Effects of phenytoin on refractoriness and conduction in the human heart

Management of COVID-19 in people with epilepsy: drug considerations

People with epilepsy (PWE) are neither more likely to be infected by the coronavirus nor are they more likely to have severe COVID-19 manifestations because they suffer from epilepsy. However, management of COVID-19 in PWE may be more complicated than that in other individuals. Drug-drug interactions could pose significant challenges and cardiac, hepatic, or renal problems, which may happen in patients with severe COVID-19, may require adjustment to antiepileptic drugs (AEDs). In this review, we first summarize the potential drug-drug interactions between AEDs and drugs currently used in the management of COVID-19. We then summarize other challenging issues that may happen in PWE, who have COVID-19 and are receiving treatment.

Fosphenytoin may cause hemodynamically unstable bradydysrhythmias

The prodrug fosphenytoin (FOS) was recently introduced as an alternative to phenytoin (PTN) and has since become a first line therapy for status epilepticus. Unlike PTN, FOS generally has been considered to be safe from cardiac toxicity. To better characterize cardiac toxicity associated with FOS administration, we performed a review of the Food and Drug Administration's Adverse Event Reporting System databank for reports of possible FOS toxicity from 1997-2002. There were 29 applicable reports of adverse cardiac events likely related to FOS infusion, including 10 cardiac deaths. Among survivors, there were four cases of high-grade atrioventricular block, and five cases of transient sinus arrest. Our data suggest that FOS may produce more cardiac toxicity than previously thought. Clinicians should consider administering intravenous FOS in a monitored setting for selected high-risk patients.

Effects of cardioactive medications on retrograde conduction: Continuing relevance for current devices

INTRODUCTION

Retroconduction (ventriculo-atrial conduction) remains a problem for patients with implanted cardiac rhythm devices. Pacemaker algorithms can detect and terminate endless loop tachycardia (ELT), but actual prevention of ELT may require anti-arrhythmic drugs (AADs). Similarly, AADs can affect ICD rhythm discrimination algorithms that depend on atrio-ventricular ratios. There is concern whether these drugs remain effective during stress situations.

METHODS

Electrophysiologic studies that included retroconduction testing using slow ramp pacing were done in 1332 patients. The presence or absence of retroconduction at baseline and with drug was recorded, as was the rate at block. As a stress surrogate, isoproterenol was used to test retroconduction and reversal of drug-induced block.

RESULTS

Procainamide, mexiletine, phenytoin, disopyramide, quinidine, beta-blockers, encainide, and amiodarone caused complete retrograde block or decreased the rate at which block occurred (mean 76% of patients, p < 0.008), whereas digoxin, lidocaine, diltiazem, and verapamil did not. Isoproterenol (in the absence of AADs) increased the rate at block in 82% of 404 patients with retroconduction at baseline (p < 0.005). Of 319 patients without retroconduction at baseline, 134 (42%) developed retroconduction after isoproterenol. Isoproterenol reversed retrograde block in 39% of patients with block on an AAD. Amiodarone, digoxin, and the combination of digoxin plus a beta-blocker were most effective at resisting this reversal of ventriculo-atrial block (80%, 68%, and 75% respectively).

CONCLUSION

Most of the AADs reviewed increase the cycle length at block or abolish retroconduction, while isoproterenol has the opposite effect. Anti-arrhythmic medications, particularly amiodarone, digoxin, and the combination of digoxin plus a beta-blocker may be considered for a patient with multiple ELT episodes or certain ICD detection problems.

Shortening of the intraventricular conduction time in premature ventricular beats during ventricular extrastimulation: possible role of dimensional shortening of the conducting distance during ventricular systole

INTRODUCTION

Conduction time (CT) is given by the formula: conducting distance divided by conduction velocity. Based on this formula, we hypothesized that CT shortening (i.e., supernormal conduction) may result from dimensional shortening of the distance of impulse propagation, which naturally occurs during ventricular systole.

METHODS AND RESULTS

To test the above, two separate groups of patients were studied, group A (14 patients) for electrophysiologic study and group B (12 patients) for echocardiographic study. In group A patients, CT from the stimulus artifact to the basal lateral wall of the left ventricle (LV) (S-LV interval) was measured using right ventricular (RV) apical extrastimulus testing. S-LV interval shortening in premature RV beats was demonstrated in all 14 patients. The maximum shortening was 20 +/- 9 msec (range 10 to 40), and the maximum % shortening was 16% +/- 6% (7% to 27%). In group B patients with implanted pacemakers, the major (long) and minor (short) axis dimensions of the LV were measured with echocardiography. The major axis dimension was used as an approximate measure of the linear length from the RV apex to the basal lateral wall of LV. The maximum % shortening of the major axis dimensions was 15% +/- 4%, 16% +/- 2%, and 11% +/- 4% during VVI pacing, respectively, at paced cycle lengths of 1,000 (11 patients), 800 (5 patients), and 600 msec (12 patients). The maximum % shortening of the S-LV intervals was comparable in magnitude with that of the major axis dimensions: 20% versus 15% +/- 4%, 15% +/- 7% versus 16% +/- 2% and 16% +/- 6% versus 11% +/- 4%, respectively, at paced cycle lengths of 1,000, 800, and 600 msec. There was also a good temporal correlation between the electrophysiologic (CT shortening) versus echocardiographic (dimensional shortening) parameters. Thus, the intraventricular CT and the major axis dimension of the LV were shortened in a similar magnitude and also at a similar timing in the cardiac cycle.

CONCLUSION

These findings suggest the possibility that supernormal conduction may result, at least in part, from dimensional shortening of the pathway length of impulse propagation from the stimulating to recording electrodes, which naturally occurs during ventricular systole.

Effect of amiodarone on retrograde conduction and refractoriness of the His-Purkinje system in man

The effects of long term treatment with oral amiodarone on retrograde conduction ( S2H2 interval) and refractoriness of the His-Purkinje system were studied in 11 patients using His bundle electrograms and the ventricular extrastimulus method. Ten patients had ventricular tachycardia and one supraventricular tachycardia. Electrophysiological studies were carried out before and after the patients had been taking their maintenance dose for a mean duration of 84 days. After amiodarone treatment the HV interval was prolonged in seven patients and unchanged in four. At comparable S1S2 intervals, the S2H2 intervals were longer after treatment with amiodarone in all patients than before. Similarly, the longest S2H2 intervals achieved after amiodarone were longer than the control values. Amiodarone significantly increased the relative, effective, and functional refractory periods of the His-Purkinje system. Thus amiodarone exerts important effects on the His-Purkinje system.