Canadian Cardiovascular Society commentary on implantable cardioverter defibrillators in Canada: waiting times and access to care issues

The Canadian Cardiovascular Society is the national professional society for cardiovascular specialists and researchers in Canada. In the spring of 2004, the Canadian Cardiovascular Society Council formed an Access to Care Working Group in an effort to use the best science and information to establish reasonable triage categories and safe wait times for access to common cardiovascular services and procedures. The Working Group has elected to publish a series of commentaries to initiate a structured national discussion on this very important issue. Access to treatment with implantable cardioverter defibrillators is the subject of the present commentary. The prevalence pool of potentially eligible patients is discussed, along with access barriers, regional disparities and waiting times. A maximum recommended waiting time is proposed and the framework for a solution-oriented approach is presented.

Progression to chronic atrial fibrillation after pacing: the Canadian Trial of Physiologic Pacing. CTOPP Investigators

OBJECTIVES

This study examined the effect of physiologic pacing on the development of chronic atrial fibrillation (CAF) in the Canadian Trial Of Physiologic Pacing (CTOPP).

BACKGROUND

The role of physiologic pacing to prevent CAF remains unclear. Small randomized studies have suggested a benefit for patients with sick sinus syndrome. No data from a large randomized trial are available.

METHODS

The CTOPP randomized patients undergoing first pacemaker implant to ventricular-based or physiologic pacing (AAI or DDD). Patients who were prospectively found to have persistent atrial fibrillation (AF) lasting greater than or equal to one week were defined as having CAF. Kaplan-Meier plots for the development of CAF were compared by log-rank test. The effect of baseline variables on the benefit of physiologic pacing was evaluated by Cox proportional hazards modeling.

RESULTS

Physiologic pacing reduced the development of CAF by 27.1%, from 3.84% per year to 2.8% per year (p = 0.016). Three clinical factors predicted the development of CAF: age > or =74 years (p = 0.057), sinoatrial (SA) node disease (p < 0.001) and prior AF (p < 0.001). Subgroup analysis demonstrated a trend for patients with no history of myocardial infarction or coronary disease (p = 0.09) as well as apparently normal left ventricular function (p = 0.11) to derive greatest benefit.

CONCLUSIONS

Physiologic pacing reduces the annual rate of development of chronic AF in patients undergoing first pacemaker implant. Age > or =74 years, SA node disease and prior AF predicted the development of CAF. Patients with structurally normal hearts appear to derive greatest benefits.

New-Onset Atrial Fibrillation

Background —Although sex differences in coronary artery disease have received considerable attention, few studies have dealt with sex differences in the most common sustained cardiac arrhythmia, atrial fibrillation (AF). Differences in presentation and clinical course may dictate different approaches to detection and management. We sought to examine sex-related differences in presentation, treatment, and outcome in patients presenting with new-onset AF.

Methods and Results —The Canadian Registry of Atrial Fibrillation (CARAF) enrolled subjects at the time of first ECG-confirmed diagnosis of AF. Participants were followed at 3 months, at 1 year, and annually thereafter. Treatment was at the discretion of the patients’ physicians and was not directed by CARAF investigators. Baseline and follow-up data collection included a detailed medical history, clinical, ECG, and echocardiographic measures, medication history, and therapeutic interventions. Three hundred thirty-nine women and 560 men were followed for 4.14±1.39 years. Compared with men, women were older at the time of presentation, more likely to seek medical advice because of symptoms, and experienced significantly higher heart rates during AF. Compared with older men, older women were half as likely to receive warfarin and twice as likely to receive acetylsalicylic acid. Compared with men on warfarin, women on warfarin were 3.35 times more likely to experience a major bleed.

Conclusions —Anticoagulants are underused in older women with AF relative to older men with AF, despite comparable risk profiles. Women receiving warfarin have a significantly higher risk of major bleeding, suggesting the need for careful monitoring of anticoagulant intensity in women.

Circadian variation of paroxysmal atrial fibrillation. PA3 Investigators. Atrial Pacing Peri-ablation for Prevention of Atrial Fibrillation Trial

The circadian variation of paroxysmal atrial fibrillation (AF) was studied in 67 patients who received a dual-chamber pacemaker 3 months before a planned atrioventricular node ablation. A distinct circadian variation of AF was observed with 2 time peaks in initiation (1 in the early morning and 1 in the early evening hours), which was modulated by atrial pacing, the duration of AF, and the use of beta-adrenergic blocking agents.

Randomized crossover comparison of DDDR versus VDD pacing after atrioventricular junction ablation for prevention of atrial fibrillation. The atrial pacing peri-ablation for paroxysmal atrial fibrillation (PA (3)) study investigators

BACKGROUND

Some clinical data suggest that atrial-based pacing prevents paroxysmal atrial fibrillation (AF). This study tested the hypothesis that DDDR pacing compared with VDD pacing prevents AF after atrioventricular (AV) junction ablation.

METHODS AND RESULTS

Patients were randomized to DDDR pacing (n=33) or to VDD pacing (n=34) after AV junction ablation and followed every 2 months for 6 months. Patients then crossed over to the alternate pacing mode and were followed for an additional 6 months. Primary analysis included the time to first recurrence of sustained AF (duration >5 minutes), total AF burden, and the development of permanent AF. The time to first episode of AF was similar in the DDDR group (0.37 days, 95% CI 0.1 to 1.3 days) and the VDD pacing group (0.5 days, 95% CI 0.2 to 1.7 days, P=NS). AF burden increased over time in both groups (P<0.01). At the 6-month follow-up, AF burden was 6.93 h/d (95% CI 4. 37 to 10.96 h/d) in the DDDR group and 6.30 h/d (95% CI 3.99 to 9.94 h/d) in the VDD group (P=NS). Twelve (35%) patients in the DDDR group and 11 (32%) patients in the VDD group had permanent AF within 6 months of ablation. Within 1 year of follow-up, 43% of patients had permanent AF.

