摘要:

The upstrokes of monophasic action potentials (MAPs) recorded with an extracellular pressure electrode were characterized in isolated canine tissue preparations in vitro. The characteristics of the MAP upstroke were compared with those of the local action potential foot as well as with the characteristics of approaching electrical activation during uniform and asynchronous conduction. The upstroke of the MAP was exponential during uniform conduction. The time constant of rise of the MAP upstroke (TMAP) correlated with that of the action potential foot (Tf,o0): TMAP + 1.01 TfO,t + 0.50; r2 = .80. Furthermore, changes in Tf0Ot with alterations in cycle length were associated with similar changes in TMAP: Tfoot = 1.06 TMAP − 0. 1 1; r2 = .78. In addition, TMAP and Tfootboth deviated from exponential during asynchronous activation; the inflections that developed in the MAP upstroke correlated in time with intracellular action potential upstrokes that were asynchronous in onset in these tissues. Finally, the field of view of the MAP was determined and was found to be dependent in part on tissue architecture and the space constant. Specifically, the field of view of the MAP was found to be greater parallel compared with transverse to fiber orientation (6.02 + 1.74 vs 3.03 + 1.10 mm; p < .01). These data suggest that the MAP upstroke may be used to define and characterize local electrical activation. The relatively large field of view of the MAP suggests that this technique may be a sensitive means to record focal membrane phenomena in vivo.

ISSN:0009-7322    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

Calcium-channel blockers are known to affect slow inward current in a frequencydependent fashion. The purpose of these experiments was to study use-dependent effects of verapamil, diltiazem, and nifedipine on atrioventricular conduction in vivo. Loading and maintenance infusion techniques were developed to study each drug at a series of stable plasma concentrations in autonomically blocked dogs anesthetized with morphine and a-chloralose. All three agents produced changes in atrioventricular conduction and refractoriness that increased with increasing stimulation frequency. The time dependence of drug-induced changes in atrioventricular conduction was characterized both by varying the coupling of single test stimuli and by abruptly changing activation frequency. The time constants for onset Of (T0n) and recovery from (T0ff) block were typical for each drug, with nifedipine having a faster time constant (T0ff = 0.36 + 0.12 sec) than verapamil (toff = 3.2 + 1.0 sec, ton = 28 ± 8 sec) or diltiazem(Tcff = 2.7 + 1 2sec, Ton = 13 ± 4 sec). The time constants for each drug were independent of concentration but the magnitude of time-dependent change increased with increasing drug concentration. We conclude that calcium-channel blockers have important frequency-dependent effects on atrioventricular conduction in vivo. This frequency dependence may result in selective depression of atrioventricular conduction in the presence of supraventricular tachyarrhythmias, with important potential implications for the clinical use of these agents.

ISSN:0009-7322    

出版商:OVID    

年代:1986

数据来源: OVID

摘要:

We studied epinephrine-induced delayed afterdepolarizations and triggered activity in atrial fibers from the canine coronary sinus to determine whether their responses to cardiac pacing would aid in formulating a uniform set of guidelines for differentiating this triggered activity from other arrhythmogenic mechanisms. We used standard microelectrode techniques and compared the delayed afterdepolarizations and triggered activity with those occurring in ouabain-superfused Purkinje fibers. Like Purkinje fibers, the frequency of triggering in the coronary sinus and the coupling interval of the first triggered beat were related directly to the basic drive cycle length, and the delayed afterdepolarization amplitude and frequency of triggering were related to the coupling interval of premature stimuli (S2). However, unlike Purkinje fibers, the coupling interval of the delayed afterdepolarization and of the first triggered beat were independent of the S2. Once initiated, triggered activity in the coronary sinus followed one of four rhythm patterns: in all four, the minimum and equilibrium cycle lengths were independent of the initiating cycle length. Triggered activity was terminated by overdrive and S2 pacing, especially by long episodes of overdrive at short cycle length. The first escape beat after overdrive was linearly related to the overdrive cycle length, resulting in overdrive acceleration. The return cycle length after S2 was linearly related to the S2 coupling interval. Because delayed afterdepolarizations and triggered activity in the coronary sinus respond differently to pacing from those in ouabain-superfused Purkinje fibers, triggered activity in general may not be identified by a uniform set of guidelines.

ISSN:0009-7322    

出版商:OVID    

年代:1986

数据来源: OVID

ISSN:0009-7322    

出版商:OVID    

年代:1986

数据来源: OVID