Elsevier

Journal of Electrocardiology

Volume 46, Issue 4, July–August 2013, Pages 302-311
Journal of Electrocardiology

Difference vectors to describe dynamics of the ST segment and the ventricular gradient in acute ischemia

https://doi.org/10.1016/j.jelectrocard.2013.04.004Get rights and content

Abstract

Background

The ECG is important in the diagnosis and triage of the acute coronary syndrome (ACS), especially in the hyperacute phase, the “golden hours,” during which myocardial salvage possibilities are largest. An important triaging decision to be taken is whether or not a patient requires primary PCI, for which, as mentioned in the guidelines, the presence of an ST elevation (STE) pattern in the ECG is a major criterion. However, preexisting non-zero ST amplitudes (diagnostic, but also non-diagnostic) can obscure or even preclude this diagnosis.

Methods

In this study, we investigated the potential diagnostic possibilities of ischemia detection by means of changes in the ST vector, ΔST, and changes in the VG (QRST integral) vector, ΔVG. We studied the vectorcardiograms (VCGs) synthesized of the ECGs of 84 patients who underwent elective PTCA. Mean ± SD balloon occlusion times were 260 ± 76 s. The ECG ischemia diagnosis (ST elevation, STE, or non-ST-elevation, NSTE), magnitudes and orientations of the ST and VG vectors, and the differences ΔST and ΔVG with the baseline ECG were measured after 3 min of balloon occlusion.

Results

Planar angles between the ΔST and ΔVG vectors were 14.9 ± 14.0°. Linear regression of ΔVG on ΔST yielded ΔVG = 324·ΔST (r = 0.85; P < 0.0001, ΔST in mV). We adopted ΔST > 0.05 mV, and the corresponding ΔVG > 16.2 mV·ms as ischemia thresholds. The classical criteria characterized the ECGs of 46/84 (55%) patients after 3 min of occlusion as STE ECGs. Combined application of the ΔST and ΔVG criteria identified 73/84 (87%) of the patients as ischemic.

Conclusion

Differential diagnosis by ΔST and ΔVG (requiring an earlier made non-ischemic baseline ECG) could dramatically improve ECG guided detection of patients who urgently require catheter intervention.

Introduction

Acute ischemia causes shortening of action potentials, decrease of action potential amplitude and decrease of maximum diastolic potential.1 These cellular changes in the ischemic region of the heart cause systolic and diastolic injury currents2 that flow between the ischemic and the surrounding, uncompromised tissue. Both the systolic and diastolic injury currents influence the ST segment in the ECG. Systolic injury current causes primary ST changes, while diastolic injury current causes TQ voltage changes that lead to compensatory ST changes as a consequence of the baseline shift.2 Moreover, the changes in the action potential morphology imply changes throughout the QRS-T complex, which become evident in changes in the spatial ventricular gradient (VG, spatial QRST integral).3

The ECG is of major importance in diagnosis of and triaging in acute coronary syndrome (ACS), especially in the hyperacute phase. According to the guidelines, an important triaging decision concerning the initial treatment is based on the presence of a new ST-elevation (STE) pattern in the ECG. In that case, the current guidelines4 mention primary percutaneous coronary intervention (PCI) as the therapy of first choice (and thrombolytic therapy when there is no, or delayed, access to PCI). In case of acute coronary syndromes without persistent ST elevation (non-ST elevation, NSTE) the current guidelines5 recommend antithrombotic (anticoagulant, antiplatelet) therapy rather than emergency PCI. The guidelines4., 5. mention, however, situations in which the ECG is non-diagnostic while there is still an urgent indication for PCI (like the ST depression without ST elevation that can be seen in left main disease). The guidelines4 read: “In any case, ongoing suspicion of myocardial ischemia— despite medical therapy—is an indication for emergency coronary angiography with a view to revascularization, even in patients without diagnostic ST-segment elevation.” The percentage of patients with NSTE admission ECGs that require PCI may be considerable: Koyama and colleagues6 found a completely occluded (TIMI flow grade 0) culprit artery in 47% of patients with an NSTE admission ECG (vs. 57% in patients with a STE admission ECG).

