Congenital heart disease caused by mutations in the transcription factor NKX2-5.

Mutations in the gene encoding the homeobox transcription factor NKX2-5 were found to cause nonsyndromic, human congenital heart disease. A dominant disease locus associated with cardiac malformations and atrioventricular conduction abnormalities was mapped to chromosome 5q35, where NKX2-5, a Drosophila tinman homolog, is located. Three different NKX2-5 mutations were identified. Two are predicted to impair binding of NKX2-5 to target DNA, resulting in haploinsufficiency, and a third potentially augments target-DNA binding. These data indicate that NKX2-5 is important for regulation of septation during cardiac morphogenesis and for maturation and maintenance of atrioventricular node function throughout life.

Loss of function and inhibitory effects of human CSX/NKX2.5 homeoprotein mutations associated with congenital heart disease.

CSX/NKX2.5 is an evolutionarily conserved homeodomain-containing (HD-containing) transcription factor that is essential for early cardiac development. Recently, ten different heterozygous CSX/NKX2.5 mutations were found in patients with congenital heart defects that are transmitted in an autosomal dominant fashion. To determine the consequence of these mutations, we analyzed nuclear localization, DNA binding, transcriptional activation, and dimerization of mutant CSX/NKX2.5 proteins. All mutant proteins were translated and located to the nucleus, except one splice-donor site mutant whose protein did not accumulate in the cell. All mutants that had truncation or missense mutations in the HD had severely reduced DNA binding activity and little or no transcriptional activation function. In contrast, mutants with intact HDs exhibit normal DNA binding to the monomeric binding site but had three- to ninefold reduction in DNA binding to the dimeric binding sites. HD missense mutations that preserved homodimerization ability inhibited the activation of atrial natriuretic factor by wild-type CSX/NKX2.5. Although our studies do not characterize the genotype-phenotype relationship of the ten human mutations, they identify specific abnormalities of CSX/NKX2.5 function essential for transactivation of target genes.

Mutations in the cardiac transcription factor NKX2.5 affect diverse cardiac developmental pathways.

Heterozygous mutations in NKX2.5, a homeobox transcription factor, were reported to cause secundum atrial septal defects and result in atrioventricular (AV) conduction block during postnatal life. To further characterize the role of NKX2.5 in cardiac morphogenesis, we sought additional mutations in groups of probands with cardiac anomalies and first-degree AV block, idiopathic AV block, or tetralogy of Fallot. We identified 7 novel mutations by sequence analysis of the NKX2.5-coding region in 26 individuals. Associated phenotypes included AV block, which was the primary manifestation of cardiac disease in nearly a quarter of affected individuals, as well as atrial septal defect and ventricular septal defect. Ventricular septal defect was associated with tetralogy of Fallot or double-outlet right ventricle in 3 individuals. Ebstein's anomaly and other tricuspid valve abnormalities were also present. Mutations in human NKX2.5 cause a variety of cardiac anomalies and may account for a clinically significant portion of tetralogy of Fallot and idiopathic AV block. The coinheritance of NKX2.5 mutations with various congenital heart defects suggests that this transcription factor contributes to diverse cardiac developmental pathways.

Heart sounds at home: feasibility of an ambulatory fetal heart rhythm surveillance program for anti-SSA-positive pregnancies.

OBJECTIVE: Fetuses exposed to anti-SSA (Sjögren's) antibodies are at risk of developing irreversible complete atrioventricular block (CAVB), resulting in death or permanent cardiac pacing. Anti-inflammatory treatment during the transition period from normal heart rhythm (fetal heart rhythm (FHR)) to CAVB (emergent CAVB) can restore sinus rhythm, but detection of emergent CAVB is challenging, because it can develop in ⩽24 h. We tested the feasibility of a new technique that relies on home FHR monitoring by the mother, to surveil for emergent CAVB. STUDY DESIGN: We recruited anti-SSA-positive mothers at 16 to 18 weeks gestation (baseline) from 8 centers and instructed them to monitor FHR two times a day until 26 weeks, using a Doppler device at home. FHR was also surveilled by weekly or every other week fetal echo. If FHR was irregular, the mother underwent additional fetal echo. We compared maternal stress/anxiety before and after monitoring. Postnatally, infants underwent a 12-lead electrocardiogram. RESULTS: Among 133 recruited, 125 (94%) enrolled. Among those enrolled, 96% completed the study. Reasons for withdrawal (n=5) were as follows: termination of pregnancy, monitoring too time consuming or moved away. During home monitoring, 9 (7.5%) mothers detected irregular FHR diagnosed by fetal echo as normal (false positive, n=2) or benign atrial arrhythmia (n=7). No CAVB was undetected or developed after monitoring. Questionnaire analysis indicated mothers felt comforted by the experience and would monitor again in future pregnancies. CONCLUSION: These data suggest ambulatory FHR surveillance of anti-SSA-positive pregnancies is feasible, has a low false positive rate and is empowering to mothers.