CONCLUSIONS

DDDR pacing compared with VDD pacing does not prevent paroxysmal AF over the long term in patients in the absence of antiarrhythmic drug therapy after total AV junction ablation. Many patients have permanent AF within the first year after ablation.

Effects of physiologic pacing versus ventricular pacing on the risk of stroke and death due to cardiovascular causes. Canadian Trial of Physiologic Pacing Investigators

BACKGROUND

Evidence suggests that physiologic pacing (dual-chamber or atrial) may be superior to single-chamber (ventricular) pacing because it is associated with lower risks of atrial fibrillation, stroke, and death. These benefits have not been evaluated in a large, randomized, controlled trial.

METHODS

At 32 Canadian centers, patients without chronic atrial fibrillation who were scheduled for a first implantation of a pacemaker to treat symptomatic bradycardia were eligible for enrollment. We randomly assigned patients to receive either a ventricular pacemaker or a physiologic pacemaker and followed them for an average of three years. The primary outcome was stroke or death due to cardiovascular causes. Secondary outcomes were death from any cause, atrial fibrillation, and hospitalization for heart failure.

RESULTS

A total of 1474 patients were randomly assigned to receive a ventricular pacemaker and 1094 to receive a physiologic pacemaker. The annual rate of stroke or death due to cardiovascular causes was 5.5 percent with ventricular pacing, as compared with 4.9 percent with physiologic pacing (reduction in relative risk, 9.4 percent; 95 percent confidence interval, -10.5 to 25.7 percent [the negative value indicates an increase in risk]; P=0.33). The annual rate of atrial fibrillation was significantly lower among the patients in the physiologic-pacing group (5.3 percent) than among those in the ventricular-pacing group (6.6 percent), for a reduction in relative risk of 18.0 percent (95 percent confidence interval, 0.3 to 32.6 percent; P=0.05). The effect on the rate of atrial fibrillation was not apparent until two years after implantation. The observed annual rates of death from all causes and of hospitalization for heart failure were lower among the patients with a physiologic pacemaker than among those with a ventricular pacemaker, but not significantly so (annual rates of death, 6.6 percent with ventricular pacing and 6.3 percent with physiologic pacing; annual rates of hospitalization for heart failure, 3.5 percent and 3.1 percent, respectively). There were significantly more perioperative complications with physiologic pacing than with ventricular pacing (9.0 percent vs. 3.8 percent, P<0.001).

CONCLUSIONS

Physiologic pacing provides little benefit over ventricular pacing for the prevention of stroke or death due to cardiovascular causes.

Clinical trials of pacing mode selection

Current recommendations in favor of dual-chamber over single-chamber ventricular pacing for patients with sinus node dysfunction or AV conduction disorders were made largely based on observational data and expert opinions. The first randomized pacing mode selection study was relatively small and suggested survival advantage with physiologic pacing only after an extended follow-up duration of 5.5 years. Preliminary results of the first large-scale multicenter randomized pacing mode selection trial revealed only modest reduction in atrial fibrillation without survival advantage after 3 years of physiologic pacing. Two other large-scale multicenter randomized trials comparing physiologic versus ventricular pacing are currently ongoing. They may provide further scientific evidence based on which more objective recommendations can be made with respect to pacing mode selection.

Pacing in right ventricular dysplasia after disconnection surgery

This report describes a 33-year-old patient with arrhythmogenic right ventricular (RV) dysplasia who had a dual chamber pacemaker implanted at age 23 years for drug-induced bradycardia. Pacing was continued after right ventricular free-wall disconnection (RVFWD) at age 24 years. Her pacemaker was not replaced after battery depletion 7 years later. She presented the following year in severe right-sided heart failure. Her old pacemaker generator was replaced. This was followed by rapid resolution of her clinical failure and return to a full, active, physical lifestyle. This observation suggests the potential benefit of dual chamber pacing in patients with RV dysplasia after RVFWD.

The Canadian Registry of Atrial Fibrillation: a noninterventional follow-up of patients after the first diagnosis of atrial fibrillation

The Canadian Registry of Atrial Fibrillation (CARAF) is a nondirected, follow-up study of 1,086 patients who are enrolled at 6 centers across Canada at the time of initial electrocardiographically documented diagnosis of atrial fibrillation (AF). Enrollment commenced in 1991 with an intended 10-year follow-up. Comprehensive baseline data, including clinical history, laboratory, and echocardiographic variables were collected. The patients were treated by their own referring physicians and CARAF did not direct their care. Detailed follow-up was performed at 3 months, 1 year, then yearly, with echocardiograms repeated every 2 years. Several studies, which evaluated patient populations, predictors of events, and cardiac structure and functioning, have been performed and are ongoing. Thyroid function was evaluated at baseline, and, of 707 patients evaluated, only 6 patients were found to be hyperthyroid. Symptoms during AF were evaluated and a profile of the types of symptoms and the predictors of symptoms was compiled. Antiarrhythmic drug use is being followed. Sotalol and propafenone were the most commonly used medications, with the use of antiarrhythmic drugs increasing with recurrence of AF. The use of anticoagulants was assessed. The overall use of warfarin was relatively low, but its use increased dramatically with the presence of various risk factors including congestive heart failure, hypertension, and previous stroke. The one risk factor that did not result in increased use of warfarin was hypertension. Therefore, CARAF was able to identify that hypertension appears to be under-recognized and undertreated in its risk for thromboembolic events. CARAF is just now reaching maturity, with the majority of patients having > or=4 years of follow-up. Therefore, extensive investigations are currently under way that will evaluate the baseline characteristics and utilize these as predictors of recurrence of AF, progression to chronicity, and the occurrence of major events such as stroke and death. A very large cohort of patients with serial echocardiograms over 4 years will permit an understanding of the progression of structural and valvular disease. Therefore, CARAF offers a unique opportunity for comprehensive, nondirected follow-up of patients from their initial diagnosis of AF.

