Effects of amiodarone on wave front dynamics during ventricular fibrillation in isolated swine right ventricle

The effects of acute amiodarone infusion on dynamics of ventricular fibrillation (VF) are unclear. Six isolated swine right ventricles (RVs) were studied in vitro. Activation patterns during VF were mapped optically, whereas action potentials were recorded with a glass microelectrode. At baseline, VF was associated with frequent spontaneous wave breaks. Amiodarone (2.5 microg/ml) reduced spontaneous wave breaks and increased the cycle length (CL) of VF from 83.3 +/- 17.8 ms at baseline to 118.4 +/- 25.8 ms during infusion (P < 0.05). Amiodarone increased the reentrant wave front CL (114.4 +/- 15.5 vs. 78.2 +/- 19.0 ms, P < 0.05) and central core area (4.1 +/- 3.8 vs. 0.9 +/- 0.3 mm2, P < 0.05). Within 30 min of infusion, VF terminated (n = 1), converted to ventricular tachycardia (VT) (n = 1) or continued at a slower rate (n = 4). Amiodarone flattened the APD restitution curves. We conclude that amiodarone reduced spontaneous wave breaks. It might terminate VF or convert VF to VT. These effects were associated with the flattening of APD restitution slope and increased core size of reentrant wave fronts.     

Lifetimes of epicardial rotors in panoramic optical maps of fibrillating swine ventricles

During ventricular fibrillation (VF), electrical activation waves are fragmented, and the heart cannot contract in synchrony. It has been proposed that VF waves emanate from stable periodic sources (often called "mother rotors"). The objective of the present study was to determine if stable rotors are consistently present on the epicardial surface of hearts comparable in size to human hearts. Using new optical mapping technology, we imaged VF from nearly the entire ventricular surface of six isolated swine hearts. Using newly developed pattern analysis algorithms, we identified and tracked VF wave fronts and phase singularities (PS; the pivot point of a reentrant wave front). We introduce the notion of a compound rotor in which the rotor's central PS can change and describe an algorithm for automatically identifying such patterns. This prevents rotor lifetimes from being inappropriately abbreviated by wave front fragmentation and collision events near the PS. We found that stable epicardial rotors were not consistently present during VF: only 1 of 17 VF episodes contained a compound rotor that lasted for the entire mapped interval of 4 s. However, shorter-lived rotors were common; 12.2 (SD 3.3) compound rotors with lifetime >200 ms were visible on the epicardium at any given instant. We conclude that epicardial mother rotors do not drive VF in this experimental model; if mother rotors do exist, they are intramural or septal. This paucity of persistent rotors suggests that individual rotors will eventually terminate by themselves and therefore that the continual formation of new rotors is critical for VF maintenance.     

Effects of diacetyl monoxime and cytochalasin D on ventricular fibrillation in swine right ventricles

Whether or not the excitation-contraction (E-C) uncoupler diacetyl monoxime (DAM) and cytochalacin D (Cyto D) alter the ventricular fibrillation (VF) activation patterns is unclear. We recorded single cell action potentials and performed optical mapping in isolated perfused swine right ventricles (RV) at different concentrations of DAM and Cyto D. Increasing the concentration of DAM results in progressively shortened action potential duration (APD) measured to 90% repolarization, reduced the slope of the APD restitition curve, decreased Kolmogorov-Sinai entropy, and reduced the number of VF wave fronts. In all RVs, 15-20 mmol/l DAM converted VF to ventricular tachycardia (VT). The VF could be reinduced after the DAM was washed out. In comparison, Cyto D (10-40 micromol/l) has no effects on APD restitution curve or the dynamics of VF. The effects of DAM on VF are associated with a reduced number of wave fronts and dynamic complexities in VF. These results are compatible with the restitution hypothesis of VF and suggest that DAM may be unsuitable as an E-C uncoupler for optical mapping studies of VF in the swine RVs.     