NKX2.5 mutations in patients with tetralogy of fallot.

BACKGROUND: Recent reports have implicated mutations in the transcription factor NKX2.5 as a cause of tetralogy of Fallot (TOF). To estimate the frequency of NKX2.5 mutations in TOF patients and to further investigate the genotype-phenotype correlation of NKX2.5 mutations, we genotyped 114 TOF patients. METHODS AND RESULTS: Patients were recruited prospectively (November 1992 through June 1999) and tested for a 22q11 deletion; those with 22q11 deletion or recognized chromosomal alteration were excluded from the present study. Patients were screened for NKX2.5 alterations by conformation-sensitive gel electrophoresis and sequencing of fragments with aberrant mobility. Four heterozygous mutations were identified in 6 unrelated patients with cases of TOF, including 3 with pulmonary atresia and 5 with right aortic arch; none had ECG evidence of PR interval prolongation. Three of 4 mutations (Glu21Gln, Arg216Cys, and Ala219Val) altered highly conserved amino acids, of which 2 mapped in the conserved NK2 domain. The fourth mutation (Arg25Cys) was identified in 3 unrelated probands in the present study and has been previously reported. No homeodomain mutations were identified. CONCLUSIONS: NKX2.5 mutations are the first gene defects identified in nonsyndromic TOF patients. NKX2.5 mutation is present in >/=4% of TOF patients. Mutations identified in the present study mapped outside of the homeodomain, were not associated with atrioventricular conduction disturbances, and were not fully penetrant, in contrast to mutations previously reported that impair homeodomain function.

Familial dilated cardiomyopathy locus maps to chromosome 2q31.

BACKGROUND: Inherited gene defects are an important cause of dilated cardiomyopathy. Although the chromosome locations of some defects and 1 disease gene (actin) have been identified, the genetic etiologies of most cases of familial dilated cardiomyopathy remain unknown. METHODS AND RESULTS: We clinically evaluated 3 generations of a kindred with autosomal dominant transmission of dilated cardiomyopathy. Nine surviving and affected individuals had early-onset disease (ventricular chamber dilation during the teenage years and congestive heart failure during the third decade of life). The disease was nonpenetrant in 2 obligate carriers. To identify the causal gene defect, linkage studies were performed. A new dilated cardiomyopathy locus was identified on chromosome 2 between loci GCG and D2S72 (maximum logarithm of odds [LOD] score=4.86 at theta=0). Because the massive gene encoding titin, a cytoskeletal muscle protein, resides in this disease interval, sequences encoding 900 amino acid residues of the cardiac-specific (N2-B) domain were analyzed. Five sequence variants were identified, but none segregated with disease in this family. CONCLUSIONS: A dilated cardiomyopathy locus (designated CMD1G) is located on chromosome 2q31 and causes early-onset congestive heart failure. Although titin remains an intriguing candidate gene for this disorder, a disease-causing mutation is not present in its cardiac-specific N2-B domain.

Comparison of transesophageal and intracardiac electrophysiologic studies in characterization of supraventricular tachycardia in pediatric patients.