Atrial fibrillation: extending basic science to clinical frontiers

Basic research in atrial fibrillation is advancing with enormous speed, extending the boundaries from research in intact tissues, to cellular electrophysiology and to molecular biology. Never before has the need been greater for a true 'bench to bedside' approach to research. The basic researchers need to understand the potential clinical relevance of their work so that their efforts may be directed to areas of clinical impact. On the other hand, the clinician needs the aid of the basic researcher to help solve some of the vexing clinical problems. Emphasizing the need for this liaison, this paper discusses problems confronted by the clinician and suggests areas of basic research that may help answer the frustration of the clinician in dealing with patients with atrial fibrillation.

Characteristics of restitution kinetics in repolarization of rabbit atrium

The study was designed to characterize restitution kinetics in atrial repolarization of rabbits and to examine effects of K+ or Ca2+ channel blockers on restitution. Action potentials were recorded from rabbit atrial tissue. Restitution curves of phase I amplitude and action potential duration at 50 and 90% repolarization (APD50, APD90) were defined at a basic cycle length of 0.5 s during control and with interventions. Restitution of phase I amplitude had a monoexponential function with a time constant of 2.8 +/- 0.2 s. The curves of APD50 frequently had a monoexponential function and time constants were 1.8 +/- 0.1 s. Restitution curves of APD90 were biphasic: a descending phase followed by an ascending phase. The blocker of Ito1 (a 4-aminopyridine-sensitive component of the transient outward current), 4-aminopyridine, flattened the restitution curves of phase I amplitude, and APD50 and APD90 curves became monophasic. Sotalol, a selective IKr (a rapid component of the delayed rectifier K+ current) blocker, did not alter curves of phase I amplitude and APD50 but shifted APD90 curves upward. Cadmium, a Ca2+ blocker shifted curves of phase I amplitude and APD50 downward and abolished the ascending phase of APD90 curves. We conclude that kinetics of Ito1 and ICa (calcium current) may account for characteristics of restitution of atrial repolarization in rabbit.

How useful is thyroid function testing in patients with recent-onset atrial fibrillation? The Canadian Registry of Atrial Fibrillation Investigators

BACKGROUND

Patients with recent-onset atrial fibrillation often undergo routine thyroid function screening to rule out thyroid disease as a cause of atrial fibrillation.

METHODS

Patients with recent (< 3 months) onset of documented atrial fibrillation or flutter were enrolled in the Canadian Registry of Atrial Fibrillation from outpatient clinics, emergency departments, and hospital wards across Canada. Seven hundred twenty-six patients underwent baseline thyroid function screening and were assessed for presence of clinical thyroid disease. Serum thyrotropin level (TSH) was measured in 707 patients (97%), and thyroxine level (T4) in 407 patients (56%).

RESULTS

A TSH level less than 0.1 mU/L was present in 5 patients (0.7%). A TSH level less than normal but more than 0.1 mU/L was present in 34 patients (4.7%). No patient had definite hypothyroidism (TSH > 20 mU/L), but 56 patients (7.7%) had an elevated TSH level that was less than 20 mU/L. During 1.7 years of follow-up, only 7 patients were found to have clinical hyperthyroidism, and 11 patients (1.5%) had hypothyroidism. Logistic regression analysis showed that palpitations (odds ratio, 4.9; 95% confidence interval, 1.7-14.0) and asymptomatic presentation (odds ratio, 5.5; 95% confidence interval, 1.9-16.2) were risk factors for low TSH level, and increasing age (odds ratio, 1.32 every 10 years; 95% confidence interval, 1.01-1.66) was a risk factor for high TSH level. The positive predictive value of palpitations and asymptomatic presentation for low TSH level were 9% and 8%, respectively.

CONCLUSIONS

An abnormal TSH level is common in patients with recent-onset atrial fibrillation. However, clinical thyroid disease is uncommon. Routine TSH screening of patients who have atrial fibrillation has a low yield and may be better applied to those patients at higher risk of having undiagnosed clinical thyroid disease.

pH-dependent actions of 4-aminopyridine on atrial repolarization: effects on the transient outward current

Effects of extracellular pH (pHo) were examined on the changes in atrial repolarization induced by 4-aminopyridine (4AP), which is a selective blocker of the transient outward potassium channel, I(to). Action potential parameters were measured, using the conventional microelectrode technique, in the absence and presence of 4AP (0.1-3.0 mM) at pHo 6.5, 7.25, and 8.0. Phase 1 amplitude served as an index of I(to). The onset and recovery kinetics of phase 1 amplitude were assessed at a basic cycle length of 0.5 s, and time constants (tau on and tau r) were computed. Both onset and recovery kinetics had monoexponential functions. Tonic blockade was influenced by external pH, and Kd for half block was 0.19, 0.44, and 2.43 mM for pHo 8.0, 7.25, and 6.5, respectively. Phasic block was defined and exhibited cycle length dependence. Phasic block was also modified by external pH with the greatest effect at pHo 8.0. 4AP (0.3 mM) accelerated tau on, 0.62 +/- 0.2, 0.55 +/- 0.1, and 2.0 +/- 0.8 beats for pHo 8.0, 7.25, and 6.5 compared with control 6.8 +/- 1.9, 6.3 +/- 1.9, and 5.1 +/- 1.4 beats. In contrast, 4AP slowed tau r by about 1 s from control value to 1.5 +/- 0.5 s at pHo 6.5, 4.8 +/- 1.5 s at pHo 7.25 (p < 0.05), and 5.7 +/- 2.0 s at pHo 8.0. We conclude that an increase in extracellular pH enhances block of Ito induced by 4AP, whereas low pHo attenuates the block.

Atrial fibrillation--future directions

AF has finally been reviewed with an importance compatible with its huge burden on the health care system. Careful evaluation of the knowledge to date reveals many areas that require further research. Many hurdles remain to be overcome in order to make a significant impact on this arrhythmia that is both troublesome and carries significant morbidity and mortality.

