Biomedical subjects

J H Lawrence

Publications and source records attributed to J H Lawrence.

At least 19 recordsLinked to original sources

PubMed1998

Predictors of fluoroscopy time and estimated radiation exposure during radiofrequency catheter ablation procedures.

The objective of this study was to identify factors that predict fluoroscopy duration and radiation exposure during catheter ablation procedures. The patient population included 859 patients who participated in the Atakr Ablation System clinical trial at 1 of 9 centers (398 male and 461 female patients, aged 36 +/- 21 years). Each patient underwent catheter ablation of an accessory pathway, the atrioventricular junction, or atrioventricular nodal reentrant tachycardia using standard techniques. The duration of fluoroscopy was 53 +/- 50 minutes. Factors identified as independent predictors of fluoroscopy duration included patient age and sex, the success or failure of the ablation procedure, and the institution at which the ablation was performed. Catheter ablation in adults required longer fluoroscopy exposure than it did in children. Men required longer durations of fluoroscopy exposure than did women. The mean estimated "entrance" radiation dose was 1.3 +/- 1.3 Sv. The dose needed to cause radiation skin injury was exceeded during 22% of procedures. The overall mean effective absorbed dose from catheter ablation procedures was 0.025 Sv for female patients and 0.017 Sv for male patients. This degree of radiation exposure would result in an estimated 1,400 excess fatal malignancies in female patients and 2,600 excess fatal malignancies in male patients per 1 million patients.

Adolescent↗

PubMed1998

Comparison of adenosine effects on atrioventricular node reentry and atrioventricular reciprocating tachycardias.

BACKGROUND: Adenosine is an established first line therapy for the treatment of narrow complex tachycardias. The two most common etiologies of paroxysmal supraventricular tachycardia (SVT) are atrioventricular node reentry tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT). HYPOTHESIS: We postulated that adenosine might have different effects on the termination of AVNRT vs. AVRT, and that these differences might assist in the noninvasive differentiation between these diagnoses. METHODS: Fifty-nine patients referred for the diagnosis and treatment of SVT were included in the study. All patients had SVT induced during electrophysiology testing, and each patient received adenosine during SVT. The adenosine dose, time to tachycardia termination, and site of tachycardia termination were recorded. Seventeen patients required isoproterenol administration to initiate SVT. This subset of patients was compared with those not requiring isoproterenol. RESULTS: There was no statistically significant difference in the adenosine dose or time to tachycardia termination when comparing patients with AVNRT with those with AVRT. All patients with AVNRT had termination of tachycardia in the antegrade direction with final activation in the atria. Patients requiring isoproterenol for tachycardia initiation experienced tachycardia termination significantly faster than those not requiring isoproterenol, although there was no difference in the dose of adenosine required for termination. CONCLUSION: These data demonstrate that patients with dual AV node physiology and AVNRT do not have altered sensitivity to adenosine compared with patients with AVRT and normal AV nodes. Further investigation will be required to determine the clinical utility of the significantly shorter time to tachycardia termination for patients receiving isoproterenol.

Adenosine↗

PubMed1998

Relation between impedance and electrode temperature during radiofrequency catheter ablation of accessory pathways and atrioventricular nodal reentrant tachycardia.

OBJECTIVES: Impedance monitoring has been proposed as a method to assess the adequacy of tissue heating during catheter ablation procedures. The purpose of this study was to evaluate the relation among initial impedance, fall in impedance, and electrode temperature during catheter ablation procedures. METHODS AND RESULTS: Data from 248 applications of radiofrequency energy in 45 consecutive patients (26 with accessory pathways and 19 with atrioventricular nodal reentrant tachycardia) referred for catheter ablation were analyzed. The initial impedance was higher during ablation of accessory pathways than during atrioventricular nodal reentrant tachycardia (116+/-66 versus 106+/-80 omega, P < .001). In both groups, a significant correlation was observed between the initial impedance and temperature (R = 0.98, P < .001). After accounting for differences between patients and ablation targets, an even closer correlation was observed (accessory pathways: R = 0.95, P < .0001; atrioventricular nodal reentrant tachycardia: R = 0.94, P < .0001). CONCLUSION: These data suggest that monitoring of the initial impedance and the fall in impedance during ablation procedures may provide clinically valuable information to assess the efficacy of tissue heating and lesion formation.

