Difference vectors to describe dynamics of the ST segment and the ventricular gradient in acute ischemia
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)
- et al.
Elucidation of the spatial ventricular gradient and its link with dispersion of repolarization
Heart Rhythm
(2006) - et al.
Prevalence of coronary occlusion and outcome of an immediate invasive strategy in suspected acute myocardial infarction with and without ST-segment elevation
Am J Cardiol
(2002) - et al.
Normal limits of the spatial QRS-T angle and ventricular gradient in 12-lead electrocardiograms of young adults: dependence on sex and heart rate
J Electrocardiol
(2008) - et al.
Detection and quantification of acute myocardial ischemia by morphologic evaluation of QRS changes by an angle-based method
J Electrocardiol
(2013) - et al.
Enhancing the precision of ECG baseline correction: selective filtering and removal of residual error
Comput Biomed Res
(2000) - et al.
The ventricular complex in left ventricular hypertrophy as obtained by unipolar precordial and limb leads
Am Heart J
(1949) - et al.
The effect of acute coronary artery occlusion on subepicardial transmembrane potentials in the intact porcine heart
Circulation
(1977) - et al.
Electrocardiography
Cited by (26)
Context-independent identification of myocardial ischemia in the prehospital ECG of chest pain patients
2024, Journal of ElectrocardiologyPost-spastic flow recovery time to document vasospasm induced ischemia during acetylcholine provocation testing
2023, IJC Heart and VasculatureECG evaluation in patients with pacemaker and suspected acute coronary syndrome: Which score should we apply?
2016, Journal of ElectrocardiologyCitation 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.