Pharmacokinetics of mexiletine enantiomers in healthy human subjects. A study of the in vivo serum protein binding, salivary excretion and red blood cell distribution of the enantiomers

1. The disposition kinetics of serum free (unbound) and total mexiletine enantiomers were studied in 12 healthy subjects following oral administration of 200 mg racemic mexiletine hydrochloride. The disposition of the enantiomers of mexiletine in urine, saliva, and red blood cells was also examined. 2. The mean peak serum total mexiletine concentration of 217 +/- 69 ng/ml for R(-)-mexiletine was found to be significantly greater than a mean of 197 +/- 56 ng/ml for S(+)-mexiletine. The mean serum total R(-)-mexiletine concentrations were also found to be significantly greater than those for S(+)-mexiletine during the first 6 h following drug administration. The oral absorption, as well as the rapid and the terminal disposition kinetic parameters between the mexiletine enantiomers, were not significantly different. 3. Comparative in vitro serum protein binding of mexiletine enantiomers examined by ultrafiltration and equilibrium dialysis indicated a pH-dependent stereoselective binding of the enantiomers to serum proteins. A serum pH ranging from 6.3 to 9.4 was found to correlate with serum protein binding of the enantiomers from approximately 30-80% respectively. Within the same serum pH range, the serum free drug R(-)/S(+) ratios were found to decrease from 1.0 to 0.7 respectively. At serum pH7.4, a mean serum free fraction of 0.57 +/- 0.7 and 0.56 +/- 0.6 were observed for R(-) and S(+)-mexiletine respectively. 4. The overall mean saliva/serum-free mexiletine enantiomer area under the concentration-time curve ratios of 6.10 +/- 2.82 and 7.49 +/- 3.48 for R(-)- and S(+)-mexiletine respectively were found to be significantly different. The overall mean saliva R(-)/S(+) enantiomer ratio of 0.89 +/- 0.02 (mean +/- SE) over 48 h suggested a stereoselective disposition of the mexiletine enantiomers in saliva. 5. The mean mexiletine red blood cells to serum-free drug concentration ratios among 11 subjects studied were found to range from 0.6 to 1.4 for R(-)-mexiletine and from 0.6 to 1.8 for S(+)-mexiletine. The overall mean ratios of 0.85 +/- 0.06 and 0.84 +/- 0.08 (mean +/- SE) over 48 h for R(-)- and S(+)-mexiletine respectively were both slightly but significantly different from unity. This data together with an overall red blood cell mean R(-)/S(+)-mexiletine concentration ratio of 0.91 +/- 0.13 suggested a non-stereoselective and passive diffusion of the enantiomers into red blood cells. 6. The cumulative amounts of unchanged R(-)- and S(+)-mexiletine in the urine were found to be variable among the 12 subjects with a mean percent urinary recovery of 3.49 +/- 3.35% for R(-)-mexiletine and 3.68 +/- 3.94% for S(+)-mexiletine.

Characterization of junctional rhythm after atrioventricular node ablation

BACKGROUND

Catheter ablation of the atrioventricular (AV) node with radiofrequency current (RFC) is associated with the short-term onset of a junctional escape rhythm (JER) in nearly all patients. However, the origin of the JER and short-term thermal effects of RFC on this junctional pacemaker activity are ill defined.

METHODS AND RESULTS

Short-term and noninvasive long-term follow-up studies were performed to examine the electrophysiological characteristics of the underlying JER in 45 patients who had undergone AV nodal ablation with RFC. Baseline characteristics and responses to overdrive ventricular pacing and intravenous atropine followed by an incremental isoproterenol infusion were determined. Short- and long-term responses were compared. HV intervals before and after ablation were 49 +/- 9 and 48 +/- 9 milliseconds, respectively (P = NS). Follow-up was 11 +/- 8.3 months. JER cycle length was 1526 +/- 298 milliseconds in the short-term setting and was present in 44 patients (98%) in the long-term setting, measuring 1426 +/- 223 milliseconds (P < .005). Junctional recovery times increased exponentially as overdrive pacing rates increased-there was no difference between short-term and long-term responses. Drug responses within each study were all significant when compared with baseline. However, there was no significant difference between short- and long-term responses, except at the highest dose of isoproterenol. Intravenous atropine (1 mg) caused an 8.6 +/- 9.3% decrease in JER cycle length in the short-term setting compared with a 7.6 +/- 7.3% decrease in the long-term setting. The decreases in JER cycle length with isoproterenol infusion (short-term versus long-term) were 10.1 +/- 9.6% versus 9.6 +/- 7.4% with 1 microgram/min, 15.8 +/- 11.7% versus 17.4 +/- 8.5% with 2 micrograms/min, 17.9 +/- 11.2% versus 21.4 +/- 9.1% with 3 micrograms/min (all P = NS), and 20.6 +/- 12.1% versus 24.8 +/- 9.1% with 4 micrograms/min (P < .01).

CONCLUSIONS

Radiofrequency ablation of the AV node is associated with development of a JER that is stable in the long-term setting. The lack of change in HV interval after ablation locates the junctional pacemaker proximal to the central fibrous body. The pattern of drug responses suggests an origin within the proximal His bundle at its junction with the AV node rather than the AV node itself. The overall similarity between short- and long-term characteristics of junctional pacemaker activity mitigates against any reversible thermal effects of RFC on this pacemaker focus.

High-performance liquid chromatographic analysis using a highly sensitive fluorogenic reagent, 2-anthroyl chloride, and stereoselective determination of the enantiomers of mexiletine in human serum

A stereoselective and highly sensitive HPLC assay was developed for mexiletine enantiomers using a new fluorogenic derivatization reagent, 2-anthroyl chloride. The reagent was synthesized and utilized for the fluorescent detection (excitation at 270 nm, emission at 400 nm) of mexiletine enantiomers as their N-anthroyl derivatives on a Pirkle phenylglycine ionic HPLC column. The assay had a lower limit of quantitation at 2.5 ng/ml with a limit of detection measured at 0.5 ng/ml for each enantiomer in serum with a signal-to-noise ratio of 5:1. In a preliminary pharmacokinetic study, 200 mg of racemic mexiletine hydrochloride were administered orally to two healthy volunteers. Serum samples were collected at timed intervals over 48 h. The terminal elimination half-lives determined for total R(-)- and S(+)-mexiletine were 10.9 and 11.5 h, respectively. The serum free fractions for R(-)- and S(+)-mexiletine were found to be 0.56 and 0.53, respectively.

