Chapter-19 Myocardial Infarction

Retrospective evaluation of the ST segment electrocardiographic features in 180 healthy dogs

“…On the electrocardiogram, the ST segment encompasses the time between the end of ventricular depolarisation (i.e., the end of the QRS complex, namely the J point) and the beginning of ventricular repolarisation (i.e., beginning of the T wave). Hemodynamically, the ST segment corresponds to ventricular systole, whereas electrophysiologically, it correlates with phase 2 (plateau phase) of the transmembrane action potential (Tilley 1992b, Rautaharju et al 2009, Chugh & Chugh 2012. This phase occurs after phase 0 and phase 1, during which the majority of myocardial cells have gone through rapid depolarisation and early repolarisation, respectively.…”

“…In other words, in this phase, all ventricular myocytes attain about the same membrane potential. Therefore, no significant difference in potential is recorded by the surface electrocardiogram, which consequently often displays an almost horizontal segment on the isoelectric line (Tilley 1992b, Rautaharju et al 2009, Chugh & Chugh 2012). Thus, any significant change in voltage gradients during the plateau phase of the action potential can result in variations in the ST segment (Rautaharju et al 2009, Chugh & Chugh 2012, Klabunde 2017.…”

“…Therefore, no significant difference in potential is recorded by the surface electrocardiogram, which consequently often displays an almost horizontal segment on the isoelectric line (Tilley 1992b, Rautaharju et al 2009, Chugh & Chugh 2012). Thus, any significant change in voltage gradients during the plateau phase of the action potential can result in variations in the ST segment (Rautaharju et al 2009, Chugh & Chugh 2012, Klabunde 2017. A perfect example of this is represented by acute myocardial ischemia, as it induces a loss of membrane integrity and leakage of potassium from the cell to the extracellular region, moving down its concentration gradient.…”

“…This creates a voltage gradient between ischemic and non-ischemic cardiomyocytes, which, in turn, leads to an abnormal electrical current (also known as "current of injury" or "injury current"). During diastole, this current flows intracellularly from the ischemic region toward normal myocytes and tends to depolarise the latter, leading to the emergence of the ST segment deviation on the surface electrocardiogram (Rautaharju et al 2009, Chugh & Chugh 2012, Klabunde 2017.…”

“…Subsequently, according to the aim of this study, particular attention was given to ST segment evaluation. For this purpose, pertinent measurements and morphological analysis were performed following indications from human medicine (Brady et al 2001, Smith 2006, Rautaharju et al 2009, Jeong et al Jeong et al 2010, Chugh & Chugh 2012. Specifically, the J point (i.e., the point of intersection between the descending branch of the R wave, or the ascending branch of the S wave when present and the baseline) was used as the start point of the ST segment; the endpoint of the ST segment was the start point of the T wave.…”