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Biomedical subjects

T Mazgalev

Publications and source records attributed to T Mazgalev.

At least 37 records · Page 2Linked to original sources
PubMed1986

Phasic effects of postganglionic vagal stimulation on atrioventricular nodal conduction.

The effects of postganglionic vagal stimulation (PGVS) on atrioventricular nodal conduction were studied in 15 rabbit atrial-atrioventricular nodal preparations. PGVS was introduced, and sinus cycle length was scanned as independent bursts of subthreshold stimuli were produced in the sinus node and atrioventricular node (AVN). Changes in conduction of atrial impulses to the bundle of His were studied under the following experimental conditions: changes in sinus cycle length resulting from vagal influence on the sinus node, direct vagal stimulation exclusively to the AVN, and during both simultaneous or nonsimultaneous vagal stimulation to sinus node and AVN. The results of the present study showed that the direct effect of PGVS on AVN conduction time at a constant sinus cycle length is phase dependent with maximal prolongation achieved in the first or second beat after introduction of the burst. The interval between the onset of PGVS producing maximal prolongation of conduction time and the following atrial beat was designated the "optimal effective phase." It was shown that the optimal effective phase was a constant parameter for a given preparation and in the present experiments was 321 +/- 16 ms. However, when PGVS was introduced in combination to both nodes while scanning the cycle length, AVN conduction was variable, reflecting both the direct effects of PGVS on the AVN as well as the indirect effects resulting from changes in the sinus cycle length. Notably, it was found that simultaneous PGVS to both the sinus node and AVN usually diminished, whereas appropriate nonsimultaneous PGVS accentuated the typical phasic dependency of AVN conduction time. Additionally, vagally induced prolongation of the sinus cycle length was found to be accompanied by changes in the time of depolarization of the inputs to the AVN, thus influencing AVN conduction and facilitating reentry. These interactions between changes in the sinus cycle length and concomitant changes in the effectiveness of vagal influence on the AVN can be used to explain complexities of AVN conduction during increased vagal activity.

Animals↗
PubMed1986

Vagally induced hyperpolarization in atrioventricular node.

The effects of postganglionic vagal stimulation on atrioventricular nodal conduction were studied in 12 rabbit atrial-atrioventricular nodal preparations. Vagal stimulation was introduced in the sinus and atrioventricular nodes, separately or in combination, using single bursts of subthreshold stimuli. The sinus cycle length was scanned to identify the phasic effect of vagal stimulation. Action potentials from cells in the AN, N, and NH regions of the atrioventricular node were recorded by microelectrode techniques. Vagally induced hyperpolarization of cells in the atrioventricular node resulted in a phase-dependent prolongation of conduction time and reflected the level of residual hyperpolarization at the moment of arrival of the next atrial beat at the atrioventricular nodal input region. Vagally induced hyperpolarization was membrane potential dependent, although its overall time course was similar at different phases. Increased diastolic depolarization followed the maximal hyperpolarization. This "rebound" observed at certain phases was responsible for paradoxical shortening of the conduction time after vagal stimulation. The predominant effects of local vagal stimulation in the atrioventricular node were observed in cells in or near the N region. Slower rate of rise, shorter amplitude and duration, as well as step formations were among the changes in action potentials recorded from these cells. The effects of vagal stimulation were inhomogeneous between different regions of the atrioventricular node as well as within the N region, producing alternative pathways of conduction and the potential for reentry. The concomitant changes in sinus cycle length resulting from vagal stimulation in the sinus node region altered the phasic effects of vagal stimulation introduced in the atrioventricular node. This was related to a direct influence of the prolonged sinus cycle length on atrioventricular nodal refractoriness as well as an indirect effect on the degree of residual vagally induced hyperpolarization at the moment of arrival of the delayed atrial beat. These findings provide mechanistic explanations for the complex effects of vagal stimulation on atrioventricular nodal conduction.

Animals↗
PubMed1986

Effect of postganglionic vagal stimulation on the organization of atrioventricular nodal conduction in isolated rabbit heart tissue.

