Relationship between different recovery curves representing rate-dependent AV nodal function in rabbit heart

Integrated rate-dependent and dual pathway AV nodal functions: principles and assessment framework

The atrioventricular (AV) node conducts slowly and has a long refractory period. These features sustain the filtering of atrial impulses and hence are often modulated to optimize ventricular rate during supraventricular tachyarrhythmias. The AV node is also the site of a clinically common reentrant arrhythmia. Its function is assessed for a variety of purposes from its responses to a premature protocol (S1S2, test beats introduced at different cycle lengths) repeatedly performed at different basic rates and/or to an incremental pacing protocol (increasingly faster rates). Puzzlingly, resulting data and interpretation differ with protocols as well as with chosen recovery and refractory indexes, and are further complicated by the presence of built-in fast and slow pathways. This problem applies to endocavitary investigations of arrhythmias as well as to many experimental functional studies. This review supports an integrated framework of rate-dependent and dual pathway AV nodal function that can account for these puzzling characteristics. The framework was established from AV nodal responses to S1S2S3 protocols that, compared with standard S1S2 protocols, allow for an orderly quantitative dissociation of the different factors involved in changes in AV nodal conduction and refractory indexes under rate-dependent and dual pathway function. Although largely based on data from experimental studies, the proposed framework may well apply to the human AV node. In conclusion, the rate-dependent and dual pathway properties of the AV node can be integrated within a common functional framework the contribution of which to individual responses can be quantitatively determined with properly designed protocols and analytic tools.

Rate-dependent AV nodal refractoriness: a new functional framework based on concurrent effects of basic and pretest cycle length

The atrioventricular (AV) node filters atrial impulses. Underlying rate-dependent refractory properties are assessed with the effective (ERPN; longest nonconducted atrial cycle length) and functional (FRPN; shortest His bundle cycle) refractory period determined with premature protocols at different basic rates. Fast rates prolong ERPN and shorten FRPN, but these effects vary with subjects, age, and species. We propose that these opposite and variable effects reflect the net sum of concurrent cumulative and noncumulative effects associated with basic (BCL) and pretest cycle length (PTCL), respectively. To test this hypothesis, we assessed selective and combined effects of five BCL (S1S1) and six PTCL (S1S2) on ERPN, FRPN, and their subintervals (ERPN = A2H2 + H2A3 and FRPN = H2A3 + A3H3, where A is atrium and H is His bundle) with S1S2S3 protocols in six rabbit heart preparations. At control BCL, PTCL shortenings prolonged ERPN (113 ± 12 vs. 101 ± 14 ms, P < 0.01) as a net result of prolonged A2H2 and curtailed H2A3. At control PTCL, BCL shortenings increased ERPN (127 ± 20 vs. 101 ± 14 ms, P < 0.01) by prolonging A2H2. FRPN did not vary with BCL but decreased (163 ± 6 vs. 175 ± 10 ms, P < 0.01) with PTCL that curtailed H2A3. Equal BCL and PTCL shortenings as in standard protocols prolonged ERPN but left FRPN unchanged. Notably, ERPN and FRPN significantly correlated through their H2A3 subinterval. In conclusion, BCL and PTCL are both important determinants of AV nodal refractoriness and together account for rate-induced changes in ERPN and FRPN observed during standard premature protocols. ERPN and FRPN are related variables. Similar functional rules may govern nodal refractory behavior during supraventricular tachyarrhythmias.

Dependence of AV Nodal Function Curves on the Selected Recovery Index: Pivotal Role of Pretest Conduction Time

INTRODUCTION

Rate-dependent AV nodal function is often assessed with premature protocols. Conduction curves generated from nodal responses differ with the selected recovery index (atrial-atrial, AA, or His-atrial interval, HA). We propose that these differences arise from changes in pretest conduction time that affect nodal recovery time assessment.

