Effects of adenosine and adenine nucleotides on the atrioventricular node of isolated guinea pig hearts

The primary goal of this study was to determine whether the slowing of atrioventricular (AV) conduction by ATP is caused by ATP per se or is mediated by adenosine formed from ATP degradation. We assessed the effects of ATP, beta, gamma-methylene ATP, ADP, AMP, and adenosine on AV conduction time in the isolated perfused guinea pig heart. The cardiac effluent was collected and analyzed for its content of adenine nucleotides and nucleosides. Perfused ATP was rapidly and almost completely broken down to AMP and adenosine; only 2.5 +/- 0.5% of the infused ATP was recoverable in the effluent. A significant correlation was found between the effluent concentration of adenosine and atria-to-His bundle (A-H) conduction time. Compounds that altered the effect of adenosine on A-H conduction likewise altered the effect of ATP: (1) aminophylline, a competitive antagonist of adenosine, antagonized the ATP-induced A-H prolongation; (2) adenosine deaminase, the enzyme responsible for the deamination of adenosine to inosine, reduced the effect of ATP by 82%; (3) the adenosine transport blockers NBMPR and dipyridamole markedly enhanced the effect of ATP; and (4) EHNA, an inhibitor of adenosine deaminase, potentiated the effect of ATP. Furthermore, the less hydrolyzable ATP analog, beta, gamma-methylene ATP, was less potent than ATP in causing A-H prolongation. We conclude that the adenosine-like action of ATP on the guinea pig AV node requires that ATP first be degraded to adenosine.

Acute electrophysiologic effects of bethanidine sulfate in patients with ventricular tachycardia or fibrillation

Ten patients with a history of ventricular tachycardia or ventricular fibrillation underwent electrophysiologic study with programmed stimulation before and 90 minutes after oral administration of bethanidine sulfate, 20 mg/kg. Mean plasma bethanidine concentration was 2.62 +/- 2.2 (+/- SD) micrograms/ml at the start of repeat testing. This dose of bethanidine produced no effect on sinus node function, atrioventricular conduction, or atrial or ventricular refractoriness. Ventricular tachycardia or fibrillation, inducible in all patients during the control study, could still be initiated by ventricular stimulation in 9 of 10 patients after bethanidine. Bethanidine suppressed the ability to initiate an arrhythmia in one patient with ventricular fibrillation during control stimulation. Orthostatic hypotension was seen in all patients despite pretreatment with the tricyclic antidepressant, protriptyline, 15 mg every 8 hours. The results suggest that bethanidine has few electrophysiologic effects and is of limited efficacy during electrophysiologic testing in patients with life-threatening ventricular arrhythmias.

Unanticipated postoperative ventricular tachyarrhythmias

Eighteen (1.4%) of 1,251 patients who underwent cardiac operations during a three-year period had new sustained ventricular tachycardia (12 patients) or ventricular fibrillation (6 patients) not caused by but resulting in hemodynamic compromise. In 13 patients, the initial arrhythmia occurred in the first 48 hours postoperatively. Lidocaine was being administered to 10 of these patients for suppression of previously noted ventricular ectopy, but it did not prevent the occurrence of the arrhythmia. The initial episode was fatal for 5 patients. Two of these deaths were directly related to the adverse effects of the antiarrhythmic agents used to suppress ventricular tachycardia or fibrillation. Five of 10 survivors underwent electrophysiological studies after initial resuscitation. In all 5, programmed ventricular stimulation reproduced the clinical arrhythmia. There have been 2 late sudden deaths in patients who either did not undergo or remained uncontrolled at electrophysiological study during serial drug trials. Our experience suggests that a cardiac operation may unmask or induce potentially lethal arrhythmias that previously had not been apparent. Pharmacological suppression of ventricular ectopy does not necessarily prevent ventricular tachycardia or ventricular fibrillation in the early postoperative period. Electrophysiological study may be helpful in determining the appropriate prophylactic therapy in such patients.

