Antiarrhythmic drug therapy in the Multicenter UnSustained Tachycardia Trial (MUSTT): drug testing and as-treated analysis.

OBJECTIVES: Using data from the Multicenter UnSustained Tachycardia Trial (MUSTT), we examined the factors used to select antiarrhythmic drug therapy and their impact on outcomes. BACKGROUND: The MUSTT examined the use of programmed ventricular stimulation (PVS) to guide antiarrhythmic therapy in patients with coronary arteriosclerosis, left ventricular dysfunction and asymptomatic, unsustained ventricular tachycardia (VT). Trial outcomes may reflect factors used to select antiarrhythmic drug therapy. METHODS: We compared subgroups of patients with inducible sustained VT randomized to PVS-guided antiarrhythmic therapy (n = 351), in particular those receiving PVS-guided antiarrhythmic drug therapy (n = 142) versus no antiarrhythmic therapy (controls, n = 353). RESULTS: "Effective" antiarrhythmic drug therapy (i.e., the term "effective" was used to denote therapy that resulted in noninducible VT or hemodynamically stable induced VT) was found for 142 of the 351 patients (43%), most often at the first or second PVS session (125/142, 88%). Mortality among the 142 patients did not differ from that among control patients. Of these 142 patients, the PVS end point was noninducibility in 91 patients and stable VT in 51 patients. Mortality did not differ between these two groups either, but arrhythmia was numerically more frequent in the PVS-induced stable VT group. Mortality was greatest in the few patients receiving propafenone (unadjusted p = 0.07, adjusted p = 0.14 vs. controls), but mortality with all agents did not differ from that of controls, even after adjustment. CONCLUSIONS: Even when presenting the results as favorably as possible, we found no benefit with PVS-guided drug therapy in patients with clinical unsustained VT who had inducible sustained VT. These findings are unaltered by using different end points for PVS or considering the response to individual drugs.

Outcome of men with ischemic cardiomyopathy, asymptomatic nonsustained ventricular tachycardia, and negative electrophysiologic studies.

The natural history of patients with ischemic cardiomyopathy, asymptomatic nonsustained ventricular tachycardia, and negative electrophysiologic studies was assessed in a retrospective analysis of 55 consecutive patients. The results suggest that the prognosis for arrhythmic death in these patients is not necessarily benign.

Interaction between autonomic tone and the negative chronotropic effect of adenosine in humans.

Prior studies have demonstrated that sympathetic tone may influence the effects of adenosine on His-Purkinje automaticity, and that enhanced vagal tone may influence its effects on the sinus node. However, the interaction between autonomic tone and the effects of adenosine on the sinus node in humans remains unknown. Therefore, this study was designed to investigate the interaction between different states of autonomic tone and the bradycardiac response of the sinus node to adenosine. In 11 patients without structural heart disease who underwent a clinically indicated electrophysiology procedure, the sinus cycle length was measured before and after a 12-mg bolus of adenosine in the baseline state, during an infusion of 2 mcg/min of isoproterenol, after the administration of 0.2 mg/kg of propranolol, and again after the administration of 0.04 mg/kg of atropine. Adenosine significantly lengthened the sinus cycle length in the baseline state (760 +/- 165 vs 909 +/- 188 ms, P < 0.05), during isoproterenol infusion (516 +/- 67 vs 766 +/- 146 ms, P < 0.05), after propranolol (850 +/- 153 vs 914 +/- 143 ms, P < 0.05) and after the combination of propranolol and atropine (662 +/- 76 vs 801 +/- 121 ms, P < 0.05). The degree of lengthening in sinus cycle length was significantly greater (P < 0.05) during isoproterenol infusion (253 +/- 157 ms, or 51% +/- 40%) than in the baseline state (149 +/- 85 ms, or 20% +/- 12%), after propranolol (68 +/- 53 ms, or 8% +/- 8%), and after propranolol and atropine (140 +/- 110 ms, or 21% +/- 18%). The negative chronotropic effect of adenosine is influenced by autonomic tone. The effect of adenosine on the sinus node is accentuated by beta-adrenergic stimulation and unaffected by beta-adrenergic blockade or combined beta-adrenergic and cholinergic blockade.

Immediate reproducibility of tilt-table test results in elderly patients referred for evaluation of syncope or presyncope.

