Characterization of immunoglobulin heavy chain knockout rats

The rat is a species frequently used in immunological studies but, until now, there were no models with introduced gene-specific mutations. In a recent study, we described for the first time the generation of novel rat lines with targeted mutations using zinc-finger nucleases. In this study, we compare immune development in two Ig heavy-chain KO lines; one with truncated Cμ and a new line with removed JH segments. Rats homozygous for IgM mutation generate truncated Cμ mRNA with a de novo stop codon and no Cγ mRNA. JH-deletion rats showed undetectable mRNA for all H-chain transcripts. No serum IgM, IgG, IgA and IgE were detected in these rat lines. In both lines, lymphoid B-cell numbers were reduced >95% versus WT animals. In rats homozygous for IgM mutation, no Ab-mediated hyperacute allograft rejection was encountered. Similarities in B-cell differentiation seen in Ig KO rats and ES cell-derived Ig KO mice are discussed. These Ig and B-cell-deficient rats obtained using zinc-finger nucleases-technology should be useful as biomedical research models and a powerful platform for transgenic animals expressing a human Ab repertoire.

Breath-holding spells associated with significant bradycardia: successful treatment with permanent pacemaker implantation

OBJECTIVE

To determine whether children with pallid breath-holding spells associated with bradycardia can be treated safely and successfully with permanent pacemaker implantation.

METHODS

The records of pediatric patients who had apparent breath-holding spells and associated bradycardia and were treated with permanent pacemaker implantation were reviewed.

RESULTS

Ten pediatric patients with apparent breath-holding spells associated with bradycardia were treated with a permanent ventricular demand pacemaker at the Mayo Clinic between 1985 and 1995. Patients had onset of symptoms between ages 6 days and 12 months and presented for evaluation between ages 12 months and 5 years. Duration of spells was 15 seconds to 10 minutes. Medications to prevent spells were unsuccessful. Electrocardiograms documented asystolic pauses of 1.7 to 24 seconds (mean: 11.9 seconds). Permanent ventricular demand pacemakers were implanted at 10 months to 5 years of age (median: 14.5 months): 9 endocardial and 1 epicardial. Three patients required pacemaker revision. At follow-up of 38 to 170 months (median: 65.5), 5 patients had complete resolution of spells, 2 had only mild color change without loss of consciousness or seizure activity, and 3 continued to have minor brief spells.

CONCLUSIONS

Permanent pacemaker therapy for children with pallid breath-holding spells associated with severe bradycardia is safe, efficacious, and warranted.

Is sinus node modification appropriate for inappropriate sinus tachycardia with features of postural orthostatic tachycardia syndrome?

Inappropriate sinus tachycardia and postural orthostatic tachycardia are ill-defined syndromes with overlapping features. Although sinus node modification has been reported to effectively slow the sinus rate, long-term clinical response has not been adequately assessed. Furthermore, whether patients with postural orthostatic tachycardia would benefit from sinus node modification is unknown. The study prospectively assessed the short- and long-term clinical outcomes of seven consecutive female patients with postural orthostatic tachycardia syndrome and inappropriate sinus tachycardia who were treated with sinus node modification. The study was conducted in a tertiary care center. The electrophysiological and clinical responses were prospectively assessed as defined by autonomic function testing, including Valsalva maneuver, deep breathing, tilt table testing, and quantitative sudomotor axonal reflex testing. Among the study population (mean age was 41+/-6 years), 5 (71%) patients had successful sinus node modification. At baseline, heart rates were 101+/-12 beats/min before modification and 77+/-9 beats/min after modification (P = 0.001). With isoproterenol, heart rates were 136+/-9 and 105+/-12 beats/min (P = 0.002) before and after modification, respectively. The mean heart rate during 24-hour Holter monitoring was also significantly reduced: 96+/-9 and 72+/-6 beats/min (P = 0.005) before and after modification, respectively. Despite the significant reduction in heart rate, autonomic symptom score index (based on ten categories of clinical symptoms) was unchanged before (15.6+/-4.1) and after (14.6+/-3.6) sinus node modification (P = 0.38). Sinus rate can be effectively slowed by sinus node modification. Clinical symptoms are not significantly improved after sinus node modification in patients with inappropriate sinus tachycardia and postural orthostatic tachycardia. A primary subtle autonomic disregulation is frequently present in this population. Sinus node modification is not recommended in this patient population.

Evaluation of the protein truncation test and mutation detection in the NF1 gene: mutational analysis of 15 known and 40 unknown mutations

The neurofibromatosis type 1 (NF1) gene located at 17q 11.2 contains 60 exons and spans 350 kb of genomic DNA. Mutation analysis has been hampered by the large size of the gene, the high rate of new mutations, a lack of mutational clustering and the presence of numerous homologous loci. Mutation detection methods based on the direct analysis of a gene's RNA transcript permit the rapid screening of large multi-exonic genes. However, the detection of frame-shift or nonsense mutations can be limited by instability of the mutant mRNA species due to nonsense-mediated decay. In order to determine the frequency of this allelic exclusion, total lymphocyte RNA was analysed from 15 NF1 patients with known truncating mutations and a panel of 40 NF1 patients with unknown mutations. The level of expression of the mutant message was greatly reduced in 2 of the 15 samples (13%), and 3 of the 18 informative samples from the panel of 40. A coupled reverse-transcription polymerase chain reaction and protein truncation test method was subsequently applied to screen RNA from the panel of 40 unrelated NF1 patients. Aberrant polypeptide bands were identified and characterised in 21 samples (53%). The mutations identified were 479del107;ins31, 495delTGTT, 1127delTGAT, R416X, R440X, 1446del 62, 1541delAG, 2252del 74, 2537insTG, 3456delACTC, R1276X, R1362X, 5749ins171, 6084del280, 6487insA, R2214X, 6791insA, 6858del141, 7458delC, 7676 2A-G and 8081delC. These mutations were uniformly distributed across the gene and 14 represent novel changes that contribute to the germline mutational spectrum of the NF1 gene.

Arrhythmogenic right ventricular dysplasia masquerading as dilated cardiomyopathy

This is a case report of 2 patients with severe left ventricular dysfunction and ventricular arrhythmias. At the time of cardiac transplantation they were found to have right ventricular dysplasia with left ventricular involvement.

Effects of slow pathway ablation on fast pathway function in patients with atrioventricular nodal reentrant tachycardia

INTRODUCTION

This study investigated whether fast pathway conduction properties are altered by slow pathway ablation in patients with AV nodal reentrant tachycardia.

