The Impact of Hospital Size on CMS Hospital Profiling.

BACKGROUND: The Centers for Medicare & Medicaid Services (CMS) profile hospitals using a set of 30-day risk-standardized mortality and readmission rates as a basis for public reporting. These measures are affected by hospital patient volume, raising concerns about uniformity of standards applied to providers with different volumes. OBJECTIVES: To quantitatively determine whether CMS uniformly profile hospitals that have equal performance levels but different volumes. RESEARCH DESIGN: Retrospective analysis of patient-level and hospital-level data using hierarchical logistic regression models with hospital random effects. Simulation of samples including a subset of hospitals with different volumes but equal poor performance (hospital effects=+3 SD in random-effect logistic model). SUBJECTS: A total of 1,085,568 Medicare fee-for-service patients undergoing 1,494,993 heart failure admissions in 4930 hospitals between July 1, 2005 and June 30, 2008. MEASURES: CMS methodology was used to determine the rank and proportion (by volume) of hospitals reported to perform "Worse than US National Rate." RESULTS: Percent of hospitals performing "Worse than US National Rate" was ∼40 times higher in the largest (fifth quintile by volume) compared with the smallest hospitals (first quintile). A similar gradient was seen in a cohort of 100 hospitals with simulated equal poor performance (0%, 0%, 5%, 20%, and 85% in quintiles 1 to 5) effectively leaving 78% of poor performers undetected. CONCLUSIONS: Our results illustrate the disparity of impact that the current CMS method of hospital profiling has on hospitals with higher volumes, translating into lower thresholds for detection and reporting of poor performance.

pH (low) insertion peptide (pHLIP) targets ischemic myocardium.

The pH (low) insertion peptide (pHLIP) family enables targeting of cells in tissues with low extracellular pH. Here, we show that ischemic myocardium is targeted, potentially opening a new route to diagnosis and therapy. The experiments were performed using two murine ischemia models: regional ischemia induced by coronary artery occlusion and global low-flow ischemia in isolated hearts. In both models, pH-sensitive pHLIPs [wild type (WT) and Var7] or WT-pHLIP-coated liposomes bind ischemic but not normal regions of myocardium, whereas pH-insensitive, kVar7, and liposomes coated with PEG showed no preference. pHLIP did not influence either the mechanical or the electrical activity of ischemic myocardium. In contrast to other known targeting strategies, the pHLIP-based binding does not require severe myocardial damage. Thus, pHLIP could be used for delivery of pharmaceutical agents or imaging probes to the myocardial regions undergoing brief restrictions of blood supply that do not induce irreversible changes in myocytes.

Payer status and access to laparoscopic subtotal colectomy for ulcerative colitis.

BACKGROUND: Medicaid populations have been shown to have inferior surgical outcomes, but less is known about their access to advanced surgical procedures. OBJECTIVE: The aim of this study was to evaluate if patients with Medicaid and ulcerative colitis who presented for subtotal colectomy would have reduced access to the laparoscopic approach in comparison with a similar population with private insurance. DESIGN/SETTINGS/PATIENTS: Using the Nationwide Inpatient Sample database from 2008 to 2010, we identified all patients who underwent subtotal colectomy for ulcerative colitis. The χ test and multivariable logistic regression were used to identify predictors for laparoscopic subtotal colectomy for ulcerative colitis. MAIN OUTCOME MEASURES: The primary end point was the use of open or laparoscopic subtotal colectomy. Secondary end points included hospital length of stay and surgical outcomes. RESULTS: We identified a total of 2589 subtotal colectomy hospitalizations for ulcerative colitis (435 with Medicaid and 2154 with private insurance). The private insurance and Medicaid groups did not have significantly different mean age, sex, or Charlson scores (p > 0.05). Although 43% of the private insurance cohort received laparoscopic subtotal colectomy during their hospitalization, only 23% of the Medicaid population received equivalent care (p < 0.001). In a multivariate analysis that included age, sex, emergency status, hospital location, hospital size, teaching status, income, and Charlson score, urban teaching hospital status (p < 0.01), emergency status (p = 0.045), age <40 (p < 0.01), northeast location (p = 0.01), and private insurance status (p < 0.01) were independent predictors of the laparoscopic approach. LIMITATIONS: Administrative data have the potential for unrecognized miscoding or incomplete risk adjustment. Disease severity is not accounted for in the Nationwide Inpatient Sample database. CONCLUSION: Medicaid payer status was associated with reduced use of laparoscopic subtotal colectomy for ulcerative colitis. Although this finding may be due in part to physician preference or patient characteristics, health system factors appear to contribute to selection of the surgical approach.

