Cardiac excitability, the electrophysiologic matrix and electrically induced ventricular arrhythmias: order and reproducibility in seeming electrophysiologic chaos

Temporal signals drive the emergence of multicellular information networks

Coordinated responses to environmental stimuli are critical for multicellular organisms. To overcome the obstacles of cell-to-cell heterogeneity and noisy signaling dynamics within individual cells, cells must effectively exchange information with peers. However, the dynamics and mechanisms of collective information transfer driven by external signals are poorly understood. Here we investigate the calcium dynamics of neuronal cells that form confluent monolayers and respond to cyclic ATP stimuli in microfluidic devices. Using Granger inference to reconstruct the underlying causal relations between the cells, we find that the cells self-organize into spatially decentralized and temporally stationary networks to support information transfer via gap junction channels. The connectivity of the causal networks depends on the temporal profile of the external stimuli, where short periods, or long periods with small duty fractions, lead to reduced connectivity and fractured network topology. We build a theoretical model based on communicating excitable units that reproduces our observations. The model further predicts that connectivity of the causal network is maximal at an optimal communication strength, which is confirmed by the experiments. Together, our results show that information transfer between neuronal cells is externally regulated by the temporal profile of the stimuli and internally regulated by cell-cell communication.

Flecainide and the electrophysiologic matrix: the effects of flecainide acetate on the determinants of cardiac excitability in sheep Purkinje fibers

Flecainide was associated with excess mortality distributed virtually equally throughout the period of the Cardiac Arrhythmia Suppression Trial, suggesting the intersection of two events, drug effect and perhaps ischemia. Flecainide's effect on active properties has been studied extensively, but nothing is known of its effects on passive properties or on the balance among active and passive cellular properties that determines cardiac excitability. The multiple microelectrode method of intracellular current application and transmembrane voltage recording was used in sheep Purkinje fibers to determines strength- and charge-duration as well as constant current-voltage relationships and to estimate active properties, liminal length, and cable properties at a normal [K+]o and in a setting of hyperkalemia analogous to that of ischemia. A computer tracked in time the alterations in the active and passive properties relevant to excitability. Flecainide slightly decreased excitability at a normal [K+]o, primarily by depressing the sodium system with some contributory effect of passive properties. At high [K+]o, flecainide caused a frequency-dependent decrease in excitability and conduction, the latter best interpreted as a failure of the fiber to attain the liminal length requirements to produce a local action potential due primarily to an effect on sodium conductance. Together, the observations suggest that the action potential is the local phenomenon and that the propagated event is the sequential fulfillment of liminal length requirements. The data were interpreted in terms of the electrophysiologic matrix first proposed in detail in this Journal, which indicated that the electrophysiologic universe moved as a system in response to the drug and a change in [K+]o, the presumed antiarrhythmic and proarrhythmic electrophysiologic matrices for flecainide were quite similar, and the matrical configuration shared characteristics with the matrices of other drugs with known proarrhythmic potential.

Long-term reproducibility of ventricular tachycardia induction in patients with implantable cardioverter/defibrillators. Serial noninvasive studies

BACKGROUND

Noninvasive electrophysiological studies (EPSs) can be performed in current implantable antitachycardia pacemaker/cardioverter/defibrillators (ICDs). Thus, these devices may be used as tools to study changes in the electrophysiological substrate and ventricular tachycardia characteristics over time.

METHODS AND RESULTS

Fifty-five patients receiving an ICD for treatment of sustained ventricular tachyarrhythmias underwent serial EPSs after implantation of the ICD. Studies were performed before hospital discharge and 1, 3, 5, 9, 12, 18, 24, and 36 months after ICD implantation. Sustained monomorphic ventricular tachycardia (VT) was induced in 37 patients (group 1) at the predischarge EPS, whereas no sustained arrhythmia could be induced in 18 patients (group 2) at baseline. Group 1 patients underwent 165 noninvasive EPSs after discharge. Sustained monomorphic VT was induced during 72% of the follow-up EPSs, ventricular fibrillation (VF) was induced during 11% of follow-up EPSs, and no sustained VT or VF was induced during 17% of follow-up visits. Sustained VT was induced at every follow-up EPS in 23 patients (62%), whereas no sustained VT/VF could be induced at least once during follow-up in 14 patients (38%). Clinical or electrophysiological variables did not predict noninducibility during follow-up. However, the probability that a patient would experience spontaneous VT decreased significantly over time in patients in whom VT was not inducible during at least 1 follow-up EPS (P = .05). Group 2 patients underwent 86 noninvasive EPSs after discharge. Sustained monomorphic VT was induced during 22% of follow-up EPSs, VF was induced during 19% of follow-up EPSs, and no sustained VT/VF could be induced during 68% of follow-up EPSs. No sustained VT/VF could be induced during every follow-up EPS in 9 patients (50%), whereas sustained monomorphic VT was induced at least once during follow-up in 7 patients (34%). Persistent noninducibility of VT during follow-up was associated with low probability of occurrence of spontaneous VT (11%), whereas inducibility of VT at least once during follow-up was associated with the occurrence of spontaneous VT (89%, P = .003).