Adolescent↗

PubMed1998

Complete correction of acid alpha-glucosidase deficiency in Pompe disease fibroblasts in vitro, and lysosomally targeted expression in neonatal rat cardiac and skeletal muscle.

The enzyme acid alpha-glucosidase catalyzes the breakdown of lysosomal glycogen. Absence of this enzyme results in infantile Pompe disease, characterized by hypertrophic cardiomyopathy, skeletal muscle weakness and fatal heart failure by 2 years of age. We have examined the possibility of gene replacement therapy for this disease, by constructing an E1-deleted recombinant adenovirus encoding human acid alpha-glucosidase (Ad-GAA). The dose-response in fibroblasts from patients with Pompe disease transduced with this vector is linear over the range tested (one to 2000 plaque forming units (p.f.u.) of Ad-GAA per cell), and acid alpha-glucosidase activity comparable to that of normal fibroblasts is achieved at 100 p.f.u. per cell. Targeting of the recombinant protein to the lysosomal compartment was confirmed by immunocytochemistry. In vivo expression was examined by injecting Ad-GAA into newborn rats; intracardiac administration produced 10 times the normal level of acid alpha-glucosidase activity in whole heart lysates, while a hind-limb i.m. injection increased activity in that muscle to six times the normal level. Western blotting of these tissues defected species at 76 kDa consistent with the size of processed lysosomal enzyme, and levels of expression as high as 1.0 mg recombinant protein per gram of tissue wet weight were produced. These data demonstrate high-level, lysosomal expression of recombinant acid alpha-glucosidase in treated target tissues and support the feasibility of gene replacement strategies for Pompe disease.

Adenoviridae↗

PubMed1998

Acceleration of widespread adenoviral gene transfer to intact rabbit hearts by coronary perfusion with low calcium and serotonin.

Previous attempts at adenoviral gene transfer to the intact heart have been limited by the requirement for prolonged exposure to high virus concentrations. In an ex vivo coronary perfusion model of intact adult rabbit hearts, we previously reported gene transfer to 96% of cardiac myocytes after a 60 min exposure to 1.6 x 10(9) p.f.u./ml Ad beta gal, a recombinant adenovirus encoding beta-galactosidase. Here we sought to decrease the virus exposure time by enhancing microvascular permeability to increase the efficiency of adenoviral gene transfer. Baseline perfusion with 1.0 x 10(8) p.f.u./ml Ad beta gal in normal Krebs solution (1 mM calcium) caused infection of 22% of myocytes at 30 min and 40% at 60 and 120 min. Increasing the virus concentration, decreasing perfusate calcium concentration, or pretreating with serotonin or bradykinin in Krebs solution or L-NAME in heparinized rabbit blood significantly decreased the necessary exposure time. Under optimal conditions of serotonin pretreatment, 50 mumol/l perfusate calcium, and a virus concentration of 1.6 x 10(9) p.f.u./ml, 2 min of coronary perfusion sufficed to produce near-total infection. This profound enhancement of infection parameters has important implications for in vivo myocardial gene transfer, where a similar strategy could facilitate gene therapy for common myocardial disorders.

Adenoviridae↗

PubMed1998

Beat-to-beat repolarization lability identifies patients at risk for sudden cardiac death.