METHODS AND RESULTS

Forty consecutive patients who underwent successful ablation of the slow pathway were prospective subjects for the study. Isoproterenol was used to enhance conduction and to differentiate interactive mechanisms. Potential electrotonic interactions were assessed by comparing patients with and those without residual dual AV node physiology after slow pathway ablation. Paired and unpaired t-tests were used when appropriate P < 0.05 was considered statistically significant. In the entire study population, heart rates were not significantly different before and after slow pathway ablation (RR = 770 +/- 114 msec before and 745 +/- 99 msec after, P = 0.07). Anterograde fast pathway conduction properties were unchanged after slow pathway ablation (effective refractory period, 348 +/- 84 msec before and 336 +/- 86 msec after, P = 0.13; shortest 1:1 conduction, 410 +/- 93 msec before and 400 +/- 82 msec after, P = 0.39). Retrograde fast pathway characteristics also were similar before and after ablation. Neither anterograde nor retrograde fast pathway conduction properties during isoproterenol infusion were changed by slow pathway ablation. When the study population was further divided into patients with (n = 13) or without (n = 27) residual dual AV node physiology, no significant change was detected in fast pathway function in either group after slow pathway ablation.

CONCLUSIONS

Fast pathway conduction characteristics were not affected by slow pathway ablation. In patients with AV nodal reentrant tachycardia, observations suggest that fast and slow pathways are functionally distinct.

Mutational and functional analysis of the neurofibromatosis type 1 (NF1) gene

Neurofibromatosis type 1 (NF1) is one of the most common autosomal dominant disorders. It is caused by mutations in the NF1 gene which comprises 60 exons and is located on chromosome 17q. The NF1 gene product, neurofibromin, displays partial homology to GTPase-activating protein (GAP). The GAP-related domain (GRD), encoded by exons 20-27a, is the only region of neurofibromin to which a biological function has been ascribed. A total of 320 unrelated NF1 patients were screened for mutations in the GRD-encoding region of the NF1 gene. Sixteen different lesions in the NF1 GRD region were identified in a total of 20 patients. Of these lesions, 14 are novel and together comprise three missense, two nonsense and three splice site mutations plus six deletions of between 1 and 4 bp. The effect of one of the missense mutations (R1391S) was studied by in vitro expression of a site-directed mutant and GAP activity assay. The mutant protein, R1391S, was found to be some 300-fold less active than wild-type NF1 GRD. The mutations reported in this study therefore provide further material for the functional analysis of neurofibromin as well as an insight into the mutational spectrum of the NF1 GRD.

Adenosine: potential modulator for vasovagal syncope

OBJECTIVES

This study examined the hypothesis that adenosine could provoke a vasovagal response in susceptible patients. Mechanisms of the vasovagal response were further explored by studying the adenosine-mediated reactions.

BACKGROUND

Increased sympathetic activity is frequently observed before vasovagal syncope. Recent studies have demonstrated that adenosine, in addition to its direct bradycardiac and vasodilatory effects, can increase sympathetic discharge by activating cardiovascular afferent nerves.

METHODS

The effects of adenosine and head-up tilt-table testing with or without isoproterenol were prospectively evaluated in 85 patients examined for syncope after negative results of electrophysiologic testing (51 men and 34 women, mean [+/- SD] age 61 +/- 17 years). Adenosine bolus injections of 6 mg and 12 mg were sequentially administered to patients in the upright position. The same protocol was implemented in 14 normal control subjects (7 men and 7 women, mean [+/- SD] age 38 +/- 10 years).

RESULTS

Transient hypertension or tachycardia was observed in 57 (67%) and 20 (24%) patients after administration of 6 mg and 12 mg of adenosine, respectively, during the immediate phase (first 15 s), suggesting direct sympathetic activation. Hypotension and reflex tachycardia were observed in all patients during the delayed phase (15 to 60 s after adenosine injection), suggesting baroreceptor unloading. A vasovagal response was induced in 22 (26%) and 29 (34%) patients after adenosine administration and during tilt-table testing. Inducibility of a vasovagal response by these two methods was comparable (p = 0.12). Of the control subjects, one (7%) had a vasovagal response after adenosine administration and one (7%) had a positive response during tilt-table testing.

CONCLUSIONS

These observations support the idea that adenosine is an endogenous modulator of the cardiac excitatory afferent nerves. Sympathetic activation by adenosine can be direct (i.e., cardiac excitatory afferent nerves) and indirect (i.e., vasodilation and reflex sympathetic activation). Adenosine could be an important modulator in triggering a vasovagal response in susceptible patients during examination for syncope.

Extraction of the Telectronics Accufix 330-801 atrial lead: the Mayo Clinic experience

OBJECTIVE

To document a cumulative experience with removal of the Telectronics 330-801 Accufix atrial lead after it was withdrawn from the market.

DESIGN

We reviewed our results with 96 attempted lead extraction procedures through Dec. 1, 1995.

MATERIAL AND METHODS

All patients underwent preoperative assessment, including fluoroscopic screening of the lead. Patients had blood typing and screening done before the extraction procedure, and extractions were performed with cardiac surgical intervention available. Pacing dependence was assessed to determine the need for temporary pacing.

RESULTS

The mean patient age was 65.7 +/- 1.6 years and the mean duration of lead implantation was 31.0 +/- 1.4 months. Of the 96 lead extractions attempted, 94 were successful. Of the 96 cases, the retention wires were normal in 29, fractured without protrusion in 34, and fractured with protrusion in 33. Fifty-three leads were removed with use of simple traction only, and seven leads were removed by using a locking stylet and telescoping sheaths. The Cook workstation was used in the removal of 32 leads through the femoral vein; in 1 of these 32 patients, the retention wire was removed before lead extraction. The retention wire was removed but the lead was left intact in one patient. One lead was removed during an opertive maze procedure. Associated complications were minimal.

CONCLUSION

Although lead extraction is a potentially fatal procedure, the Telectronics 330-801 Accufix atrial lead can be extracted safely with minimal morbidity when performed by experienced personnel.