CONCLUSIONS

Considerable variability of VT induction is observed over a lengthy period in patients presenting with sustained VT/VF. Persistent noninducibility of VT is associated with a reduced probability of spontaneous VT. These observations suggest that the substrates for inducible and spontaneous VT change in parallel over time.

Action potential alternans and irregular dynamics in quinidine-intoxicated ventricular muscle cells. Implications for ventricular proarrhythmia

BACKGROUND

Cardiac cells display rate-dependent beat-to-beat variations in action-potential duration (APD), action potential amplitude (APA), and excitability during periodic stimulation. We hypothesized that quinidine causes a marked increase in the variability of APD, APA, and excitability of ventricular cells isolated from quinidine-toxic, arrhythmic ventricles.

METHODS AND RESULTS

Action potentials were recorded from right ventricular endocardial tissues (2 x 1 cm, < 2 mm thick) isolated from dogs in which ventricular tachycardia and ventricular fibrillation (VT/VF) were induced with intravenous quinidine (80-100 mg/kg) over a 5-hour period in vivo (n = 7). As the basic cycle length (BCL) of stimulation was progressively shortened, rate-dependent variations in APD and APA occurred. The initial dynamic change was alternans of APD and APA that could be either in or out of phase between two cells. The magnitude of alternans was a function of the BCL and the strength of the stimulation current. At critically short BCLs, irregular APD and APA behavior emerged in the quinidine-intoxicated cells. In control cells (n = 16) isolated from three nontreated dogs, APD and APA remained constant at all BCLs tested (2,000-300 msec). Quinidine increased the slope of the APD restitution curve compared with control. The observed quinidine APD restitution curve was fitted with a biexponential equation, and computer simulation using the fitted restitution curve reproduced the aperiodic APD seen in the quinidine toxic cells during periodic stimulation. Thus, the observed irregular APD behavior was predictable from the restitution curve.

CONCLUSIONS

Quinidine toxicity increases the temporal and spatial variability of APD and APA in the ventricle that may promote the initiation of reentrant VT/VF in vivo. The slope of the APD restitution curve provides a method to quantitate inhomogeneities in repolarization time and could be a useful marker for proarrhythmia.

Phase plane plot of electrograms as a marker of ventricular electrical instability during acute ischemia: initial experimental results and potential clinical applications

We used a phase plane plots method (a plot of voltage [V(t)] vs rate of change of voltage [dV/dt]) derived from nonlinear dynamics to transform a sinus rhythm strip into a geometric form. The geometric form thus obtained detects variations in the ECG over time and therefore may aid in identifying patterns of vulnerability to ventricular fibrillation (VF). The trajectories of consecutive ventricular depolarizations and repolarizations of sinus rhythm in dogs that evolve to VF showed characteristic differential band thickening (inhomogeneities) that were absent in the dogs that did not develop VF. This method of analysis can be a useful complementary tool to detect cardiac electrical instability, and with simple hardware, can easily be displayed in real time.

Reproducibility of Successful Drug Trials in Patients With Inducible Sustained Ventricular Tachycardia

Patients whose inducible sustained ventricular tachycardia is suppressed during serial electrophysiological testing have a small but gradually increasing actuarial incidence of recurrent arrhythmias despite therapy with the "successful" drug. In an effort to improve the predictive value of a drug response, in 1990 we began to require that our full stimulation protocol be repeated successfully several times before considering a drug to be effective. In 23 consecutive patients who had inducible sustained ventricular tachycardia which was suppressed by at least one drug during invasive serial drug testing using a standard stimulation protocol, the identical stimulation protocol was performed six times during therapy with the initially successful drug (three trials on Day 1 and three trials on Day 2). Repeat trials were completed (i.e., either all six trials were successfully finished or sustained tachycardia was induced) for 29 initially successful drugs in these 23 patients. With 18 of these 29 initially successful drugs (62%), sustained ventricular tachycardia was eventually induced during repeat trials. The eventual drug failures could not be correlated with specific drugs tested, subtherapeutic or falling serum drug levels, marked fluctuations in autonomic tone, or changes in anatomic substrate. The proportion of patients failing each repeat trial was relatively constant: 4/29 (14%) failed Trial 2, 2/25 (8%) failed Trial 3, 7/23 (30%) failed Trial 4, 2/16 (13%) failed Trial 5, and 3/14 (21%) failed Trial 6. The increase in the cumulative incidence of drug failure during repeat trials was nearly linear. Inducibility of ventricular tachycardia appears to be a probability function; a successful drug study should not be regarded as an absolute phenomenon.