METHODS AND RESULTS

We varied the basic (S(1)S(1)), pretest (S(1)S(2)), and test (S(2)S(3)) cycle length with S(1)S(2)S(3) protocols, and analyzed nodal responses as AA (A(3)H(3) vs A(2)A(3)) and HA (A(3)H(3) vs H(2)A(3)) curves in six rabbit heart preparations. Any A(2)H(2) (pretest conduction time) prolongation bodily shifted AA curve rightward and HA curve leftward, regardless of prevailing basic cycle length. A prolonged A(2)H(2) caused all A(3)H(3) to occur at longer A(2)A(3) and shorter H(2)A(3,) as compared with control. When corrected for these shifts, AA and HA curves displayed similar recovery and fatigue properties. To further investigate the possibility that nodal recovery time varies beyond that imposed by pacing interval, six additional preparations were subjected to 5-minute frequency step protocols during which a long cycle was introduced after every 30th short cycle. After each long cycle, nodal conduction time increased progressively despite the constant short cycle length and fatigue level.

CONCLUSIONS

Increases in the pretest conduction time play a pivotal role in apparent recovery-index-dependent differences in rate-dependent AV nodal function by shifting AA and HA curves in opposite directions along the x-axis. When corrected for pretest effects, AA and HA curves display similar rate-dependent AV nodal function with recovery and fatigue as main properties.

Combined Bispectral and Bicoherency approach for Catastrophic Arrhythmia Classification

Quantitative classification of cardiac arrhythmia is an important tool in ICU and CCU that enables on line monitoring of the cardiac activities. Among fatal arrhythmias are atrial fibraliation (AF), ventricular tachycardia (VT), and ventricular fibrillation that require special algorithms for detection and so for direct medical actions. In this paper, a combined bispectrum and bicoherency classification algorithm is introduced. It is based on extracting diagnostic features from the bispectrum contours and the bicoherency indices that better describe the arrhythmia. A simple classification scheme utilizing these features showed notable sensitivity and specificity. The obtained results are found comparable to the state of the art algorithms with the ability of being integrated for on line monitoring and classification.

Unified rate-dependent atrioventricular nodal function: Consistent recovery and fatigue properties revealed with S1S2S3 protocols and different recovery indexes

BACKGROUND

Rate-dependent nodal properties are commonly assessed with premature protocols performed at different basic rates. Because characteristics of responses differ with recovery time index, the true nature of nodal rate-dependent properties is elusive.

OBJECTIVES

The purpose of this study was to reveal consistent nodal rate-dependent properties regardless of selected recovery index.

METHODS

With S(1)S(2)S(3) protocols, we independently varied basic and pretest cycle lengths and thereby distinguished cumulative from noncumulative effects of rate on nodal conduction time in rabbit heart preparations. Nodal responses to 30 basic and pretest cycle length combinations (five with identical basic and pretest cycles as in standard protocols) were analyzed using both atrial (AA) and His-atrial (HA) intervals as recovery index.

RESULTS

AA and HA curves had an identical shape for any of 30 steady-state conditions. When assessed with constant pretest cycle lengths, cumulative effects (fatigue) of shortened basic cycle lengths were also independent of recovery index. Shortening of pretest cycle length at fixed basic rates led to apparent inhibitory and facilitatory effects when assessed with AA and HA curves, respectively. These effects vanished when a single long cycle was inserted after the pretest cycle. In all responses including those obtained with standard protocols, combined effects of basic and pretest cycle lengths set nodal conduction time.

CONCLUSION

S(1)S(2)S(3) protocols reveal consistent nodal recovery and fatigue properties regardless of recovery index used. Changes in nodal function curves arising from the use of different recovery indexes mainly depend on pretest effects. This study provides a new approach to a unified interpretation of nodal recovery and fatigue properties.

Role of Compact Node and Posterior Extension in Direction-Dependent Changes in Atrioventricular Nodal Function in Rabbit

INTRODUCTION

AV nodal conduction properties differ in the anterograde versus the retrograde direction. The underlying substrate remains unclear. We propose that direction-dependent changes in AV nodal function are the net result of those occurring in the slow and fast pathways.

