Correlation between total atrial conduction time estimated via tissue Doppler imaging (PA-TDI Interval), structural atrial remodeling and new-onset of atrial fibrillation after cardiac surgery.

BACKGROUND: Recent studies identified total atrial conduction time (TACT) as an independent and powerful predictor of new-onset atrial fibrillation (AF). The purpose of this study was to assess the association between the degree of atrial fibrosis, TACT, and frequency of postoperative atrial fibrillation (POAF) among patients undergoing cardiac surgery. METHODS AND RESULTS: Sixty patients in sinus rhythm (mean ± SD age 66 ± 10 years; 22% women) and without a history of AF undergoing cardiac surgery were prospectively enrolled. TACT was measured preoperatively in the left atrium by tissue-Doppler Imaging (PA-TDI interval). Holter-ECG/telemetry was used to screen for POAF throughout 10 days after cardiac surgery. Right atrial appendages (RAA) were obtained in 33 patients during surgery; atrial fibrosis was assessed by visual quantification (% area of positive van Gieson elastic staining). POAF occurred in 23 patients (38%). Fibrosis extent of RAA was higher in patients with POAF as compared to those without (27.5 ± 1.93 vs 15.8 ± 0.81% area; mean ± SEM; P < 0.001). PA-TDI interval was longer in patients with POAF versus patients who maintained in sinus rhythm (152.1 ± 3.0 vs 120.8 ± 1.8 milliseconds; P < 0.001) and correlated with the degree of atrial fibrosis (r = 0.73; P < 0.01). At the cut-off value of 133 milliseconds, TACT sensitivity and specificity related to POAF were 100% and 86%, respectively. CONCLUSION: PA-TDI interval is useful to identify patients at risk for POAF undergoing cardiac surgery and correlates with the degree of atrial fibrosis.

Termination of ventricular tachycardia by far-field stimulation in humans: a feasibility study.

BACKGROUND: Although a low-energy cardioversion (LEC) shock from an implantable cardioverter-defibrillator (ICD) can terminate ventricular tachycardia (VT), it frequently triggers ventricular fibrillation (VF) and is therefore not used in clinical practice. We tested whether a modified LEC shock with a very short duration (0.12-0.36 ms), termed "field stimulus," can terminate VT without triggering VF. METHODS: In 13 sedated patients with implanted ICDs, we attempted to induce VT and to terminate the arrhythmias by field stimuli during hospital predischarge tests. RESULTS: In eight patients, 27 VT episodes were induced and treated with a total of 46 high-voltage (25-200 V) field stimuli, which terminated 11 VT episodes (41% efficacy) and never accelerated VT into VF. VT episodes slower than 230 beats per minute (bpm) (median rate) were terminated more successfully than faster arrhythmia episodes (69% vs 15%, P < 0.01). The strength of the field stimulus had no major influence on the effectiveness. We therefore postulate that suboptimal timing of field stimuli (delivered simultaneously with a sensed event in the right ventricular apex) was the main reason for failed VT terminations. CONCLUSION: A short (0.12-0.36 ms), high-voltage (50-100 V) field stimulus delivered from the shock coil of an implanted ICD system can safely terminate VT, especially for VT rates below 230 bpm. We believe that it would be reasonable to test the effectiveness of automatic field-stimulus therapy from implanted ICDs in VT episodes up to 230 bpm that are not susceptible to termination by antitachycardia pacing.

G protein-activated (GIRK) current in rat ventricular myocytes is masked by constitutive inward rectifier current (I(K1)).

