POTENTIAL CHANGES IN AN INJURED REGION OF CARDIAC MUSCLE

Genesis of ischemic ST segment changes: A study using precordial bipolar leads and regional vectorcardiograms

BACKGROUND

Precordial Bipolar Leads (PBLs), Weighted Unipolar Leads (WUL), and Regional Vectorcardiograms (RVCGs) are constructed using the same data recorded by a standard 12‑lead ECG, but they provide additional information not visible in the standard 12‑lead ECG (ECG) tracings.

OBJECTIVES

In previous studies during balloon occlusion of the LAD and RCA, we observed a complete ischemic inversion of the QRS waves, with folding of the loop and ST segment shift. In the present study, we aim to investigate this abnormality using new ECG methods. We hypothesize that utilizing PBL, WUL, and RVCG in patients with ischemia caused by total acute coronary artery occlusion enables the detection of specific abnormalities-such as changes in waveform time/amplitude, the presence of the omega sign, distortion and folding of RVCG loops, and alterations in loop direction in both the transverse and frontal planes-that are not easily discernible with a standard 12‑lead ECG. This enhanced detection aids in understanding the mechanisms underlying electrocardiographic changes and may assist in managing patients when diagnostic uncertainties arise.

METHODS

Thirty-three patients undergoing elective PTCA were studied before and after acute LAD (16 patients) or RCA (17 patients) occlusion, and their data were processed with new methods based on PBLs, WULs, and RVCGs.

RESULTS

1. In acute ischemia due to occlusion of the LAD and RCA, the most important current of injury occurs in the right to left axis. This axis is underestimated by the standard 12‑lead ECG and only partially complemented by leads V3R and V4R. 2. The two-dimensional presentation detects a new sign (the omega sign), not detectable in the classic ECG, but almost always present in ischemia. It also allows for an accurate identification of the J point. 3. Ischemic R wave peak delay results in distortion and folding of the RVCG loop and causes displacement of the J point and the ST segment. 4. Wave inversion changes the loop direction in the transverse and frontal plane.

CONCLUSIONS

Precordial bipolar leads, weighted unipolar leads, and regional vectorcardiograms provide essential information omitted by the standard 12‑lead ECG.

EDEN (Electrocardiographic Radial Depth Navigation): A Novel Approach to Navigate Inside Heart Muscle

BACKGROUND

Intramyocardial guidewire navigation is a novel technique that allows free transcatheter movement within ventricular muscle. Guidewire radial depth, between endocardial and epicardial surfaces, is ambiguous by x-ray and echocardiography.

OBJECTIVES

The aim of this study was to develop a simple tool, EDEN (Electrocardiographic Radial Depth Navigation), to indicate radial depth during intramyocardial guidewire navigation. Combined with routine imaging, EDEN facilitates a new family of intramyocardial catheter procedures to slice, reshape, pace, and ablate the heart.

METHODS

We mapped intramyocardial electrograms of left and right ventricular walls and septum during open- and closed-chest swine procedures (N = 53), including MIRTH (Myocardial Intramural Remodeling by Transvenous Tether) ventriculoplasty. We identified radial depth-dependent features on unipolar electrograms. We developed a machine learning-based classifier to indicate categorical position, and modeled the findings in silico to test understanding of the physiology.

RESULTS

EDEN signatures distinguished 5 depth zones throughout left and right ventricular free walls and interventricular septum. Relative ST-segment elevation magnitude best discriminated position and was maximum (40.1 ± 6.5 mV) in the midmyocardium. Subendocardial positions exhibited dominant Q waves with lower-amplitude ST segments (16.8 ± 5.8 mV), whereas subepicardial positions exhibited dominant R waves with lower-amplitude ST segments (15.7 ± 4.8 mV). EDEN was unaffected by pacing-induced left bundle branch block. ST-segment elevation declined over minutes and reappeared after submillimeter guidewire manipulation. Modeling recapitulated EDEN features. The machine learning-based classifier was 97% accurate. EDEN successfully guided MIRTH ventriculoplasty.

CONCLUSIONS

EDEN provides a simple and reproducible real-time reflection of categorical guidewire-tip radial depth during intramyocardial guidewire navigation. Used in tandem with x-ray, EDEN enables novel, transcatheter, intramyocardial therapies such as MIRTH, SESAME (Septal Surfing Along Midline Endocardium), and cerclage ventriculoplasty.