We studied the immediate reproducibility of the results of the head-up tilt-table test in elderly patients (age > or =60 years). Twenty-seven consecutive men underwent 51 tests. The overall reproducibility was 98%, including 92% of a positive test and 100% of a negative test. The finding of this study validates the use of intravenous pharmacologic intervention in the elderly population.

Comparison of early and late complications in patients undergoing coronary artery bypass graft surgery with and without concomitant placement of an implantable cardioverter defibrillator.

Previous studies have reported a significant morbidity and mortality associated with coronary artery bypass graft (CABG) surgery in conjunction with the placement of an implantable cardioverter defibrillator (ICD) with an epicardial lead system. In the absence of a control group, how significantly the component of concomitant placement of the ICD system contributes to these untoward outcomes remains unknown. The purpose of this study was to assess the short- and long-term complications in patients undergoing CABG surgery in conjunction with the placement of an ICD with epicardial leads and to compare these complications with those of patients who had only CABG surgery (control group). The study group (group A) consisted of 56 patients who underwent CABG surgery and placement of an ICD pulse generator with epicardial leads. A control group (group B) consisted of 56 patients who underwent CABG surgery only during the same time period. The two groups were matched for age, sex distribution, left ventricular function, surgical approach, number of bypass grafts per patient, bypass pump time, and length of follow-up period. The early mortality for group A was 7.1% versus 1.8% for group B (p > 0.05). The incidence of early morbidity (congestive heart failure, infection, supraventricular and ventricular arrhythmias) for groups A and B was similar (26.8% vs 25.0%, p > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Incidence of implantable defibrillator discharges after coronary revascularization in survivors of ischemic sudden cardiac death.

Coronary revascularization has been suggested as sole therapy for secondary prevention of sudden cardiac arrest associated with ischemia. The use of implantable defibrillators (ICD) in combination with coronary revascularization for this patient population is unclear. Among 412 consecutive patients receiving an ICD, 23 (6%) were identified as sudden cardiac arrest survivors who were noninducible with programmed stimulation and had unstable angina or ischemia on a functional study; they underwent successful coronary revascularization. During a follow-up of 34 +/- 18 months, 10 (43%) of the 23 patients received ICD shocks (8 +/- 8 per patient, range 1 to 22 shocks), and nine of the 10 patients had syncope/presyncope associated with at least one ICD discharge. Patients with ICD discharges were compared with those without ICD discharges, and no clinical characteristics were statistically different between the two groups. In conclusion, revascularization alone may be inadequate therapy for survivors of sudden cardiac arrest associated with ischemia who are noninducible with programmed stimulation, and clinical variables cannot predict which patients are likely to have recurrent malignant ventricular arrhythmias.

Incidence, presentation, diagnosis, and management of malfunctioning implantable cardioverter-defibrillator rate-sensing leads.

Recognition of tachyarrhythmia by an implantable cardioverter-defibrillator (ICD) requires an intact rate-sensing lead. We retrospectively examined 266 consecutive patients requiring an ICD to characterize the incidence, clinical presentation, diagnosis, and management of a defective rate-sensing lead. To identify clinical parameters that may contribute to lead complications, we also assessed the effects of age, gender, type of rate-sensing lead, manufacturer of the lead, and surgeon. Over a follow-up period of 30 +/- 22 months (mean +/- standard deviation), a defective lead was found in 9 (3.4%) patients, in 9 (1.7%) of 514 leads over a period of 2 to 39 (mean 17 +/- 15) months after implantation. Except for 1 patient, in whom a lead fracture was incidently found during ICD generator replacement, these patients had multiple inappropriate shocks of recent onset. Clinical parameters were not helpful in identifying patients at risk for lead complication. An abnormal beeping signal obtained while the patients performed various maneuvers was helpful in confirming a defect. All of the defective leads were epicardial. These cases were managed by placement of a transvenous endocardial lead.

Iodixanol, a new isosmotic nonionic contrast agent compared with iohexol in cardiac angiography.

