt-level, subthreshold deviation of the ST segment developed gradually, over 15 to 20 minutes, until it reached the maximum, superthreshold level. Despite the different ischemia localizations, the root-mean-square curve allowed accurate detection of significant changes in the ST segment in the studied group (Friedman analysis of variance for repeated measurements over a 1-hour interval). Changes in body position could be identified by tracking dynamics of the QRS pattern/axis. Adaptive algorithms for tracking of the ST dynamics with simultaneous tracking of the patterns of QRS complexes to discriminate the true and "false"-positive events are presented and discussed.

Figure 1

Electrocardiographic waveforms in leads I, II, V1, V3, and V6 (columns 1-5, respectively) obtained from a 46-year old male with transient chest pain thought to be due to coronary vasospasm during positional tests (rows 1-4) and an ischemic event (row 5).

Figure 2

Electrocardiographic changes during positional tests (Row 1-4) and cardiac ischemia (Row 5-8) in the patient whose ECG waveforms are shown in Figure 1: QRS area in lead V3 (1^st row), QRS area in lead V6 (2^nd row), ST-amplitude in lead V3 (3^rd row), ST-slope in lead V3 (4^th row). ECG data in the same channels during an ischemic event are shown in rows 4-8. The time series ends at the time of maximum ST-elevation.

Figure 3

Electrocardiographic changes during positional tests (Row 1-4) and cardiac ischemia (Row 5-8) in a 74-year old female with transient ischemic events: QRS area in lead V3 (1^st row), QRS area in lead V6 (2^nd row), ST-amplitude in lead V6 (3^rd row), ST-slope in lead V6 (4^th row). ECG data in the same channels during an ischemic event is shown in rows 4-8. Note that ST-segment depression in lead V6 was sub-threshold for approximately 15 min (<.1mV), before becoming super-threshold during the last 5 min (>.1mV). The time series ends at the time of maximum ST-depression.