Abnormal myocardial activation as a cause of ST elevation: A study using Precordial Bipolar Leads (PBL)

OBJECTIVES

The purpose of the study was to describe the ischemic changes occurring during percutaneous transluminal coronary angioplasty (PTCA) using a new method based on Precordial Bipolar Leads (PBL) and Precordial Unipolar Leads (PUL).

BACKGROUND

Ischemic ECG changes have been attributed to both systolic and diastolic injury currents. The relation between ST segment shift and QRS changes is unclear and there is discussion about its significance.

METHODS

Twelve-lead electrocardiograms (ECGs) were performed in 16 patients before PTCA balloon inflation and immediately after balloon deflation in the proximal left anterior descending coronary artery (LAD). Also, ECG data was used to generate V2-V1 PBL, average V1+V2 lead and the correspondent loop in order to explore ECG and spatial vector changes.

RESULTS

1) The V2-V1 vs. Average V1+V2 loop rotation changed from counter clockwise (CCW) to clockwise (CW) in 14/15 patients (93%). 2) In 12/16 patients (75%), there was an abrupt change of QRS vector direction, producing a "folding" of the loop. In 10 of these cases, the change occurred between 32 and 49 milliseconds after the QRS initiation. 3) In 3/16 patients the final part of the loop was "transported", without folding, to the turning point. 4) The "folding" of the loop changed the direction of the final QRS forces and the J point and ST segment were displaced to the left and forward. 5) For this reason, repolarization began from an abnormal anterior location.

CONCLUSIONS

1) Ischemic changes in QRS loop have a cornerstone point in which the whole loop changes. 2) Once the loop has changed its direction, there are no major modifications in the loop development but the forces do not aim anymore to the isoelectric point. 3) Alterations of myocardial activation appear to be responsible of ST elevation in hyperacute ischemia.

The Genesis of Injury Potentials The Role of Recording Electrodes at Different Locations

The objective of the present study was to elucidate the mechanisms underlying the so-called injury potentials, including the origin of monophasic action potentials and the role of recording electrodes. Two-dimensional computer simulation was performed for cardiac tissue containing an inactivated region due to high extracellular K concentration. Myocardial activation was reproduced using a membrane model. The bidomain model was utilized for the calculation of intra-and extracellular potentials. A bipolar lead from electrodes at injured and intact regions showed a monophasic curve corresponding to the transmembrane potential of the fiber under the electrode of the intact region. Unipolar leads from injured and intact regions showed monophasic and biphasic curves, respectively. Lowering the extracellular conductivity was associated with an increase in the wave amplitude. The injured region of myocardium was associated with monophasic potential variations. A bipolar lead with electrodes at injured and intact regions reflected the activity of the intact region.

Mechanisms of ischemia-induced ST-segment changes

Many aspects of ischemia-induced changes in the electrocardiogram lack solid biophysical underpinnings although variations in ST segments form the predominant basis for diagnostic and monitoring of patients. This incomplete knowledge certainly plays a role in the poor performance of some forms of electrocardiogram-based detection and characterization of ischemia, especially when it is limited to the subendocardium. The focus of our recent studies has been to develop a comprehensive mechanistic model of the electrocardiographic effects of ischemia. The computational component of this model is based on highly realistic heart geometry with anisotropic fiber structure and allows us to assign ischemic action potentials to contiguous regions that can span a prescribed thickness of the ventricles. A separate, high-resolution model of myocardial tissue provides us with a means of setting electrical characteristics of the heart, including the status of gap junctional coupling between cells. The experimental counterpart of this model consists of dog hearts, either in situ or isolated and perfused with blood, in which we control coronary blood flow by means of a cannula and blood pump. By reducing blood flow through the cannula for various durations, we can replicate any phase of ischemia from hyper acute to early infarction. Based on the results of these models, there is emerging a mechanism of the electrocardiographic response to ischemia that depends strongly on the anisotropic conductivity of the myocardium. Ischemic injury currents flow across the boundary between healthy and ischemic tissue, but it is their interaction with local fiber orientation and the associated conductivity that generates secondary currents that determine epicardial ST-segment potentials. Results from experiments support qualitatively the findings of the simulations and underscore the role of myocardial anisotropy in electrocardiography.

