Site of origin of the monophasic action potential: Which electrode, the “potassium” or the “indifferent,” records monophasic action potential?

CREATIVE CONCEPTS

Site of origin of the monophasic action potential: Which electrode, the “potassium” or the “indifferent,” records monophasic action potential? Youhua Zhang, MD, PhD, Todor N. Mazgalev, PhD, FHRS From the Department of Molecular Cardiology, Cleveland Clinic, Cleveland, Ohio.

Introduction The monophasic action potential (MAP) is a useful tool in basic as well as clinical electrophysiology. However, its site of origin has been an issue of heated debates.1– 4 As with any bioelectric signals, two electrode poles are needed for the MAP recording. The essence of the argument is which of the two electrode poles records the MAP; in other words, the tissue under which pole participates in the genesis of the component of the MAP signal representing the time course of the transmembrane potential? This question is not superficial since it is important to know the precise source of the recorded MAP.2,5 One opinion2,5 is based on the classic understanding that MAP represents electrical activity of healthy tissue in the immediate proximity of the depolarizing potassium (“tip,” “contact”) electrode pole. The opposing alternative opinion1 proposes that MAP is picked up by the indifferent (far field) electrode pole. Clarification of these mutually exclusive propositions is needed since it affects the proper interpretation of MAP-derived information, especially when the two electrode poles are placed apart to record MAP.1,3 In previous studies designed to support the alternative indifferent pole concept,1 both the indifferent and the potassium chloride (KCl) poles were placed in the same cardiac (e.g., ventricular wedge) tissue. Subsequent tissue modifications in the vicinity of the MAP poles by drugs or cooling were used to induce characteristic MAP changes.1 However, the interpretation of the results has been complicated by the superposition of closely timed electrical signals arising from the poles’ vicinity.4,6,7 We hypothesized that by placing each of the two MAP poles in cardiac locations with distinct electrophysiological characteristics (e.g., atrial and ventricular myocardium), one could easily differentiate This study was supported by the Atrial Fibrillation Innovation Center, a State of Ohio Wright Center of Innovation Award, and a Biomedical Research and Technology Transfer Partnership Award (Ohio’s Third Frontier Project). Address reprint requests and correspondence: Todor N. Mazgalev, Ph.D., Cleveland Clinic, 9500 Euclid Avenue, Research Institute, Building NE-61, Cleveland, Ohio 44195. E-mail address: [email protected]. (Received November 7, 2008; accepted December 8, 2008.)

the true recording electrode pole by the morphology and timing of the resulting MAP.

Methods This study was approved by the Institutional Animal Care and Use Committee and is in compliance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health.

Surgical preparation The experiments were performed on six adult mongrel dogs (body weight 21–30 kg) that were instrumented as described elsewhere.8 Briefly, dogs were premedicated with thiopental sodium 20 mg/kg intravenously, intubated, and ventilated with room air supplemented with oxygen as needed to maintain normal arterial blood gases by a respirator (Integra II SP Anesthesia Machine, DRE, Inc., Louisville, KY). Anesthesia was then maintained with 1%–2% isoflurane throughout the experiment. Normal saline was infused through a peripheral vein at 100 –200 mL/hour to replace spontaneous fluid loses. Standard surface electrocardiogram leads I, II, and III were monitored continuously throughout the entire study. Intermittent arterial blood gas measurements were taken, and, if needed, ventilator adjustments were made to correct metabolic abnormalities. Body temperature was monitored with a rectal probe, and an electrical heating pad placed under the animal and operating room lamps were used to maintain a body temperature of 36 –37°C. After the chest was opened through a median sternotomy, a pericardium cradle was created to support the heart. Custom-made bipolar plate electrodes were sutured to the right atrium and right ventricular apex for epicardial signal recording. To record MAP, we used a cylindrical electrode pole (DRIREF-2, World Precision Instruments, Sarasota, FL) with a contact diameter of 2 mm, which was made of special porous glass saturated with 3M-KCl. Minute leakage of 3M-KCl from the porous glass produced local myocardial depolarization under the pole. A second pole made of 2 mm Ag-AgCl disk (World Precision Instruments) was used as an indifferent electrode. Both poles were placed gently against the epicardium of right atrium (RA) or right ventricle (RV) using mechanical manipulators and avoiding