Postganglionic stimulation of vagal terminals (PGVS) in the isolated rabbit heart atrioventricular (AV) node was used to study the effects of cholinergic influence on AV nodal conduction. Standard microelectrode techniques were used to record action potentials, predominantly from cells located in the N region of the AV node. In addition, programmed stimulation was used in conjunction with PGVS to initiate or terminate AVN reentry. The introduction of a single short burst of PGVS (total duration 50 to 100 msec, impulse duration 1 msec, and interimpulse interval 6 msec) with subthreshold amplitude for AV node fibers caused reproducible disorganization of the prevailing excitation front. This was manifest as local nonuniform depression of conduction, hump formations in the action potentials, and alteration in the sequence of depolarization. The introduction of repetitive bursts of PGVS revealed a triphasic time course of changes in AV nodal conduction time, representing initial maximal prolongation, relative shortening, and secondary inhibition. It was found that these phases corresponded to vagally induced initial disorganization and a subsequent rebound process. Vagally induced disorganization of the sequence of action potential depolarization was also a triggering mechanism for concealed as well as manifest AV nodal reentry. In the latter case the reentry circuit usually involved the AN region and perinodal atrial tissue. PGVS-induced depression of the N region was also able to block the retrograde wavefront, thereby terminating reentry. The possible relationship of PGVS-induced disorganization of conduction and the inhomogeneous structure of AV node are discussed. The present results provide additional information for better understanding of the AV nodal conduction abnormalities observed clinically and particularly those related to AV node-vagus interaction.

Action Potentials↗
PubMed1985

Changes in the conductivity and in the functional refractory period of the atrioventricular node during hypoxia.

The experiments were carried out on preparations from rabbit right atrium, containing the sinoatrial node, the interatrial barrier, the atrioventricular node and the His-bundle. Surface electrodes were used to record the atrial and His-electrograms. The conduction time (AH-interval) was measured and the functional refractory period of the node was determined during normoxia (95% O2, 5% CO2) and hypoxia (30% O2, 65% N2 and 5% CO2) for 45 min. Hypoxia causes a considerable delay in conduction. This delay is greater for the shorter coupling-intervals and at the higher frequencies of the imposed rhythm. The shifting of the conduction times towards the higher values during hypoxia is not parallel compared with hypoxia. In the case of hypoxia the refractory curve of the A-V node is situated higher than the curve during normoxia, owing to the greater duration of the H1H2 intervals. The functional refractory period is increased and shifted toward the longer coupling-intervals. The results obtained show that during hypoxia the functional refractory properties of the atrioventricular node are impaired, which, combined with the delayed conduction through it, creates conditions for serious disturbances in the function of the atrioventricular conduction system.

Animals↗
PubMed1985

Interaction of the input atrial excitatory waves and conduction through the atrioventricular node. I. Changes in the cellular electrical activity.

Extrasystoles were applied only at the posterior input, only at the anterior input and jointly at both AVN-inputs in preparations from rabbit right atrium, after 10 basic beats. The effect of the interaction of the input excitatory waves was evaluated by the changes in the cellular electrical activity and in the output H1H2-interval. It has been demonstrated that both the summation effects (shortening of the H1H2-interval) and the inhibition effects (prolongation of the H1H2-interval) are accompanied by substantial changes in the spatial-temporal organization of conduction within the node. The summation of excitatory waves leads to higher velocity of AP increase (most frequently in the N-zone), to changes in the shape of AP as a result of electrotonic influences, to changes in the sequence of activation of cellular group forming different conduction pathways. The inhibition of the excitatory waves is related to the increase in the inhomogeneity of conduction, manifested in the formation of competitive conduction pathways. In addition to the changes in the AP-front, a typical characteristic is the hill-structure reflecting the effect of the excitatory fronts. The formation of conduction pathways under the effect of each of the input excitatory waves probably takes place on the background (spatial and temporal) of the disappearing refractoriness, related to another excitatory wave. The interaction between the input excitatory waves, taking places through the proposed model, could also be present in a normally functioning heart provided the phasic correlations between the moments of activation of the posterior and anterior inputs are optimal for such an interaction.

Action Potentials↗
PubMed1985

Filtering role of the atrioventricular node in atrial fibrillations.