Familial sudden death syndrome with an abnormal signal-averaged electrocardiogram as a potential marker

Most familial sudden cardiac death syndromes are associated with structural heart disease or 12-lead electrocardiographic abnormalities. Additionally, the utility of signal-averaged electrocardiograms in patients with familial sudden death syndromes has not been examined. We studied a kindred with sudden death to determine whether they could be classified into any of the previously described syndromes and whether an abnormal signal-averaged electrocardiogram is a marker for this trait. Surviving family members had normal 12-lead electrocardiograms and echocardiograms. Two of the patients who died from ventricular arrhythmias had normal hearts on autopsy. Two surviving family members had a clinical history of arrhythmic events; both had abnormal signal-averaged electrocardiograms and inducible ventricular arrhythmias during electrophysiologic studies. The other family members had normal signal-averaged electrocardiograms. This familial sudden death syndrome appears to be unique because the patients have anatomically normal hearts and normal 12-lead electrocardiograms. An abnormal signal-averaged electrocardiogram may be a marker for the sudden death trait.

Multicenter experience with a bipolar tined polyurethane ventricular lead

A multicenter study was undertaken to determine the failure rate of a specific bipolar tined polyurethane ventricular pacing lead, the Medtronic 4004/4004M pacing lead. Seven centers in the United States and Canada implanted 586 Medtronic 4004/4004M pacing leads. The study was designed to determine the probability and clinical manifestations of lead failure. Only failures compatible with an insulation problem were included. The Kaplan-Meier estimate of the percentage of 4004/4004M lead failures within 4 years after implantation was 14.1% (95% confidence interval: 8.5%-19.3%). Failures were manifested as sensing abnormalities, failure to capture, early battery depletion, and significant decrease in measured impedance compared with previous impedance measurements. The observed rate of failure is unacceptable, and strong consideration should be given to replacing the 4004/4004M pacing lead in pacemaker dependent patients and closely monitoring nondependent patients.

Consistent subcutaneous prepectoral implantation of a new implantable cardioverter defibrillator

OBJECTIVE

To describe the use of a new implantable cardioverter defibrillator (ICD) that can be placed in the prepectoral region rather than implanted in the abdominal wall.

DESIGN

We report the experience of placement of this new ICD in the prepectoral region in 13 patients from Sept. 28, 1993, through Jan. 10, 1994, at the Mayo Clinic.

MATERIAL AND METHODS

Thirteen consecutive patients offered this new ICD underwent placement of transvenous defibrillation leads, and the pulse generator was placed in a pocket formed in the subcutaneous, prepectoral space. Testing ensured a defibrillation threshold of 24 J or less.

RESULTS

In all 13 patients, the pulse generator could be placed in the subcutaneous, prepectoral space. In all except one patient, acceptable defibrillation thresholds were achieved by using lead systems placed totally transvenously. Only one patient required placement of a subcutaneous patch. All but two patients were dismissed from the hospital within 3 days after the ICD implantation.

CONCLUSION

Consistent subcutaneous, prepectoral placement of this new ICD pulse generator is possible. Because the entire procedure can be performed in the pacemaker implantation room, the potential exists for decreasing the duration of the hospitalization and associated costs.

The Dotter retriever and pigtail catheter: efficacy in extraction of chronic transvenous pacemaker leads

Several techniques exist for percutaneous extraction of chronic pacemaker leads. To establish the efficacy of the Dotter retriever and pigtail catheter, we reviewed the removal of 59 endocardial pacemaker leads in 42 patients (mean age 71 years). The mean duration of lead implantation was 44 months (range 1-169 months). Thirty-two leads were withdrawn with simple traction alone, and five leads were abandoned when traction failed. The remaining 22 leads were manipulated with a Dotter retriever or pigtail catheter, or both. Twelve leads were dislodged from the endocardium with simple traction (10) or with traction transmitted through an entwining pigtail catheter (2), but they could not be fully withdrawn. Eleven of these leads (92%) were then successfully extracted with the Dotter retriever. Seven of the remaining 10 leads were successfully dislodged and removed by the Dotter retriever. Overall, 9 of 12 leads (75%) that could not be dislodged from the endocardium with simple traction were removed with a Dotter retriever or pigtail catheter, or both. Three patients in whom no catheter method worked required thoracotomy for removal of infected leads. No complications resulted from use of the Dotter retriever or pigtail catheter. We conclude that the Dotter retriever and pigtail catheter have moderate efficacy for dislodging chronic endocardial leads. Once mobilized, however, the leads can be withdrawn with great success with the Dotter retriever. Newer technology should not result in the abandonment of this proven technique.

A multicenter experience with a bipolar tined polyurethane ventricular lead

A multicenter study was undertaken to determine the failure rate of a specific polyurethane bipolar tined pacing lead, the Medtronic 4012 pacing lead. Six centers in the United States and Canada implanted 1,190 Medtronic 4012 pacing leads. The study was designed to determine the probability and clinical manifestations of lead failure. Only failures compatible with an insulation problem were included. The probability of a 4012 lead failure by Kaplan-Meier analysis was 20.9% at 6 years after implantation. Failures were manifested as sensing abnormalities, failure to capture, early battery depletion, and significant decrease in measured impedance compared with the previous impedance measurements. Of the 95 definite lead failures, 16 (16.8%) were associated with symptoms similar to those experienced before pacemaker placement. The observed failure rate is unacceptable, and strong consideration should be given to replacing the 4012 pacing lead in pacemaker-dependent patients and closely monitoring nondependent patients.

Pyopneumopericardium attributed to an esophagopericardial fistula: report of a survivor and review of the literature

Herein we describe a case of pyopneumopericardium that resulted from formation of an acquired esophagopericardial fistula in a patient with silent, benign esophageal ulcer disease. Atypical features on initial examination suggested congestive heart failure or a pneumonic process (or both). The delayed development of pneumopericardium disclosed on a chest roentgenogram led to the clinical recognition of the esophagopericardial fistula. Subsequent emergent pericardiocentesis relieved cardiac tamponade and enabled us to diagnose pyopneumopericardium. A radiographic contrast study with use of meglumine diatrizoate revealed the site of the fistula in the midesophagus. The esophagopericardial fistula was surgically closed, and our patient had a good final result. Formation of an esophagopericardial fistula is a relatively uncommon finding; of the 60 previously reported cases, only 10 patients have survived. As illustrated in the current case, early diagnosis and treatment, including pericardial drainage and intense antibiotic therapy followed by a well-planned operative closure of the fistula, are paramount for the successful management of esophagopericardial fistulas.