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doi:10.1016/j.hrthm.2008.12.022

562 excessive contact pressure. The poles were connected to a differential amplifier (the indifferent pole to the negative input), and the signals were properly filtered (0.05–1000 Hz), amplified, displayed, and saved on Prucka Cardiolab EP System (GE Medical Systems, Fairfield, CT). The following pole configurations were tested: (1) both the KCl pole and the indifferent pole were placed in proximity to each other (⬇10 mm) either in the RA or in the RV; (2) one pole was placed in the RA and the other in the RV; (3) MAP was recorded using 2 KCl poles, one placed in the RA and the other in the RV, connected to the amplifier’s inputs. To further distinguish the RA and RV electrical activity, complete atrioventricular (AV) block was created by radiofrequency ablation of the AV node resulting in AV dissociation.

Results Exploring (KCl) and indifferent (reference) poles placed in close proximity on the same cardiac chamber It was consistently found in all six hearts that in configuration (1) when both poles were placed in proximity on the same tissue (RA or RV), either typical RA (Figure 1A) or RV (Figure 1B) MAP, respectively, was recorded. These trivial observations are in agreement with the customary use of MAP technology but could not distinguish the precise site of origin of the recorded signals, that is, whether the MAP reflected activation of tissue under the KCl or under the indifferent pole.

Exploring (KCl) and indifferent (reference) poles placed separately on atrium and ventricle In all experiments using configuration (2) with the KCl pole in the RA and the indifferent pole in the RV, the MAP was of RA origin (Figure 2A) as clearly evidenced by its shape and timing; the trace was overlapped with RV electrogram contaminant (arrow). In contrast, with the KCl pole in the RV and the indifferent pole in the RA, the MAP was ventricular and the

Figure 1 Two examples of MAP recorded with the indifferent and the KCl pole located in same tissue. A: When the poles were placed in the RA, a typical RA-MAP was recorded as further confirmed by the timing of the MAP versus the RA electrogram. B: When the poles were placed in the RV, a typical RV-MAP was recorded as further confirmed by the timing of the MAP versus the RV electrogram. Note the presence of A-V dissociation.

Heart Rhythm, Vol 6, No 4, April 2009

Figure 2 Two examples of MAP recorded with the indifferent and the KCl pole located in different tissues. A: When the KCl pole was in the RA and the indifferent electrode in the RV, a RA-MAP was recorded (see timing versus RA electrogram) overlapped with RV contaminants (arrow). B: When the KCl pole was in the RV and the indifferent electrode in the RA, an RV-MAP was recorded (see timing versus RV electrogram) overlapped with RA contaminants (arrows).

trace was overlapped with RA electrogram contaminant (Figure 2B, arrows). These experiments identified the tissue under the KCl pole as the source of the MAP. Notably, the precise position of the reference pole was not critical. Thus placement of the reference pole in various atrial spots did not affect the recorded ventricular MAP but only the RA electrogram. The same was true when the reference pole was in ventricular tissue: the atrial MAP remained unchanged.

Two exploring (KCl) poles placed separately on atrium and ventricle Finally, in all experiments using configuration (3) with two KCl poles, the MAP signal was complex and consisted of both RA and RV components that could be easily distinguished (Figure 3). The pole connected to the positive input recorded a positive (here ventricular) MAP. These experi-

Figure 3 An example of MAP recorded with two KCl poles located in different tissues. In this case, one pole was placed in the RA and connected to the negative amplifier input, and the other was placed in the RV. The recorded MAP was complex and displayed distinct atrial (A) and ventricular components (V). Note the timing of these components versus the RA and RV electrograms, respectively.