Experiments on rabbit heart preparations were carried out with the aim of studying the causal relation between the signals at the input and at the output of the atrioventricular node (AVN) during atrial fibrillations (AF). Electrograms were recorded simultaneously from the posterior and anterior inputs and from the output of AVN, together with cellular action potentials from one or two structures of the node. AVN is shown to play the role of a filtering unit for the excitatory waves passing from the atria to the ventricles during AF. The random and fragmentary character of the atrial excitatory front during AF causes summation of excitatory waves, local and manifested re-entry, and conduction along competitive pathways. These processes accompany both the successful conduction through AVN and the blocking of the excitation at different levels in the node. The filtration process takes place mainly in the AN- and N-zones of the node. In the NH-zone conduction does not differ compared to that in the case of spontaneous rhythm. The results obtained show that conduction through AVN during AF does not take place through triggering of specific mechanisms, but through a complex combination of the mechanisms characterizing the deteriorated conduction under extreme conditions.

Animals↗
PubMed1985

Interaction between atrial excitation waves and conduction in the atrioventricular node. II. Differences in the conduction of basic rhythms with different phasic correlation between the posterior and anterior inputs.

During in vitro experiments on the right atrium of rabbit heart microelectrodes were used to record simultaneously action potentials from different cells in the atrioventricular node (AVN) under stationary basic rhythms, imposed separately or jointly at the posterior and anterior inputs. A comparison was made of the pattern of conduction in the three different phase correlations between the input excitatory waves. Causal relation is found between the changes in the phase correlation and the conduction pattern with unchanged duration of the basic cycle. The formation of more than one excitatory front in AVN has no priority connection with the anterior input. Although stimulation at the posterior input is connected with more homogeneous conduction, even then the formation and interaction of two excitatory fronts is possible; on the other hand, during independent stimulation of the anterior input the formation of a second excitatory wave is possible, which, upon entering the node posteriorly, may have a dominating influence on conduction. A special case is the differentiation of an excitatory front which enters AVN only through one of the inputs. For example, upon anterior stimulation it is possible to induce one direct excitatory front to the N-zone and a second one which first activates the posterior AN-zone and then turns antegradely. The interaction of these two fronts creates different patterns concerning the characteristics of the action potentials and the organization of conduction, being related to the complex spatial inhomogeneous structure of excitable elements in different refractory phases.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Effects of the site and timing of atrioventricular nodal input on atrioventricular conduction in the isolated perfused rabbit heart.

Programmed stimulation was used to study patterns of atrioventricular nodal propagation in an isolated rabbit heart preparation. Stimulation was done at the two major atrioventricular nodal input regions, the crista terminalis and interatrial septum, by use of various sequencing protocols. The influence of stimulation input interactions on atrioventricular nodal propagation was then evaluated with the use of simultaneous extracellular and intracellular recordings from the various regions of the atrioventricular node and His bundle. Activation of the atrioventricular node occurred predominantly via the stimulated input site, although perinodal conduction to the opposite input also occurred and could modify the atrioventricular conduction pattern. Engagement patterns of both AN and N fibers were dependent on the sequence of activation at the major input sites, and similar conduction times to the His bundle were often associated with different local activation times, depending on the site of premature stimulation. Conversely, if premature stimulation was timed to produce constant local activation times but the input site varied, then the time to His bundle activation could also vary. Atrioventricular nodal functional and effective refractory periods and conduction patterns were dependent on both the timing and pattern of engagement at the principal inputs to the atrioventricular node as well as the subsequent organization of activation of the various regions of the atrioventricular node. Furthermore, depending on the sequence of stimulation at the crista terminalis and interatrial septum, activation patterns were either organized or inhomogeneous in the AN and N regions of the atrioventricular node, consequently influencing the H-H interval, and in some instances resulting in conduction block to the His bundle. It is concluded that the relative timing of activation at the principal input regions of the atrioventricular node is critical to patterns of atrioventricular nodal propagation and subsequent conduction to the His bundle. Our results emphasize the complexity of atrioventricular nodal propagation during premature stimulation.

Animals↗
PubMed1984

The role of the phase relation between the atrial excitatory waves in determining the functional refractory properties of the atrioventricular node.