Iodixanol, a new ratio 6 nonionic iodinated contrast agent with an osmolality equal to serum, was compared with iohexol in a randomized, double-blind, parallel study. Two hundred patients undergoing elective diagnostic cardiac angiography were randomized to iodixanol (n = 101) or iohexol (n = 99). There were no differences noted between the 2 agents in the mean changes in systolic or diastolic blood pressure or heart rate during or immediately after any angiography. However, significantly more patients had a decrease in diastolic blood pressure of > 20 mm Hg during left coronary angiography with iodixanol. The only significant differences in any electrophysiologic parameter were slightly more PR prolongation during left coronary angiography with iodixanol and more ST-segment depression with iohexol during coronary angiography. Neither was clinically significant. Injection-associated discomfort occurred with both agents, but more patients experienced moderate to severe discomfort with iohexol (52%) than with iodixanol (17%) (p < 0.001). Only 1 potentially serious adverse event, ventricular fibrillation with iohexol, was considered related to contrast, and there were no differences noted between the agents. Overall, angiographic quality was equal with all angiograms being assessed as good or excellent in both groups (p = 0.885). In this low-risk population undergoing cardiac angiography, iodixanol is safe and effective without clinically important differences from iohexol. Additional studies in patients at high risk for complications should help further define the role of iodixanol in cardiac angiography.

Incidence of loss of consciousness during automatic implantable cardioverter-defibrillator shocks.

OBJECTIVE: To determine the incidence of loss of consciousness occurring in association with shocks delivered by automatic implantable cardioverter-defibrillators (AICD) in patients who had undergone implantation as treatment for ventricular tachycardia or ventricular fibrillation. DESIGN: Cohort study. SETTING: Two major tertiary medical care facilities. PATIENTS: A total of 180 patients who had undergone implantation of an AICD for treatment of ventricular tachycardia or ventricular fibrillation. INTERVENTION: Implantation of an AICD that delivered only high-energy shock. MEASUREMENTS: During a mean (+/- SD) follow-up period of 16 +/- 12 months, the incidence of loss of consciousness occurring in association with spontaneous AICD shocks was determined. Various clinical factors were analyzed to identify predictors of loss of consciousness that occurred during AICD shocks. MAIN RESULTS: Of the 180 patients who received an AICD, 106 patients (59%) experienced AICD shocks during follow-up. Sixteen of the 180 patients (9%) experienced loss of consciousness; 13 of these 16 patients had syncope and 3 died suddenly, in association with AICD shocks. The absence of syncope during one AICD shock did not always predict the absence of syncope during subsequent shocks. Syncope could not be predicted by age, sex, history of syncope, left ventricular function, type of underlying heart disease, electrophysiologic findings, rate of ventricular tachycardia, antiarrhythmic medications, and type of pulse generator implanted. CONCLUSIONS: Patients with sustained ventricular tachycardia or ventricular fibrillation who receive an AICD that delivers only high-energy shock therapy are at moderate risk for experiencing loss of consciousness during AICD shocks. No clinical variables were found to be predictors of syncope. Therefore, driving and other activities that require patients to be extra vigilant should not be assumed to be safe after implantation of an AICD that delivers only high-energy shock.

Reduction in medical care cost associated with radiofrequency catheter ablation of accessory pathways.

The cost of definitive therapy was compared in 25 patients who underwent radiofrequency catheter ablation of accessory pathways in 1990 and 25 patients who underwent surgical ablation of accessory pathways in 1989. In the radiofrequency group, 23 of 25 patients had a single accessory pathway and the remaining 2 patients each had 2 accessory pathways. In the surgical group, 20 patients had a single accessory pathway and 5 patients each had 2 accessory pathways. The success rate was 96% in each group. The mean duration of hospitalization was 3 +/- 1 days in the radiofrequency group and 9 +/- 4 days in the surgical group (p less than 0.0001). All the cost data are expressed in fiscal year 1990/1991 dollar values. The total cost of therapy in the radiofrequency group was $14,919 +/- $6,740 compared with $53,265 +/- $12,755 in the surgical group (p less than 0.0001). The cost of radiofrequency ablation consisted of a hospital charge of $7,753 +/- $3,472 and physician fees of $7,166 +/- $3,439. The hospital charge included charges for use of the electrophysiology laboratory, hospital stay, electrocardiograms, echocardiograms and blood studies. The cost of surgery consisted of a hospital charge of $37,708 +/- $10,179 and physician fees of $15,557 +/- $3,149. The hospital charge in the surgical group included the costs of a baseline electrophysiology study, in-hospital care and a follow-up office visit. In conclusion, radiofrequency catheter ablation of accessory pathways results in a dramatic reduction in the cost of definitive therapy in patients with the Wolff-Parkinson-White syndrome.

Effect of antiarrhythmic drug therapy on the incidence of shocks in patients who receive an implantable cardioverter defibrillator after a single episode of sustained ventricular tachycardia/fibrillation.

