Temporal and positional variability of the ST segment during continuous vectorcardiography monitoring in healthy subjects

Dynamic tracking of ischemia in the surface electrocardiogram

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.

Observer variability and optimal criteria of transient ischemia during ST monitoring with continuous 12-lead ECG

BACKGROUND

ST monitoring with continuous 12-lead ECG is a well-established method in patients with unstable coronary artery disease (CAD). However, the method lacks documentation on optimal criteria for episodes of transient ischemia and on observer variability.

METHODS

Observer variability was evaluated in 24-hour recordings from 100 patients with unstable CAD with monitoring in the coronary care unit. Influence on ST changes by variations in body position were evaluated by monitoring 50 patients in different body positions. Different criteria of transient ischemia and their predictive importance were evaluated in 630 patients with unstable CAD who underwent 12 hours of monitoring and thereafter were followed for 1 to13 months. Two sets of criteria were tested: (1) ST deviation > or = 0.1 mV for at least 1 minute, and (2) ST depression > or = 0.05 mV or elevation > or = 0.1 mV for at least 1 minute.

RESULTS

When the first set of criteria were used, the interobserver agreement was good (kappa = 0.72) and 8 (16%) had significant ST changes in at least one body position. Out of 100 patients with symptoms suggestive of unstable CAD and such ischemia, 24 (24%) had a cardiac event during follow-up. When the second set of criteria were used, the interobserver agreement was poor (kappa = 0.32) and 21 (42%) had significant ST changes in at least one body position. Patients fulfilling the second but not the first set of criteria did not have a higher risk of cardiac event than those without transient ischemia (5.3 vs 4.3%).

CONCLUSIONS

During 12-lead ECG monitoring, transient ischemic episodes should be defined as ST deviations > or = 0.1 mV for at least 1 minute, based on a low observer variability, minor problems with postural ST changes and an important predictive value.

Computerized vectorcardiography telemetry: a new device for continuous multilead ST-segment monitoring of ambulatory patients. A preliminary report

BACKGROUND

Continuous vectorcardiography ST-segment monitoring has become a well-established method in the surveillance of patients with acute myocardial ischemia. However, immobility of the vectorcardiography technique prevents monitoring of patients during ambulatory activities. Computerized vectorcardiography telemetry (CVT) with the capacity of real-time ST-segment analysis has been developed in an attempt to overcome this shortcoming. Recent data, however, indicate that changes in body position occasionally lead to pseudo-ischemic ST-segment changes during continuous ST-segment monitoring.

AIMS

This report describes the technical features of the CVT system, presents clinical examples using CVT, and assesses the influence of changes in body position on ST-vector magnitude (ST-VM) during CVT, respectively.

METHODS

Clinical cases involving CVT are presented. The influence of changing body position during CVT monitoring was evaluated on 24 patients with suspected acute coronary syndromes, i.e., unstable angina or acute myocardial infarction. Each patient performed a specific body positional schedule.

RESULTS

We present three discrete clinical cases where CVT provided early and valuable evidence of ongoing myocardial ischemia. The consequences of different recumbent and ambulatory body positions on ST-VM during CVT monitoring appear to be limited.

CONCLUSION

Computerized vectorcardiography telemetry is a promising new tool for disclosing residual myocardial ischemic activity during the mobilization phase of patients with acute coronary syndromes. The clinical value of CVT needs further investigation in future trials.

Vectorcardiographic ST deviations related to increased heart rate in the absence of ischemia in an experimental pig model

The electrocardiographic ST segment may change when heart rate (HR) increases. We aimed to analyze vectorcardiographic ST relation and myocardial conditions during controlled HR increases in anesthetized pigs. The relative parameters ST change vector magnitude and ST change vector angle were calculated at paced HRs ranging from 85 to 175 beats per minute. ST change vector magnitude increased from baseline 6.3 +/- 1.3 to 26.0 +/- 3.1 microV (P < .01; range, 4-50 microV) at HR 175 beats per minute with similar changes in ST change vector angle, whereas the absolute parameter ST vector magnitude demonstrated a heterogeneous pattern without any systematic relation to HR changes. Microdialysis results from left ventricular wall, with analysis of glucose, lactate, and pyruvate, showed no sign of ischemia during pacing. Potassium concentrations did not change during pacing. We conclude that significant HR-related ST vector changes can occur in the absence of myocardial ischemia.

Comparison of Signal Quality Between Easi and Mason-Likar 12-Lead Electrocardiograms During Physical Activity

• Background Myoelectric noise and baseline wander, artifacts that appear when patients move during electrocardiographic monitoring, can cause false alarms. This problem can be addressed by using a reduced lead set and placing electrodes on the anterior part of the torso only. The Mason-Likar modification of the standard 12-lead electrocardiogram and the EASI lead system are 2 alternative systems for lead placement.

• Objectives To test the hypothesis that the EASI lead system is less susceptible to artifacts than is the Mason-Likar modification of the standard 12-lead electrocardiogram.

• Methods Baseline wander and myoelectric noise amplitudes of EASI and Mason-Likar 12-lead electrocardiograms were compared. Twenty healthy volunteers participated. Both lead systems were recorded simultaneously for different types of physical activities. For each lead in each subject, baseline wander and myoelectric noise were measured for both systems, at rest and during each physical activity.

• Results The outcome for baseline wander was mixed. For myoelectric noise content, the EASI system performed better for the limb leads in the different physical activities. In the precordial leads, the differences were minimal or mixed. However, for supine-to-right turning, EASI performed worse than the Mason-Likar system.

• Conclusions The 2 systems have similar susceptibilities to baseline wander. The EASI system is, however, less susceptible to myoelectric noise than is the Mason-Likar system. EASI performed worse than Mason-Likar for turning supine to right, because only the EASI system uses an electrode in the right-midaxillary line.

Positional changes of spatial QRS- and ST-segment variables in normal subjects: implications for continuous vectorcardiography monitoring during myocardial ischemia

Electrocardiographic QRS- and ST-segment changes are to be expected during changes in body posture. We prospectively analyzed the influence of changes in body position on continuous vectorcardiography monitoring of QRS-vector difference (QRS-VD) and ST change-vector magnitude (STC-VM) according to the currently used criteria of myocardial ischemia in 21 normal subjects. Fifteen (71%) and 6 (29%) subjects had significant positional QRS-VD and STC-VM changes, respectively. Vectorcardiography changes were most frequent and pronounced in the left lateral position. An alternative to the existing criterion of ischemia is proposed to improve the specificity of STC-VM. Subjects with positional QRS-VD changes had higher mean STC-VM values as compared with those without such changes. Otherwise no characteristics among those with positional vectorcardiography changes could be identified. There was no statistically significant association between positional QRS-VD and STC-VM changes (R = .13, P = .57). We conclude that the clinical use of QRS-VD in its present form for continuous vectorcardiography monitoring of myocardial ischemia seems to be of limited practical value, because of the presence of frequent "pseudo-ischemic" changes. STC-VM seems to have a significant potential of continuous vectorcardiography monitoring. However, an indicator of body position change or even an algorithm enabling on-line correction for positional vectorcardiography changes seems to be essential to improve the accuracy of this technique in identifying myocardial ischemia.