Inwardly-rectifying K+ channel subunits are not homogenously expressed in different cardiac tissues. In ventricular myocytes (VM) the background current-voltage relation is dominated by I(K1), carried by channels composed of Kir2.x subunits, which is less important in atrial myocytes (AM). On the other hand in AM a large G protein gated current carried by Kir3.1/3.4 complexes can be activated by stimulation of muscarinic M(2) receptors (I(K(ACh))), which is assumed to be marginal in VM. Recent evidence suggests that total current carried by cardiac inward-rectifiers (I(K(ATP)), I(K(ACh)), I(K1)) in both, AM and VM is limited, due to K+ accumulation/depletion. This lead us to hypothesize that in conventional whole celI recordings I(K(ACh)) in VM is underestimated as a consequence of constitutive I(K1). In that case a reduction in density of I(K1) should be paralleled by an increase in density of I(K(ACh)). Three different experimental strategies have been used to test for this hypothesis: (i) Adenovirus-driven expression of a dominant-negative mutant of Kir2.1, one of the subunits supposed to form I(K1) channels, in VM caused a reduction in I(K1)-density by about 80 %. In those cells I(K(ACh)) was increased about 4 fold. (ii) A comparable increase in I(K(ACh)) was observed upon reduction of I(K1) caused by adenovirus-mediated RNA interference.(iii) Ba2+ in a concentration of 2 microM blocks I(K1) in VM by about 60 % without affecting atrial I(K(ACh)). The reduction in I(K1) by 2 microM Ba2+ is paralleled by a reversible increase in I(K(ACh)) by about 100%. These data demonstrate that the increase in K+ conductance underlying ventricular I(K(ACh)) is largely underestimated, suggesting that it might be of greater physiological relevance than previously thought.

Initial Experience With Novel Oral Anticoagulants During the First 45 Days After Left Atrial Appendage Closure With the Watchman Device.

BACKGROUND: The use of oral anticoagulation or dual antiplatelet therapy (DAPT) is recommended within the first 45 days after left atrial appendage (LAA) closure using the Watchman device because of incomplete device endothelialization. This study reports for the first time the feasibility of novel oral anticoagulants (NOAC) in these patients. HYPOTHESIS: NOAC therapy is safe and effective after LAA closure. METHODS: Interventional LAA closure was performed successfully in 45 patients. Of these, 18 patients received NOAC during the first 45 days after implantation and 27 patients received DAPT. Transesophageal echocardiography was conducted 45 days after implantation. The primary study endpoint was abnormal thrombus apposition 45 days after implantation. Secondary study endpoints were death from any cause, major adverse cardiac and cerebrovascular events (MACCE), and major bleedings. RESULTS: After 45 days, transesophageal echocardiography revealed no abnormal thrombus apposition. During a follow-up of 417 ± 323 days, 7 patients died. No stroke or transient ischemic attack occurred. Nonfatal myocardial infarction occurred in 1 patient. There was a nonsignificant trend for lower all-cause mortality (P = 0.159) and occurrence of MACCE (P = 0.096) in the NOAC group compared with the DAPT group. Overall, 6 patients suffered from a major bleeding (NOAC, n = 3; DAPT, n = 3). In NOAC group, major bleedings (at day 205, 688, and 736) occurred long after termination of NOAC therapy. There was no significant difference in the frequency of major bleedings in different groups. CONCLUSIONS: Our pilot study suggests that NOAC therapy within the first 45 days after interventional LAA closure is safe and effective.

Generation of a constitutive Na+-dependent inward-rectifier current in rat adult atrial myocytes by overexpression of Kir3.4.

Apart from gating by interaction with betagamma subunits from heterotrimeric G proteins upon stimulation of appropriate receptors, Kir.3 channels have been shown to be gated by intracellular Na+. However, no information is available on how Na+-dependent gating affects endogenous Kir3.1/Kir3.4 channels in mammalian atrial myocytes. We therefore studied how loading of adult atrial myocytes from rat hearts via the patch pipette filling solution with different concentrations of Na+ ([Na+]pip) affects Kir3 current. Surprisingly, in a range between 0 and 60 mm, Na+ neither had an effect on basal inward-rectifier current nor on the current activated by acetylcholine. Overexpression of Kir3.4 in adult atrial myocytes forced by adenoviral gene transfer results in formation of functional homomeric channels that interact with betagamma subunits upon activation of endogenous muscarinic receptors. These channels are activated at [Na+]pip >or= 15 mm, resulting in a receptor-independent basal inward rectifier current (I bir). I bir was neither affected by pertussis toxin nor by GDP-beta-S, suggesting G-protein-independent activation. PIP(2) depletion via endogenous PLC-coupled alpha1 adrenergic receptors causes inhibition of endogenous Kir3.1/3.4 channel currents by about 75%. In contrast, inhibition of Na+-activated I bir amounts to < 20%. The effect of the Kir3 channel blocker tertiapin-Q can be described using an IC50 of 12 nm (endogenous I K(ACh)) and 0.61 nm (I bir). These data clearly identify I bir as a homotetrameric Kir3.4 channel current with novel properties of regulation and pharmacology. Ibir shares some properties with a basal current recently described in atrial myocytes from an animal model of atrial fibrillation (AF) and AF patients.