Efficacy of sotalol guided by programmed electrical stimulation for sustained ventricular arrhythmias secondary to coronary artery disease

Sotalol is a class III antiarrhythmic drug with additional beta-blocker activity that has been shown to be effective in supraventricular and ventricular arrhythmias. Its long-term efficacy for ventricular arrhythmias is not as well described. Patients with documented sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) who had their clinical arrhythmia inducible at baseline electrophysiologic study received sotalol 320 to 640 mg/day. Repeat programmed stimulation was performed after a minimum of 72 hours while receiving the final dose. Of 28 patients (25 men and 3 women) whose arrhythmias were inducible at baseline, 15 had their arrhythmias suppressed with sotalol. Sotalol had greater success in suppressing arrhythmias in those with VF (8 of 9, 89%) than in those with VT (7 of 19, 37%, p < 0.01). In patients with a history of coronary artery disease but no history of myocardial infarction the arrhythmia was suppressed in 7 of 8 (88%) compared with 8 of 20 (40%, p < 0.05) patients with a history of myocardial infarction. All 15 patients in whom ventricular arrhythmias were suppressed continued to take long-term sotalol, and at a follow-up of 10.3 +/- 6.4 months none has had arrhythmia recurrence. Thus, sotalol is an effective drug for the suppression of ventricular arrhythmias as judged by programmed electrical stimulation. It appears to be more effective in patients in whom the clinical arrhythmia is VF rather than VT.

Electrophysiologic properties of a narrow isthmus in rabbit atrial tissue: cycle length dependent effect of quinidine

The modulation of quinidine's effect by pacing cycle lengths was assessed over an isthmus of atrial myocardium, simulating the Wolff-Parkinson-White syndrome. Isolated rabbit atria were dissected so that two tissue blocks, A and B, were linked by an isthmus, 1 mm in width. Effective refractory period in the tissue blocks and over the isthmus was measured at cycle lengths of 1,000, 600, and 400 ms, and the minimum cycle length to sustain 1:1 conduction over the isthmus was measured before treatment, during quinidine superfusion (4 mg/L), and after washout. Longitudinal velocity over the isthmus was also measured. The increment in the effective refractory period in the tissue blocks by quinidine appeared to be similar, about 10% at three pacing cycle lengths (p > 0.05). However, the increment of the effective refractory period over the isthmus was modulated by pacing cycle lengths: greater increase at shorter cycle lengths (p < 0.001). Quinidine prolonged the minimum cycle length over the isthmus by 44 +/- 17%. Regression analysis showed that after quinidine there was a correlationship between conduction velocity and refractoriness over the isthmus (R = 0.85, p < 0.001). Intracellular implements showed stable action potentials, confirming the integrity of the preparation. We conclude that (i) quinidine preferentially prolongs refractoriness over the isthmus and (ii) quinidine's effect on refractoriness over the isthmus is cycle length dependent.

Atrioventricular nodal reentrant tachycardia in patients with ventriculo-atrial conduction block

OBJECTIVE

To demonstrate the reversibility of retrograde ventriculo-atrial block by isoproterenol in patients with atrioventricular nodal reentrant tachycardia (AVNRT).

DESIGN

Three case reports and their electrophysiological features.

PATIENTS

Three patients with documented or suspected paroxysmal supraventricular tachycardia.

INTERVENTIONS

At routine electrophysiology study, no supraventricular tachycardia was inducible in the baseline state. Infusion of isoproterenol (1 to 5 micrograms/min) was given and stimulation procedures were repeated.

RESULTS

At baseline, all three patients had discontinuous antegrade atrioventricular (AV) nodal conduction, but very poor (two patients) or absent (one patient) ventriculo atrial conduction prevented induction of AVNRT. During infusion of isoproterenol, retrograde conduction was enhanced so that 1:1 retrograde occurred to cycle lengths of 300, 340 and 260 ms. AVNRT was then inducible in all patients, reproducing their clinical symptoms.

CONCLUSION

Absent or poor ventriculo-atrial conduction in patients with suspected AV node reentry does not preclude the development of tachycardia with sympathomimetic enhancement. Isoproterenol should be given to attempt reversal of retrograde block in these patients.

Regional differences in rabbit atrial repolarization: importance of transient outward current

Regional differences in rabbit atrial repolarization were investigated using a conventional microelectrode technique. A more rapid phase 1 repolarization (lower phase 1 amplitude) was seen in the left atrial (LA) roof area compared with the right atrial (RA) roof area: 54 +/- 10 vs. 82 +/- 6 mV at 1,000 ms (P < 0.001). In addition, action potential duration at 40 mV above the resting potential (APD40) was shorter in LA and was associated with a slower phase 3 repolarization rate. Furthermore, the recovery time constant of phase 1 amplitude at 500 ms was 0.9 +/- 0.2 s in LA and 3.5 +/- 1.5 s in RA (P < 0.001). Pacing cycle lengths (2,000, 1,500, 1,000, 800, and 500 ms) modulated phase 1 amplitude, APD40, and phase 3 rate in both regions. 4-Aminopyridine (4-AP; 1 mM), a selective transient outward current (I(to)) blocker, abolished cycle length dependence of the above action potential parameters and diminished the differences in electrophysiological properties between the two regions. 4-AP also flattened the restitution curve of phase 1 amplitude in both regions. In conclusion, the findings suggest that the different kinetics of I(to) play an important role in regional differences of atrial repolarization.