Long-term clinical outcome of ventricular tachycardia or fibrillation treated with amiodarone

The determinants of long-term clinical outcome were studied in 42 patients with recurrent ventricular tachycardia (VT) or ventricular fibrillation (VF) who were treated with amiodarone as the sole antiarrhythmic agent. Of the 42 patients, 11 (26%) either died suddenly or had recurrent, symptomatic, sustained VT during a mean follow-up period of 10 months (range 0.3 to 45). Of the 19 patients without inducible VT/VF during electrophysiologic study while receiving amiodarone, 1 patient died suddenly but no patient had recurrent VT/VF. Ten of the 23 patients (43%) with persistently inducible arrhythmia have died suddenly or have had recurrent VT/VF. Using survival and stepwise logistic regression analyses, 2 significant independent predictors of recurrent arrhythmia were identified; persistently inducible VT during electrophysiologic testing in patients receiving amiodarone therapy (p less than 0.002) and the left ventricular ejection fraction at rest (p less than 0.05). The predictive accuracy of the response to serial electrophysiologic testing during amiodarone therapy was 67%, the sensitivity was 58% and the specificity was 91%. Thus, serial electrophysiologic testing is useful for determining the prognosis in patients with inducible VT/VF treated with amiodarone.

Paroxysmal supraventricular tachycardia with Wenckebach block: evidence for reentry within the upper portion of the atrioventricular node

Paroxysmal supraventricular tachycardia with Wenckebach block is an uncommon electrocardiographic finding. Electrophysiologic studies performed in a patient who exhibited this phenomenon indicated that the tachycardia was not caused by either sinus node or intraatrial reentry or abnormal automaticity. The tachycardia cycle length decreased after atropine administration and increased in response to propranolol. Administration of either adenosine (75 to 112.5 micrograms/kg) or verapamil (10 mg) terminated individual episodes of tachycardia. After verapamil and when atrial extrastimuli were introduced, dissociation of atrial activity from His-ventricular activity could be observed even though atrioventricular (AV) nodal block with a clear Wenckebach periodicity could also occur. These findings suggest that paroxysmal supraventricular tachycardia may be produced by reentry located solely within the upper portion of the AV node.

Relationship of potassium and magnesium concentrations in serum to cardiac arrhythmias

Low concentrations of potassium and magnesium in serum have been implicated in cardiac arrhythmias; the importance of mild hypokalemia or hypomagnesemia is uncertain. To investigate possible associations among use of diuretics, the concentration of these ions in serum, and the onset of clinically important arrhythmias, we reviewed records of 103 patients admitted to our Coronary Care Unit during three months and found mild to moderate hypokalemia and hypo- magnesemia in 18 and 24%, respectively. The significant correlation between the concentrations of magnesium and potassium in serum at admission (r = 0.27, p less than 0.007) remained constant in patients, whether they were receiving diuretics or not. Potassium concentrations were significantly lower (p less than 0.05) in patients receiving diuretics (3.93 mmol/L) than in those who were not (4.21 mmol/L), but the mean concentrations of magnesium did not differ significantly. Except for myocardial infarction, no single variable or combination of variables was highly predictive of cardiac arrhythmias in these patients. We conclude that there is no strong predictive relationship between mildly decreased concentrations of magnesium or potassium in serum and onset of cardiac arrhythmias.

Sinusoidal ventricular tachycardia associated with flecainide acetate

New pharmacologic agents have been developed which are highly effective in suppressing ventricular premature beats, but their chronic efficacy in patients with sustained ventricular arrhythmias remains uncertain. We observed the appearance of ventricular tachycardia with an unusual sinusoidal QRS complex in three of five such patients treated with flecainide acetate, 200 mg twice daily. Our experience suggests that the early reports of successful suppression of ventricular premature beats in stable patients may not be directly applicable to sicker patients with recurrent sustained ventricular arrhythmias. In these patients, life-threatening arrhythmias may appear as the first manifestation of drug toxicity.

Relationship of potassium and magnesium concentrations in serum to cardiac arrhythmias

Low concentrations of potassium and magnesium in serum have been implicated in cardiac arrhythmias; the importance of mild hypokalemia or hypomagnesemia is uncertain. To investigate possible associations among use of diuretics, the concentration of these ions in serum, and the onset of clinically important arrhythmias, we reviewed records of 103 patients admitted to our Coronary Care Unit during three months and found mild to moderate hypokalemia and hypo- magnesemia in 18 and 24%, respectively. The significant correlation between the concentrations of magnesium and potassium in serum at admission (r = 0.27, p less than 0.007) remained constant in patients, whether they were receiving diuretics or not. Potassium concentrations were significantly lower (p less than 0.05) in patients receiving diuretics (3.93 mmol/L) than in those who were not (4.21 mmol/L), but the mean concentrations of magnesium did not differ significantly. Except for myocardial infarction, no single variable or combination of variables was highly predictive of cardiac arrhythmias in these patients. We conclude that there is no strong predictive relationship between mildly decreased concentrations of magnesium or potassium in serum and onset of cardiac arrhythmias.

