R wave amplitude in ischemia, injury, and infarction. Plenary address

Shark Fin Electrocardiogram: A Deadly Electrocardiogram Pattern in ST-Elevation Myocardial Infarction (STEMI)

Shark fin electrocardiographic (ECG) pattern, also known as 'Lambda-wave', 'giant R waves', or 'triangular QRS-ST-T waveform' is a dangerous ECG pattern associated with ST-elevation myocardial infarction (STEMI). It is formed by the fusion of QRS, ST, and T waves and predicts the high risk of mortality due to cardiogenic shock and ventricular fibrillation. The management should be aggressive with reperfusion via thrombolysis or percutaneous intervention, ideally in the intensive care unit with ventricular assist devices. This ECG pattern may be misdiagnosed as wide complex tachycardia or the ECG changes of hyperkalemia. Thus, differentiating it from other conditions causing similar ECG changes and prompt management is highly important to save the patient from serious complications. Here we have presented a case of STEMI with shark fin ECG associated with pulmonary edema (Killip class III acute myocardial infarction).

Acute myocardial ischemia: cellular mechanisms underlying ST segment elevation

The electrocardiogram (ECG) is an essential tool for the diagnosis of acute myocardial ischemia in the emergency department, as well as for that of an evolving acute myocardial infarction (AMI). Changes in the surface ECG in leads whose positive poles face the ischemic region are known to be related to injury currents flowing across the boundaries between the ischemic and the surrounding normal myocardium. Although experimental studies have also shown an endocardium to epicardium differential sensitivity to the effect of acute ischemia, the important contribution of this transmural heterogeneous response to the changes observed in the surface ECG is less appreciated by the clinical cardiologist. This review briefly discusses our current knowledge regarding the electrophysiology of the ischemic myocardium focusing primarily on the electrophysiologic changes underlying the ECG alterations observed at the onset of a transmural AMI.

J wave syndromes: molecular and cellular mechanisms

An early repolarization (ER) pattern in the ECG, consisting of J point elevation, distinct J wave with or without ST segment elevation or slurring of the terminal part of the QRS, was long considered a benign electrocardiographic manifestation. Experimental studies a dozen years ago suggested that an ER is not always benign, but may be associated with malignant arrhythmias. Validation of this hypothesis derives from recent case-control and population-based studies showing that an ER pattern in inferior or infero-lateral leads is associated with increased risk for life-threatening arrhythmias, termed early repolarization syndrome (ERS). Because accentuated J waves characterize both Brugada syndrome (BrS) and ERS, these syndromes have been grouped under the heading of J wave syndromes. BrS and ERS appear to share common ECG characteristics, clinical outcomes, risk factors as well as a common arrhythmic platform related to amplification of Ito-mediated J waves. However, they differ with respect to the magnitude and lead location of abnormal J waves and can be considered to represent a continuous spectrum of phenotypic expression. Recent studies support the hypothesis that BrS and ERS are caused by a preferential accentuation of the AP notch in right or left ventricular epicardium, respectively, and that this repolarization defect is accentuated by cholinergic agonists. Quinidine, cilostazol and isoproterenol exert ameliorative effects by reversing these repolarization abnormalities. Identifying subjects truly at risk is the challenge ahead. Our goal here is to review the clinical and genetic aspects as well as the cellular and molecular mechanisms underlying the J wave syndromes.

The pathophysiological mechanism underlying Brugada syndrome: depolarization versus repolarization

This Point/Counterpoint presents a scholarly debate of the mechanisms underlying the electrocardiographic and arrhythmic manifestations of Brugada syndrome (BrS), exploring in detail the available evidence in support of the repolarization vs. depolarization hypothesis.

R-wave amplitude changes measured by electrocardiography during early transmural ischemia

BACKGROUND

Changes in the amplitude of the R wave (RWA) on the electrocardiogram (ECG) have been described during acute myocardial ischemia and infarction. However, this has not been well studied in a controlled setting. We hypothesized that significant increase in RWA occurs during early transmural myocardial ischemia.

METHODS

We prospectively evaluated changes in RWA in 50 patients during brief episodes of transmural ischemia induced by first balloon occlusion (mean, 38 seconds at 6-10 atmospheric pressures) during elective percutaneous coronary intervention. We recorded 12-lead ECGs at 20-second intervals before and during balloon inflation in 16 right coronary arteries, 14 left circumflex arteries, and 20 left anterior descending arteries. R wave amplitude was digitally measured in each of the 12 leads in every ECG using the ECG interval editor (General Electric HC, Menomonee Falls, WI). Intracoronary (IC) ECGs were also recorded in 4 patients. The mean of the RWA in each lead before balloon inflation was compared to the mean RWA during balloon inflation.