Seventy-four patients (16 women, 58 men, age 58 +/- 11 years, mean +/- standard deviation) who received an implantable cardioverter defibrillator (ICD) after experiencing a single episode of ventricular tachycardia or ventricular fibrillation were followed to determine if antiarrhythmic drug therapy affects the incidence of ICD discharges. Thirty-three patients (group A) were treated with an antiarrhythmic drug that was either untested or previously demonstrated during electropharmacological testing to be ineffective in suppressing the induction of ventricular tachycardia. Forty-one patients (group B) were not treated with an antiarrhythmic drug. There were no significant differences between the two groups in regards to age, sex, incidence of coronary artery disease, left ventricular function or the type of ICD pulse generator used. During a mean follow-up of 14 months for the entire cohort, 15 patients (46%) in group A and 18 patients (44%) in group B experienced at least one ICD shock. The time to the first appropriate shock (5 +/- 5 months in both groups) and the frequency of ICD shocks (0.3 +/- 0.2/month in group A vs 0.4 +/- 0.5/month in group B) were similar in both groups. The incidence of syncope at the time of ICD discharge was higher in group A than group B patients (31% vs 5%, P less than 0.05). In conclusion, antiarrhythmic drugs that are untested or have failed electropharmacological testing do not appear to reduce the probability of ICD discharge over a short-term (mean 14 months) follow-up in patients who have had only one clinical episode of VT/VF and may increase the risk of syncope during ICD discharge.(ABSTRACT TRUNCATED AT 250 WORDS)

Diagnosis and cure of the Wolff-Parkinson-White syndrome or paroxysmal supraventricular tachycardias during a single electrophysiologic test.

BACKGROUND: We conducted this study to determine the feasibility of an abbreviated therapeutic approach to the Wolff-Parkinson-White syndrome or paroxysmal supraventricular tachycardia, in which the diagnosis is established and radiofrequency ablation carried out during a single electrophysiologic test. METHODS: One hundred six consecutive patients were referred for the management of documented, symptomatic paroxysmal supraventricular tachycardias (66 patients) or the Wolff-Parkinson-White syndrome (40 patients). All agreed to undergo a diagnostic electrophysiologic test and catheter ablation with radiofrequency current. No patient had had such a test previously. RESULTS: Among the 66 patients with paroxysmal supraventricular tachycardias, the mechanism was found to be atrioventricular nodal reentry in 46 (70 percent) (typical in 44 and atypical in 2), atrioventricular reciprocating tachycardia involving a concealed accessory pathway in 16 (24 percent), atrial tachycardia in 2 (3 percent), and noninducible paroxysmal supraventricular tachycardia in 2 (3 percent). A successful long-term outcome was achieved in 57 of 62 patients (92 percent) with paroxysmal supraventricular tachycardia in whom ablation was attempted and in 37 of 40 patients (93 percent) with the Wolff-Parkinson-White syndrome. The only complications were one instance of occlusion of the left circumflex coronary artery, leading to acute myocardial infarction, and one instance of complete atrioventricular block. The mean (+/- SD) duration of the electrophysiologic procedures was 114 +/- 55 minutes. CONCLUSIONS: The diagnosis and cure of paroxysmal supraventricular tachycardia or the Wolff-Parkinson-White syndrome during a single electrophysiologic test are feasible and practical and have a favorable risk-benefit ratio. This abbreviated therapeutic approach may eliminate the need for serial electropharmacologic testing, long-term drug therapy, antitachycardia pacemakers, and surgical ablation.

Concealed entrainment as a guide for catheter ablation of ventricular tachycardia in patients with prior myocardial infarction.