Near-fatal Kearns-Sayre syndrome. A case report and review of clinical manifestations

The Kearns-Sayre syndrome is a relatively uncommon condition which occurs in childhood and is characterized by neurologic dysfunction and abnormalities in atrioventricular (AV) conduction. The high mortality rate is directly attributable to the cardiovascular manifestations of this syndrome. We report a case, with documentation of a "near-fatal" episode, and review the pertinent clinical features of the syndrome.

Adenosine: electrophysiologic effects and therapeutic use for terminating paroxysmal supraventricular tachycardia

Adenosine was administered intravenously to 17 patients undergoing intracardiac electrophysiologic studies. At a mean dose of 179 +/- 88 micrograms/kg (+/- SD), adenosine suppressed sinus node automaticity and depressed atrioventricular (AV) nodal conduction. These effects were less than 20 sec in duration and were not influenced by muscarinic blockade with atropine (0.02 to 0.03 mg/kg). Adenosine at this dose had no effect on antegrade conduction over accessory pathways in patients with Wolff-Parkinson-White syndrome. No independent hemodynamic effects were observed. In six patients, adenosine was administered intravenously during stimulation-induced paroxysmal supraventricular tachycardia. In the five patients in whom the reentry loop of their tachycardia included the AV node, adenosine at a mean dose of 83 +/- 35 micrograms/kg (+/- SD) terminated their tachycardia within 20 sec after peripheral intravenous injection. The dose of adenosine required to terminate these tachycardias did not produce manifest sinus node suppression, and sinus rhythm promptly resumed in all patients. Adenosine did not terminate either supraventricular tachycardia due to intra-atrial reentry or atrial flutter, but did produce transient AV block during these arrhythmias. Our findings demonstrate that the human sinus and AV nodes are sensitive to physiologic doses of adenosine and that adenosine may be used safely and effectively to terminate acute episodes of supraventricular tachycardia that involve the AV node in the reentry pathway.

Determination of bethanidine in plasma by liquid-chromatography with a microbore reversed-phase column

In this novel method for quantifying bethanidine in plasma, after a multi-step extraction of bethanidine and internal standard from 2.0 mL of plasma, the drugs are separated on a "microbore" C18 reversed-phase column and quantified by their ultraviolet absorbance at 210 nm. The isocratic chromatographic separation takes about 15 min with use of an ion-pairing regent in the mobile phase (acetate buffer/acetonitrile, 9/1 by vol) and a flow rate of 0.25 mL/min. Sensitivity is increased relative to conventional columns, and solvent consumption is reduced by 90%. The standard curve is linear to at least 5 mg/L, and the detection limit is 0.02 mg/L. The within-run precision of the method is excellent (CV 4%) at a midrange concentration of 1.25 mg/L.

Determination of bethanidine in plasma by liquid-chromatography with a microbore reversed-phase column

In this novel method for quantifying bethanidine in plasma, after a multi-step extraction of bethanidine and internal standard from 2.0 mL of plasma, the drugs are separated on a "microbore" C18 reversed-phase column and quantified by their ultraviolet absorbance at 210 nm. The isocratic chromatographic separation takes about 15 min with use of an ion-pairing regent in the mobile phase (acetate buffer/acetonitrile, 9/1 by vol) and a flow rate of 0.25 mL/min. Sensitivity is increased relative to conventional columns, and solvent consumption is reduced by 90%. The standard curve is linear to at least 5 mg/L, and the detection limit is 0.02 mg/L. The within-run precision of the method is excellent (CV 4%) at a midrange concentration of 1.25 mg/L.