Differential effects of ivabradine and ryanodine on pacemaker activity in canine sinus node and purkinje fibers.

INTRODUCTION: It is generally accepted that at least 2 major mechanisms contribute to sinus node (SN) pacemaking: a membrane voltage (mainly I(f) ) clock and a calcium (Ca) clock (localized submembrane sarcoplasmic reticulum Ca(2+) release during late diastolic depolarization). The aim of this study was to compare the contributions of each mechanism to pacemaker activity in SN and Purkinje fibers (PFs) exhibiting normal or abnormal automaticity. METHODS AND RESULTS: Conventional microelectrodes were used to record action potentials in isolated spontaneously beating canine SN and free running PF in control and in the presence of 0.1 µM isoproterenol. Ryanodine (0.1-3 µM) and ivabradine (3 µM) were used to inhibit sarcoplasmic reticulum Ca(2+) release or I(f), respectively. To induce automaticity at low membrane potentials, PFs were superfused with BaCl(2). In SN, ivabradine reduced the rate whereas ryanodine had no effect. Isoproterenol significantly accelerated automatic rate, which was decreased by ivabradine and ryanodine. In normally polarized PFs, ryanodine had no effects on the automatic rate in the absence or presence of isoproterenol, whereas ivabradine inhibited both control and isoproterenol-accelerated automaticity. In PF depolarized with BaCl(2), ivabradine decreased BaCl(2) -induced automatic rate while ryanodine had no effect. CONCLUSION: In canine SN, I(f) contributes to both basal automaticity and ß-adrenergic-induced rate acceleration while the ryanodine-inhibited Ca clock appears more involved in ß-adrenergic regulation of pacemaker rate. In PF, normal automaticity depends mainly on I(f). Inhibition of basal potassium conductance results in high automatic rates at depolarized membrane potentials with SN-like responses to inhibition of membrane and Ca clocks.

Analysis of Florida and New York state hospital discharges suggests that carotid stenting in symptomatic women is associated with significant increase in mortality and perioperative morbidity compared with carotid endarterectomy.

BACKGROUND: Although large randomized studies have established the efficacy and safety of carotid endarterectomy (CEA) and, recently, carotid artery stenting (CAS), the under-representation of women in these trials leaves the comparison of risks to benefits of performing these procedures on women an open question. To address this issue, we reviewed the hospital outcomes and delineated patient characteristics predicting outcome in women undergoing carotid interventions using New York and Florida statewide hospital discharge databases. METHODS: We analyzed in-hospital mortality, postoperative stroke, cardiac postoperative complications, and combined postoperative stoke and mortality in 20,613 CEA or CAS hospitalizations for the years 2007 to 2009. Univariate and multiple logistic regression analyses of variables were performed. RESULTS: CEA was performed in 16,576 asymptomatic and 1744 symptomatic women and CAS in 1943 asymptomatic and 350 symptomatic women. Compared with CAS, CEA rates, in asymptomatic vs symptomatic, were significantly lower for in-hospital mortality (0.3% vs 0.8% and 0.4% vs 3.4%), stroke (1.5% vs 2.6% and 3.5% vs 9.4%), and combined stroke/mortality (1.7% vs 3.1% and 3.8% vs 10.9%). In cohorts matched by propensity scores, the same trend favoring CEA remained significant in symptomatic women. There was no difference in cardiac complication rates among asymptomatic women, but among symptomatic woman cardiac complications were more frequent after CAS (10.6% vs 6.5%; P = .0077). Among symptomatic women, the presence of renal disease, coronary artery disease, or age ≥80 years increased the risk of CAS over CEA threefold for the composite end point of stroke or death. For asymptomatic women only in those with coronary artery disease or diabetes, there was a statistical difference in the composite mortality/stroke rates favoring CEA (1.9% vs 3.3% and 1.7% vs 3.4%, respectively). After adjusting for relevant clinical and demographic risk factors and hospital annual volume, for CAS vs CEA, the risk of the composite end point of stroke or mortality was 1.7-fold higher in symptomatic and 3.4-fold higher in asymptomatic patients. Medicaid insurance, symptomatic patient, history of cancer, and presence of heart failure on admission were among other strong predictors of composite stroke/mortality outcome. CONCLUSIONS: Databases reflecting real-world practice performance and management of carotid disease in women suggest that CEA compared with CAS has overall better perioperative outcomes in women. Importantly, CAS is associated with significantly higher morbidity in certain clinical settings and this should be taken into account when choosing a revascularization procedure.