Gene silencing in adult rat cardiac myocytes in vitro by adenovirus-mediated RNA interference.

RNA interference (RNAi) by short double stranded RNA (siRNA) represents an efficient and frequently used tool for gene silencing to study gene function. Whereas efficient ablation of genes has been demonstrated in neonatal cardiac myocytes, thus far information on successful application of this technique in adult cardiac myocytes (ACM), a standard experimental model in cardiac physiology and pathophysiology, is sparse. Here we demonstrate efficient ablation of a transgene encoding for enhanced green fluorescent protein (EGFP) and a cell specific endogenous gene encoding for an inward-rectifier channel subunit (Kir2.1) in ACM in vitro using adenovirus driven transcription of siRNA hairpins. EGFP fluorescence and density of background inward rectifier current (IK1) were reduced by > 90% within about 6-8 days after transformation with the corresponding virus. In Kir2.1-silenced myocytes resting membrane potential was significantly reduced. Survival of these cells in culture was compromised, presumably due to Ca2+ -overload caused by the depolarization. The sequence-specific knockdowns of EGFP and Kir2.1 were confirmed on the RNA level using real-time RT-PCR. In Kir2.1-silenced myocytes density of transient outward current, carried predominantly by Kv4.x subunits remained unaffected. This communication for the first time demonstrates proof of principle of efficient RNA interference using adenovirus-based vectors and demonstrates its large potential in phenotyping of ACM.

Transfection with 5-HT1A receptor gene and antisense directed against muscarinic M2 receptors reveal a mutual influence between Gi/o-coupled receptors in rat atrial myocytes.

A recently described reduction in sensitivity of G protein-activated inward-rectifying K(+) (GIRK) channels to stimulation of muscarinic M(2) receptors (M(2)AChR) in atrial myocytes overexpressing purinergic A(1) receptors (A(1)AdoR) was further investigated by heterologous expression of a 5-HT(1A) receptor (5-HT(1A)R) and by reducing the expression level of endogenous M(2)AChR receptors using antisense. In 5-HT(1A)R-expressing myocytes, in line with previous studies, sizable GIRK currents could be activated by 5-HT. In these cells, the mean current density and activation rate of M(2)AChR-activated current were significantly reduced, supporting the notion that signalling via this receptor is negatively regulated by other G protein-coupled receptors (GPCR) coupling to the same class (G(i/o)) of G proteins. To study if reducing M(2)AChR expression affects sensitivity of GIRK current to stimulation of A(1)AdoR, antisense oligodinucleotides (AsODN) against the M(2)AChR were used. Incubation of myocytes with M(2)AChR-specific AsODN resulted in a significant reduction in mean amplitude and activation rate of ACh-induced currents. This was paralleled by an increase in mean amplitude and activation rate of current activated by stimulation of A(1)AdoR. Plotting amplitudes of 5-HT- or Ado-induced currents from individual manipulated cells against the amplitude of ACh-induced current yielded a positive correlation between these data. Although difficult to interpret in mechanistic terms, this argues against a competition of receptors for a common pool of G(i/o). The mutual interaction between G(i/o)-coupled receptors depends on manipulation of the expression level, since long-term desensitization or down regulation of M(2)AChR by treatment with carbachol did not affect sensitivity of GIRK current to A(1)AdoR stimulation, despite a substantial reduction in amplitude and activation rate of M(2)AChR-activated currents. These data suggest a novel crosstalk between parallel receptors converging on the same class of G proteins.