Through programme stimulation of the two basic inputs of the atrioventricular node (AVN)--crista terminalis and interatrial septum--different spatial and temporal relations between the excitatory waves passing from the atrium into the node were modelled in intact rabbit heart preparation. The essential role of the spatial-temporal organization for the determination of the refractory properties is demonstrated. The functional refractory curves depend not only on the basic rhythm, but also on the direction of the atrial excitation front. It is shown that interaction of the input waves is possible in intact preparation as well. At optimum phase regimes combined stimulation of the two inputs leads to prolongation or shortening of the initial H1H2 interval. The qualitative characteristic of the observed effects is very varied. It is defined predominantly by the stimulation programme, but it does not depend on which of the two inputs the basic rhythm is applied. The view is expressed that in order to obtain a more accurate characterization of the AVN-conduction, it is necessary to consider the interaction of the atrial excitatory waves arriving at the posterior and anterior inputs of AVN and the reflection of this interaction on its refractory properties.

Animals↗
PubMed1984

Comparison of the posterior and anterior activation of the atrioventricular node in rabbit heart with identical prematurity--microelectrode investigation.

The present work evaluates the input prematurity of AVN-activation as a determinant atrioventricular condition. The cellular organization of conduction is studied by consecutive stimulation of the posterior input (crista terminalis) and the anterior input (interatrial septum) with identical prematurity. The results obtained demonstrate the existence of essential differences in conduction upon posterior and anterior stimulation. Above all, separate activation of the two inputs leads to different timing of the cell excitation from the adjacent AN-zones; different sections of the AN-area may prove to be excluded from the conduction process to the His-bundle. Moreover, upon changing the direction of the excitatory wave (even if the prematurity of the extrastimulus is preserved unchanged), temporal and spatial reorganization occurs in the N-zone of the node, the concurrent changes in the shape of the action potentials and the intervals between them. The activation of only one AVN input (by electrical stimulation) could cause passive or active excitation of the other. In the first case the passively activated input may participate in the retrograde conduction of the excitation to the atrium (re-entry); in the second case, under definite conditions, the two antegrade input waves may interact within the AVN. Knowledge only of the absolute values of the stimulation prematurity is not sufficient for description of the conduction through AVN.

Animals↗
PubMed1983

Inert characteristic and functional refractory period of the atrioventricular node in rabbit heart preparation.

The effect of heart rate on the functional refractory period (FRP) of the atrioventricular node (AVN) is studied. A conclusion is reached that FRP, being a complex characteristic of AVN, should not be directly related to conduction time, but it can serve as a measure of the maximum moment acceleration which AVN is capable of transmitting to the lower-lying structures. It is proposed to use the coefficient of relative acceleration for quantitative estimation of the inert (dynamic) properties of the node at different heart rates. It is concluded that for a longer basic cycle length (BCL) AVN is characterized by greater reserve possibilities for transformation of accelerations of the input rhythm.

Animals↗
PubMed1982

Atrioventricular nodal conduction during atrial fibrillation in rabbit heart.

Atrial fibrillation was induced in 15 superfused rabbit atrial-atrioventricular nodal preparations in which surface bipolar electrograms were recorded simultaneously from the crista terminalis, interatrial septum, and His bundle along with microelectrode action potentials from cells in the atrionodal (AN), nodal (N), and nodal-His (NH) regions of the atrioventricular node. Effective engagement of the atrioventricular node with propagation to the His bundle was critically dependent on the relative timing of activation at the crista terminalis and interatrial septal input regions of the atrioventricular node. Conduction through the AN and N regions appeared dependent on the relative timing of activation wave fronts emerging from the two input regions. Asynchronous engagement of AN and N regions resulted in both distortion of action potentials and concealed conduction, with delayed conduction and block to the NH region and His bundle. Successful engagement of the NH region always produced a 1:1 NH-to-His bundle relationship. It is concluded that during atrial fibrillation 1) activation of the AN region occurs as a result of the variable interaction of inputs from the crista terminalis and interatrial septum; 2) predictably, effective synchronous engagement of the AN and consequently the N region is responsible for conduction to the NH and His bundle regions; 3) conversely, asynchronous activation inputs from the crista terminalis and interatrial septum result in fragmented, asynchronous as well as concealed conduction within the AN and N regions with block in the atrioventricular node and variable conduction to the His bundle.