Epicardial border zone overexpression of skeletal muscle sodium channel SkM1 normalizes activation, preserves conduction, and suppresses ventricular arrhythmia: an in silico, in vivo, in vitro study.

BACKGROUND: In depolarized myocardial infarct epicardial border zones, the cardiac sodium channel (SCN5A) is largely inactivated, contributing to low action potential upstroke velocity (V(max)), slow conduction, and reentry. We hypothesized that a fast inward current such as the skeletal muscle sodium channel (SkM1) operating more effectively at depolarized membrane potentials might restore fast conduction in epicardial border zones and be antiarrhythmic. METHODS AND RESULTS: Computer simulations were done with a modified Hund-Rudy model. Canine myocardial infarcts were created by coronary ligation. Adenovirus expressing SkM1 and green fluorescent protein or green fluorescent protein alone (sham) was injected into epicardial border zones. After 5 to 7 days, dogs were studied with epicardial mapping, programmed premature stimulation in vivo, and cellular electrophysiology in vitro. Infarct size was determined, and tissues were immunostained for SkM1 and green fluorescent protein. In the computational model, modest SkM1 expression preserved fast conduction at potentials as positive as -60 mV; overexpression of SCN5A did not. In vivo epicardial border zone electrograms were broad and fragmented in shams (31.5 +/- 2.3 ms) and narrower in SkM1 (22.6 +/- 2.8 ms; P=0.03). Premature stimulation induced ventricular tachyarrhythmia/fibrillation >60 seconds in 6 of 8 shams versus 2 of 12 SkM1 (P=0.02). Microelectrode studies of epicardial border zones from SkM1 showed membrane potentials equal to that of shams and V(max) greater than that of shams as membrane potential depolarized (P<0.01). Infarct sizes were similar (sham, 30 +/- 2.8%; SkM1, 30 +/- 2.6%; P=0.86). SkM1 expression in injected epicardium was confirmed immunohistochemically. CONCLUSIONS: SkM1 increases V(max) of depolarized myocardium and reduces the incidence of inducible sustained ventricular tachyarrhythmia/fibrillation in canine infarcts. Gene therapy to normalize activation by increasing V(max) at depolarized potentials may be a promising antiarrhythmic strategy.

Altered ventricular stretch contributes to initiation of cardiac memory.

BACKGROUND: Cardiac memory is a change in T-wave morphology induced by ventricular pacing or arrhythmias that persist after resumption of normal AV conduction. Changing the pacemaker site from atrium to ventricle alters ventricular activation and the mechanical pattern of ventricular contraction. Either or both alterations affect T-wave configuration. OBJECTIVE: The purpose of this study was to study the role of altered contractile patterns on initiation of cardiac memory. METHODS: Isolated rabbit hearts were immersed in Tyrode's solution (37 degrees C) and aortically perfused at a constant pressure of 70 mmHg. Three orthogonal quasi-ECG leads were recorded via six Ag-AgCl electrodes located on the walls of the bath. Hearts were paced at a constant cycle length from either the right atrial appendage or left ventricle lateral wall. The pulmonary artery was sealed, and both ventricles contracted isovolumetrically. Cardiac memory was quantified as T-wave vector displacement expressed as distance between T-wave vector peaks during atrial pacing before and after ventricular pacing. RESULTS: Five minutes of ventricular pacing induced significant T-wave vector displacement that returned to control in 5 to 10 minutes. No significant changes in intraventricular pressure occurred during and after ventricular pacing. Interventions that decreased ventricular load (shunting both ventricles to the bath) or contractility (excitation-contraction uncoupler blebbistatin) significantly decreased developed pressure and eliminated T-wave vector displacement. Neither intervention affected ventricular activation during ventricular pacing. Locally applied left ventricular epicardial stretch induced T-wave vector displacement similar to that induced by ventricular pacing. CONCLUSION: Altered ventricular activation during ventricular pacing initiates cardiac memory via induction of altered contractile patterns and altered stretch.

Early electrical remodeling in rabbit pulmonary vein results from trafficking of intracellular SK2 channels to membrane sites.