OBJECTIVES: This study sought to determine the accuracy of transesophageal electrophysiologic studies in diagnosing and characterizing various mechanisms of supraventricular tachycardia in pediatric patients. BACKGROUND: Transesophageal electrophysiologic studies are a relatively noninvasive means of characterizing supraventricular tachycardia. Although widely used, to our knowledge no data exist that directly compare information obtained from transesophageal electrophysiologic studies with that from intracardiac electrophysiologic studies. METHODS: We reviewed the records of 57 pediatric patients undergoing both transesophageal and intracardiac electrophysiologic studies at our institution. The results of these studies were compared with respect to mechanism of tachycardia, localization of accessory atrioventricular (AV) connections (if present) and characterization of anterograde accessory connection conduction properties. RESULTS: Tachycardia mechanisms were concordant in 56 of 57 patients: orthodromic reciprocating tachycardia in 43, antidromic reciprocating tachycardia in 1, both orthodromic and antidromic tachycardia in 2, AV node reentrant tachycardia in 5, atrial reentrant tachycardia in 4 and ectopic atrial tachycardia in 2. Of 29 patients with orthodromic reciprocating tachycardia using a concealed accessory connection, transesophageal study predicted the accessory connection site through changes induced by transient bundle branch block in 12. By the Bland-Altman method in 14 patients with pre-excitation, the anterograde accessory connection effective refractory period determined by transesophageal study compared favorably with that determined by intracardiac study (mean difference 5.0 ms, limits of agreement -55 and 65 ms). CONCLUSIONS: Transesophageal electrophysiologic studies are a highly accurate means of diagnosing and characterizing various mechanisms of supraventricular tachycardia in pediatric patients.

Proarrhythmia, cardiac arrest and death in young patients receiving encainide and flecainide. The Pediatric Electrophysiology Group.

The potential for proarrhythmic responses to the class IC sodium channel-blocking drugs encainide and flecainide has not been well described in young patients. Therefore, data were retrospectively collected from 36 institutions regarding 579 young patients who were administered encainide or flecainide for treatment of supraventricular tachycardias (encainide 86 patients, flecainide 369 patients) or ventricular arrhythmias (encainide 21 patients, flecainide 103 patients) to assess the frequency of proarrhythmia, cardiac arrest and death during therapy (adverse events). The two drugs were similar in regard to efficacy (flecainide 71.4%, encainide 59.8%) and rate of proarrhythmic responses (flecainide 7.4%; encainide 7.5%). However, patients receiving encainide more frequently experienced cardiac arrest (encainide 7.5% vs. flecainide 2.3%, p less than 0.05) or died during treatment (encainide 7.5% vs. flecainide 2.1%, p less than 0.05). Detailed data were provided for 44 patients experiencing one or more adverse events. Patient age, previous drug trials, concomitant therapy and days of inpatient monitoring were similar for patients receiving encainide or flecainide. However, echocardiographic left ventricular shortening before treatment was lower among patients receiving encainide (0.23 +/- 0.09) than among those receiving flecainide (0.34 +/- 0.06, p less than 0.05). Plasma drug concentrations were rarely elevated. Cardiac arrest (12 patients) and deaths (13 patients) occurred predominantly among patients with underlying heart disease, particularly among patients receiving flecainide for supraventricular tachycardia (8.3% vs. 0.3%, p less than 0.001). Fifteen patients with an ostensibly normal heart and normal ventricular function experienced proarrhythmia during treatment for supraventricular tachycardia, but only 3 of the 15 had a cardiac arrest or died. The relatively high incidence of adverse events should be considered when contemplating treatment with encainide or flecainide, particularly among patients with underlying heart disease.

Relation between preoperative left ventricular muscle mass and outcome of the Fontan procedure in patients with tricuspid atresia.

The relation between preoperative left ventricular muscle mass and clinical outcome of the Fontan procedure was evaluated retrospectively in 22 patients with tricuspid atresia who were selected for this physiologic surgical correction by conventional hemodynamic criteria. Patients were divided into two groups: group A (excellent or good outcome) and group B (poor outcome or death) based on the clinical course assessed up to 9.5 years postoperatively. Thirteen of 22 group A patients did not have prolonged, clinically significant, systemic venous hypertension and were not on long-term diuretic drug therapy. Nine of 22 group B patients either had clinically significant systemic venous hypertension, required long-term diuretic drug therapy or died (3 patients). Age at surgery, pulmonary arteriolar resistance, left ventricular ejection fraction, end-diastolic volume, end-diastolic pressure, systemic oxygen saturation and pulmonary to systemic blood flow ratio (Qp/Qs) were not statistically different between the two groups. Left ventricular muscle mass, both in group A patients (92 +/- 31 g/m2) and in group B patients (146 +/- 61 g/m2), was greater than the normal mean value (p less than 0.01 and p less than 0.001, respectively). Left ventricular muscle mass in group B was significantly greater than in group A (p less than 0.01). Furthermore, left ventricular muscle mass/end-diastolic volume (mass/volume) ratio, reflecting the extent of left ventricular hypertrophy relative to volume overload, was significantly greater in group B (1.1 +/- 0.28) than in group A (0.84 +/- 0.21) (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Heart rate and blood pressure response to upright tilt in young patients with unexplained syncope.