INTRODUCTION: Recent studies have implicated repolarization lability in the genesis of malignant ventricular arrhythmias. However, few data exist on assessment of temporal QT interval variability and its relation to arrhythmogenesis. We tested the ability of the QT variability index (QTVI), a measure of beat-to-beat QT interval fluctuations measured on a single ECG lead, to identify patients presenting with malignant ventricular arrhythmias and predict their subsequent occurrences. METHODS AND RESULTS: We measured the QTVI in 95 patients presenting for electrophysiologic study (EPS). The ability of the QTVI to identify patients with sudden cardiac death (SCD) or sustained monomorphic ventricular tachycardia (MVT) on presentation and during follow-up of 23.7+/-14.3 months was compared with spatial QT dispersion, T wave alternans ratio during atrial pacing, MVT inducibility at EPS, signal-averaged ECG, heart rate variability, and ejection fraction. The QTVI was higher in patients with heart disease than in controls (-0.7+/-0.7 vs -1.1+/-0.5, P < 0.05), and higher in patients presenting with SCD than in other patients with heart disease (0.0+/-0.6 vs -0.8+/-0.5, P < 0.05). The QTVI was the only clinical variable that identified patients who presented with SCD (P = 0.004, odds ratio = 12.5) on stepwise, logistic multiple regression. Fourteen patients had arrhythmic events during follow-up. In a Kaplan-Meier analysis of arrhythmic events, QTVI> or =0.1 was a discriminator for higher risk of arrhythmic events (P < 0.05). CONCLUSIONS: (1) This noninvasive measure of temporal repolarization lability identified patients with SCD and predicted arrhythmia-free survival. (2) Further studies are needed to determine the mechanisms that mediate beat-to-beat QT interval variability.

Death, Sudden, Cardiac↗

PubMed1997

Safety and effectiveness of placement of pacemaker and defibrillator leads in the axillary vein guided by contrast venography.

Despite evidence of an increased incidence of lead fracture, the infraclavicular subclavian approach remains the dominant approach for placement of pacemaker and implantable defibrillator leads. Although this complication can be prevented by lead placement in the cephalic vein or by recently described approaches for lead placement in the axillary vein, these approaches have not gained widespread acceptance. The purpose of this study was to evaluate the safety and efficacy of an alternative technique for lead placement that uses contrast-guided venipuncture of the axillary vein with a 5Fr micropuncture introducer set. A total of 50 patients underwent an attempt at placement of pacemaker or implantable defibrillator leads via the axillary vein using this new technique. Patients were randomized into 2 groups based on whether the initial attempt at axillary vein access was performed medial or lateral to the rib cage margin. Lead placement was successfully accomplished in 49 of the 50 patients using this technique. Initial success was achieved in each of 25 patients randomized to the medial approach compared with 18 of 24 patients randomized to the lateral approach to the axillary vein (75%). In each of the 6 patients in whom the initial technique failed, lead placement was subsequently achieved with the medial approach. In addition to a higher initial success rate, the medial approach was determined to be preferable as evidenced by a shorter lead placement time, a smaller number of contrast injections, and a reduced requirement for additional micropuncture guidewires. There were no major complications associated with either approach. Contrast-guided venipuncture of the axillary vein is a safe and effective approach to placement of endocardial leads.

Aged↗

PubMed1997

Ultrarapid, highly efficient viral gene transfer to the heart.

Gene therapy for common myocardial diseases will require effective and homogeneous gene delivery throughout the intact heart. We created two experimental models to identify and optimize parameters important for adenovirus-mediated cardiac gene transfer. In cultured rabbit ventricular myocytes, the percentage of infected cells increased with higher absolute numbers of virus particles, longer durations of virus exposure, physiological temperatures, and specific culture media compositions. Simulating the in vitro conditions, we delivered adenovirus to intact rabbit hearts by intracoronary perfusion. The percentage of infected cells increased with higher coronary flow rates, longer virus exposure times, and higher virus concentrations. Under optimal conditions, nearly 100% of myocytes expressed the reporter gene beta-galactosidase after ex vivo infection. This novel delivery method, the first to demonstrate virtually complete transduction of any intact organ, could be adapted to achieve widespread gene transfer in vivo.

Adenoviridae↗

PubMed1997

Dispersion of ventricular activation and refractoriness in patients with idiopathic dilated cardiomyopathy.

We sought to evaluate the electrophysiologic substrate for ventricular arrhythmias and sudden cardiac death in patients with idiopathic dilated cardiomyopathy. When compared with controls, patients with cardiomyopathy had prolonged activation times, increased dispersion of activation and recovery, and prolonged duration of monophasic action potential recordings at 70%, but not at 90%, of repolarization.