OBJECTIVE: Atrial fibrillation is often initiated by bursts of ectopic activity arising in the pulmonary veins. We have previously shown that a 3-h intermittent burst pacing protocol (BPP), mimicking ectopic pulmonary vein foci, shortens action potential duration (APD) locally at the pulmonary vein-atrial interface (PV) while having no effect elsewhere in rabbit atrium. This shortening is Ca(2+) dependent and is prevented by apamin, which blocks small conductance Ca(2+)-activated K(+) channels (SK(Ca)). The present study investigates the ionic and molecular mechanisms whereby two apamin-sensitive SK(Ca) channels, SK2 and SK3, might contribute to the regional APD changes. METHODS: Microelectrode and patch clamp techniques were used to record APDs and apamin-sensitive currents in isolated rabbit left atria and cells dispersed from PV and Bachmann's bundle (BB) regions. SK2 and SK3 mRNA and protein levels were quantified, and immunofluorescence was used to observe channel protein distribution. RESULTS: There was a direct relationship between APD shortening and apamin-sensitive current in burst-paced but not sham-paced PV. Moreover, apamin-sensitive current density increased in PV but not BB after BPP. SK2 mRNA, protein, and current were increased in PV after BPP, while SK2 immunostaining shifted from a perinuclear pattern in sham atria to predominance at sites near or at the PV membrane. CONCLUSIONS: BPP-induced acceleration of repolarization in PV results from SK2 channel trafficking to the membrane, leading to increased apamin-sensitive outward current. This is the first indication of involvement of Ca(2+)-activated K(+) currents in atrial remodeling and provides a possible basis for evolution of an arrhythmogenic substrate.

Long-term cardiac memory in canine heart is associated with the evolution of a transmural repolarization gradient.

OBJECTIVE: The contribution of regional electrophysiologic heterogeneity to the T-wave changes of long-term cardiac memory (CM) is not known. We mapped activation and repolarization in dogs after induction of CM and in sham animals. METHODS AND RESULTS: CM was induced by three weeks of AV-sequential pacing at the anterior free wall of the left ventricle (LV), midway between apex and base in 5 dogs. In 4 sham controls a pacemaker was implanted but ventricular pacing was not performed. At 3 weeks, unipolar electrograms were recorded (98 epicardial, 120 intramural and endocardial electrodes) during atrial stimulation (cycle length 450 ms). Activation times (AT) and repolarization times (RT) were measured and activation recovery intervals (ARIs) calculated. CM was associated with 1) deeper T waves on ECG, with no change in QT interval; 2) longer activation time at the site of stimulation in CM (29.7+/-1.0, X+/-SEM) than sham (23.9+/-1.3 ms p<0.01); 3) an LV transmural gradient in repolarization time such that repolarization at the epicardium terminated 12.4+/-2.4 ms later than at the endocardium p<0.01), in contrast to no gradient in shams (2.7+/-4.2 ms); in memory dogs, the repolarization time gradient was greatest at sites around the pacing electrode varying from 13.1+/-2.3 ms to 25.5+/-3.8 ms; 4) more negative left ventricular potentials at the peak of the body surface T wave (-4.9+/-0.8 vs -2.2+/-0.4 mV; p<0.05) but no altered right ventricular epicardial T-wave potentials. ARIs did not differ between groups. Right ventricular activation was delayed but was not associated with altered repolarization because of compensatory shortening of the right ventricular ARIs. CONCLUSION: CM-induced T-wave changes are caused by evolution of transmural repolarization gradients manifested during atrial stimulation that are maximal near the site of ventricular pacing.

Ionic mechanisms underlying region-specific remodeling of rabbit atrial action potentials caused by intermittent burst stimulation.