In vitro protein binding of propafenone and 5-hydroxypropafenone in serum, in solutions of isolated serum proteins, and to red blood cells

The binding of propafenone (PF) and 5-hydroxypropafenone (5-OH-PF) in serum and in solutions of isolated serum proteins was examined by equilibrium dialysis. Both PF and 5-OH-PF displayed pH-dependent binding in serum and in a solution of alpha-1-acid glycoprotein (AAG). PF displayed extensive binding to AAG (i.e., free fraction of 0.08 +/- 0.02), whereas the binding of 5-OH-PF to AAG was moderate (i.e., free fraction of 0.54 +/- 0.10). The removal of lipoproteins from serum did not alter the free fraction of PF but significantly increased the free fraction of 5-OH-PF compared with that in intact serum. Both PF and 5-OH-PF displayed concentration-dependent binding in a 19.3-mumol AAG solution. Concentration-independent binding was apparent in solutions of human serum albumin, high-density lipoproteins, low-density lipoproteins, and very low density lipoproteins over the PF and 5-OH-PF concentration ranges examined. By use of previously determined binding parameters (affinities and capacities), the binding model of PF provided an estimate of the free fraction in serum that was similar to the observed free fraction, although the free fraction of 5-OH-PF was overestimated. The distribution of PF and 5-OH-PF into red blood cells was extensive when buffer was used as the supernatant; however, when serum was used as supernatant, the amounts of PF and 5-OH-PF that were distributed into red blood cells decreased substantially. PF and 5-OH-PF interacted with all of the proteins examined.

Propafenone-mexiletine combination for the treatment of sustained ventricular tachycardia

OBJECTIVES

The purpose of this study was to explore the efficacy of combined therapy with propafenone and mexiletine for control of sustained ventricular tachycardia.

BACKGROUND

Combination antiarrhythmic drug therapy may enhance efficacy and lead to control of ventricular arrhythmias in some patients. Few reports have studied the combination of class IB and class IC drugs. Thus, this study was designed to investigate a combination of mexiletine and propafenone in patients with refractory ventricular tachycardia.

METHODS

Sixteen patients with sustained ventricular tachycardia had their clinical arrhythmia induced by programmed stimulation. Procainamide and propafenone alone failed to prevent reinduction of tachycardia in all. Mexiletine was subsequently added to propafenone and programmed stimulation was repeated.

RESULTS

With combination therapy ventricular tachycardia was noninducible in three patients (19%). A fourth who had presented with polymorphic ventricular tachycardia had slow bundle branch reentry (cycle length 500 ms) induced. In the other 12, tachycardia cycle length increased from 262 +/- 60 ms at baseline to 350 +/- 82 ms with propafenone and to 390 +/- 80 ms with propafenone plus mexiletine (p less than 0.0001 compared with baseline). Hemodynamic deterioration requiring defibrillation occurred in six patients at baseline study, in five taking propafenone and in two taking both drugs.

CONCLUSIONS

The combination of propafenone and mexiletine is effective in suppressing the induction of ventricular tachycardia in some patients refractory to procainamide and propafenone alone. In those in whom ventricular tachycardia could still be induced, the rate was slower and hemodynamically tolerated.

High success rate of atrioventricular node ablation with radiofrequency energy

Radiofrequency current was introduced as an alternative energy source for transcatheter ablation of cardiac arrhythmias to avoid the complications associated with direct current shocks. Initial use of radiofrequency current for complete ablation of the atrioventricular (AV) node yielded only moderate success rates, presumably because of the small size of electrodes and difficulty in localizing the AV node. The use of a larger 4-mm tip electrode for delivery of radiofrequency current and a method to better localize the AV node were prospectively studied in 32 patients undergoing catheter ablation of the AV node. There were 21 men and 11 women with a mean age of 62 +/- 12 years. Complete AV block was achieved immediately in 31 patients (97%) and it persisted in 28 patients (88%) during a mean follow-up period of 12 +/- 6 months. Three patients who had return of AV condition required no drug therapy for control of ventricular rate during atrial fibrillation. The number of radiofrequency pulses used to achieve complete AV block ranged from 1 to 5 (mean 1.9 +/- 1.1). In greater than 50% of the cases, only one radiofrequency pulse was required. The mean power and duration of radiofrequency pulses were 21.2 +/- 4.5 W and 33 +/- 15 s, respectively. All patients developed a stable junctional escape rhythm within 45 min of successful ablation. The QRS configuration was unchanged in 30 patients. One patient had a new right bundle branch block after ablation. There were no complications related to the ablation procedure.(ABSTRACT TRUNCATED AT 250 WORDS)

Long term efficacy of propafenone for prevention of atrial fibrillation

OBJECTIVE

Propafenone, a class IC antiarrhythmic drug, has been successful in the treatment of ventricular and supraventricular arrhythmias. This study retrospectively evaluated the efficacy of propafenone in the prevention of recurrent atrial fibrillation.

DESIGN

Propafenone was given to 81 patients (49 males and 32 females, mean age 61 +/- 16 years) with recurrent atrial fibrillation. The mean dose of propafenone was 701 +/- 235 mg. Patients were monitored for recurrent arrhythmias.

MAIN RESULTS

Long term follow-up over 30 +/- 1.7 months showed 31 patients (38%) remained on propafenone with complete or partial control of atrial fibrillation. The drug was stopped in 35 due to inefficacy, in 12 due to adverse effects, and in three due to desire for ablation therapy.

CONCLUSION

Propafenone may be effective in some patients for long term prevention of atrial fibrillation, although efficacy may decrease over time.