G protein-independent inhibition of GIRK current by adenosine in rat atrial myocytes overexpressing A1 receptors after adenovirus-mediated gene transfer.

G protein-activated inwardly rectifying K+ (GIRK) channels, important regulators of membrane excitability in the heart and central nervous system, are activated by interaction with betagamma subunits from heterotrimeric G proteins upon receptor stimulation. In atrial myocytes various endogenous receptors couple to GIRK channels, including the canonical muscarinic M2 receptor (M2AChR) and the A1 adenosine receptor (A1AdoR). Saturating stimulation of A1AdoR in atrial myocytes activates only a fraction of the GIRK current that is activated via M2AChR, which reflects a lower density of A1AdoR. In the present study A1AdoR were overexpressed by means of adenovirus-mediated gene transfer using green fluorescent protein (GFP) as the reporter. Confirmatory to a previous study, this resulted in an increased sensitivity of macroscopic GIRK current (ACh-activated K+ current (IK(ACh))) to stimulation by Ado. However, in the majority of GFP-positive myocytes, exposure to Ado at concentrations > or =1 microM resulted in activation of IK(ACh) followed by a rapid inhibition. In those cells a rebound activation of current was recorded upon washout of Ado. The inhibitory component could be recorded in isolation when IK(ACh) was activated by M2AChR-stimulation and brief pulses of Ado were superimposed. In myocytes loaded with GTP-gamma-S, IK(ACh), irreversibly activated by brief exposure to agonist, was still reversibly inhibited by Ado, suggesting that inhibition is independent of G protein cycling. In myocytes co-transfected with adenoviral vectors encoding A1AdoR and GIRK4 subunit, no inhibition of GIRK current by Ado was observed. As acute desensitization of atrial GIRK current, which is reminiscent of the inhibition described here, has been shown to be absent in myocytes overexpressing GIRK4, this suggests that acute desensitization and the novel inhibition might share a common pathway whose target is the GIRK channel complex or its GIRK1 subunit.

Acute desensitization of GIRK current in rat atrial myocytes is related to K+ current flow.

We have investigated the acute desensitization of acetylcholine-activated GIRK current (I(K(ACh))) in cultured adult rat atrial myocytes. Acute desensitization of I(K(ACh)) is observed as a partial relaxation of current with a half-time of < 5 s when muscarinic M2 receptors are stimulated by a high concentration (> 2 micromol l(-1)) of ACh. Under this condition experimental manoeuvres that cause a decrease in the amplitude of I(K(ACh)), such as partial block of M2 receptors by atropine, intracellular loading with GDP-beta-S, or exposure to Ba2+, caused a reduction in desensitization. Acute desensitization was also identified as a decrease in current amplitude and a blunting of the response to saturating [ACh] (20 micromol l(-1)) when the current had been partially activated by a low concentration of ACh or by stimulation of adenosine A1 receptors. A reduction in current analogous to acute desensitization was observed when ATP-dependent K+ current (I(K(ATP))) was activated either by mitochondrial uncoupling using 2,4-dinitrophenole (DNP) or by the channel opener rilmakalim. Adenovirus-driven overexpression of Kir2.1, a subunit of constitutively active inwardly rectifying K+ channels, resulted in a large Ba2+-sensitive background K+ current and a dramatic reduction of ACh-activated current. Adenovirus-driven overexpression of GIRK4 (Kir3.4) subunits resulted in an increased agonist-independent GIRK current paralleled by a reduction in I(K(ACh)) and removal of the desensitizing component. These data indicate that acute desensitization depends on K+ current flow, independent of the K+ channel species, suggesting that it reflects a reduction in electrochemical driving force rather than a bona fide signalling mechanism. This is supported by the observation that desensitization is paralleled by a significant negative shift in reversal potential of I(K(ACh)). Since the ACh-induced hyperpolarization shows comparable desensitization properties as I(K(ACh)), this novel current-dependent desensitization is a physiologically relevant process, shaping the time course of parasympathetic bradycardia.