Ventricular Depolarization in Ischemic Heart Disease.Value of Electrocardiography in Assessment of Severity and Extent of Acute Myocardial Ischemia

Abstract: Background In patients with symptoms compatible with acute myocardial infarction (MI), early triage by ECG in the pre-hospital phase by ST-segment elevation myocardial infarction (STEMI) criteria is important for direct transport of these patients to a regional center for primary percutaneous coronary intervention (pPCI). The time from first medical contact to pPCI should, due to present guidelines, be no longer than two hours. One main determinant of final infarct size (IS), in addition to myocardium at risk (MaR) and time to treatment, is the severity of ischemia, which relates to the rate of progression of the infarction wavefront. Presently, no assessment of severity is made. Patients with severe ischemia may have changes within the QRS complex in addition to ST-T changes, making it possible to identify these high-risk patients. QRS changes are, however more difficult to determine and to quantify correctly as compared to the changes within the ST segment. Aims and methods The overall objective was to increase the understanding of depolarization changes during myocardial ischemia and to evaluate whether these changes have possible clinical implications in patients with acute MI. Different QRS methods are applied in patients during ischemia produced by elective, prolonged PCI as well as during STEMI, and comparisons are made with conventional ECG parameters as well as single-photon emission computed tomography (SPECT) images. Results and conclusions Study I compared the computer-derived high-frequency QRS components (HF-QRS) in patients with and without standard ECG changes indicative of old MI. In contrast to previous findings we found that HF-QRS cannot differentiate between patients with and without old MI. Study II tested the ability of HF-QRS versus conventional ST-segment measurements to detect and quantify myocardial ischemia, as determined by SPECT, in a group of patients undergoing elective balloon PCI. We showed that HF-QRS can provide valuable information both for detecting acute ischemia and for quantifying MaR and its severity. Study III evaluated a potentially more readily available (compared with HF-QRS) new marker of ventricular depolarization distortion, which is based on calculation of up- and downslope within the QRS complex, in patients undergoing coronary intervention that includes temporary occlusion of a coronary artery. We found that in particular the downward slope between R and S waves better correlates with ischemia than conventional QRS parameters, as quantified by SPECT, and thus can be of value in risk stratification of patients with ischemia in addition to conventional ST-segment analysis. Study IV, in a large cohort of STEMI patients, assessed the value of the conventional Sclarovsky-Birnbaum ischemia grading system that includes terminal QRS distortion in addition to ST elevation, on pre-hospital ECG and its dynamic behavior during transport time to the PCI center for prediction of final infarct size and salvage, as estimated by SPECT imaging. The study explored the temporal behavior of the ischemia grading and showed the strong association of ischemia grade assessed from pre-hospital ECG, as well as the dynamic patterns, with infarct size, independent of ST-segment analysis. It also demonstrated the importance of early intervention, which was found to be particularly important in patients who had advanced, QRS-based ischemia grade.