Relation of therapeutic response to nifedipine to coronary anatomy and motion of S-T segment during unstable angina pectoris

Of 77 patients hospitalized for unstable angina pectoris and failure of oral, dermal, or intravenous nitrates and/or beta blockade, 81 percent with negligible or single-vessel disease and 55 percent with two- or three-vessel disease showed response (p less than 0.05) to nifedipine therapy. Patients with either S-T elevation or no change during pain responded better (31 of 45) than those with any S-T depression (16 of 32; p less than 0.05). Patients with negligible or single-vessel disease had a higher prevalence of S-T elevation (13 of 16) than patients with two- or three-vessel disease (15 of 31; p = 0.004). S-T motion did not predict response in patients with two- or three-vessel disease, but did predict response in patients with negligible or single-vessel disease. On follow-up study at 9 +/- 8 (range one to 33) months, 39 of 42 who had shown response were free from pain. Three died from infarction without unstable angina. (range one to 33) months, 39 of 42 who had shown response were free from pain. Three died from infarction without unstable angina. Five who showed response had elective bypass surgery. The addition of nifedipine abolished or reduced pain episodes by more than 50 percent in 61 percent of patients with refractory unstable angina pectoris. Patients with negligible or single-vessel disease with S-T elevation benefit most. In patients with two- or three-vessel disease, the type of S-T motion did not predict response. Follow-up of all those with response indicated sustained amelioration by nifedipine therapy. Failure of nifedipine therapy should not be accepted until a dose of 120 mg per day has been achieved, or until intolerable side effects appear.

Electrophysiologic studies before and after myocardial revascularization in patients with life-threatening ventricular arrhythmias

Electrophysiologic studies with programmed cardiac stimulation were performed in a selected group of 17 patients with severe proximal coronary artery disease involving at least 2 major vessels and left ventricular ejection fractions greater than 30% who were undergoing coronary artery bypass graft surgery after prehospital cardiac arrest or ventricular tachycardia (VT) unassociated with acute myocardial infarction. Before surgery and without antiarrhythmic drug therapy, programmed cardiac stimulation induced ventricular fibrillation (VF) in 4 patients, and VT (greater than or equal to 5 beats) in 11 patients. Inducible VT or VF was suppressed by antiarrhythmic drugs in 7 of 13 patients in whom they were tried. Patients underwent coronary artery bypass graft surgery unassociated with perioperative myocardial infarction. When studied again an average of 19 days after surgery, 10 patients had no inducible VT or VF without antiarrhythmic drug therapy; 6 had induced VT. One patient had spontaneous VT. An effective antiarrhythmic regimen that suppressed inducible or spontaneous VT, or both, was defined by serial electrophysiologic studies in 4 patients, whereas 3 patients continued to manifest electrically inducible VT with all antiarrhythmic regimens tested. All but 1 patient, in whom postoperative VT could not be suppressed, are free of arrhythmias after a mean follow-up period of 23 months (range 6 to 53). It is concluded that myocardial revascularization alone may improve the abnormal electrophysiologic findings in certain patients; however, this effect of coronary artery bypass graft surgery is unpredictable, and pre- and postoperative electrophysiologic studies are recommended as part of the evaluation of these patients.

Quinidine for ventricular arrhythmias: value of electrophysiologic testing

Quinidine was evaluated during serial electrophysiologic testing with programmed ventricular stimulation in 89 patients with life-threatening ventricular arrhythmias. In 30 of the 89 patients, quinidine therapy prevented the initiation of ventricular tachycardia (VT) during programmed ventricular stimulation. In 8 additional patients no single drug tested was effective, and quinidine in combination with either mexiletine (7 patients) or propranolol (1 patient) prevented the initiation of VT during electrophysiologic testing. The mean serum concentrations of quinidine in the patients who responded and those who failed to respond were 2.9 +/- 0.8 and 2.8 +/- 1.1 micrograms/ml, respectively; however, but nonresponders were characterized by more severe congestive heart failure and an increased incidence of digitalis use. During chronic therapy (24 +/- 3 months) with quinidine either alone or in combination with a second antiarrhythmic drug in the 38 patients whose arrhythmia had been suppressed during electrophysiologic testing, 32 (84%) remain symptom-free while 3 have had recurrent arrhythmia and 3 discontinued quinidine because of adverse effects. These data demonstrate that quinidine, when selected on the basis of electrophysiologic testing, provides effective long-term prophylaxis against recurrent ventricular arrhythmia and that approximately 40% of patients tested are likely to respond either to quinidine alone or quinidine in combination with another antiarrhythmic agent.