Fifteen consecutive patients with drug-refractory, recurrent, sustained, monomorphic ventricular tachycardia and a history of remote myocardial infarction underwent catheter ablation of ventricular tachycardia. Shocks of 100 to 300 J were delivered to sites at which pacing during ventricular tachycardia resulted in concealed entrainment, in which the ventricular tachycardia accelerated to the pacing rate, there was a long stimulus to QRS interval and there was no change in the configuration of the QRS complex during pacing at several rates compared with the configuration during ventricular tachycardia, thus identifying a zone of slow conduction in the reentrant circuit. Concealed entrainment was demonstrated in nine (60%) of 15 patients, and the stimulus to QRS intervals were 90 to 400 ms. At sites of concealed entrainment, the endocardial activation time relative to the QRS complex during ventricular tachycardia ranged from -125 to +50 ms, the timing of the local electrogram relative to the QRS complex was the same during entrainment as during ventricular tachycardia and the pace map during sinus rhythm was discordant with that of the ventricular tachycardia in seven patients. In the six patients in whom a site of concealed entrainment could not be identified, the target site for ablation was selected on the basis of identification of an isolated mid-diastolic potential, activation mapping and pace mapping. The mean (+/- SD) cumulative number of joules delivered to the target site was 306 +/- 140. A successful long-term clinical outcome was achieved in 9 of the 15 patients (mean follow-up 20 +/- 7 months). The clinical success rate was the same whether the target site was selected on the basis of concealed entrainment (five of nine, 56%) or on the basis of the other mapping techniques (four of six, 67%). In conclusion, the responses to pacing suggest that sites at which there is concealed entrainment may be located within a zone of slow conduction in the ventricular tachycardia reentry circuit, although not necessarily in an area critical for the maintenance of reentry. The long-term clinical efficacy of catheter ablation targeted to sites of concealed entrainment is about 60%, similar to the results achieved when conventional mapping techniques are used.

Prospective comparison of a conventional and an accelerated protocol for programmed ventricular stimulation in patients with coronary artery disease.

BACKGROUND: This study compared the sensitivity, specificity, and efficiency of a "conventional" and "accelerated" programmed stimulation protocol in 293 patients with coronary artery disease who had a history of sustained or nonsustained monomorphic ventricular tachycardia (VT). METHODS AND RESULTS: In the conventional protocol, one and two extrastimuli were introduced during sinus rhythm and during basic drive trains at cycle lengths of 600 and 400 msec at the right ventricular apex and then at the outflow tract or septum. In the accelerated protocol, one, two, and then three extrastimuli were introduced at each of three basic drive train cycle lengths (350, 400, and 600 msec) at the right ventricular apex; the procedure was repeated at a second right ventricular site. Six hundred thirty-four electrophysiological tests were performed using one of these two protocols either in the baseline state (293 tests) or during drug testing (341 tests). The yield of sustained, monomorphic VT was 89% with the conventional protocol and 92% with the accelerated protocol during baseline tests in patients who had a history of sustained VT (p = 0.05); 20% and 34%, respectively, during baseline tests in patients with a history of nonsustained VT (p = 0.06); and 70% and 77%, respectively, during drug testing (p = 0.2). To induce sustained, monomorphic VT, 10.1 +/- 5.0 (mean +/- SD) protocol steps and 14.4 +/- 8.7 minutes were required with the conventional protocol, compared with 4.0 +/- 3.7 steps and 5.6 +/- 6.1 minutes with the accelerated protocol (p less than 0.001 for each comparison). Among the tests in which sustained, monomorphic VT was induced, sustained polymorphic VT or ventricular fibrillation was induced more often with the conventional protocol (3.6%) than with the accelerated protocol (0.9%, p = 0.05). CONCLUSIONS: The efficiency of programmed stimulation can be improved by the early use of a basic drive train cycle length of 350 msec and three extrastimuli. Compared with a conventional stimulation protocol, the accelerated protocol used in this study reduces the number of protocol steps and duration of time required to induce monomorphic VT by an average of more than 50% and improves the specificity of programmed stimulation without impairing the yield of monomorphic VT.

Long-term results of catheter ablation of idiopathic right ventricular tachycardia.

Ten consecutive patients with recurrent episodes of symptomatic, idiopathic, sustained monomorphic ventricular tachycardia (VT) originating in the right ventricle underwent an attempt at catheter ablation of the ventricular tachycardia. There were seven women and three men, with a mean age of 39 +/- 14 years (+/- SD). None of the patients had any evidence of structural heart disease. The VT had a left bundle branch block configuration and an inferior axis in each patient, and the mean cycle length was 313 +/- 75 msec. Based on the methods of induction of the VT and the response of the VT to verapamil, the VT mechanism was presumed to be reentry in six patients, triggered activity in three patients, and catecholamine-sensitive automaticity in one patient. Sites for ablation were guided by pace mapping, and an appropriate target site was identified in the right ventricular outflow tract in each patient. From one to three shocks of 100-360 J (mean total, 336 +/- 195 J) were delivered from a defibrillator between the tip of the ablation catheter (cathode) and a patch electrode on the anterior chest (anode). An electrophysiology test 7-9 days after ablation demonstrated that VT was still inducible in only one patient, who was treated with amiodarone. One other patient had a recurrence of VT 3 weeks after ablation and was treated with verapamil. Eight of 10 patients were not treated with antiarrhythmic medications and have had no episodes of symptomatic VT during 15-68 months of follow-up (mean follow-up, 33 +/- 18 months). There were no acute or long-term complications.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of atrioventricular relationship on ventricular refractoriness in humans.