METHODS AND RESULTS

Anterograde and retrograde AV nodal properties were determined with a premature protocol before and after posterior extension (slow pathway) ablation, and before and after upper compact node (fast pathway) ablation. Each ablation was performed in a different group of six rabbit heart preparations. In control, nodal minimum conduction time (NCTmin) and effective refractory period (ERPN) typically were longer, and maximum conduction time (NCTmax) was shorter in the retrograde compared to the anterograde direction. Posterior extension ablation prolonged anterograde ERPN from 91 +/- 10 ms to 141 +/- 15 ms (P < 0.01) and shortened NCTmax from 150 +/- 13 ms to 82 +/- 7 ms (P < 0.01) but did not affect retrograde conduction. Thus, the posterior extension normally contributes to the anterograde but not retrograde recovery curve. Compact node ablation prolonged anterograde conduction (NCTmin increased from 57 +/- 2 ms to 73 +/- 7 ms, P < 0.01) but did not alter ERPN and NCTmax. This ablation abolished retrograde conduction in two preparations and resulted in retrograde slow pathway conduction in four, the latter being interrupted by posterior extension ablation. Thus, the compact node accounts for the baseline of the recovery curve in both directions. Ablation of the compact node results in anterograde slow pathway conduction over the entire cycle length range and may result in retrograde slow pathway conduction.

CONCLUSION

Direction-dependent properties of the AV node arise from those of the compact node-based fast pathway and posterior extension-based slow pathway. Normal AV node has bidirectional dual pathways.

Complex AV nodal dynamics during ventricular-triggered atrial pacing in humans

In vitro experiments have shown that the complexity of atrioventricular nodal (AVN) conduction dynamics increases with heart rate. Although complex AVN dynamics (e.g., alternans) have been observed clinically, human AVN dynamics during rapid pacing have not been systematically investigated. We studied such dynamics during ventricular-triggered atrial pacing in 37 patients with normal AVN function (18 patients with dual AVN pathway physiology and 19 patients without). Alternans, which always resulted from single pathway conduction, occurred in 18 patients. In 16 patients (3 of whom also had alternans), quasisinusoidal AVN conduction oscillations occurred (mean frequency 0.02 Hz); such oscillations have not been previously reported. There were no significant differences in the dynamics for patients with or without dual AVN pathways. To illuminate the governing dynamic mechanism, a second atrial pacing trial was performed on 12 patients after autonomic blockade. Blockade facilitated alternans but inhibited oscillations. This study suggests that rapid AVN excitation in vivo can lead to autonomically mediated AVN conduction oscillations or single pathway alternans that are a function of inherent nonlinear dynamic AVN tissue properties.

Role of the compact node and its posterior extension in normal atrioventricular nodal conduction, refractory, and dual pathway properties

INTRODUCTION

The functional origin of AV nodal conduction, refractory, and dual pathway properties remains debated. The hypothesis that normal conduction and refractory properties of the compact node and its posterior nodal extension (PNE) play a critical role in the slow and the fast pathway, respectively, is tested with ablation lesions targeting these structures.

METHODS AND RESULTS

A premature atrial stimulation protocol was performed before and after PNE ablation in six isolated rabbit heart preparations. Discrete (approximately 300 microm) histologically controlled PNE lesions amputated the AV nodal recovery curve from its left steep portion reflecting slow pathway conduction and prevented reentry without affecting the right smooth fast pathway portion of the curve. The ablation shortened A2H2max from 159 +/- 16 ms to 123 +/- 11 msec (P < 0.01) and prolonged the effective refractory period from 104 +/- 6 msec to 119 +/- 11 msec (P < 0.01) without affecting A2H2min (55 +/- 9 msec vs 55 +/- 8 msec; P = NS) and functional refractory period (174 +/- 7 msec vs 175 +/- 6 msec; P = NS). These results did not vary with the input reference used. In six other preparations, lesions applied to the compact node after PNE ablation shifted the fast pathway portion of the recovery curve to longer conduction times and prolonged the functional refractory period, suggesting a compact node involvement in the fast pathway.

CONCLUSION

The normal AV nodal conduction and refractory properties reflect the net result of the interaction between a slow and a fast pathway, which primarily arise from the asymmetric properties of the PNE and compact node, respectively.

Anatomic and functional characteristics of a slow posterior AV nodal pathway: role in dual-pathway physiology and reentry

BACKGROUND

The AV node is frequently the site of reentrant rhythms. These rhythms arise from a slow and a fast pathway for which the anatomic and functional substratum remain debated. This study proposes a new explanation for dual-pathway physiology in which the posterior nodal extension (PNE) provides the substratum for the slow pathway.