Zhang and Mazgalev

Site of Origin of the MAP

ments conclusively confirmed that it is the healthy tissue under the depolarizing KCl pole that is the site generating the MAP. As one should expect, when the above procedures were repeated while replacing the KCl poles with two indifferent (reference) poles, only bipolar electrograms (atrial and ventricular) were recorded (not shown).

Discussion Major findings By employing an unorthodox technique of placing the MAP recording poles in cardiac tissues that are not activated simultaneously and have distinct cellular action potentials, we were able to demonstrate that electrical activity at the potassium electrode pole is responsible for the timing and morphology of the MAP signal. The MAP signal most likely originates from healthy tissue under the depolarizing potassium electrode pole. The indifferent electrode, especially when placed away from the potassium pole, can contaminate the recorded MAP by electrical activity arising in its vicinity. These findings support previous interpretations and analysis of MAP genesis.5,9 –11

Role of the exploring (KCl) and reference (indifferent) poles during MAP recording There is still a heated debate on the origin of the MAP attempting to elucidate which of the two electrode poles, the tip (KCl, depolarizing, inactivating) pole or the indifferent (distant-fromtip, grounding) pole, records MAP.1– 4,9,10,12 For example, in the report by Weissenburger et al,3 the KCl pole was located subepicardially and the other (the indifferent) needle poles were plunged at different depths into the ventricular wall. The investigators suggested that the MAPs were recorded from different depths of the ventricular wall. However, Franz4 and Jungschleger and Vos6 argued that the MAPs recorded by Weissenburger et al3 were not intramural MAPs but rather hybrid MAPs. These hybrid MAPs reflect “the single template MAP at a remote cardiac site (the KCl site creating the MAP current) and the superimposition of T waves derived primarily from the unipolar intramural electrograms.”4 A common feature of previous studies is that both the indifferent and the KCl poles were placed on the same cardiac (e.g., ventricular) tissue. This should not be considered a disadvantage; in fact it represents the normal way MAP recordings are made. However, by introducing subtle contaminating imprints such as unipolar electrograms, the traditionally obtained MAP proved to be complex and permitted seemingly endless argument concerning the site of their origin. We now provide an almost simplistic but effective approach by separating the two MAP poles and placing each of them in cardiac locations with distinct electrophysiological characteristics (e.g., atrial and ventricular myocardium). One could easily differentiate the active recording electrode pole just by observing the morphology and timing of the resulting MAP. Our data clearly indicate that the MAP is recorded from the KCl pole. When both the KCl pole and the indifferent pole

563 were placed in close proximity in the atrium or ventricle, a typical atrial or ventricular MAP was recorded (Figure 1A, 1B), just as expected from the classical Franz contact electrode theory5,13,14 in which unipolar contaminations are largely canceled by the close proximity of the indifferent electrode and differential amplification. However, when the KCl pole and the indifferent pole were placed in two different locations (RA and RV), the MAP was always recorded by the KCl pole superimposed with an electrogram picked up by the indifferent pole (Figure 2A, 2B). Our results support the findings of Coronel et al15 who also found that when the KCl electrode was placed onto atrial tissue and paired with silver multiterminal electrode on the ventricular surface in a pig heart, an atrial MAP could be detected. Our results clearly illustrate (Figure 2) that an arbitrary location of the reference electrode can contaminate the complex MAP trace with either atrial or ventricular electrograms. However, positioning the reference electrode at variable atrial spots did not change the ventricular MAP, and, likewise, the same atrial MAP was recorded regardless of the precise positioning of the reference electrode in the ventricle. Finally, our study shows that in an unorthodox configuration when two KCl poles were used, one in the RA and the other in the RV, both RA and RV MAPs were simultaneously recorded (Figure 3), further demonstrating that each of the actual MAP signals was recorded with a depolarizing KCl-electrode.

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