Right ventricular refractory periods obtained using atrial and ventricular pacing with an atrioventricular (AV) interval of 160 ms were compared with those obtained with an AV interval of 0 ms in a group of 53 patients. The mean right ventricular effective refractory periods were longer at an AV interval of 160 ms than at an AV interval of 0 ms at pacing cycle lengths of 600 (mean 2.5 +/- 1.8 ms difference) and 400 ms (mean 3.8 +/- 2.4 ms difference; P less than 0.01). In a subset of 10 patients, left ventricular volumes measured by two-dimensional echocardiography demonstrated that pacing with an AV interval of 160 ms was associated with a higher end-diastolic volume and stroke volume than pacing with an AV interval of 0 ms (P less than 0.001). Autonomic blockade did not alter the relationship between AV interval and ventricular refractoriness. We conclude that 1) right ventricular refractory periods are shorter at an AV interval of 0 ms than at an AV interval of 160 ms; 2) these differences are not caused by changes in autonomic tone but are associated with differences in ventricular volumes; and 3) if precise determinations of ventricular refractoriness are desired, then the timing of atrial systole should be controlled by atrial pacing.

The ventricular excitability-interval relationship in early diastole in humans: the influence of the electrode configuration during bipolar stimulation.

The purpose of this study was to determine the relationship between ventricular excitability and the extrastimulus (S2) coupling interval in early diastole in humans. The influence of the electrode configuration during bipolar stimulation was examined. Ventricular strength-interval curves were constructed using an eight beat drive train at a cycle length of 600 msec, 2-msec decrements in the S2 coupling interval, and 0.2-mA increments in the current intensity of S2 at each coupling interval. A transient dip in the excitation threshold was observed in early diastole during bipolar pacing when the distal electrode was the anode in six of 17 patients (35%). In contrast, this type of dip was not observed when the distal electrode was the cathode. The excitation threshold at the trough of the dip ranged from 1.0 to 2.2 mA. Unipolar strength-interval curves indicated that a dip occurred with anodal but not cathodal pacing. We postulated that the dip might interfere with the accurate determination of the ventricular effective refractory period by resulting in transient loss of ventricular capture during decremental scanning of diastole with S2. A gap in ventricular capture was produced in all six patients who demonstrated a dip in the bipolar strength-interval curve by selecting an S2 current intensity that fell within the dip. In conclusion, the bipolar strength-interval curve may display an early diastolic dip in the excitation threshold, depending on the electrode configuration.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute changes in pacing threshold and R- or P-wave amplitude during permanent pacemaker implantation.

This study examines the changes in pacing threshold and R- or P-wave amplitude during the first 30 minutes after implantation of tined and screw-in leads. The leads examined were those of 1 manufacturer (Medtronic) and consisted of 3 ventricular pacing leads (model numbers 6957 unipolar screw-in [11 patients], 6961 unipolar tined [12 patients] and 6962 bipolar tined [7 patients]) and 1 atrial lead (model number 6957J unipolar screw-in [10 patients]). After optimal lead position was obtained fluoroscopically in the right ventricular apex or right atrium, the pacing threshold and R- or P-wave amplitudes were measured at 5-minute intervals for 30 minutes. The acute ventricular pacing threshold with the screw-in lead was significantly higher than with the tined lead (0.84 +/- 0.17 vs 0.58 +/- 0.15 volts; p less than 0.001). There was a significant (p less than 0.001) acute decrease in the ventricular pacing threshold with both lead types, with the maximum decrease occurring 5 minutes after lead implantation. There was a significant acute increase in R-wave size with the ventricular screw-in lead that peaked 20 minutes after lead implantation (11.9 +/- 3.0 to 14.7 +/- 4.1 mV; p less than 0.001). The atrial screw-in lead behaved in a manner identical to its counterpart in the ventricle. In conclusion, there are acute changes in the pacing threshold and R- or P-wave amplitude obtained with tined and screw-in pacing leads.(ABSTRACT TRUNCATED AT 250 WORDS)