Focal extracellular potential: a means to monitor electrical activity in single cardiac myocytes

The focal extracellular potential (FEP) described in this study is an electrophysiological signal related to the transmembrane potential (V(m)) of cardiac myocytes that avoids the mechanical fragility, interference with contraction, and intracellular contact associated with conventional whole cell recording. One end of a frog ventricular myocyte was secured into a glass holding pipette. The FEP was measured differentially between this pipette and a bath pipette while the cell was voltage- or current-clamped by a third whole cell pipette. The FEP appeared as an amplitude-truncated action potential, while FEP duration accurately reflected the action potential duration (APD) at 90% repolarization (APD(90)). FEP magnitude increased as the holding pipette K(+) concentration ([K(+)]) was increased. The FEP-voltage relation was quasi-linear at negative V(m) with a slope that increased with elevated holding pipette [K(+)]. Increasing the membrane conductance inside the holding pipette by adding amphotericin B or cromakalim linearized the FEP-voltage relation across all V(m). The FEP accurately reported electrical activation and APD(90) during changes of stimulation frequency and episodes of cellular stretch.

Long-term recording of monophasic action potentials from human endocardium

In 36 patients undergoing routine cardiac catheterization, a new "contact electrode" catheter technique was used to record monophasic action potentials (MAPs) from right atrial and right and left ventricular endocardial sites without the application of suction. Although of smaller amplitude, typically ranging from 15 to 40 mV, and of different reversal ratio (33 +/- 3%), MAP recordings closely resembled transmembrane action potentials in configuration and duration. Continuous MAP recordings of stable amplitude and, during regular pacing, of constant duration (+/- 1% at 90% repolarization) could be made from the same endocardial site for test periods of 1 hour (n = 4), permitting direct evaluation of the effect of cycle length alterations on local myocardial repolarization. A linear relation was found between MAP duration and basic cycle length varying from 350 to 700 ms. These rate-dependent changes in MAP duration were caused by a change in the slow phase of repolarization (phase 2), whereas the slope of rapid repolarization (phase 3) was unaltered. Single premature MAPs or MAPs after a pause showed changes in both phases. No MAPs could be recorded in areas of infarcted, aneurysmal myocardium, indicating that local viable myocardium is a prerequisite for the generation of the monophasic signal. Thus, in human subjects this catheter permits safe, long-term recording of MAPs which, although of smaller amplitude than transmembrane action potentials, bear appropriate and predictable phase relations. Such recordings may be useful in evaluating changes in local myocardial electrical activity induced by pacing or resulting from myocardial disease, or both.

Electrocardiography in myocardial infarction

Electrocardiography is an evolving clinical diagnostic modality for detection of acute myocardial infarction. Animal studies and electrocardiographic-clinical-pathological correlations have provided experience currently used for detection and rough localization of myocardial infarcts. Additions to the conventional 12 electrocardiographic leads have been utilized to increase the diagnostic sensitivity of the ECG in the setting of myocardial infarction. Mapping of ST-segment elevation an QRS complex from several chest wall loci have been employed for purposes of quantitating serially myocardial ischemic injury and eventual necrosis. These multiple lead electrocardiographic systems have also been utilized in assessing therapeutic interventions in the Coronary Care Unit. Usefulness of standard and multiple-lead recording systems is enhanced by awareness of their limitations when applied to patients with acute myocardial infarction.

Computer processing of intracardiac electrograms for conduction studies

Computer techniques developed to process intracardiac signals recorded in dogs are presented. The signals under measurement are the auricular and ventricular monophasic action potentials and the His bundle electrogram. Computerized measurement of significant timing parameters on simultaneous recordings of these signals can assess quite precisely changes in the normal conduction scheme of the heart provoked by different experimental protocols. Increased accuracy is mainly due to the objective way of defining wave onsets and the processing power of the system used. Signal recording, signal acquisition, automatic waveform measurements, interactive process and production of end result graphs by computer are all described.