BACKGROUND: Pulmonary veins (PVs) and the coronary sinus (CS) play pivotal roles in triggering some episodes of atrial fibrillation. In isolated rabbit right or left atrial preparations, a 3-hour intermittent burst pacing protocol shortens action potential duration (APD) in CS and PV, but not in sinus node (SN) and left Bachmann bundle (BB) regions. OBJECTIVE: The purpose of this study was to use patch clamp techniques to study the rapidly inactivating (I(to)) and sustained (I(sus)) K(+) currents as well as Ca(2+) currents (I(Ca)) in cells dispersed from intermittent burst pacing and sham PV, BB, CS, and SN regions to determine whether changes in these currents contributed to APD shortening. METHODS: Real-time polymerase chain reaction was performed for transient outward K(+) and Ca(2+) channel subunit mRNAs to determine if intermittent burst pacing affected expression levels. RESULTS: I(to) densities were unaffected by intermittent burst pacing in PV and Bachmann bundle cells. mRNA levels of K(V)4.3, K(V)4.2, K(V)1.4, and KChIP2 subunits of I(to) in both regions were stable. In CS cells, I(to) densities in intermittent burst pacing were greater than in sham (P <.05), but there were no parallel mRNA changes. I(Ca) density of PV cells was reduced from 14.27 +/- 2.08 pA/pF (at -5 mV) in sham to 7.52 +/- 1.65 pA/pF in intermittent burst pacing PV cells (P <.05) due to a significant shift in voltage dependence of activation. These results were seen in the absence of mRNA changes in alpha(1C) and alpha(1D) Ca(2+) channel subunits. In contrast, intermittent burst pacing had no effect on Ca(2+) current densities and kinetics of CS cells, but decreased alpha(1)C and alpha(1)D mRNA levels. CONCLUSION: There is region-specific remodeling of I(to) and I(Ca) by intermittent burst pacing protocols in rabbit atrium. Increased I(to) in CS cells could account for the APD shortening observed with intermittent burst pacing, whereas an intermittent burst pacing-induced shift in voltage dependence of activation may contribute to APD shortening in PV cells.

Application of blebbistatin as an excitation-contraction uncoupler for electrophysiologic study of rat and rabbit hearts.

BACKGROUND: Application of fluorescence imaging of cardiac electrical activity is limited by motion artifacts and/or side effects of currently available pharmacologic excitation-contraction uncoupling agents. OBJECTIVES: The purpose of this study was to test whether blebbistatin, a recently discovered inhibitor of myosin II isoforms, can be used as an excitation-contraction uncoupler. METHODS: The specificity and potency of blebbistatin were examined by assaying the effects of blebbistatin on the contraction and basic cardiac electrophysiologic parameters of Langendorff-perfused rabbit hearts, isolated rabbit right ventricle and right atrium, and single rat ventricular myocytes using conventional ECG, surface electrograms, microelectrode recordings, and optical imaging with voltage-sensitive and Ca(2+)-sensitive dyes. Action potential morphology, ECG parameters, cardiac conduction, and refractoriness were determined after perfusion with 0.1-10 microM blebbistatin. RESULTS: Blebbistatin 5-10 microM completely eliminated contraction in all cardiac preparations but did not have any effect on electrical activity, including ECG parameters, atrial and ventricular effective refractory periods, and atrial and ventricular activation patterns. Blebbistatin 10 microM had no effects on action potential morphology in rabbit cardiac tissue. Blebbistatin inhibited single cellular contraction in a dose-dependent manner with half-maximal inhibitory concentration (IC(50)) = 0.43 microM, without altering the morphologies of intracellular calcium transients. The blebbistatin effect was completely reversible by simultaneous washout and photobleaching by ultraviolet light CONCLUSION: Blebbistatin is a promising novel selective excitation-contraction uncoupler that can be used for optical imaging of cardiac tissues.

I(Kr) contributes to the altered ventricular repolarization that determines long-term cardiac memory.

OBJECTIVE: Cardiac memory (CM) is characterized by an altered T-wave morphology, which reflects altered repolarization gradients. We hypothesized that the delayed rectifier currents, I(Kr) and I(Ks), might contribute to these repolarization changes. METHODS: We studied conscious, chronically instrumented dogs paced from the postero-lateral left ventricular (LV) wall at rates 5-10% faster than sinus rate for 3 weeks. ECGs during sinus rhythm were recorded on days 0, 7, 14 and 21 of pacing. Within 3 weeks, CM achieved steady state, hearts were excised, and epicardial and endocardial tissues and myocytes were studied. RESULTS: In unpaced controls, action potential duration to 50% and 90% repolarization (APD) in epicardium was shorter than in endocardium (P < 0.05); in CM epicardial APD increased at CL > or = 500 ms, while endocardial APD was either unchanged or decreased such that the transmural gradient seen in controls diminished (P < 0.05). A transmural I(Kr) gradient occurred in controls (epicardium>endocardium, P < 0.05) and was reversed in CM. No I(Ks) transmural gradient was found in controls, while in CM endocardial I(Ks) was greater than epicardial at greater than +50 mV. Canine ERG (cERG) mRNA and protein in epicardium > endocardium in controls (P < 0.05), and this difference was lost in CM. Expression levels of KCNQ1 and KCNE1 protein were similar in all groups. CONCLUSIONS: A transcriptionally induced change in epicardial I(Kr) contributes to the altered ventricular repolarization that characterizes CM.