Stereoselective pharmacokinetics of tocainide in human uraemic patients and in healthy subjects

The disposition of tocainide enantiomers were examined in healthy human subjects and uraemic patients following a single i.v. dose (200 mg) of racemic tocainide hydrochloride. In the healthy subjects, the total body clearance of R(-)-tocainide was significantly greater than that of S(+)-tocainide (2.62 vs 1.70 ml.min-1.kg-1). Renal clearance also favoured R(-)-tocainide and appeared to contribute significantly to the stereoselective total body clearance. The volume of distribution of the enantiomers did not differ significantly. Uraemia produced a marked decrease in the total body clearance with no apparent effect on the volume of distribution of both enantiomers. The S/R ratio for total body clearance decreased significantly from 0.66 in healthy subjects to 0.54 in the uraemics, while the ratio for terminal elimination half-life significantly increased from 1.43 to 1.59. These results indicate that uraemia alters the degree of stereoselectivity in the pharmacokinetic parameters of tocainide enantiomers.

Effects of propafenone on sinus node function in the rabbit heart

Propafenone is a type 1C antiarrhythmic drug with efficacy for both ventricular and supraventricular arrhythmias. We investigated the effects of propafenone on properties of sinus node function in an in vitro preparation of rabbit sinus node and surrounding atrium. Spontaneous sinus cycle length (SCL), atriosinus conduction time (ASCT), and sinus node effective refractory period (SNERP) at multiple pacing cycle lengths were measured in the control state and during superfusion with propafenone (2.3 microM). SNERP prolonged from 175 +/- 25 ms in the control state to 220 +/- 45 ms (p less than 0.001) with propafenone. ASCT also prolonged significantly (p less than 0.01) from 50 +/- 20 to 65 +/- 20 ms whereas SCL did not change. In four experiments, multiple concentrations of propafenone were utilized and there appeared to a dose-dependent prolongation of SNERP. Thus, propafenone has a significant effect on SNERP and ASCT in an isolated rabbit sinus node preparation.

Effects of acetylcholine, propranolol, and verapamil on sinus node refractoriness of the rabbit

At a critical premature interval, atrial premature beats encounter sinus node refractoriness and are blocked on entering and fail to reset the sinus node, resulting in interpolation of the premature beat. The transition from reset to interpolated response has been used to define the effective refractory period of the sinus node (SNERP). In an in vitro preparation of rabbit sinus node, we evaluated the effects of acetylcholine, propranolol, and verapamil on SNERP. Results obtained in the control state were compared with those obtained during superfusion with drugs, all of which prolonged refractoriness: acetylcholine from 233 +/- 41 (SD) to 325 +/- 88 ms; propranolol from 215 +/- 60 to 241 +/- 67 ms; and verapamil from 192 +/- 69 to 254 +/- 79 ms (p less than 0.005 with all drugs). The site of block of premature beats was mapped between sinus node and crista terminalis with an intracellular microelectrode. All three drugs resulted in block of premature beats at sites farther from the primary pacemaker site. Thus, acetylcholine, propranolol, and verapamil prolong sinus node refractoriness.

Nonsustained ventricular tachycardia arising from the right ventricular outflow tract

Characteristics of left bundle branch block morphology, inferiorly directed frontal plane QRS axis and repetitive nonsustained salvos were used to define a discrete subgroup of patients with ventricular tachycardia (VT). The origin of this tachycardia was thought to be the right ventricular outflow tract. Twenty-six patients with this definition (group 1) were compared with 29 consecutive patients with all other forms of VT (group 2). When compared with patients in group 2, group 1 patients were younger (average age 37 vs 51 years, p less than 0.005), had less structural heart disease (2 of 26 vs 25 of 29 patients, p less than 0.005) and had a better prognosis (no deaths) after an average follow-up time of 28 months in comparison with 5 deaths after an average follow-up of 35 months (p less than 0.05). Induction of VT was possible using isoproterenol infusion in 14 of 20 group 1 patients, but no VT could be induced in 9 group 2 patients (p less than 0.05). Exercise stress testing induced VT in 11 of 21 group 1 patients and 2 of 9 group 2 patients (p greater than 0.05). Programmed electrical stimulation failed to induce VT in 9 group 1 patients, but did induce it in 15 of 20 group 2 patients (p less than 0.005). Successful therapy in group 1 patients was achieved by beta blockers alone (7 patients), beta blockers plus type 1A antiarrhythmic drugs (9 patients), procainamide alone (2 patients), sotalol (3 patients) and amiodarone (2 patients). Three patients were not treated.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial pacing: efficacy and safety

Atrial pacing has the advantages of simplicity, maintenance of AV synchrony, and economy. The major detraction has been the potential for deterioration of atrioventricular conduction. In this study, we followed 43 patients with sick sinus syndrome treated with atrial (AAI) pacing. Excellent initial implant parameters were obtained in all. Three early lead repositionings were required. Minor sensing and pacing problems could all otherwise be handled by reprogramming. Follow-up for a mean of 25 +/- 20 months demonstrated excellent performance of the pacing systems. Pacing and sensing thresholds and lead impedance indicated excellent lead performance. There were no late lead failures. Nine patients have had mild deterioration in atrioventricular conduction and one of these had a change to DDD pacing at the time of elective battery change. All patients are asymptomatic. Thus, chronic atrial pacing in selected patients is safe and reliable with good chronic lead performance and low risk of subsequent conduction system disease.

Impedance to transesophageal atrial pacing: significance regarding power sources

Transesophageal stimulation is an expeditious method of atrial pacing. Using pulse widths of 10 msec results in reduction of current requirement to values that are usually less than 15 mA. An unknown variable in transesophageal atrial pacing has been impedance. In this study, we investigated the impedance to transesophageal atrial pacing in ten patients using a stimulator with a 63 V power source capable of delivering constant current to 20 mA against an impedance of 2,000 ohms. A bipolar electrode was used to deliver stimuli with a current of 15 mA. Voltage across a known resistance and current were measured on an oscilloscope and the impedance was calculated. Pacing thresholds were also performed and ranged from 6.2 to 16.5 mA (mean 9.4 +/- 2.9 mA, SD). Impedance varied between 720 and 2,670 ohms (mean 1,750 +/- 540 ohms). The stimulator used to measure impedance in man and two other commercially available stimulators were bench tested against known resistances of 500 to 2,000 ohms. The other stimulators with power sources of 12.5 and 15 V had attenuation of the delivered current at resistances of between 1,000 and 2,000 ohms. Thus, this study has demonstrated that transesophageal atrial pacing incurs impedances two to five times greater than incurred with intracardiac pacing leads. Therefore stimulators with high power sources are required to deliver the programmed current against these impedances.

