Intra-individual ECG changes over 25 years: How long can elective ECGs be used as reference for acute ischemia detection?

- Detection of acute ischemia in triage ECGs of patients with chest pain is of utmost importance but remains problematic because of the limited sensitivity.
- Comparison of an acute ECG with a previously electively made non-ischemic ECG of the same patient can possibly increase sensitivity; especially changes in the ST amplitude and in the ventricular gradient are useful for the detection of acute ischemia.
- However, ageing and disease affect the electrical heart function of a patient, thus rendering an old ECG unsuitable as a reference ECG.
- Our current study makes an inventory of the changes in the ST amplitude and in the ventricular gradient over 25 years in patients with cardiovascular disease, in an attempt to assess how long an elective ECG remains valid as reference ECG for ischemia detection.
- The results of our study suggest that serial comparison for ischemia detection in patients with known cardiovascular disease requires a recent ECG (e.g., one year old or less) as a reference ECG.

BackgroundThe guidelines advocate, in patients with chest pain, comparison of the acute ECG with a previously made, non-ischemic ECG that serves as a reference, to corroborate the working diagnosis of acute coronary syndrome (ACS). Our approach of this serial comparison is to compute the differences between the ST vectors at the J point and 60 ms thereafter (∆ST(J + 0), ∆ST(J + 60)) and between the ventricular gradient (VG) vectors (∆VG). In the current study, we investigate if reference ECGs remain valid in time.MethodsWe studied 6 elective non-ischemic ECGs (ECG0, ECG1, …, ECG5), 5 years apart, in 88 patients. Within each patient, serial comparisons were done 1) between all successive ECGs, and 2) between each of ECG1, ECG2, …, ECG5 and ECG0, computing, in addition to ∆ST(J + 0), ∆ST(J + 60) and ∆VG, the difference in heart rates, ∆HR. Additionally, relevant clinical events and the diagnoses associated with each ECG were collected. Linear regression was used to assess trends in ∆ST(J + 0), ∆ST(J + 60) and ∆VG; multiple linear regression was used to assess the influence of the clinical events and diagnoses on ∆ST(J + 0), ∆ST(J + 60) and ∆VG.ResultsThere were no trends in the differences between successive ECGs. Positive trends were seen with increasing time lapses between ECGs: ∆ST(J + 0), ∆ST(J + 60) and ∆VG increased per year by 0.65 μV, 1.45 μV and 3.69 mV∙ms, respectively. Extrapolation to a time lapse of 0 yielded 50.92 μV, 36.63 μV and 20.91 mV∙ms for the short-term reproducibility of ∆ST(J + 0), ∆ST(J + 60) and ∆VG, respectively. Multiple linear regression revealed that clinical variables could explain only 10%, 17% and 13% of the variability in ∆ST(J + 0), ∆ST(J + 60) and ∆VG, respectively.ConclusionWith a view on ischemia detection thresholds in the order of magnitude of 58 μV for ∆ST and 26 mV·ms for ∆VG, our study suggests that it is important to have a recent ECG available for the detection of myocardial ischemia, as an “aged” ECG may have lost its validity as a reference.