Repolarization gradients in the canine left ventricle before and after induction of short-term cardiac memory.

BACKGROUND: Questions remain about the contributions of transmural versus apicobasal repolarization gradients to the configuration of the T wave in control settings and after the induction of short-term cardiac memory. METHODS AND RESULTS: Short-term cardiac memory is seen as T-wave changes induced by altered ventricular activation that persists after restoration of sinus rhythm. We studied cardiac memory in anesthetized, open-chest dogs paced from the ventricle for 2 hours. Unipolar electrograms were recorded from as many as 98 epicardial and 144 intramural sites, and activation times and activation-recovery intervals (ARIs) were measured. In separate experiments, epicardial monophasic action potentials were recorded. We found no appreciable left ventricular intramural gradients in repolarization times (activation time+ARI) in either control conditions or after the induction of memory. In controls, there was a left ventricular apicobasal gradient, with the shortest repolarization times in anterobasal regions and longest repolarization times posteroapically. After induction of memory, repolarization times shortened uniformly throughout the ventricular wall. Monophasic action potential duration at 90% repolarization decreased by approximately 10 ms after induction of memory. CONCLUSIONS: In the intact canine left ventricle at physiological rates, there is no transmural gradient in repolarization. Apicobasal gradients in repolarization time, with shortest repolarization times in anterobasal areas and longest repolarization times in posteroapical regions, are important in the genesis of the T wave. Repolarization times and monophasic action potentials at the 90% repolarization level shorten after the induction of memory. The deeper T wave in the ECG after induction of memory may be explained by the more rapid phase 3 of the action potential.

Age-associated changes in electrophysiologic remodeling: a potential contributor to initiation of atrial fibrillation.

OBJECTIVE: Although the incidence of atrial fibrillation (AF) increases with age, the cellular electrophysiological changes that render the atria of aged individuals more susceptible to AF remain poorly understood. We hypothesized that dispersion of atrial repolarization increases with aging, creating a substrate for initiation of AF. METHODS: Four groups of dogs were studied: adult and old dogs in normal sinus rhythm (SR) and adult and old dogs with chronic AF (CAF) induced by rapid atrial pacing. In each dog, action potentials (AP) were recorded with microelectrodes from isolated endocardial preparations of four regions of right atrium and three regions of left atrium. Two indices of AP duration (APD) heterogeneity were obtained in each dog by calculating standard deviation (SD) and the coefficient of variation (COV=[SD/mean] x 100%). RESULTS: In SR groups, APD averaged across all regions was significantly longer in old than in adult tissues. Both indices of APD heterogeneity were higher in old dogs in comparison to adult. At both ages, CAF was associated with significant APD shortening and a decrease in APD adaptation to rate. While CAF significantly increased both indices of APD heterogeneity in adult dogs, it significantly decreased them in old dogs. CONCLUSIONS: The increase of spatial variability in repolarization in old atria may contribute to the initiation of AF in the aged. CAF-induced APD shortening and a decrease in APD adaptation appear to be important for the maintenance of sustained AF in both adult and old atria. The CAF-induced increase in dispersion of repolarization may be important for AF stabilization in adults, while previously reported fibrosis and slowed conduction of premature beats may be important in the old for both AF initiation during SR and subsequent stabilization of AF.

Region-specific, pacing-induced changes in repolarization in rabbit atrium: an example of sensitivity to the rare.

OBJECTIVE: In subsets of patients paroxysmal firing of ectopic foci in pulmonary veins or coronary sinus is an important cause of atrial fibrillation. This appears to represent a rare event overriding a dominant sinus mechanism to alter the rhythmic firing of the atrium. Hence, we tested the hypothesis that a rare stimulation pattern might alter the myocardial substrate, making it more susceptible to the initiation of arrhythmias. METHODS: In isolated right and left rabbit atria, a "rare" burst pacing protocol (BPP) was applied as follows: over 3 h, preparations were driven for 4.5 min from sinus node (SN) or Bachmann's bundle (BB) regions at cycle length (CL)=400 ms followed by 30 s of stimulation from coronary sinus (CS) or pulmonary vein (PV) at CL=200 ms. Microelectrodes were used to record action potentials at the end of 4.5 min of pacing at CL=400 ms. We then intervened with 5-min bigeminal pacing to probe atrial vulnerability to arrhythmias: S1 was delivered from SN or BB and S2 from CS or PV, respectively. S1-S2 interval was the shortest eliciting a propagated response. RESULTS: BPP shortened repolarization in CS and PV regions but not in SN or BB, resulting in increased dispersion of repolarization in right and decreased in left atria. Propranolol, atropine and losartan failed to alter the decrease in repolarization induced by BPP whereas apamin, nifedipine and ryanodine prevented BPP effects. Before BPP, bigeminy did not induce arrhythmias in either atrium, but after BPP, bigeminy significantly increased the incidence of arrhythmias in the right atrium. CONCLUSIONS: BPP from foci outside the regions of dominant activation alters dispersion of atrial repolarization. Modulation of apamin-sensitive channels may contribute to the shortening of repolarization in CS and PV regions. Alterations of atrial repolarization gradient create an arrhythmogenic substrate and may be an early step in atrial electrophysiologic remodeling.