Evidence for energy-dependent transposition of core lipopolysaccharide across the inner membrane of Salmonella typhimurium

The uncoupler 2,4-dinitrophenol blocks the final step of lipopolysaccharide assembly--transfer of O antigen from undecaprenyl pyrophosphate to core lipopolysaccharide--in intact Salmonella typhimurium but not in isolated membrane fractions. The O-antigen ligase enzyme is not inhibited by dinitrophenol in vitro, and core lipopolysaccharide synthesized in the presence of uncoupler in vivo is functional as acceptor of O antigen in vitro. The evidence strongly suggests that maintenance of proton motive force is required for transmembrane transposition of core lipopolysaccharide to the active site of O-antigen ligase at the periplasmic face of the inner membrane.

Energy dependence of O-antigen synthesis in Salmonella typhimurium

The uncoupler 2,4-dinitrophenol prevents in vivo synthesis of O antigen in Salmonella typhimurium by inhibiting the first reaction of the pathway, formation of galactosyl-pyrophosphoryl-undecaprenol. Inhibition was observed only in intact cells; dinitrophenol had no effect on activity of the synthase enzyme in isolated membrane fractions. In vivo inhibition could not be explained by changes in intracellular nucleotide pools or a shift in the equilibrium of the reaction and appeared to be specific for the first step in the pathway. Neither the subsequent mannosyl transferase, which catalyzes formation of the trisaccharide-lipid intermediate, mannosyl-rhamnosyl-galactosyl-pyrophosphoryl-undecaprenol, nor O-antigen polymerase was inhibited. In addition, incorporation of galactose into core lipopolysaccharide was only modestly inhibited under conditions in which O-antigen synthesis was abolished. The results suggest that maintenance of proton motive force is required for access of substrate, UDP-galactose and/or undecaprenyl phosphate, to the active site of the galactosyl-pyrophosphoryl-undecaprenol synthase enzyme.

Molecular heterogeneity at the phenylalanine hydroxylase locus in the population of the south-west of England

The phenylalanine hydroxylase gene locus has been studied in 35 independent phenylketonuric families in the south-west of England using RFLP haplotype patterns and allele specific oligonucleotide probes. Haplotype 3 was the most common pattern on mutant chromosomes and there was strict linkage disequilibrium between this haplotype and the splice mutation in exon 12. The R408W mutation in exon 12 occurred on both haplotypes 1 and 2. The R126Q mutation in exon 7 was found only on a rare haplotype 28 pattern. No gene carried the R158Q mutation. More than 60% of mutant genes did not carry these four mutations which were originally described in other European populations. We suggest that the splice mutation arose as a single event and spread throughout northern Europe by population migration and admixture. In addition, we believe the haplotype/mutation associations seen in our population are a reflection of the mixed ancestry of the inhabitants of the British Isles.

Antitachycardia pacemaker treatment of postoperative arrhythmias in pediatric patients

An automatic antitachycardia pulse generator (Intertach 262-12) was implanted in each of six pediatric patients (mean age, 10 years) with drug-resistant and persistent postoperative supraventricular arrhythmias. Four had bradycardia-tachycardia syndrome, two after a Mustard procedure for transposition of the great arteries, one after a Senning procedure for the same anomaly, and one after a Fontan procedure for univentricular heart with transposition of the great arteries. Of the two remaining patients, one had atrial flutter after a modified Fontan procedure for univentricular heart and one had intra-atrial reentry tachycardia after a modified Fontan procedure for double-outlet right ventricle with pulmonary stenosis. During a mean follow-up interval of 31 months after implantation, pacemakers were activated on multiple occasions and functioned appropriately in all six patients. Complications necessitated six invasive interventions in three patients: erosion or infection of the system, adaptor fracture, and connector block fracture on one occasion each and lead dislodgment on three occasions. Four of the six patients continued to take drugs at the end of this study; however, all patients had their drug therapy reduced and one was taking digoxin only. The number of hospital admissions decreased after implantation. Despite a number of technical challenges, this newer multiprogrammable antitachycardia pacemaker appears to be a valuable addition to the treatment of refractory postoperative supraventricular tachyarrhythmias in pediatric patients.

Genetic analysis of treated and untreated phenylketonuria in one family

We describe a family in which four subjects in two generations have a disorder of phenylalanine metabolism. Two first cousins had different biochemical presentations in the neonatal period. The older child was thought to have a more severe form of phenylketonuria (PKU), and the younger child a milder form. While carrying out family studies we discovered that their mutual grandfather and his older unmarried brother, both of normal intelligence, had a marked and previously undiagnosed hyperphenylalaninaemia. DNA analysis using RFLP haplotypes has shown that there are four independent mutant PKU alleles in this family which are found on three haplotype patterns. None of the affected family members carries a previously defined mutation at the phenylalanine hydroxylase (PAH) locus and so DNA analysis was not able to explain the apparently different biochemical phenotypes in the affected members of this family.

Influence of ventricular function and presence or absence of coronary artery disease on results of electrophysiologic testing for asymptomatic nonsustained ventricular tachycardia

One hundred ten patients with asymptomatic nonsustained ventricular tachycardia (VT) were evaluated prospectively to assess the value of electrophysiologic testing. This testing consisted of up to 3 extrastimuli delivered during 3 drive cycle lengths from 2 right ventricular sites. A positive study was defined as monomorphic VT lasting 30 seconds or requiring cardioversion. Patients with a positive study were treated, and serial drug testing was done. An event during follow-up was sustained VT or cardiac arrest. The mean follow-up was 15 months. Of 57 patients with an ejection fraction greater than or equal to 40%, 6 had a positive electrophysiologic test with 1 event and 51 had a negative test with 1 event. Twenty-eight patients had an ejection fraction less than 40% and coronary artery disease: 14 had a positive test with 1 event, and 14 had a negative test with 3 events. Twenty-five patients had an ejection fraction less than 40% and no coronary artery disease: 1 had a positive test with no events, and 24 had a negative test with 8 events. Only ejection fraction and congestive heart failure class were found to be independent predictors of outcome. Patients with an ejection fraction greater than 40% had low inducibility (11%), had few events (3.5%) and did not require electrophysiologic testing. In patients with an ejection fraction less than 40% and coronary artery disease, inducibility was high (50%) and a negative study was of no value. Patients with an ejection fraction less than 40% and no coronary artery disease had low inducibility (4%), had frequent events (33%) and did not benefit from electrophysiologic testing.

Clinical application of the signal-averaged electrocardiogram and "late potentials"

The signal-averaged ECG has proven to be a valuable tool for identifying patients at risk of ventricular arrhythmias. This computerized method of analyzing standard ECGs identifies microvolt-level late potentials that represent delayed conduction through diseased myocardium. This diseased myocardium is a potential substrate for reentrant ventricular arrhythmias. In select patient groups, the signal-averaged ECG predicts electrophysiologic testing results. Problems remain and continued development is needed to evaluate patients with conduction system disease, the patient without coronary artery disease but at risk of sudden death, and proper general application of the technique.