RESULTS

R wave amplitude significantly increased during balloon inflation from baseline in limb leads I, II, aVL, and all the precordial leads with the exception of lead V(1). The RWA increase did not reach statistical significance in leads III, aVF, and V(1). Mean RWA increase was consistent in all leads except aVR during the brief episodes of ischemia during initial balloon inflation because of the inverse polarity of this lead. The increase in RWA was seen in most patients (mean, 75%) in whom transmural ischemia was induced by first balloon inflation. Besides, the RWA showed an increase from baseline in 3 patients who had IC-lead recordings.

CONCLUSION

R wave amplitude increases significantly in precordial leads (V(2)-V(6)) and limb leads (I, II, aVL) of the surface ECG during brief episodes of transmural ischemia. The increase in RWA might be consistent with the expansion of the left ventricular cavity during ischemia and/or alterations in conduction that are intrinsic to the myocardium.

Cellular basis for ST-segment changes observed during ischemia

This study probes the cellular basis for ischemia-induced ST-segment elevation with the isolated arterially perfused canine ventricular wedge preparation. Transmembrane action potentials (AP) from epicardial (Epi) and endocardial (Endo) regions, a pseudo-electrocardiogram (ECG), and 5 intramural unipolar electrograms were simultaneously recorded at a basic cycle length of 800 or 2,000 ms. Global ischemia was induced by an abrupt interruption of coronary flow for 30 minutes. Under control conditions, the ST segment was isoelectric because of the absence of voltage gradients at the level of AP plateau among the cells spanning the ventricular wall. Global ischemia could cause an all-or-none repolarization at the end of phase 1 of the AP in Epi but not Endo leading to ST-segment elevation and extrasystolic activity secondary to phase 2 re-entry. In the majority of preparations, global ischemia resulted in a progressive increase in transmural conduction time after 25 to 30 minutes of interruption of flow caused by a step delay of impulse transmission in the midmyocardium. The ECG assumed a "tombstone" configuration. Correlation of the APs and ECG activity revealed that the apparent severe ST-segment elevation encountered under these conditions is actually a markedly prolonged R wave. In control, Endo repolarized after Epi yielding upright T waves in the ECG. After 30 minutes of ischemia Epi repolarized after Endo causing reversal of repolarization gradients and T-wave inversion. The ischemia-induced electrophysiologic changes returned to nearly control values within 5 minutes of reperfusion. Our results indicate that 2 distinctly different mechanisms involving 1) loss of the epicardial action potential dome and 2) markedly delayed transmural conduction underlie the apparent ST-segment elevation encountered during acute ischemia.

The infarct Q wave: foretaste and aftertaste; presentiment and recall

Since acute myocardial infarction involves a progressive disturbance of intramyocardial propagation the process might be regarded as a conduction disturbance: incomplete in the evolving phase; complete with Q wave inscription. Premature atrial excitation (PAE) can induce fascicular or bundle branch conduction delay, or complete functional block. Considered here are the concepts of foretaste: in which PAE reveals the infarct Q before it has appeared in regular sinus beats, and aftertaste: in which, following the healing of the myocardial infarction, the Q wave is recalled by PAE after it has disappeared from regular beats. Foretaste was observed in a small number of acutely infarcted dogs. Aftertaste is substantially more problematic since Q wave inscription is known to occur, however uncommonly, in a small minority of noninfarcted aberration morphologies. Only lateral infarct Q waves appear to be recallable by PAE.

Bedside diagnosis of myocardial ischemia with ST-segment monitoring technology: measurement issues for real-time clinical decision making and trial designs

Monitoring of the ST segment is a valuable tool for guiding clinical decision making and evaluating anti-ischemia interventions in clinical trials; however, measurement issues hamper its diagnostic accuracy. This study reports the frequency and type of false positives and other measurement issues we have encountered during 12-lead ST-segment monitoring of patients in a cardiac care unit. Of 292 patients, 117 (40%) had one or more false positive events during an average of 41 hours of ST-segment monitoring, for a total of 506 false positive events. The 506 false positive events included 167 (36%) due to body positional change; 132 (26%) due to sudden increase in QRS complex/ST-segment voltage; 96 (19%) due to transient arrhythmia or pacing; 80 (16%) due to heart rate change in steeply sloped ST-segment contours; 26 (5%) due to a noisy signal; and 5 (1%) due to lead misplacement. It is concluded that many conditions in addition to myocardial ischemia can cause transient ST-segment deviation in patients with unstable coronary syndromes. Accurate ST-segment monitoring requires expertise in electrocardiogram interpretation, an understanding of the patient's clinical situation, and knowledge of the functions and limitations of the ST-segment monitoring system.