Syncope in apparently normal patients has been attributed to an inhibitory reflex originating in cardiac sensory receptors. The reflex may be elicited by upright tilt with or without isoproterenol infusion. In this study, an upright 90 degree tilt protocol was evaluated in 20 young patients aged 7 to 22 years with syncope but with normal cardiac and neurologic evaluations. The electrocardiogram and blood pressure were noninvasively recorded at 1 min intervals while the patient was supine (5 to 10 min) and during tilt (15 min) in the baseline state. The protocol was repeated during isoproterenol infusion at increasing doses until symptoms of syncope or near syncope were provoked or the maximal isoproterenol dose was achieved (0.07 to 0.1 microgram/kg per min). Mean heart rate, mean blood pressure and RR interval variability, expressed as the standard deviation and the mean of the absolute difference between consecutive RR intervals, were assessed. Symptoms were elicited during tilt in 16 of the 20 patients (in 4 at baseline and in 12 with isoproterenol infusion); no symptoms were induced in 4 patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracavitary electrode catheter cardioversion of atrial tachyarrhythmias in the dog.

This study examined factors determining efficacy of intracavitary cardioversion of atrial tachyarrhythmias in closed chest, anesthetized dogs with talc pericarditis. Electrode catheters were positioned transvenously with the cathode in the right atrial appendage. In Group 1 dogs (n = 6), three anode sites (superior and inferior venae cavae ostia and mid-right atrium) were tested with graded energy shocks to determine the lowest effective cardioversion energy at each anode position. In Group 2 dogs (n = 9), multiple cardioversion attempts with energy levels of 0.01 to 5.0 J were used to evaluate reproducibility of energy thresholds. In Group 3 dogs (n = 6) without talc-induced pericarditis, atrial pathologic study was done after five intracavitary shocks (0.5 or 5.0 J). In Group 1, cardioversion was achieved with 0.75 J or less with no significant difference in minimal effective cardioversion energies among the three anode positions tested. In Group 2, 98 (26%) of 372 cardioversion attempts were successful. Intra-animal minimal effective cardioversion energies varied widely, and timing of shocks relative to atrial electrograms did not influence efficacy. Complications were infrequent and included delayed sinus rhythm recovery, transient atrioventricular block and ventricular fibrillation. Ventricular fibrillation occurred in 9 (2.4%) of 372 shocks, and was associated with higher delivered energies (6 of 9 with greater than or equal to 1.0 J) and with shocks delivered 116 to 180 ms after onset of the QRS complex. In Group 3, two dogs had no histologic damage, three dogs had multiple small foci of subendocardial necrosis and in one dog these foci coalesced to involve half the atrial wall thickness. Thus, low energy cardioversion of atrial tachyarrhythmias is feasible using intracavitary electrodes. Synchronization of energy delivery to the QRS complex is important to minimize risk of ventricular fibrillation.

Fetal tachycardia: mechanisms and predictors of hydrops fetalis.

OBJECTIVES: This study had three objectives: 1) to determine the electrophysiologic mechanisms of fetal supraventricular tachycardia at presentation and postnatally; 2) to identify the clinical and electrophysiologic predictors of hydrops fetalis; and 3) to describe the medium-term follow-up (1 to 7 years) of patients with fetal supraventricular tachycardia. BACKGROUND: Fetal supraventricular tachycardia causes significant fetal and neonatal morbidity and mortality. Prenatal analysis and postnatal confirmation of fetal supraventricular tachycardia mechanisms have been limited. METHODS: Supraventricular tachycardia mechanisms were evaluated by prenatal Doppler/M-mode echocardiography, immediate neonatal surface electrocardiography and postnatal transesophageal electrophysiologic procedures in 30 consecutive patients presenting with fetal supraventricular tachycardia (17 managed prenatally, 13 first managed postnatally). RESULTS: The fetal supraventricular tachycardia mechanism was 1:1 atrioventricular conduction in 22 patients and supraventricular tachycardia with atrioventricular block (atrial flutter) in 8. At the postnatal transesophageal electrophysiologic procedure, tachycardia was induced in 27 of 30 patients; atrioventricular reentrant tachycardia in 25 (93%) of 27 and intraatrial reentrant tachycardia in only 2 (7%) of 27. Hydrops was present in 12 of 30 fetuses. Sustained supraventricular tachycardia (> 12 h) and lower gestation at presentation correlated with hydrops (p < 0.02, p < 0.05), but mechanism of tachycardia and heart rate did not. Gestational age at delivery was significantly greater in those who received intrauterine management (39 +/- 1.3 vs. 37 +/- 2.9 weeks, p = 0.04) despite earlier presentation (32.6 vs. 37.1 weeks). Cesarean section deliveries were reduced in the same group (3 of 17 vs. 11 of 13, p = 0.0006). CONCLUSIONS: Atrioventricular reentrant tachycardia was the predominant mechanism of supraventricular tachycardia in the fetus. There was a high association of supraventricular tachycardia with atrioventricular block in utero and accessory atrioventricular connections. Outcome at 1 to 7 years was excellent regardless of severity of illness at clinical presentation.