Cardiac Catheterization↗

PubMed1997

Acute radiation dermatitis following radiofrequency catheter ablation of atrioventricular nodal reentrant tachycardia.

Radiation exposure during fluoroscopic imaging poses potential risks to patients and physicians, especially during protracted cardiovascular or radiological interventional procedures. We describe a woman with refractory paroxysmal supraventricular tachycardia who underwent radiofrequency catheter ablation of the slow pathway involved in atrioventricular nodal reentrant tachycardia. The patient subsequently returned 4 weeks later with acute radiation dermatitis that was retrospectively attributed to a malfunction in the fluoroscopy unit that lacked a maximum current output cut-off switch. Using dose reconstruction studies and her estimated biological response, we determined that she received between 15 and 20 Gy (1 Gy = 100 rads) to the skin on her back during the procedure. The exposure will result in an increase in her lifelong risk of skin and lung cancer. This article underscores the potential for radiation-induced injury during lengthy therapeutic procedures using x-ray equipment.

Acute Disease↗

PubMed1996

Local anesthetics as effectors of allosteric gating. Lidocaine effects on inactivation-deficient rat skeletal muscle Na channels.

Time- and voltage-dependent local anesthetic effects on sodium (Na) currents are generally interpreted using modulated receptor models that require formation of drug-associated nonconducting states with high affinity for the inactivated channel. The availability of inactivation-deficient Na channels has enabled us to test this traditional view of the drug-channel interaction. Rat skeletal muscle Na channels were mutated in the III-IV linker to disable fast inactivation (F1304Q: FQ). Lidocaine accelerated the decay of whole-cell FQ currents in Xenopus oocytes, reestablishing the wild-type phenotype; peak inward current at -20 mV was blocked with an IC50 of 513 microM, while plateau current was blocked with an IC50 of only 74 microM (P < 0.005 vs. peak). In single-channel experiments, mean open time was unaltered and unitary current was only reduced at higher drug concentrations, suggesting that open-channel block does not explain the effect of lidocaine on FQ plateau current. We considered a simple model in which lidocaine reduced the free energy for inactivation, causing altered coupling between activation and inactivation. This model readily simulated macroscopic Na current kinetics over a range of lidocaine concentrations. Traditional modulated receptor models which did not modify coupling between gating processes could not reproduce the effects of lidocaine with rate constants constrained by single-channel data. Our results support a reinterpretation of local anesthetic action whereby lidocaine functions as an allosteric effector to enhance Na channel inactivation.

Allosteric Regulation↗

PubMed1996

Coupling between fast and slow inactivation revealed by analysis of a point mutation (F1304Q) in mu 1 rat skeletal muscle sodium channels.

1. We sought to elucidate the mechanism of the defective inactivation that characterizes sodium channels containing mutations in the cytoplasmic loop between the third and fourth domains (the III-IV linker). Specifically, we measured whole-cell and single-channel currents through wild-type and F1304Q mutant mu 1 rat skeletal muscle Na+ channels expressed in Xenopus laevis oocytes. 2. In wild-type channels, inactivation is complete and the faster of two decay components predominates. In F1304Q, inactivation is incomplete; the slow decay component is larger in amplitude and slower than in wild-type. The fraction of non-inactivating current is substantial (37 +/- 2% of peak current at -20 mV) in F1304Q. 3. Cell-attached patch recordings confirmed the profound kinetic differences and indicated that permeation was not altered by the F1304Q mutation. The F1304Q phenotype must be conferred entirely by changes in gating properties and is not remedied by coexpression with the beta 1-subunit. 4. Recovery from inactivation of F1304Q channels is faster than for wild-type channels and three exponentials are required to describe recovery adequately following long (5 s) depolarizations. Thus, there are three inactivated states even in 'inactivation-deficient' F1304Q channels. 5. The steady-state voltage dependence of F1304Q inactivation is right-shifted by 26 +/- 2 mV. 6. A gating model incorporating three inactivated states, all directly accessible from multiple closed states or the open state, was constrained to fit wild-type and F1304Q inactivation (h infinitive) data and repriming data simultaneously. While it was necessary to alter the rate constants entering and exiting all three inactivated states, the model accounted for the F1304Q-induced rightward shift in steady-state inactivation without imposing voltage dependence on the inactivation rate constants. 7. We conclude that the F1304Q mutation in mu 1 sodium channels modifies several inactivation processes simultaneously. The fact that a single amino acid substitution profoundly alters both fast and slow inactivation indicates that these processes share physical determinants in Na+ channels.