The role of frequency of atrial contraction versus atrial pressure in atrial natriuretic peptide release

This study was designed to investigate the role of frequency of atrial contraction compared to acute increases in right atrial pressure in the regulation of atrial natriuretic peptide (ANP) release in humans. The studies were performed in patients undergoing electrophysiological study. In group 1 (n = 12) the rate of atrial contraction was increased by continuous rapid right atrial pacing at a rate of 120 beats/min (bpm; group 1A; n = 6) or 176 bpm (group lb; n = 6) for 5 min. No increases in atrial pressure or circulating ANP occurred in response to atrial tachycardia. In contrast, continuous rapid right ventricular pacing (group II: n = 12) at ventricular rates of 120 bpm (group IIa; n = 6) and 150 bpm (group IIb; n = 6) increased both right atrial pressure and circulating ANP. These results demonstrate that, in contrast to studies in vitro, increases in the frequency of atrial contraction in the absence of increases in atrial pressure do not release atrial natriuretic peptide. These studies, therefore, support the conclusion that atrial pressure is the primary physiological stimulus for ANP.

Identification of the haplotype pattern associated with the mutant PKU allele in the Gypsy population of Wales

Using the full length cDNA probe, the RFLP haplotype patterns at the phenylalanine hydroxylase locus have been studied in the extensive and highly consanguineous Welsh Gypsy population. The pattern associated with the mutant PKU allele is identical to haplotype 4 in the northern European population. Two children with classical PKU are homozygous for this haplotype. We have tracked the mutant allele through four generations to a great grandfather who died 22 years ago. Both affected children almost certainly have inherited a double dose of the same mutant PKU allele from one common ancestor. It should be possible to identify the specific mutation that is associated with haplotype 4 which results in the more serious form of PKU.

Follow-up of an activity-sensing, rate-modulated pacing device, including transtelephonic exercise assessment

Single-chamber rate-modulated (ventricular or atrial demand [VVIR or AAIR]) pacemakers are now an accepted and frequently used pacing modality. Although several sensor-driven pacemakers are currently under investigation, activity-sensing pacemakers have been used most frequently to date. We assessed our implantation and follow-up experience with 156 activity-sensing pacemakers. Analysis of the patient population revealed an age range of 6 to 91 years and a variety of indications for pacing, the most common being atrioventricular block. Specific programming techniques were adopted for assessing these pacemakers. Although several episodes of reprogramming may be necessary to identify the optimal sensor settings for an individual patient, once optimal programming has been achieved, the sensing characteristics seem to have long-term stability. An important part of our follow-up procedure involves transtelephonic exercise, which allows some degree of long-term assessment of the sensor. All but six patients with pacemakers programmed to the sensor-driven mode (VVIR or AAIR) had evidence of sensor-driven pacing during transtelephonic exercise. Thus, this adjunct seems helpful for follow-up of some rate-modulated pacemakers.

Cryosurgical modification of atrioventricular conduction for treatment of atrioventricular node reentrant tachycardia

Cryosurgical modification of atrioventricular (AV) node conduction was performed in five patients with AV node reentrant tachycardia that used dual AV nodal pathways and was refractory to drug therapy. The procedure alleviated the tachycardia in all patients without the development of complete heart block and without any associated surgical morbidity or mortality. These results suggest that cryosurgical modification of AV node conduction is a promising and potentially curative method of treating AV node reentrant tachycardia.

Impact of intracardiac electrophysiologic testing on the management of elderly patients with recurrent syncope or near syncope

Seventy-five patients, 75 years of age and over, experienced recurrent syncope, with the etiology remaining unclear but presumably cardiogenic, after cardiac and neurologic examinations and noninvasive laboratory testing (including an electrocardiogram and ambulatory electrocardiographic monitoring). The mean number of previous syncopal spells was 14 (range, 1 to 64) over a mean of 36 months (range, 1 to 480 months). These patients underwent invasive electrophysiologic testing and a potential cause for syncope was identified in 68%. Abnormal findings at electrophysiologic testing included: sinus node dysfunction (55%); abnormal His-bundle conduction (39%); and ventricular tachycardia (14%), with some patients having more than one abnormality. No major complications were associated with the electrophysiologic testing. Patients were subsequently treated with permanent cardiac pacing or antiarrhythmic drugs or both, depending upon results of the electrophysiologic study. Follow-up examinations (mean of 26 months; range, 1 to 70) were possible in 90% of patients. No further syncope occurred in 84% of patients with an abnormal electrophysiologic study who received subsequent therapy to prevent the identified abnormality. Thus, in this difficult group of patients with recurrent syncope of uncertain etiology, electrophysiologic testing was safe and indicated abnormalities of conduction or rhythm in 68% of patients; treatment to correct these abnormalities prevented recurrent syncope in 84%.

Out-of-hospital cardiac arrest in patients without clinically significant coronary artery disease: comparison of clinical, electrophysiological, and survival characteristics with those in similar patients who have clinically significant coronary artery disease

Fifty nine survivors of out-of-hospital cardiac arrest unassociated with an acute myocardial infarction were referred for intracardiac electrophysiological study. Thirty patients who had no clinically significant coronary artery disease (group 1) were compared with 29 who did (group 2). Ventricular tachycardia or fibrillation was induced in significantly more patients in group 2 than in group 1 (69% vs 40%). Median duration of follow up, which was achieved in all patients, was 31 months in group 1 and 14 months in group 2. In group 1, an effective treatment was identified electrophysiologically in seven patients, and none died; an arrhythmia was induced, but no effective treatment was identified in five patients, and one patient died subsequently; an arrhythmia was not induced in 18 patients, 15 of whom were treated empirically with anti-arrhythmic drugs, and one died. In group 2, effective treatment was identified electrophysiologically in seven patients and three died (two of pump failure) during follow up. In 13 an arrhythmia was induced but no effective drug was identified, and six died or had a recurrence; in another nine patients without inducible arrhythmias, six subsequently died or had a recurrence. A Cox proportional hazards analysis identified previous myocardial infarction as the only predictor of recurrence. Patients without coronary artery disease who suffer an out-of-hospital cardiac arrest have a low inducibility rate at electrophysiological study and an excellent prognosis compared with patients who have coronary artery disease. Electrophysiological testing seemed to be of value in predicting the response to antiarrhythmic drugs, but non-inducibility of arrhythmias in patients with coronary artery disease was of no predictive value.