Intracardiac electrophysiologic techniques in recurrent syncope of unknown case

Twenty-five patients with recurrent episodes of syncope, unexplained despite thorough medical and neurologic evaluation, underwent intracardiac electrophysiologic study with programmed stimulation. Electrophysiologic study yielded a presumptive diagnosis in 17 patients: nine with rapid ventricular tachycardia by programmed stimulation, three with intra-His conduction delays, one with symptomatic atrial flutter, one with sick sinus syndrome, and three with persistent hypervagotonia manifested as atropine-reversible prolongation of atrioventricular nodal refractoriness. Therapy based on these findings provided complete symptomatic relief in 14 and improvement in one of these 17 patients during a mean follow-up of 18 +/- 10 months. Therapy based on electrophysiologic testing was ineffective in two of the 17 patients. Syncope persisted in four of the eight patients in whom electrophysiologic study did not define a probable arrhythmic mechanism. These observations indicate that full electrophysiologic evaluation with programmed stimulation is useful in the diagnosis and therapy of recurrent unexplained syncope.

Repetitive responses to single ventricular extrastimuli in patients with serious ventricular arrhythmias: incidence and clinical significance

Electrophysiologic studies were carried out in 85 patients with serious ventricular arrhythmias: 44 with recurrent sustained ventricular tachycardia (group A), 16 with recurrent nonsustained ventricular tachycardia (group B), and 25 with recent prehospital ventricular fibrillation not associated with acute myocardial infarction (group C). Programmed ventricular stimulation from the right ventricular apex included premature stimulation during normal sinus rhythm, atrial pacing, and ventricular pacing, as well as brief bursts of rapid ventricular pacing (RVP). A repetitive ventricular response (RVR) was defined as one or more non-stimulated premature ventricular depolarizations in response to a single paced premature ventricular depolarization during normal sinus rhythm or atrial pacing. RVRs were observed in seven of 44 (16%) group A patients, one of 16 (6%) group B patients, and three of 25 (12%) group C patients. In contrast, single and double premature ventricular stimuli during ventricular pacing and/or bursts of RVP resulted in the reproducible initiation of ventricular tachycardia in 40 of 44 (91%) group A patients, 10 of 16 (63%) group B patients, and 19 of 25 (76%) group C patients. We conclude that RVRs to single ventricular extrastimuli during normal sinus rhythm or atrial pacing are rare, and therefore are an insensitive index of susceptibility to serious ventricular arrhythmias in these patients.

Mexiletine for refractory-ventricular arrhythmias: results using serial electrophysiologic testing

Mexiletine was administered during serial drug testing to 35 patients with electrically inducible ventricular arrhythmias and to 6 with recurrent ventricular tachycardia that could not be induced or terminated by programmed cardiac stimulation. All patients had arrhythmias resistant to all conventionally available agents. Electrically induced arrhythmias were completely suppressed during mexiletine therapy in 13 patients. In 12 patients no antiarrhythmic regimen was completely suppressive and in 7 of these mexiletine favorably modified the response to programmed stimulation. In four of six patients with frequent episodes of spontaneous ventricular tachycardia that were not inducible by programmed cardiac stimulation, arrhythmia was controlled by mexiletine. The presence of complete arrhythmia suppression with mexiletine during acute testing accurately predicted long-term freedom from recurrent arrhythmia in 16 of 17 patients over a mean follow-up period of 12.6 +/- 6 months. Severe adverse neurologic effects were noted early during mexiletine therapy in three patients, but no patient discharged on such therapy experienced major adverse effects. The study demonstrates that mexiletine can provide well tolerated effective prophylaxis against recurrent ventricular arrhythmias in a significant proportion of patients resistant to conventional drugs.

Out-of-hospital cardiac arrest: electrophysiologic observations and selection of long-term antiarrhythmic therapy

We performed electrophysiologic studies in 31 survivors of out-of-hospital cardiac arrest not associated with acute myocardial infarction. At the time of resuscitation, eight patients had sustained ventricular tachycardia and 23 patients had ventricular fibrillation. Programmed right ventricular stimulation later revealed electrically inducible ventricular arrhythmias in 25 of the 31 patients (81 per cent). Complete suppression of electrically inducible ventricular arrhythmias was achieved with antiarrhythmic therapy in 19 of these 25 patients. None of the 19 patients in whom the inducible arrhythmias was suppressed before discharge has died suddenly or had a symptomatic arrhythmia after a mean follow-up of 15 months (range, five to 26 months). Of the six patients in whom inducible arrhythmias could not be suppressed, three died suddenly (one in the hospital) within six months. We conclude that ventricular arrhythmias can be initiated and reproduced by programmed ventricular stimulation in a majority of patients who have been resuscitated after out-of-hospital cardiac arrest and that complete suppression of these arrhythmias with anti-arrhythmic therapy is highly predictive of survival for at least one year.