Animals↗

PubMed1996

Adenovirus-mediated transfer of CCAAT/enhancer-binding protein-alpha identifies a dominant antiproliferative role for this isoform in hepatocytes.

CCAAT/enhancer-binding protein (C/EBP) isoforms are thought to be important regulators of the hepatocyte phenotype. However, the specific physiological roles of different isoforms are poorly understood because hepatocytes express multiple C/EBPs, and various isoforms have overlapping functions. To identify the functions of C/EBPalpha in mature hepatocytes, replication-defective adenovirus vectors were used to efficiently and homogeneously overexpress the mouse C/EBPalpha gene in a SV40 virus-conditionally transformed rat hepatocyte line that can be induced to express C/EBPbeta and C/EBPdelta but that has little endogenous C/EBPalpha expression. Hepatocytes were infected with a recombinant adenovirus vector carrying the cDNA for C/EBPalpha driven by Rous sarcoma virus promoter elements (AdCEBPalpha) or a similar vector carrying the Escherichia coli lacZ gene (Adbetagal). Staining for beta-galactosidase demonstrated an infection efficiency of 100% at a multiplicity of infection of 25 plaque-forming units/cell and persistence of foreign gene expression for at least 9 days. Cultures infected with AdCEBPalpha had 50-fold higher levels of C/EBPalpha mRNA and protein than those infected with Ad-beta-gal, but similar expression of C/EBP-beta. Infection with AdCEBPalpha inhibited proliferation in cells expressing little C/EBPbeta, even when proliferation was driven by the SV40 transforming antigen, and also blunted mitogenic induction of the c-myc proto-oncogene in nontransformed cells with high levels of C/EBPbeta. Although overexpression of C/EBPalpha consistently increased C/EBPalpha DNA binding activity, it was not sufficient for albumin expression. Infection with AdCEBPalpha only increased albumin mRNA levels in nontransformed cells that also expressed relatively high levels of C/EBPbeta. Thus, in hepatocytes, C/EBPalpha has a dominant antiproliferative function, but must interact with other factors to regulate hepatocyte-specific gene expression.

Adenoviridae↗

PubMed1996

Single-channel analysis of inactivation-defective rat skeletal muscle sodium channels containing the F1304Q mutation.

The intracellular linker between domains III and IV of the voltage-gated Na channel mediates fast inactivation. Targeted alteration of one or more of a triplet of hydrophobic amino acids within this linker region results in a marked slowing in the decay of ionic current. The mechanism of this defective inactivation was explored in rat skeletal muscle sodium channels (mu 1) containing the F1304Q mutation in Xenopus laevis oocytes with and without coexpression of the rat brain beta 1 subunit. Cell-attached single-channel patch-clamp recordings revealed that the mu 1-F1304Q channel reopens multiple times with open times that are prolonged compared with those of the wild-type channel. Coexpression of the beta 1 subunit stabilized a dominant nonbursting gating mode and accelerated the activation kinetics of mu 1-F1304Q but did not modify mean open time or fast-inactivation kinetics. A Markov gating model incorporating separate fast- and slow-inactivation particles reproduced the results by assuming that the F1304Q mutation specifically influences transitions to and from fast-inactivated states. These effects are independent of interactions of the mutant channel with the beta 1 subunit and do not result from a change in modal gating behavior. These results indicate that F1304Q mutant channels can still enter the inactivated state but do so reversibly and with altered kinetics.