Value of the resting 12 lead electrocardiogram and vectorcardiogram for locating the accessory pathway in patients with the Wolff-Parkinson-White syndrome

The resting 12 lead electrocardiogram and vectocardiogram were reviewed in 47 patients with the Wolff-Parkinson-White syndrome (a) who had pre-excitation on the resting 12 lead electrocardiogram, (b) who had a single anterograde conducting accessory pathway assessed and located during preoperative electrophysiological study and during epicardial mapping at operation, and (c) in whom surgical division of the accessory pathway resulted in loss of pre-excitation. The site of the accessory pathway established during operation was compared with that established by evaluating the polarity of the delta wave and QRS complex on the resting 12 lead electrocardiogram. The electrocardiogram was assessed by the Rosenbaum criteria (Wolff-Parkinson-White type A, left-sided pathway; or type B, right-sided pathway), the Gallagher criteria (atrial pacing resulting in maximal pre-excitation), and the World Health Organisation criteria (a composite of previous studies). The Gallagher and World Health Organisation criteria were derived from patients demonstrating maximal pre-excitation that often required atrial pacing. The present study was designed to determine whether these criteria could be accurately applied to the resting 12 lead electrocardiogram on which the degree of pre-excitation was variable. The Rosenbaum criteria correctly identified a left sided accessory pathway in 26 of 34 patients and a right-sided accessory pathway in nine of 13 patients. The Gallagher and World Health Organisation criteria correctly identified the location in only 15 (32%) of the 47 patients. The resting vectorcardiogram was inaccurate for locating the accessory pathway. Although published criteria are useful for identifying the site of the accessory pathway from an electrocardiogram obtained when rapid atrial pacing is being used to achieve maximal pre-excitation, they are not suitable for identifying the exact site of an accessory pathway from the resting 12 lead electrocardiogram.

Double-blind study of intravenous propafenone for paroxysmal supraventricular reentrant tachycardia

Propafenone was administered during electrophysiologic testing to determine its efficacy and safety for terminating and preventing reinduction of paroxysmal supraventricular reentrant tachycardia. Four men and 10 women (mean age 50 years, range 28 to 69) were studied. Five patients had Wolff-Parkinson-White syndrome with orthodromic atrioventricular (AV) reentrant tachycardia, three had a concealed accessory pathway with AV reentrant tachycardia and six had tachycardia due to reentry within the AV node. In the five patients with Wolff-Parkinson-White syndrome, propafenone terminated reentrant tachycardia in three (the tachycardia was reinducible in one) and had no effect in two. In the three patients with a concealed accessory pathway, propafenone terminated reentrant tachycardia in all three and prevented reinduction of the tachycardia in two. In the six patients with tachycardia due to reentry within the AV node, propafenone terminated and prevented reinduction of reentrant tachycardia. Propafenone had no effect on blood pressure, heart rate, PA interval, AV node refractoriness or rate of reentrant tachycardia. Propafenone significantly (p less than 0.05) prolonged the AH, HV, QRS and ventriculoatrial intervals and decreased the AV node Wenckebach rate. Of the nine patients receiving long-term oral propafenone therapy, eight had a reduction of at least 90% in reentrant tachycardia during a mean follow-up period of 14.5 months (range 11 to 22); all eight patients had had noninducible reentrant tachycardia after intravenous propafenone. One patient had increased frequency of reentrant tachycardia; this patient had had inducible reentrant tachycardia after intravenous propafenone. In conclusion, intravenously administered propafenone terminated reentrant tachycardia in 85% of patients and prevented reinduction in 71%, with no adverse hemodynamic effects.

Role of invasive electrophysiologic testing in patients with symptomatic bundle branch block

Electrophysiologic testing was performed in 112 symptomatic patients with bundle branch block. Abnormalities included HV interval 70 ms or longer (35 patients), infra-Hisian block with atrial pacing (6 patients) and sinus node dysfunction (23 patients). Inducible ventricular tachycardia occurred in 47 patients (42%). Therapy was based on the electrophysiologic test result: group I--16 patients with no therapy (normal study results); group II--34 patients with permanent pacing alone; group III--39 patients with antiarrhythmic therapy alone; and group IV--21 patients with both antiarrhythmic therapy and permanent pacing. Cumulative 4-year survival rates were 83% in group I, 84% in group II, 63% in group III and 84% in group IV (mean follow-up 2.5 years). Recurrent syncope occurred in 19% of group I, 6% of group II, 33% of group III and 19% of group IV. In symptomatic patients with bundle branch block and normal electrophysiologic test results, prognosis is good without treatment. In patients undergoing permanent pacing based on electrophysiologic testing, survival is good and rate of symptom recurrence is low. Electrophysiologic testing identifies patients with inducible ventricular tachycardia for whom antiarrhythmic therapy is indicated but who nevertheless have a poor prognosis.

Idiopathic lymphoplasmacytic aortic valvulitis: observations in two elderly women with unexplained aortic insufficiency

Two patients with a unique aortic valvulitis required aortic valve replacement. Both were elderly women who presented with evidence of systemic disease, including fever, arthralgia, myalgia, markedly elevated erythrocyte sedimentation rate, anemia, leukocytosis, hypoalbuminemia and renal insufficiency, in addition to progressive subacute aortic insufficiency. Histologic examination of the excised aortic valve revealed a lymphoplasmacytic infiltrate. Neither patient had evidence of other diseases that have been associated with aortic insufficiency. One should consider the judicious use of glucocorticosteroids for such patients.

Clinical intracardiac electrophysiologic testing: technique, diagnostic indications, and therapeutic uses

Clinical cardiac electrophysiologic testing has evolved rapidly since 1968, when the technique was first described. In an electrophysiologic study, electrode catheters are positioned within the heart to record electrical activity from the atrium, atrioventricular conduction tissue, and ventricle. Programmed stimulation is then performed, which involves pacing of the atrium or ventricle and introducing critically timed premature stimuli during sinus rhythm or paced rhythm. The use of programmed stimulation in conjunction with intracardiac recordings in electrophysiologic studies has facilitated the diagnosis of mechanisms of arrhythmias and the assessment of therapy. Electrophysiologic testing is useful in selected patients with sinus node dysfunction, conduction system disorders, supraventricular tachycardia, ventricular tachycardia, or ventricular fibrillation and in survivors of out-of-hospital cardiac arrest and patients with symptomatic but unsubstantiated rhythm disturbances. Therapeutic approaches that can be assessed by electrophysiologic testing include serial drug testing to determine the effectiveness of antiarrhythmic agents, antitachycardia pacing, the implantable defibrillator, transcatheter ablation, and electrophysiologically guided surgical procedures. In this review, we discuss the methods of electrophysiologic testing, its clinical applications in diagnosing the various cardiac rhythm disturbances, and its use in assessing various therapeutic modalities.