Testosterone diminishes the proarrhythmic effects of dofetilide in normal female rabbits.

BACKGROUND: Recent clinical and experimental data suggest that testosterone may protect males against the deleterious effects of repolarization-prolonging drugs. This study tests the hypothesis that 5alpha-dihydrotestosterone (DHT) protects normal females against drug-induced excessive prolongation of repolarization. METHODS AND RESULTS: We used microelectrode techniques to study isolated preparations of rabbit ventricular endocardium from age-matched normal control female rabbits and female rabbits treated with DHT for 4 weeks. Serum 17beta-estradiol levels were identical in the control and DHT-treated animals, whereas DHT levels were high (equaling those in normal males) only in the DHT-treated animals. Basal action potential duration to 90% repolarization (APD90) was significantly shorter in DHT-treated (155+/-7.4 ms, n=32) than control females (178+/-6.7 ms, n=29; P<0.05) at cycle length=1000 ms. The increase in APD90 induced by 10(-8) mol/L dofetilide at cycle length=1000 ms was significantly less in DHT-treated females than normal females (DeltaAPD90=8+/-7 and 29+/-5 ms, respectively, P<0.05). At 10(-6) mol/L dofetilide, the incidence of early afterdepolarizations was 28% in DHT-treated and 55% in normal female rabbits (P<0.05). CONCLUSIONS: Elevating DHT levels diminishes the effects of dofetilide to increase APD and induce early afterdepolarizations in females. Moreover, treatment of females with DHT results in prolongation of APD and an incidence of early afterdepolarization equal to values previously reported by us for dofetilide-treated normal males. That serum levels of 17beta-estradiol were the same in DHT-treated and untreated females suggests that estradiol is not involved in the response to dofetilide. Thus, these data suggest that DHT and perhaps other androgenic hormones may protect normal females against the risk of dofetilide-induced arrhythmia.

Heterogeneous ventricular repolarization provides a substrate for arrhythmias in a German shepherd model of spontaneous arrhythmic death.

BACKGROUND: German shepherd dogs with inherited arrhythmias and sudden death appear to be a model for catecholamine-dependent ventricular tachycardias in human subjects. We tested the hypothesis that heterogeneity of left ventricular repolarization creates an arrhythmogenic substrate for pause-dependent ventricular tachycardia in these animals. METHODS AND RESULTS: We used microelectrode techniques to record action potentials (AP) from midmyocardial sections of anteroseptal, anterobasal, and posterobasal left ventricular (LV) wall of unafflicted and afflicted dogs. There were no differences in AP duration to 90% repolarization (APD) among LV regions in unafflicted dogs. In contrast, in afflicted dogs, there was significant heterogeneity, with the longest APD in anterobasal and shortest in anteroseptal regions. Isoproterenol did not affect repolarization in unafflicted dogs, whereas in afflicted dogs, it shortened APD anterobasally and prolonged APD anteroseptally. We studied the repolarizing currents, IKr and IKs, in single anteroseptal and anterobasal LV myocytes with the use of a whole-cell voltage clamp. There were no differences in IKr and IKs between anteroseptal and anterobasal regions in unafflicted dogs, whereas in afflicted dogs, IKr was smaller anterobasally (P<0.05). Isoproterenol produced a more prominent leftward shift in IKs voltage-dependent activation in anterobasal regions of afflicted than unafflicted dogs. CONCLUSIONS: Spatial heterogeneity in expression and catecholamine responsiveness of IKr and IKs results in heterogeneous LV repolarization in afflicted German shepherd dogs, contributing importantly to the arrhythmogenic substrate.

Cardiac memory evolves with age in association with development of the transient outward current.