The pharmacokinetics of the enantiomers of mexiletine in humans

1. This study examined the pharmacokinetics of the enantiomers of mexiletine in five healthy subjects who were each given a single, 300 mg, oral dose of racemic mexiletine hydrochloride. 2. The time course of the concentration ratio between the R(-) and the S(+) enantiomers (R/S) in plasma showed a progressive decrease, with a mean +/- S.D. ratio of 1.37 +/- 0.11 at 1 h and 0.64 +/- 0.11 at 48 h. Similarly, the R/S ratios in urine were 1.38 +/- 0.42 and 0.55 +/- 0.12 at 1 h and 72 h, respectively. 3. The terminal elimination half-life of S(+)mexiletine was 11.0 +/- 3.80 h, which was significantly greater (P less than 0.05) than that of the R(-) enantiomer, 9.10 +/- 2.90 h. S(+)Mexiletine also showed a significantly greater apparent volume of distribution (P less than 0.01) and renal clearance (P less than 0.05) than R(-)mexiletine. There was no significant difference in the apparent oral total drug clearance of the enantiomers. 4. The disposition of mexiletine enantiomers in man was stereoselective, and the differences observed between the enantiomers may be due largely to differences in their serum protein binding.

In vitro protein binding of propafenone in normal and uraemic human sera

The protein binding of propafenone, a Class I antiarrhythmic agent, was studied in vitro using a selective and sensitive electron-capture detection gas-liquid capillary chromatographic assay method developed in our laboratory. The concentration-dependency of the serum protein binding of propafenone was confirmed in vitro by equilibrium dialysis, using serum obtained from healthy human subjects and patients with chronic renal failure. In normal serum the unbound fraction of propafenone was 0.027 at a propafenone concentration of 0.25 microgram.ml-1, 0.041 within the therapeutic concentration range (0.5-2 micrograms.ml-1), 0.138 at a propafenone concentration of 25 micrograms.ml-1, and 0.187 when the propafenone concentration was increased to 100 micrograms.ml-1. There was no evidence of significant concentration-dependent changes in unbound fraction within the propafenone concentration range of 0.5-1.5 micrograms.ml-1. However, concentration-dependent binding was demonstrated at concentrations greater than 1.5 micrograms.ml-1. A high-affinity, low-capacity binding site (K1 = 6.53 x 10(5) l.mol-1; n1P1 = 1.73 x 10(-4) mol.l-1) and a low-affinity, high-capacity binding site (K2 = 8.77 x 10(3) l.mol-1; n2P2 = 8.57 x 10(-3) mol. x l-1) were identified. In pooled uraemic serum the unbound fraction of propafenone was approximately 50% of that of normal serum throughout the concentration range studied (1-5 micrograms.ml-1). In sera from patients with chronic renal failure the increase in propafenone binding ratio or the decrease in unbound fraction was associated with the increase in alpha 1-acid glycoprotein concentrations, and there was a correlation (r = 0.8302) between alpha 1-acid glycoprotein concentration and the propafenone binding ratio.

Effect of pacing cycle length and autonomic blockade on sinus node refractoriness

Sinus node (SN) refractoriness can be measured indirectly by observing the return responses after the introduction of progressively earlier atrial premature beats. The SN effective refractory period (ERP) is defined as the longest premature interval resulting in an interpolated atrial return response. In the present study, SNERP was analyzed in 71 subjects--51 control persons and 20 patients with evidence of SN dysfunction. SNERP could be measured in 40 of 51 control subjects and was shown to prolong at shorter basic pacing cycle lengths. At a basic cycle length of 600 ms, SNERP was 330 +/- 40 ms, whereas at 500 ms it was 350 +/- 50 ms (p less than 0.05). At a basic cycle length of 600 ms, SNERP was measured in 31 control subjects and 7 patients with SN dysfunction. The values of 330 +/- 40 and 520 +/- 20 ms, respectively, in these 2 groups suggested that this method can be used to differentiate patients with SN dysfunction (p less than 0.001). In 12 control subjects, SNERP was measured before and after partial autonomic blockade with propranolol and atropine. SNERP shortened from 360 +/- 40 to 320 +/- 40 ms (p less than 0.05). It shortened with atropine and prolonged with propranolol. Thus, SNERP prolongs with a shorter basic pacing cycle length and is affected by autonomic manipulation, in a fashion analogous to the atrioventricular node.

Surgical therapy for sinoatrial reentrant tachycardia

Sinoatrial reentry is an uncommon cause of paroxysmal supraventricular tachycardia. This paper presents a case of supraventricular tachycardia, refractory to medical therapy, in which the sinus node formed part or all of the reentrant circuit. The mechanism of the arrhythmia was confirmed by catheter mapping during electrophysiological study and by intraoperative epicardial mapping. Cryosurgical ablation of the right atrium in the region of the sinus node has led to cure of her arrhythmia and emergence of a stable ectopic atrial pacemaker rhythm.

"Nodoventricular" accessory pathway: evidence for a distinct accessory atrioventricular pathway with atrioventricular node-like properties

Two patients are described with recurrent pre-excited tachycardia and electrophysiologic characteristics typically ascribed to a nodoventricular accessory connection. The accessory pathway in each case demonstrated rate-dependent prolongation of conduction time and a low right ventricular insertion site; it was associated with a left bundle branch block configuration during pre-excitation. Intraoperatively, the pathway was demonstrated to originate at the anterior right atrioventricular (AV) anulus and not at the AV node. These data suggest that a "typical" nodoventricular pathway, by electrophysiologic criteria, may in fact be an AV pathway with AV node-like conduction properties and a distal right ventricular insertion site.