The diagnostic possibilities of the ECG are inherently limited by the cancelation effect, which may explain how ST changes can remain limited with relatively large areas at risk, e.g., in case of left main disease. There are, however, more reasons for the limited performance of the ECG in the setting of ACS. Non-ischemic ECGs with non-diagnostic ST segments often have small, non-zero, ST amplitudes (in vectorcardiographic terms a small, non-diagnostic ST vector). When, in such ECGs, ischemia alters the ST segment, and the vector that represents the contribution by ischemia makes an acute angle with the preexistent non-diagnostic ST vector, the resulting ST vector will initially become smaller than the preexistent vector. At the same time, the direction of the ST vector will not faithfully represent the direction of the ischemic vector. Only when the ischemic component continues to increase, the resulting ST vector becomes diagnostic and the direction will more and more assume the genuine direction of the ischemia vector. Hence it would be logic to measure the ischemic change of the ST vector with respect to its baseline value instead of the ST vector itself. The concept of an ST difference vector was first published by Lundin et al.,7 and another Swedish research group has continued to explore the usefulness of this concept (first publication by Näslund et al.8).

Similarly, an ischemia difference vector can be defined for the VG. Baseline values of the VG are by definition non-zero3 and differ considerably between individuals.9 This implies that when the VG would be used in ischemia diagnosis, subtraction of the baseline value is mandatory before a reasonable estimate of the ischemia contribution can be made. See Fig. 1.

Ischemia detection on the basis of ST amplitudes in the ECG requires that these ST changes are “new, or presumably new” (guidelines,4 Table 3), and, hence, ST interpretation in patients with a preexistent non-zero ST segment (e.g., due to left ventricular hypertrophy) is difficult. We realize that in many cases of suspected ACS the hyperacute ECG made within 10 min after first medical contact will have to be judged without a non-ischemic reference ECG at hand. However, with increasing technical possibilities and the increasing use of electronic patient files, we envisage that such a comparison becomes increasingly more often possible in the near future. With this in mind, exploration of the potential clinical use of an ischemia difference vector, computed by subtraction of the baseline vector from a vector during ischemia, becomes interesting.

Also, exploration of the VG becomes of interest within this perspective. Because of its non-zero3 and highly individual9 baseline value, the VG has never been used in diagnosis and triage in acute coronary syndrome. However, individual comparison of the VG in an ischemic ECG and a baseline ECG might be interesting, because the changes in the VG during ischemia are caused by action potential morphology changes in the ischemic area, rather than the ST changes, that are strongly based on the changes in the phase 4 resting potential in the ischemic area. Moreover, VG is independent of the ventricular depolarization order.3 Thus, ST changes and VG changes are induced by different electrophysiological processes that are, however, all related to ischemia of a compromised part of the ventricular myocardium.

In the current study we sought to explore the potential clinical value of ischemia diagnosis based on ST and VG difference vectors by analyzing the ECG changes of patients during elective percutaneous transluminal coronary angioplasty (PTCA).

Section snippets

Patients and study data

We analyzed ECGs from the STAFF III database, a collection of ECGs recorded in the setting of elective PTCA procedures performed in 1995 and 1996. These ECGs are unique because of the relatively long balloon inflation times. As such, the PTCA procedure is a model of the hyperacute phase of ACS in humans. The data in the STAFF III database were collected before coronary stenting was widely available in the USA.

For the PTCA study, patients were admitted to the Charleston Area Medical Center, West

Study group

The STAFF database comprises 104 patients. After exclusion of patients because of predominant arrhythmias (e.g., atrial fibrillation), predominant low-quality ECG signal, ECG electrode misplacement or abundant dye injections throughout the occlusion episode, 84 patients constituted our study group. Table 1 shows the group characteristics. Table 2 gives an overview of the positioning of the balloon in the coronary artery tree during the initial occlusion in each patient. Mean ± SD duration of the

Discussion

To our knowledge, our study is the first to characterize differential vectorcardiographic diagnosis not only with changes in the ST vector but also with changes in the VG vector. Additionally, it is the first study that compares ischemia diagnosis by ΔST and ΔVG magnitude with the conventional 12-lead ECG diagnosis in terms of STE or NSTE.

We showed, in a group of 84 patients undergoing elective PTCA for ischemic coronary heart disease that, after 3 min of balloon occlusion the ECG changes were

References (17)

There are more references available in the full text version of this article.

Cited by (26)

  • ECG evaluation in patients with pacemaker and suspected acute coronary syndrome: Which score should we apply?

    2016, Journal of Electrocardiology
    Citation Excerpt :

    Koyama and colleagues [12] found a completely occluded coronary artery in 47% of patients without ST-segment elevation at presentation. Similarly, two other studies [13,14] showed that after 1 to 3 min of balloon occlusion only about 50% of patients revealed ST-segment elevation. Interestingly, in the study conducted by Sejersten and colleagues [14] it was found that the discriminative ability of the 12-lead ECG was only moderate (AUC = 0.668) which is very similar to the discriminative abilities of the scores studied.

View all citing articles on Scopus
View full text