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(Circulation. 2008;117:1474-1477.)
© 2008 American Heart Association, Inc.

Images in Cardiovascular Medicine

Optical Mapping of the Human Atrioventricular Junction

William J. Hucker, PhD; Vadim V. Fedorov, PhD; Kelley V. Foyil, MS; Nader Moazami, MD; Igor R. Efimov, PhD

From the Department of Biomedical Engineering (W.J.H., V.V.F., K.V.F., I.R.E.) and Department of Surgery, School of Medicine (N.M.), Washington University, St. Louis, Mo.

Correspondence to Igor R. Efimov, Washington University in St. Louis, Department of Biomedical Engineering, Campus Box 1097, 1 Brookings Dr, St. Louis, MO 63130. E-mail igor{at}wustl.edu

Fluorescent optical mapping of cardiac electrophysiology in animal models has produced a wealth of information about the function of the cardiac pacemaking and conduction system. However, expanding optical mapping studies to the human conduction system will significantly increase our understanding of clinically relevant phenomena, such as atrioventricular nodal reentrant tachycardia, that are difficult to fully reproduce in animal models. In this report, we present the first instance of optical mapping data recorded from the human atrioventricular junction, revealing its dual pathway electrophysiology, which is the basis of atrioventricular nodal reentrant tachycardia.

Explanted human hearts (n=2) were obtained at the time of cardiac transplantation and perfused with cardioplegic solution. The atrioventricular junction was cannulated, isolated from the rest of the heart, immobilized with the excitation-contraction uncoupler blebbistatin (10 µmol/L),¹ and optically mapped using the voltage sensitive dye Di-4-ANEPPS and a 16x16 photodiode array. In the first heart, explanted because of idiopathic cardiomyopathy, successful perfusion of the His bundle and ventricular septum, but not the atrioventricular (AV) node, was achieved. In this preparation, a junctional rhythm of 55 bpm originated from the His bundle (Figure 1). Optical action potentials (OAPs) from the His displayed diastolic depolarization and a slow upstroke with the maximum derivative of the fluorescent signal dF/dt_max=2.8±0.5 U/s. Pacing the surrounding working ventricular myocardium produced a sharper upstroke (dF/dt_max=37±11, P<0.001 versus His OAPs) and longer action potential duration (APD) than the His (APD₈₀: 315±23 ms in His versus 410±3ms in ventricle, P<0.001). The activation map of the His junctional rhythm demonstrated slow conduction (7 cm/s) transversely along the His bundle (Figure 1). These data provide the first optical recordings of the human His bundle.

View larger version (59K): [in this window] [in a new window]   Figure 1. Junctional rhythm in the His bundle with schematic of the atrioventricular junction shown in the right inset for orientation. Spontaneous activity began in the superior region of the His bundle and spread transversely at 7 cm/s. OAPs recorded from the His are shown in the left inset, and the bold His OAP in the inset is shown at a higher magnification on the right. Because the ventricular septum (VS) was cut to isolate the AV junction, the septal myocardium was electrically uncoupled from the His bundle. Therefore, pacing the ventricular septum produced activity that was completely independent from His activity. Ventricular OAPs had a longer action potential duration and faster upstroke than His potentials. His potentials displayed diastolic depolarization (compare the baseline of the His signal to that of the ventricular signal). Photodiode resolution was 0.5x0.5 mm. AVN indicates atrioventricular node; CS, coronary sinus; FOV, field of view; IAS, interatrial septum; INE, inferior nodal extension; RAO, right anterior oblique; TV, tricuspid valve; S, superior; I, inferior; P, posterior; and A, anterior.

In the second heart (with ischemic cardiomyopathy), the entire AV junction was perfused, enabling us to optically map AV nodal dual pathway characteristics for the first time in the human. We followed standard premature S1–S2 pacing protocols to unmask dual pathway electrophysiology: The S1–S2 interval was decreased until conduction block occurred in the fast pathway because of a long refractory period. The slow pathway then conducted the impulse to the His, as demonstrated by a longer interval between atrial and His activation. Previous rabbit studies demonstrated that slow- versus fast-pathway activation can be recognized optically² and can be induced not only by premature stimuli but also by pacing the slow pathway directly.³ We found that both methods of inducing slow-pathway conduction worked in the human.

Figure 2A illustrates atrial pacing at 60 bpm, which depolarized the endocardial atrial layer with conduction velocities ranging from 30 cm/s to 60 cm/s (atrial APD₈₀: 390±22 ms). After the AV nodal delay, His activation, seen in the optical recordings as a double-humped OAP, occurred at 126 ms.⁴ Activation between 33 ms and 114 ms was difficult to recognize in the fluorescent signals. However, nodal components of OAP upstrokes could be seen in some traces (Figure 2). Moving the pacing electrode ~1-mm closer to the AV groove (still pacing at 60 bpm) produced conduction consistent with slow-pathway activation (Figure 2B). The pacing stimulus activated the atrial myocardium as before. However, His depolarization now occurred at 206 ms, and double-humped OAPs began slightly inferior and anterior to those in Figure 2A. Once the His was activated, longitudinal conduction was rapid (80 cm/s). S1–S2 intervals of 600 ms to 550 ms produced the same slow-pathway conduction seen in Figure 2B, and S1–S2 intervals of 530 ms to 510 ms induced an extra beat consistent with typical slow-fast AV nodal reentry (Figure 3). His electrogram morphology and amplitude also changed with the change in activating pathway (compare Figure 2A to 2B and Figures 3 to 4) as shown previously in rabbits.³