Propafenone for the treatment of refractory complex ventricular ectopic activity

The results of therapy with propafenone were evaluated in 45 patients with complex ventricular ectopic activity that had been refractory to a mean of 3.8 antiarrhythmic drugs. The cardiac diagnoses were ischemic heart disease (in 16 patients), cardiomyopathy (in 7), mitral valve prolapse (in 7), mitral valve prolapse (in 7), idiopathic ventricular ectopic beats (in 6), valvular heart disease (in 5), and hypertension (in 4). The frequency of ventricular ectopic beats was established after therapy with antiarrhythmic agents had been discontinued. Patients then received propafenone during a dose-ranging protocol. An effective response was defined as a reduction in total ventricular ectopic beats of 80% or more. During dose ranging, therapy failed in four patients because of side effects, in eight because of a reduction in ventricular ectopic beats of less than 80%, and in three because of an aggravation of the arrhythmia. Thirty patients had a reduction in total ventricular ectopic beats of 80% or more. During a mean follow-up of 12.4 months, therapy failed in 1 patient because of sustained ventricular tachycardia and in 7 because of intolerable side effects; 22 patients continued to receive propafenone. PR and QRS intervals were significantly prolonged (P = 0.001), but the corrected QT interval and the heart rate were unchanged. The mean trough plasma level of propafenone associated with an effective response was 756 ng/ml, and that associated with intolerable side effects was 920 ng/ml. Thus, in patients with refractory complex ventricular ectopic beats, propafenone was effective and well tolerated initially in 67% of patients and during long-term administration in 49%, and toxicity was minor in most patients.

Increased vagal tone as an isolated finding in patients undergoing electrophysiological testing for recurrent syncope: response to long term anticholinergic agents

Features suggestive of an isolated increase in vagal tone during electrophysiological study were found in 12 patients with recurrent near syncope or syncope. Results at neurological and cardiac evaluation were otherwise normal. The increased tone or heightened sensitivity to vagal tone was manifested by abnormal atrioventricular nodal refractoriness and conduction that were reversed with atropine. The patients underwent long term treatment with an anticholinergic agent (propantheline bromide) and 75% improved. Before treatment they had experienced a median of seven episodes (range 3-28) of near syncope or syncope during 10.5 months (range 1-60). During treatment these episodes decreased to a median of one (range 0-15) during 22.5 months (range 3-67); six patients experienced no further symptoms. Three patients continued to have syncope while on treatment, and one of these required permanent cardiac pacing. No additional cause for syncope was identified in any patient. During electrophysiological assessment of patients with syncope, evidence may be obtained pointing to an increase in vagal tone. In many of these patients treatment with anticholinergic drugs seemed to improve or eliminate the symptoms.

Symptomatic "isolated" carotid sinus hypersensitivity: natural history and results of treatment with anticholinergic drugs or pacemaker

Because syncope may occur intermittently in patients with carotid sinus hypersensitivity, a knowledge of its natural history is needed as a basis for interpreting the usefulness of therapy. Fifty-six consecutive patients are described (47 men and 9 women; mean age 61 years) with carotid sinus hypersensitivity and syncope in whom 24 hour ambulatory monitoring and intracardiac electrophysiologic study revealed no other cause for the syncope. The mean duration of symptoms was 44 months (range 1 to 480) and the mean number of episodes was 4.0 (range 1 to 20). During a follow-up period of 6 to 120 months (median 40), syncope recurred in 3 of 13 patients who received no treatment, in 2 of 23 patients who received a pacemaker and in 4 of 20 patients who received anticholinergic drugs (incidences corrected for totals available at follow-up: 27, 9 and 22%, respectively). Two-thirds of the patients receiving no treatment were asymptomatic compared with all nine of the patients with syncope and a pure cardioinhibitory response to carotid sinus massage who received an atrioventricular (AV) sequential pacemaker. Although pacing was effective in abolishing syncope, its use should be reserved for recurrent episodes because of the high rate of spontaneous remission of symptoms.

Energy dependence of lipopolysaccharide translocation in Salmonella typhimurium

Energy inhibitors block translocation of pulse-labeled core lipopolysaccharide to outer membrane under conditions which allow maintenance of constant specific radioactivity of intracellular precursor pools throughout the chase period. Under the conditions used, approximately 75% of the total cellular label was membrane-bound at initiation of chase. Translocation of core lipopolysaccharide from inner to outer membrane showed apparent first order kinetics (t1/2 = 1.2 min, 32 degrees C). Translocation was blocked by arsenate (5-10 mM) under conditions where proton motive force was unchanged, while the uncouplers 2,4-dinitrophenol (0.1 mM to 0.8 mM) and carbonyl cyanide-m-chlorophenyl hydrazone (12-30 microM) inhibited translocation with no apparent effect on the ATP pool. Therefore, core lipopolysaccharide translocation appears to require maintenance of both proton motive force and high energy phosphate pools. Electron microscopic experiments show no gross disruption of zones of adhesion, the putative sites of lipopolysaccharide translocation, in the presence of arsenate or 2,4-dinitrophenol suggesting that energy is not required simply for maintenance of these structures.

Amiodarone pulmonary toxicity. Assessment by bronchoalveolar lavage

Bronchoalveolar lavage was used to demonstrate morphologic changes in alveolar macrophages characteristic of amiodarone effect in lung tissue obtained by biopsy. This procedure may be useful in the assessment of abnormal chest x-ray findings in patients suspected to have amiodarone toxicity.

