KEYWORDS
border zone, myocardial ischemia, potassium, epicardial mapping, action potential duration, potassium ionic currents
INTRODUCTION
Acute regional myocardial ischemia leads to the development of myocardial regions with different properties: an ischemic zone in the nonperfused myocardium with metabolic and electrolytic disturbances, a normal zone in the perfused area with unchanged properties, and a border zone with mild transitional changes located in the perfused area between the normal and ischemic zones. The border zone is thought to play an important role as a substrate and a source of triggers for arrhythmias1-3.
The ischemic zone characterized by a local increase in extracellular potassium concentration ([K+]) (“local hyperkalemia”). The excessive K+ leak from the affected cells and its accumulation in the extracellular space 4 are caused by the opening of the ATP-dependent potassium channels 5, 6 and inhibition of Na,K-ATPase 7 in conditions of the depletion of ATP. The increased extracellular [K+] gradually declines in the direction from the ischemic to normal myocardium 8-11. In such a case, [K+] in the transition zone depends on the K+ efflux from the ischemic zone and the [K+] in the normal zone, which is equilibrated with a plasma [K+] level.
Extracellular [K+] influences the major parameters of the action potential such as its duration, resting membrane potential (RMP), and dV/dt max 12. Hypokalemia and hyperkalemia lead to the decrease or increase in K+ channel conductance, lengthening or shortening of action potential duration (APD), hyperpolarization or depolarization of RMP, respectively. Both disturb myocardial conduction and repolarization and promote ventricular arrhythmias. The low (<3.5 mM) and high (≥4.5 mM) serum [K+] levels are known to increase the risk of ventricular fibrillation (VF) and mortality of patients with myocardial infarction 13.
Inhomogeneity of electrophysiological properties of myocardium is one of the major prerequisites of arrhythmogenesis, and the ischemia-related spatial differences of the extracellular [K+] promote action potential heterogeneity and electrical instability in the heart.
Since blood [K+] affects [K+] in the myocardial tissue, the ischemia-related myocardial electrical heterogeneities might be modified in hypo- or hyperkalemic conditions. The aim of the present study was to evaluate the effect of the variation of blood [K+] level on the distribution of depolarization and repolarization characteristics across the ischemic, border and normal zones and to test them for the association with arrhythmic outcomes.
RESULTS