View larger version (49K): [in this window] [in a new window]   Figure 2. Optical action potentials recorded from the AV junction were composed of multiple components. Pacing the atrial myocardium produced a fast wave of excitation that spread across the interatrial septum (IAS) (gray activation maps) that was responsible for the initial atrial upstroke (A) of the OAPs shown. His activation produced an additional hump in OAPs recorded near the His bundle (OAP 4), and the colored activation maps were constructed from the His component of these OAPs. The bipolar His electrogram (EGM) indicates His activation at 126 ms in panel A, and at 206 ms in panel B, which verifies that these OAP components reflect His activation. The bipolar His EGM is much larger in panel A than in panel B (same scale). The changes in His activation time and His EGM amplitude from panel A to panel B suggest that panel A was fast-pathway conduction and panel B was slow-pathway conduction. Some OAPs (such as OAP 3) contained a small change in upstroke following the atrial upstroke but preceding the His component, reflecting AV nodal conduction (N). OAPs recorded near the INE did not reflect any obvious component indicating INE activation (OAP 2). The locations of the histology slides shown in Figure 4 are indicated. The field of view (FOV) of panel B is shifted slightly from panel A, as indicated in the schematic inset. Pacing occurred at time zero at the location marked by square pulse. Photodiode resolution was 2x2 mm. FP indicates fast pathway input of the AVN; SP, slow-pathway input of the AVN. Other abbreviations as in Figure 1.

View larger version (51K): [in this window] [in a new window]   Figure 3. After pacing with an S1 cycle length of 1000 ms (60 bpm), a premature S2 stimulus at 510 ms produced an extra beat consistent with slow AVN reentry. Field of view (FOV) is the same as Figure 2A. The S2 stimulus excited the atrium but failed to conduct to the His (see His EGM). However an extra beat followed 440 ms after the S2, originating from the approximate location of the fast pathway (FP) input to the AVN. The extra beat spread across the atrium in the opposite direction of the S2 with its latest atrial activation near the INE. After a 260-ms delay, His activation occurred, shown in the colored activation map and seen in the His EGM as a much smaller signal than that recorded during the S1 beat. Gray arrows mark the proposed circuit of reentry; however, slow-pathway conduction after the S2 atrial excitation was not readily apparent in the fluorescent optical traces. Therefore, the jagged gray arrow on the S2 map marks the presumed location of the slow pathway. Photodiode resolution was 2x2 mm. Abbreviations as in Figures 1 and 2.

View larger version (65K): [in this window] [in a new window]   Figure 4. Masson Trichrome staining of sections taken throughout the AV junction, corresponding to the locations of photodiodes 2 to 4 in Figures 2 and 3. A, The INE is covered on its endocardial aspect by fibro-fatty tissue and a very thin layer of atrial myocardium, which contributed to the atrial component of the OAPs recorded from this region. OAPs recorded during slow-pathway conduction did not reflect any obvious INE component. B, The AVN is also covered with a layer of fibro-fatty tissue and atrial myocardium above its endocardial aspect, which was responsible for the initial upstroke seen in the OAPs recorded above the AVN. C, The His bundle is surrounded by the fibrous tissue of the central fibrous body (CFB). The tricuspid valve (TV) was cut during the experiment to facilitate optical mapping of the His bundle. The His electrode location is indicated as 2 dots reflecting the actual size of the bipolar wires of the electrode. The inset shows the change in His EGM amplitude that occurred with a premature stimulus. OAPs recorded from this region possessed both atrial and His components, yet only His and fibrous tissue are present, implying that the atrial component of the OAPs originated from light scattering through the translucent connective tissue of the CFB. IAS indicates interatrial septum; VS, ventricular septum.

Histology of the second heart is shown in Figure 4 at the locations indicated in Figures 2 and 3. Activation of the large inferior nodal extension in this heart (Figure 4A), which is the possible substrate of the slow pathway, was difficult to recognize (perhaps because of its depth in the tissue).

	Sources of Funding

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Dr Efimov is supported by American Heart Association grant-in-aid 0750031Z.

Disclosures

None.

	References

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1. Fedorov VV, Lozinsky IT, Sosunov EA, Anyukhovsky EP, Rosen MR, Balke CW, Efimov IR. Application of blebbistatin as an excitation-contraction uncoupler for electrophysiologic study of rat and rabbit hearts. Heart Rhythm. 2007; 4: 619–626.[CrossRef][Medline] [Order article via Infotrieve]

2. Nikolski V, Efimov I. Fluorescent imaging of a dual-pathway atrioventricular-nodal conduction system. Circ Res. 2001; 88: E23–E30.[Medline] [Order article via Infotrieve]

3. Hucker WJ, Sharma V, Nikolski VP, Efimov IR. Atrioventricular conduction with and without AV nodal delay: two pathways to the bundle of His in the rabbit heart. Am J Physiol Heart Circ Physiol. 2007; 293: H1122–H1130.[Abstract/Free Full Text]

4. Efimov IR, Mazgalev TN. High-resolution, three-dimensional fluorescent imaging reveals multilayer conduction pattern in the atrioventricular node. Circulation. 1998; 98: 54–57.[Abstract/Free Full Text]

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