Significance of S-T segment elevations in acute myocardial ischemia. Evaluation with intracoronary electrode technique

A method is described for measuring intracoronary S-T segment elevations in the closed chest, a technique that appears to provide more reliable measurements of myocardial ischemia. Electrodes were inserted through intracoronary balloon catheters that were placed within a coronary artery and its adjoining vein both proximal and at several points distal to a coronary occlusion. Intracoronary arterial and adjacent venous electrocardiograms produced equivalent tracings. The intracoronary S-T segment elevations after coronary occlusion resembled those recorded from the epicardial surface but were free of artifacts noted in open chest studies. Study of progressive alterations of the intracoronary S-T segment after proximal occlusion of the left anterior descending coronary artery in 18 closed chest dogs revealed a peak segment elevation of 3.2 +/- 0.6 mv within 5 minutes, followed within 2 to 3 hours by spontaneous reduction by more than 40% of the S-T elevation over the occluded zone. In 44% of these animals, the S-T elevation decreased spontaneously to less than 1 mv, and in 22% it decreased to the preocclusion control level within 2 hours of occlusion. This spontaneous decrease in S-T elevation was frequently followed by a secondary increase and then S-T segment fluctuations. Reperfusion of the left anterior descending coronary artery after 30 to 60 minutes of occlusion generally led to a prompt reduction in S-T elevation. In some cases S-T elevations persisted up to 14 hours of occlusion, were reduced after reperfusion and exhibited a renewed pronounced increase after subsequent reocclusion of the left anterior descending coronary artery. During the 1st hour after occlusion, the early S-T segment elevation followed by spontaneous reduction reduction generally corresponded temporally with the derangements in myocardial lactate extraction and potassium loss. However, after 1 hour of occlusion no clear-cut correlation could be established between S-T fluctuations and changes in hemodynamic or myocardial metabolic measurements. We conclude that the new closed chest intracoronary electrocardiographic S-T technique might be of use for monitoring the early ischemic myocardial derangements and to assess benefits or drawbacks of treatment in both the experimental animal and man. Correspondence of S-T segment elevation with lactate and potassium alterations in the coronary-occluded region in the 1st hour after occlusion indicates that S-T segment elevation might represent an index of early myocardial ischemia. The spontaneous S-T changes that follow coronary occlusion must be taken into consideration when investigators utilize S-T segment modification as a sign of effectiveness of treatment.

Effects of Experimental Regional Ischemia and Levarterenol on the RS-T Segment and Baseline of Ventricular Surface Electrocardiograms Obtained by Direct-Coupled Amplification

Conventional electrocardiographie equipment (capacitance-coupled amplification) cannot distinguish between RS-T segment and baseline shifts of the ventricular complex. The so-called RS-T segment displacement is the net difference between the undetermined levels of those components. Using direct-coupled amplification, the absolute changes of baseline and RS-T segment on the surface of the exposed left ventricle during levarterenol administration and during regional ischemia, and release-recovery were studied in dogs. Ten significant patterns of displacement were observed. These permitted clear differentiation of the effects of levarterenol injection and hyperperfusion from those of ischemia. The centers and borders of an ischemic area could be distinguished and the change with prolonged ischemia described. The relationship of these observations to the literature and pertinent studies of intracellular action potentials are discussed.

Electric instability of the heart; the concept of the current of oxygen differential in coronary artery disease

Normally oxygenated dog hearts and hearts rendered uniformly anoxic exhibit electric stability: there is no difference in resting electric potentials, and spontaneous ventricular fibrillation does not occur. When an area of myocardium is rendered ischemic, the resulting "trigger" may destroy the coordinated mechanism of the heart. A current of oxygen differential is produced across the zone of contact between red and blue myocardium resulting in "electric instability." In the electrically stable, uniformly anoxic heart, perfusion of an area with red blood produces a "reverse trigger" and electric instability. Application of these concepts and therapeutic implications to human coronary disease appears justified.

Delayed development of ventricular ectopic rhythms following experimental coronary occlusion

Following aseptic occlusion of the anterior descending artery of the dog's heart ectopic ventricular tachycardia develops after a latency of four and one-half to eight hours, and persists for two to four days. Large gross infarcts are found in all hearts. The duration of latency of onset of major ectopic activity approximates the minimal period of ischemia required to produce histologic signs of necrosis. It is suspected that products or processes of necrosis have excitatory effects on tissues bounding the ischemic zone. Evidences concerning various possible excitatory factors are briefly reviewed.