METHODS AND RESULTS

The anatomic and functional properties of the PNE were studied in 14 isolated rabbit heart preparations. A PNE was found in all studied preparations. It appeared as an elongated bundle of specialized tissues lying along the lower side of Koch's triangle between the coronary sinus ostium and compact node. No well-defined boundary separated the PNE, compact node, and lower nodal cell bundle. The electric properties of the PNE were characterized with a premature protocol and surface potential recordings from histologically controlled locations. The PNE showed cycle-length-dependent posteroanterior slow activation with a shorter refractory period (minimum local cycle length) than that of the compact node. During early premature beats resulting in block in transitional tissues, the markedly delayed PNE activation could propagate to maintain or resume nodal conduction and initiate reentrant beats. A shift to PNE conduction resulted in different patterns of discontinuity on conduction curves. Transmembrane action potentials recorded from PNE cells in 6 other preparations confirmed the slow nature of PNE potentials.

CONCLUSIONS

The PNE is a normal anatomic feature of the rabbit AV node. It constitutes a cycle-length-dependent slow pathway with a shorter refractory period than that of the compact node. Propagated PNE activation can account for a discontinuity in conduction curves, markedly delayed AV nodal responses, and reentry. Finally, the PNE provides a substratum for the slow pathway in dual-pathway physiology.

Electrical interactions between a rabbit atrial cell and a nodal cell model

Atrial activation involves interactions between cells with automaticity and slow-response action potentials with cells that are intrinsically quiescent with fast-response action potentials. Understanding normal and abnormal atrial activity requires an understanding of this process. We studied interactions of a cell with spontaneous activity, represented by a "real-time" simulation of a model of the rabbit sinoatrial (SA) node cell, simultaneously being electrically coupled via our "coupling clamp" circuit to a real, isolated atrial myocyte with variations in coupling conductance (Gc) or stimulus frequency. The atrial cells were able to be driven at a regular rate by a single SA node model (SAN model) cell. Critical Gc for entrainment of the SAN model cell to a nonstimulated atrial cell was 0.55 +/- 0.05 nS (n = 7), and the critical Gc that allowed entrainment when the atrial cell was directly paced at a basic cycle length of 300 ms was 0.32 +/- 0.01 nS (n = 7). For each atrial cell we found periodic phenomena of synchronization other than 1:1 entrainment when Gc was between 0.1 and 0.3 nS, below the value required for frequency entrainment, when the atrial cell was directly driven at a basic cycle length of either 300 or 600 ms. In conclusion, the high input resistance of the atrial cells allows successful entrainment of nodal and atrial cells at low values of Gc, but further uncoupling produces arrhythmic interactions.

Effects of blocked atrial beats on the atrioventricular nodal recovery property: facilitation or depression?

INTRODUCTION

Blocked atrial beats (A(B)) usually have concealed AV nodal penetration, which can change the nodal conduction time (AH) of a subsequent beat. However, without an output marker it is difficult to assess their effect on the node. In this report we used all possible parameters as nodal resting time after A(B) and plotted them against the AH of testing beats to study their effects on the node.

METHODS AND RESULTS

Atrial extrastimulation studies were done in 21 patients in whom one blocked atrial beat (A(2B)) was observed. Nodal recovery curves were obtained for basic pacing (A1), after a conducted premature beat (A2), and after A(2B). In six patients there were 2 to 3 consecutively blocked beats (A(nB)) and recovery curves were constructed after each A(nB). Nodal recovery curves were plotted with AH of the testing beat against different nodal resting parameters and fitted to a single exponential equation. We found contradicting phenomena when using different formats. (1) For recovery curves of A(2B), there was a rightward shift from that of the basic curve when using H1A3 or A1A3 as the gauge (depression phenomenon). On the contrary, there was a leftward shift of the curves when using A(2B)A3 (facilitating phenomenon). (2) For recovery curves after multiple blocked beats there was a marked rightward shift of all curves except A(n-1)(B)An-curves, which were all leftward shifted.

CONCLUSION

Because these contradicting phenomena were dictated by the presenting formats, the terms "depression" and "facilitation" cannot be considered intrinsic AV nodal properties outside of the strict context of the pacing protocol and the format of data presentation.