BACKGROUND: Calcium-insensitive transient outward current (I(to)) is important to the development of cardiac memory (CM), which itself reflects the capacity of the heart to remodel electrophysiologically. We used cardiac pacing to test the hypothesis that CM evolution can be explained by developmental maturation of I(to). METHODS AND RESULTS: Acutely anesthetized dogs from 1 day old to adult were paced from the left ventricle (VP, n=29) or left atrial appendage (AP, n=12) to induce CM. T-wave vector displacement (TVD) obtained during VP was greater than with AP (adults, 0.39+/-0.06 mV; neonates, 0.04+/-0.01 mV; P<0.05). TVD began to increase at approximately 40 days of age, reaching adult levels by approximately 200 days. Microelectrode studies performed in 18 dogs (ages 3 to 94 days) after completing the CM protocol and 20 additional dogs (1 day old to adult) revealed that the epicardial action potential notch was absent in neonates, became apparent in the young, and was deepest in adults. The relationship between TVD and epicardial notch was such that as notch magnitude increased, TVD increased (r=-0.65, P<0.05). KChIP2 and Kv4.3 mRNA (measured via reverse transcription-polymerase chain reaction) also increased with age. CONCLUSIONS: The inducibility of CM gradually increases with age in association with evolution of the epicardial action potential notch and mRNA expression for KChIP2 and Kv4.3. This suggests that the capacity of the heart to remodel electrophysiologically and to manifest memory during development depends in part on evolution of the determinants of I(to).

Biological pacemaker implanted in canine left bundle branch provides ventricular escape rhythms that have physiologically acceptable rates.

BACKGROUND: We hypothesized that administration of the HCN2 gene to the left bundle-branch (LBB) system of intact dogs would provide pacemaker function in the physiological range of heart rates. METHODS AND RESULTS: An adenoviral construct incorporating HCN2 and green fluorescent protein (GFP) as a marker was injected via catheter under fluoroscopic control into the posterior division of the LBB. Controls were injected with an adenoviral construct of GFP alone or saline. Animals were monitored electrocardiographically for up to 7 days after surgery, at which time they were anesthetized and subjected to vagal stimulation to permit emergence of escape pacemakers. Hearts were then removed and injection sites visually identified and removed for microelectrode study of action potentials, patch clamp studies of pacemaker current, and/or immunohistochemical studies of HCN2. For 48 hours postoperatively, 7 of 7 animals subjected to 24-hour ECG monitoring showed multiple ventricular premature depolarizations and/or ventricular tachycardia attributable to injection-induced injury. Thereafter, sinus rhythm prevailed. During vagal stimulation, HCN2-injected dogs showed rhythms originating from the left ventricle, the rate of which was significantly more rapid than in the controls. Excised posterior divisions of the LBB from HCN2-injected animals manifested automatic rates significantly greater than the controls. Isolated tissues showed immunohistochemical and biophysical evidence of overexpressed HCN2. CONCLUSIONS: A gene-therapy approach for induction of biological pacemaker activity within the LBB system provides ventricular escape rhythms that have physiologically acceptable rates. Long-term stability and feasibility of the approach remain to be tested.

Regional dispersion of L-type calcium current in ventricular myocytes of German shepherd dogs with lethal cardiac arrhythmias.

OBJECTIVES: The purpose of this study was to determine if regional differences in L-type Ca(2+) current (I(Ca,L)) are altered in a German shepherd model of sudden death. BACKGROUND: German shepherd dogs with inherited sudden cardiac death have reduced sympathetic innervation in the anteroseptal left ventricle that may contribute to arrhythmias in afflicted animals compared to control unafflicted animals. Differences in a number of repolarizing K(+) currents have been identified in this model, but I(Ca,L) has not been studied. METHODS: We measured action potentials in intact tissue and I(Ca,L) in isolated myocytes from anteroseptal and anterobasal left ventricle. RESULTS: Action potential plateau level and I(Ca,L) density were significantly lower in unafflicted anteroseptal than in afflicted anteroseptal, afflicted anterobasal, or unafflicted anterobasal. Isoproterenol increased I(Ca,L) density more in the unafflicted anteroseptal group than in the other groups. CONCLUSIONS: Differences in I(Ca,L) between afflicted and control animals, combined with our earlier finding of regional reductions in I(Kr), provide a likely substrate for the occurrence of pause-dependent arrhythmias in afflicted animals and for the T-wave abnormalities characterizing them.