Action Potentials↗
PubMed1981

The mechanism of AV junctional reentry: role of the atrionodal junction.

Mapping of atrial-AV nodal (AVN) activation patterns was performed in 10 superfused rabbit AVN preparations utilizing 3 bipolar electrodes placed simultaneously at the crista terminalis (CT), interatrial septum (IAS) and His bundle (H) regions in close proximity to the AVN, along with two or three microelectrodes in the AN, N and NH regions of the AVN. Basic and premature stimuli were introduced at either or both the CT and IAS regions of the AVN. The timing of the premature stimuli was varied to induce close interaction of inputs to the AVN. Summation and/or cancellation, as evidenced by changes in the morphology of the action potentials and alteration of H1-H2 intervals, were observed. Apparent reentry phenomena could be induced or terminated by appropriately timed stimulation at the CT and IAS inputs. Reentry was critically related to both the patterns and timing of the spread and excitation within AVN and surrounding atrial tissue. Slow conduction and unidirectional block were observed in various portions of the reentry circuit. It is concluded that 1) interaction between input waves engaging the AVN produced either summation or fragmentation; 2) slow conduction and reentry resulted from a critical collision of wavefronts; 3) in this preparation, AVN reentry circuits always appeared to involve conduction through atrial tissue immediately surrounding AVN; and 4) remarkably, reentry confined to the intranodal region was never observed.

Action Potentials↗
PubMed1979

Influence of temperature on the atrioventricular conduction time for spontaneous and imposed rhythms.

In the present work data are obtained about the changes in the atrioventricular conduction time under the effect of two parameters: temperature and frequency of the sinus node. The experiments are carried out on preparations from the right atrium of rabbit heart. Temperature changes are obtained by regulation of the temperature of the nutrient medium in the 23--36 degrees C range. The excitation frequency of the sinus node was changed by imposition of artificial (external) rhythms. Data are obtained about the presence of frequency- and temperature-dependent components of the atrioventricular (AV) delay. Upon cooling the preparation the influence of the temperature factor is predominant and parallel with this the capacity for assimilating a higher imposed rhythm decreases. Considerations are expressed about the behaviour of the AV-conduction system at different temperatures under conditions of saccadic changes in the imposed rhythm.

Animals↗
PubMed1979

On the effect of preceding atrial- atrial and His-atrial intervals on the atrioventricular nodal conduction time in rabbit heart.

The study of conduction in the atrioventricular node is often associated with two characteristic time intervals. The first of these intervals reflects the time between two successive atrial excitatory complexes. The second interval reflects the time from the moment of the appearance of the excitation at the output of the atrioventricular node to the next atrial complex. In different publications the value of the conduction time is associated predominantly either with one (atrial-atrial) or with the other (His-atrial) preceding intervals. The present paper makes an attempt at comparative estimation of the effect of preceding atrial-atrial and His-atrial intervals on current conduction time. The experiments are carried out on rabbit heart preparation, using micro- and macroelectrode technique. Stimulation programmes guaranteeing the imposition of rhythm with fixed atrial-atrial and His-atrial intervals are used. It is concluded that there is no strictly determined connection between the value of any of the above intervals and atrioventricular conduction time.

Animals↗
PubMed1979

On the changes in the atrioventricular conduction time under the effect of acetylcholine.

The aim of the present investigation is to study the effect of acetylcholine on the atrioventricular (AV) conduction time in preparation from rabbit heart right atrium. The artrial electrogram and the action potentials of the His-cells are recorded by means of bipolar electrodes and microelectrodes. The effect of acetylcholine is assessed by the simultaneous changes in the period of the atrial complexes and in the atrioventricular conduction time. The significance of the prolonged period of the atrial complexes for the compensation of the acetylcholine effect on the node area of the conduction system is pointed out. A considerable increase in the refractory periods of the atrioventricular node is found.

Acetylcholine↗