Developmental changes of atrioventricular nodal recovery properties

Atrioventricular (AV) nodal recovery properties can be studied by a periodic premature stimulation protocol performed at a slow basic rate. Developmental aspects of these properties have not been determined. The purpose of this study was to determine the developmental changes of AV nodal recovery properties. Forty-three children and young adults (male:female ratio 25:18) without AV nodal disease (aged 3.3 to 21.9 years) were studied by delivering premature atrial extrastimuli coupled to basic driven atrial beats. The individual recovery curve was fitted to the equation: A2H2 = A0H0 + exp(alpha -H1A2/tau) for H1A2 > or =theta, where A0H0 is the minimum AH interval, H1A2 is any recovery interval that exceeds the nodal effective refractory period, A2H2 is the corresponding nodal conduction time at any given H1A2, alpha is a constant, tau is the recovery time constant, and theta is the nodal effective refractory period. We found that: (1) A0H0 and alpha constant did not change significantly with age; (2) both tau (r = 0.324; p <0.05) and theta (r = 0.401; p <0.05) had a positive correlation with age; and (3) the maximum change in A2H2 with a 10-ms decrement in H1A2 was 32 ms and did not change significantly with age. Our results suggest that AV nodal recovery properties are age-dependent and both the recovery time constant and effective refractory period lengthen with age.

Alternation of atrioventricular nodal conduction time during atrioventricular reentrant tachycardia: are dual pathways necessary?

INTRODUCTION

Alternation of atrial cycle length and AV nodal conduction time (NCT) is often observed during AV reentrant tachycardia. Both AV nodal dual pathway and rate-dependent function have been postulated to be involved in this phenomenon. This study was designed to determine the respective role of these two mechanisms in the alternation observed in an in vitro model of orthodromic AV reentrant tachycardia.

METHODS AND RESULTS

The tachycardia was produced by detecting each His-bundle activation and stimulating the atrium after a retrograde delay, thereby simulating retrograde pathway conduction, in six isolated rabbit heart preparations. After a 5-minute stabilization period at a fast rate, the retrograde delay was decremented by 2 msec every minute until nodal blocks occurred. We observed a sequential alternation of the cycle length and NCT in four preparations in the short retrograde delay range. The magnitude of the alternation gradually increased as the retrograde delay was decreased and reached 4.6 +/- 0.5 msec during 1:1 conduction. The alternation increased further just prior to termination of the tachycardia by an AV nodal block. None of the preparations showed discontinuous AV nodal recovery curves. Moreover, an electrode positioned over the endocardial surface of the node showed that the alternation developed distally to the nodal inputs, which are believed to constitute a major component of dual pathways. A mathematical model predicted the alternation from known characteristics of rate-dependent nodal functional properties.

CONCLUSIONS

NCT and cycle length alternation can arise during orthodromic AV reentrant tachycardia when the retrograde delay is sufficiently short. The characteristics of the alternation, presence of continuous recovery curves, intranodal location of the alternation, and mathematical modeling suggest that the alternation is predictable from the known functional properties of the AV node without postulating dual pathway physiology.

Selective functional properties of dual atrioventricular nodal inputs. Role in nodal conduction, refractoriness, summation, and rate-dependent function in rabbit heart

BACKGROUND

The atrioventricular node receives its activation signal from the low crista terminalis and low interatrial septum, the summation of which is believed to favor conduction. A functional asymmetry between the inputs is also believed to be involved in nodal reentrant rhythms. We studied the selective functional characteristics of nodal inputs and determined their role in nodal conduction, refractoriness, summation, and rate-dependent function.

METHODS AND RESULTS

The nodal properties of recovery, facilitation, and fatigue were characterized with stimulation protocols applied with varying phases between the two inputs in isolated rabbit heart preparations. The effects of the input phase, nodal functional state, and input reference on the nodal conduction time, recovery time, and refractory periods were assessed with multifactorial ANOVAs. It was found that the phase of stimulation significantly affected nodal conduction time but not the refractory periods or the time constant of the recovery. Each input could show longer and shorter conduction time than the other depending on the stimulation phase, input reference, and coupling interval. These effects were similar for different nodal functional states. However, pacing and recording from the low crista resulted in similar conduction and refractory values than did pacing and recording from the low septum. Input summation did not increase the otherwise equal efficacy of individual input in activating the node. Nodal surface recordings confirmed this functional symmetry and equivalent efficacy of the inputs and showed that input effects were confined to the proximal node.

CONCLUSIONS

The two nodal inputs have equivalent functional properties and are equally effective in activating the rate-dependent portion of the node. Input interaction affects perinodal activation but not the rate-dependent nodal function.