Permanent cardiac pacing after the Fontan procedure

Permanent cardiac pacing after a Fontan procedure is complicated by complex cardiovascular anatomy. Of 332 patients undergoing the Fontan procedure at the Mayo Clinic, we evaluated 15 who postoperatively required permanent pacing (mean age 16.5 years, range 4 to 31 years). Underlying congenital cardiac defects included univentricular heart in nine patients, double-outlet right ventricle in three, and tricuspid atresia in three. The indication for pacing was postoperative heart block in seven patients, congenital heart block in five, postoperative sick sinus syndrome in two, and heart block because of previous operation in one. Pacemakers were implanted immediately postoperatively in 11 patients and 12 to 57 months later in four patients. VVI systems were used in nine patients, DDD in four, AAI in one, and a Medtronic Activitrax VVI in one. All ventricular leads were epicardial. Four atrial leads were transvenous endocardial and one was epicardial. Three patients died 4, 9, and 69 months later of causes unrelated to pacing. Among the 12 survivors, mean follow-up was 34 months (range 1 to 107 months). Two patients had a total of three episodes of loss of ventricular capture because of increased chronic thresholds. Our current approach to pacing after a Fontan procedure includes (1) intraoperative placement of temporary atrial and ventricular electrodes, (2) intraoperative attachment of a permanent ventricular epicardial lead for congenital or surgically induced high-grade atrioventricular block, (3) postoperative insertion of transvenous atrial leads if dual-chamber pacing is indicated, and (4) use of programmable pulse generators with high output capability.

Amiodarone pulmonary toxicity: report of two cases associated with rapidly progressive fatal adult respiratory distress syndrome after pulmonary angiography

Use of amiodarone, an investigational antidysrhythmic agent, has been associated with cases of pulmonary toxicity. The mechanism of amiodarone pulmonary toxicity is unknown, and recommendations for the assessment and treatment of patients with this condition continue to evolve. In two patients with clinically diagnosed amiodarone pulmonary toxicity, a rapidly progressive and fatal adult respiratory distress syndrome developed after pulmonary angiography. Physicians should be aware of the potential for serious complications of pulmonary angiography in patients with presumed or clinically diagnosed amiodarone toxicity.

Surgical treatment of accessory atrioventricular pathways and symptomatic tachycardia in children and young adults

Twenty-seven patients aged 21 years or younger (mean 15) with symptomatic tachycardia underwent operation for ablation of an accessory atrioventricular pathway. Six patients had associated Ebstein's malformation of the tricuspid valve. Supraventricular tachycardia had been present for a mean of 5 years. At electrophysiologic study, 4 patients were found to have 2 accessory pathways. Left ventricular free wall pathways were found in 14 patients, right ventricular free wall pathways in 10 and septal pathways in 6. Successful initial ablation of all the pathways was achieved in 26 of the 27 patients. No patient died perioperatively and none had persistent complete heart block. During a mean follow-up of 11 months, no patient had recurrence of an arrhythmia related to the accessory pathway. Thus, the surgical treatment of children and young adults with accessory atrioventricular pathways and symptomatic supraventricular tachycardia is safe and effective. For these patients, unless the tachycardia can be easily controlled with a minimal number of drugs and adverse effects, surgical ablation should be considered early in the clinical course.

Update on infections involving permanent pacemakers. Characterization and management

From January, 1974, to December, 1983, 75 patients with infections related to permanent pacemakers were successfully treated. Demographic characterization, mode of presentation, types of infecting organisms, potential predisposing factors, significance of a retained infected pacemaker lead, and various medical and surgical treatment methods were analyzed. Likely infecting organisms depended on the mode of presentation and the time of the infection. Dermatologic diseases accounted for a significant number of secondary infections. Removal of the entire infected pacing system was required for eradication of infection in 74 of 75 patients. In 31 patients, the infected system was removed at the same time that the new system was implanted. In 26 patients, a two-stage procedure was used that included a period of temporary pacing between explantation of the old system and implantation of the new. No difference in complications or incidence of reinfection was found between these two groups. Infections occurring within 2 weeks after operation accounted for 15% of the cases. In these patients, Staphylococcus aureus was the most common organism. In patients with later infections, Staphylococcus epidermidis was the most common.

Bipolar tined polyurethane ventricular lead: a four-year experience

With the advent of polyurethane as an insulating material for permanent pacemaker leads, concern has arisen over the integrity and long-term durability of polyurethane-insulated pacing leads. Specific concern has arisen over particular bipolar tined polyurethane ventricular leads. We have assessed our 4-year experience with this lead. This experience involves two groups of patients, those with leads manufactured before a certain date and those with leads manufactured at a later date. In the first group (judged to be at increased risk) the failure rate was 8.8%, and in the second group (judged not to be at increased risk) the rate was 3.9%. Lead failure occurred at an average of 17.5 months in the first group. Adequate follow-up on the second group is not available to determine whether or not the failure rate may eventually be as high as that in the early group. Actuarial analysis suggests that survivorship free of lead failure is probably not significantly different in the two groups. This experience points out the need for determining lead failure rates, identifying optimal lead design and configuration, and establishing a lead registry or mechanism by which the integrity of various pacing leads can be evaluated.

Cardiac histologic findings in patients with life-threatening ventricular arrhythmias of unknown origin

Percutaneous endomyocardial biopsy (right ventricle in 10, left ventricle in 2) was performed in 12 patients, aged 9 to 57 years, with serious ventricular arrhythmias occurring in the setting of normal cardiac anatomy and mechanical function. Light microscopic examination of tissue revealed histologic abnormalities in 11 patients, including myocardial cellular hypertrophy in 7, interstitial fibrosis in 5, endocardial fibrosis in 2, myocardial degenerative changes in 1 and increased interstitial cellularity in 1. One patient had histologic evidence of acute lymphocytic myocarditis. Thus, a majority of patients with serious ventricular arrhythmias and normal cardiac anatomy had histologic abnormalities, bringing into question the concept of primary electrical heart disease or idiopathic ventricular tachycardia.

Changing experience with dual chamber (DDD) pacemakers

Dual chamber (DDD) or "universal" pacemakers have had a significant impact on the advancement of artificial pacemakers by providing a more physiologic approach to cardiac pacing. However, with the early generation of DDD pacemakers (pacemakers that sense and pace in both the atrium and the ventricle), a significant number of patients experienced pacemaker-mediated tachycardia because intact ventriculoatrial conduction was sensed in the atrium and a reentrant tachycardia was induced. Newer generation DDD pacemakers have provided longer atrial refractory periods, which should correct this problem. In this study the first and second years of a 2 year experience with DDD pacemakers were compared to determine if the newer generation devices have allowed maintenance of pacing in the DDD mode as opposed to reprogramming to some alternate mode because of pacemaker-mediated tachycardia or other pacing problems. The results showed a significant decrease in pacemaker-mediated tachycardia during the second year and continuation of pacing in the DDD mode in a higher percent of patients. This improvement is attributed to improvement in the pulse generator as well as better patient selection.