Role of endomyocardial biopsy in rejection surveillance after heart transplantation in neonates and children.

OBJECTIVES: The aim of this study was to retrospectively evaluate the sensitivity of noninvasive surveillance (physical examination, echocardiography) of rejection in accurately predicting histologically documented rejection episodes. Additionally, the usefulness of routine scheduled biopsy and its safety in pediatric patients was explored. BACKGROUND: Endomyocardial biopsy has been utilized as the standard for rejection surveillance after heart transplantation in adults, but its role in documenting clinically suspected rejection and in routine surveillance of pediatric patients has not been agreed upon. METHODS: Heart transplantation was performed in 14 neonates and 21 children. The immunosuppressive regimen consisted of cyclosporine, azathioprine and prednisone. All patients underwent routine noninvasive rejection surveillance that included clinical examination and echocardiography. In the neonates, biopsy was performed quarterly beginning 6 months after transplantation, after cessation of prednisone therapy. In the children, biopsy was performed 15 times in the 1st year. A minimum of five biopsy samples were interpreted using the Working Formulation for Heart Transplant Rejection. RESULTS: In the neonates, 37 biopsies were performed. Evidence of rejection was present in only three biopsy samples obtained during eight episodes (38%) of clinically suspected rejection. In 29 biopsies performed when rejection was not clinically suspected, each biopsy was free of cellular infiltrate. In the children, 291 biopsies were performed. Evidence of rejection was present in only seven biopsies (41%) from 17 episodes of clinically suspected rejection. Cellular rejection was discovered during routine rejection surveillance biopsies in asymptomatic patients in 23 (8.4%) of 274 biopsies. CONCLUSIONS: In neonates with clinically suspected rejection, endomyocardial biopsy identified which patients did not require rejection therapy. Endomyocardial biopsy surveillance did not detect any unsuspected cases of rejection. In children, noninvasive rejection surveillance was less reliable even in asymptomatic patients, suggesting that periodic endomyocardial biopsy should be utilized.

Permanent cardiac pacing in patients with the long QT syndrome.

A permanent pacemaker was inserted in eight patients with the long QT syndrome. All had recurrent syncope or seizures, six had documented torsade de pointes and four had aborted sudden death. Among the eight patients, permanent pacing was instituted in three who were unsuccessfully treated with both a beta-adrenergic blocking agent and left cardiothoracic sympathectomy, and in two who proved refractory or intolerant to beta-blockers. Another three patients had pacemaker implantation and long-term beta-blocker therapy because of spontaneous atrioventricular (AV) block in one, aborted sudden death in one and patient preference in one. After pacing (70 to 85 beats/min), there was no significant change in the mean corrected QT interval, but the mean QT interval decreased significantly (534.4 +/- 51.4 to 425.6 +/- 18.9 ms, p less than 0.0001). Over a mean follow-up period of 35.1 +/- 18.9 months, all patients are alive and currently free of syncope. One patient without a history of stress-induced syncope had two syncopal episodes (believed to be due to hyperventilation) while under severe emotional stress, but has been symptom free for the past 5 years. One patient with an atrial demand (AAI) pacemaker developed dizziness due to documented episodes of AV block, but remains asymptomatic after conversion to atrial rate-responsive dual chamber (DDD) pacing. Either atrial or ventricular pacing combined with beta-blocker therapy appears to be effective treatment for a subset of patients with the long QT syndrome, by either preventing episodes of torsade de pointes or alleviating symptoms due to bradycardia from beta-blocker therapy.