Animals↗

PubMed1996

Reversal of potassium channel deficiency in cells from failing hearts by adenoviral gene transfer: a prototype for gene therapy for disorders of cardiac excitability and contractility.

Heart failure is a common, often lethal disorder in which conventional pharmacologic strategies have achieved limited success. Failing hearts exhibit a delay of electrical repolarization which predisposes to fatal arrhythmias. To explore the feasibility of gene therapy for this condition, we isolated myocytes from normal and failing dog hearts and quantified electrophysiologic and contractile parameters in primary culture. Action potentials were prolonged in failing cells as a result of diminished potassium currents. Exposure to AdShK, an adenovirus that overexpresses potassium channels, reversed the action potential prolongation of failing cells. The precise phenotype varied as a function of the density of expressed channels; modest increases in potassium current sufficed to mimic the non-diseased state most faithfully, while more robust expression of the transgene excessively abbreviated excitation and contraction. Our results demonstrate that viral gene transfer can modify the electrical properties of adult mammalian heart cells in a manner appropriate to reverse a fundamental disorder of excitability. Realistic application of this form of therapy will need to include a sensitive mechanism for control of the level and distribution of transgene expression.

Action Potentials↗

PubMed1995

Prospects for genetic manipulation of cardiac excitability.

Despite impressive advances in the therapy of a number of types of heart disease in the last two decades, sudden cardiac death remains a public health problem of staggering dimensions. Current treatment options include antiarrhythmic drugs that have higher than desired failure rates and implantable defibrillators that incur significant costs to the patient and society. The development of therapies that better suppress the cardiac arrhythmias responsible for sudden cardiac death requires a broad and comprehensive understanding of the basic mechanisms underlying electrical instability in the heart. This study explores the scientific basis for a molecular genetic approach to modify cardiac excitability and thereby to create animal models of sudden cardiac death. The availability of such models will open up new avenues of research in arrhythmogenesis and facilitate the development of novel antiarrhythmic agents.

Animals↗

PubMed1995

Adenovirus-mediated expression of a voltage-gated potassium channel in vitro (rat cardiac myocytes) and in vivo (rat liver). A novel strategy for modifying excitability.

Excitability is governed primarily by the complement of ion channels in the cell membrane that shape the contour of the action potential. To modify excitability by gene transfer, we created a recombinant adenovirus designed to overexpress a Drosophila Shaker potassium channel (AdShK). In vitro, a variety of mammalian cell types infected with AdShK demonstrated robust expression of the exogenous channel. Spontaneous action potentials recorded from cardiac myocytes in primary culture were abbreviated compared with noninfected myocytes. Intravascular infusion of AdShK in neonatal rats induced Shaker potassium channel mRNA expression in the liver, and large potassium currents could be recorded from explanted hepatocytes. Thus, recombinant adenovirus technology has been used for in vitro and in vivo gene transfer of ion channel genes designed to modify cellular action potentials. With appropriate targeting, such a strategy may be useful in gene therapy of arrhythmias, seizure disorders, and myotonic muscle diseases.

3T3 Cells↗

PubMed1994

Sudden cardiac death in heart failure. The role of abnormal repolarization.

Congestive heart failure is a common, highly lethal cardiovascular disorder claiming over 200,000 lives a year in the United States alone. Some 50% of the deaths in heart failure patients are sudden, and most of these are probably the result of ventricular tachyarrhythmias. Methods designed to identify patients at risk have been remarkably unrewarding, as have attempts to intervene and prevent sudden death in these patients. The failure to impact favorably on the incidence of sudden death in heart failure patients stems largely from a lack of understanding of the underlying mechanisms of arrhythmogenesis. This article explores the role of abnormalities of ventricular repolarization in heart failure patients. We will examine evidence for the hypothesis that alteration of repolarizing K+ channel expression in failing myocardium predisposes to abnormalities in repolarization that are arrhythmogenic. The possible utility of novel electrophysiological and ECG measures of altered ventricular repolarization will be explored. Understanding the mechanism of sudden death in heart failure may lead to effective therapy and more accurate identification of patients at greatest risk.

Action Potentials↗