Cardiac and skeletal muscle abnormalities in cardiomyopathy: comparison of patients with ventricular tachycardia or congestive heart failure.

Results of cardiac muscle and skeletal muscle biopsies were compared in 22 patients with cardiomyopathy; 11 patients presented with symptoms secondary to ventricular tachycardia (Group 1) and 11 had symptoms of severe congestive heart failure (Group 2). No patient had structural or ischemic cardiac disease. In Group 1 patients, hemodynamic abnormalities were subtle, but invasive study demonstrated dilated cardiomyopathy in two patients and restrictive cardiomyopathy in nine. In Group 2, eight patients had dilated cardiomyopathy and three had restrictive cardiomyopathy. Cardiac biopsy results were abnormal in all 22 patients and the abnormalities were similar for the two groups. Cardiac histologic study revealed a spectrum of abnormalities including fibrosis, dilated sarcoplasmic reticulum, increased numbers of intercalated discs and mitochondrial abnormalities. Histologic abnormalities of skeletal muscle were similar in each group, consisting of endomysial fibrosis and increased lipid deposits. Slightly more than half of the Group 1 and Group 2 patients also had a low concentration of skeletal muscle long chain acylcarnitine. These data demonstrate that abnormalities of both cardiac and skeletal muscle are common in patients with cardiomyopathy; abnormalities are similar whether initial symptoms are due to ventricular tachycardia or congestive heart failure. It is suggested that these patients with cardiomyopathy may have a generalized myopathy.

Prevention of recurrent sudden cardiac arrest: role of provocative electropharmacologic testing.

This study evaluates the usefulness of serial provocative electropharmacologic testing for predicting the efficacy of prophylactic antiarrhythmic treatment regimens in patients resuscitated from sudden cardiac arrest in the absence of acute myocardial infarction. Testing was carried out in 34 consecutive patients (28 men and 6 women) who required cardiopulmonary resuscitation and direct current countershock for treatment of primary ventricular fibrillation (28 patients), ventricular tachycardia (5 patients) or excessively rapid heart rate during atrial fibrillation with preexcitation (1 patient). In 8 (24%) of the 34 patients, drug testing either was not feasible because of absence of inducible arrhythmia or was incomplete because of patient withdrawal from study; and 3 of these 8 patients had recurrent sudden cardiac arrest within 10 to 19 months. In an additional five patients, treatment regimens failed to prevent initiation of sustained ventricular tachyarrhythmias in the catheterization laboratory, and two of these five patients had cardiac arrest recurrences within 2 weeks to 25 months of follow-up. In the remaining 21 (62%) of the 34 patients, including 3 patients with preexcitation syndrome, a drug regimen or surgical treatment, or both, was found that prevented inducible life-threatening tachyarrhythmias in the laboratory. Subsequently, only 1 (5%) of these 21 patients died suddenly within a 7 to 38 month (mean +/- standard deviation, 18 +/- 8.3) follow-up period. Thus, provocative electropharmacologic testing appears to be useful in predicting response to therapy in survivors of sudden cardiac arrest.

Bromocriptine treatment of digitalis-induced ventricular tachyarrhythmias: studies in a canine model.

Ventricular tachyarrhythmias associated with digitalis toxicity are believed to be due, in part, to cardiac glycoside-mediated increased central sympathetic neural activity. Because dopaminergic receptor agonists reduce sympathetic outflow, this study assessed effectiveness of the available dopaminergic agonist, bromocriptine, in slowing or terminating ouabain-induced ventricular tachycardia in anesthetized dogs. In all experiments, ouabain was administered intravenously (20 micrograms/kg body weight bolus injection, followed by 2.5 micrograms/kg per min infusion) until the onset of stable ventricular tachycardia. Of seven untreated dogs (Group 1), ouabain-induced ventricular tachyarrhythmias resulted in ventricular fibrillation in three, while in four dogs tachycardia persisted without significant change in rate until the study was terminated. Fourteen dogs (Group 2) received bromocriptine, either 30 micrograms/kg (Group 2A) or 50 micrograms/kg (Group 2B), after the onset of ventricular tachycardia. Tachycardia slowed in all 14 dogs and terminated with resumption of sinus rhythm in 8 of the 14. In all six dogs pretreated with the peripheral dopaminergic antagonist domperidone (Group 3), bromocriptine, 50 micrograms/kg, slowed ventricular tachycardia and in three of the six, tachycardia terminated. In contrast, of five dogs pretreated with haloperidol, a central and peripheral dopaminergic receptor antagonist (Group 4), bromocriptine, 50 micrograms/kg, failed to slow ventricular tachycardia in three, and two of the three developed ventricular fibrillation. In summary, the dopaminergic receptor agonist, bromocriptine, presumably acting at central dopaminergic receptor sites, consistently slowed and in most cases reversed ouabain-induced ventricular tachycardia in a canine model.

Canine tricuspid valve malformation, a model of human Ebstein anomaly, maps to dog chromosome 9.

BACKGROUND: Ebstein anomaly of the tricuspid valve is a congenital cardiac malformation characterised by downward displacement of the attachment of the septal and posterior leaflets of the tricuspid valve. Canine tricuspid valve malformation (CTVM) is morphologically similar to Ebstein anomaly; familial occurrence of CTVM has been described. Several observations suggest a genetic cause but most cases appear to be sporadic. METHODS: Three purebred Labrador Retriever kindreds enriched for CTVM underwent clinical examination and echocardiography. DNA was extracted from whole blood. Genotyping was carried out using polymorphic repeat markers with an average spacing of 15 cM and polymorphic information content of 0.74. RESULTS: Pedigree analysis identified CTVM segregating as an autosomal dominant trait with reduced penetrance. Genome wide linkage analysis in one kindred identified a CTVM susceptibility locus on dog chromosome 9 (CFA9) with a maximum multipoint lod score of 3.33. The two additional kindreds showed a conserved disease haplotype. CONCLUSIONS: This study identifies a CTVM susceptibility locus on CFA9 and a founder effect in apparently unrelated Labrador Retriever kindreds. These results provide the basis for a positional candidate cloning effort to identify the CTVM disease gene. Identification of the CTVM gene will permit mutation screening of patients with Ebstein anomaly, which should provide additional insights into the genetic programmes of valve development.

Ventricular relaxation in the stage 24 chick embryo following changes in volume and blockade of Na+ and Ca2+ channels.

OBJECTIVE: Early in cardiac development, regulation of cytosolic Ca2+ has been thought to depend primarily on sarcolemmal Ca2+ transport. We hypothesized that perturbation of cytosolic Ca2+ in the embryonic ventricle would result in a change in ventricular relaxation which could be quantified by a monoexponential model. We reasoned that since it has been difficult to selectively block the Na(+)-Ca2+ exchanger in vivo, that blockade of Na+ (lidocaine) or Ca2+ (verapamil) channels in the embryonic heart may perturb cytosolic Ca2+ and thereby alter ventricular relaxation. METHODS: All studies were performed in ovo in Hamilton-Hamburger stage 24 chick embryos. Isovolumic relaxation time (mean +/- standard deviation, 58 +/- 19 ms) was derived from dorsal aortic flow and atrioventricular inflow during 61 cardiac cycles in 4 embryos. Ventricular pressure was digitally recorded from 13 embryos during 188 cycles following intravenous injection of chick Ringer's solution (5 embryos), verapamil (4 embryos) or lidocaine (4 embryos). Ventricular relaxation was characterized by a monoexponential model: P(t) = P infinity + Poe-1/tau where P(t) = pressure at time (t), P infinity = pressure asymptote, Po = pressure at the onset of relaxation and tau = the isovolumic relaxation constant. Non-linear least-squares regression was used to estimate tau and P infinity during isovolumic relaxation at baseline and at 30 s and 60 s post-injection. RESULTS: Ventricular end-diastolic pressure was increased by all three interventions. Both lidocaine and verapamil prolonged cycle length. Lidocaine prolonged tau while chick Ringer's solution and verapamil did not. No significant change in P chi was observed. CONCLUSIONS: This study demonstrates that blockade of Na+ channels with lidocaine slows ventricular relaxation presumably by perturbing cytosolic Ca2+ via the Na(+)-Ca2+ exchange system. Changes following Ca2+ channel blockade with verapamil are less evident in the stage 24 chick embryo. Evaluation of ventricular relaxation may provide a useful way to study developmental aspects of Ca2+ transport.