Chemical ablation of the Purkinje system causes early termination and activation rate slowing of long-duration ventricular fibrillation in dogs

Endocardial mapping has suggested that Purkinje fibers may play a role in the maintenance of long-duration ventricular fibrillation (LDVF). To determine the influence of Purkinje fibers on LDVF, we chemically ablated the Purkinje system with Lugol solution and recorded endocardial and transmural activation during LDVF. Dog hearts were isolated and perfused, and the ventricular endocardium was exposed and treated with Lugol solution (n = 6) or normal Tyrode solution as a control (n = 6). The left anterior papillary muscle endocardium was mapped with a 504-electrode (21 x 24) plaque with electrodes spaced 1 mm apart. Transmural activation was recorded with a six-electrode plunge needle on each side of the plaque. Ventricular fibrillation (VF) was induced, and perfusion was halted. LDVF spontaneously terminated sooner in Lugol-ablated hearts than in control hearts (4.9 +/- 1.5 vs. 9.2 +/- 3.2 min, P = 0.01). After termination of VF, both the control and Lugol hearts were typically excitable, but only short episodes of VF could be reinduced. Endocardial activation rates were similar during the first 2 min of LDVF for Lugol-ablated and control hearts but were significantly slower in Lugol hearts by 3 min. In control hearts, the endocardium activated more rapidly than the epicardium after 4 min of LDVF with wave fronts propagating most often from the endocardium to epicardium. No difference in transmural activation rate or wave front direction was observed in Lugol hearts. Ablation of the subendocardium hastens VF spontaneous termination and alters VF activation sequences, suggesting that Purkinje fibers are important in the maintenance of LDVF.     

Mechanisms of destabilization and early termination of spiral wave reentry in the ventricle by a class III antiarrhythmic agent, nifekalant

Nifekalant (NF) is a novel class III antiarrhythmic agent that is effective in preventing life-threatening ventricular tachycardia/fibrillation (VT/VF). We investigated mechanisms of destabilization and early termination of spiral-type reentrant VT by NF in a two-dimensional subepicardial myocardial layer of Langendorff-perfused rabbit hearts (n = 21) using a high-resolution optical action potential mapping system. During basic stimulation, NF (0.1 microM) caused uniform prolongation of action potential duration (APD) without affecting conduction velocity and an increase of APD restitution slope. VTs induced by direct current stimulation in the presence of NF were of shorter duration (VTs > 30 s: 2/54 NF vs. 19/93 control). During VTs in control (with visible rotors), the wave front chased its own tail with a certain distance (repolarized zone), and they seldom met each other. The average number of phase singularity (PS) points was 1.31 +/- 0.14 per 665 ms (n = 7). In the presence of NF, the wave front frequently encountered its own tail, causing a transient breakup of the spiral wave or sudden movement of the rotation center (spatial jump of PS). The average number of PS was increased to 1.63 +/- 0.22 per 665 ms (n = 7, P < 0.05) after NF. The mode of spontaneous termination of rotors in control was in most cases (9/10, 90.0%) the result of mutual annihilation of counterrotating wave fronts. With NF, rotors frequently terminated by wave front collision with the atrioventricular groove (12/19, 63.2%) or by trapping the spiral tip in a refractory zone (7/19, 36.8%). Destabilization and early termination of spiral wave reentry induced by NF are the result of a limited proportion of excitable tissue after modulation of repolarization.     

Ventricular fibrillation in myopathic human hearts: mechanistic insights from in vivo global endocardial and epicardial mapping

Ventricular fibrillation (VF) is an important cause of sudden cardiac death and cardiovascular mortality in patients with cardiomyopathy. Although it was generally believed that chaotic reentrant wavefronts underlie VF in humans, there is emerging evidence of spatiotemporal organization during early VF. The mechanism of this organization of electrical activity in early VF is unknown in myopathic hearts. We studied early VF in vivo, intraoperatively in five cardiomyopathic patients. Simultaneous electrograms were obtained from the epicardium and endocardium in left ventricular cardiomyopathy and from the endocardium in right ventricular myopathy. The Hilbert transform was used to derive the phase of the electrograms. Rotors were identified by isolating phase singularity points. Rotors were present in all of the myopathic hearts studied during VF and cumulatively lasted a mean of 3.2 +/- 2.0 s of the 7.0 +/- 4.0 s of the VF segments analyzed. For each surface mapped, 3.6 +/- 2.9 rotors were identified for the duration mapped. The average number of cycles completed by these rotors was 4.9 +/- 4.9. The longest rotor lasted 10.2 +/- 6.2 rotations and lasted 2.0 +/- 1.2 s. The rotors on the endocardium had a cycle length of 192 +/- 33 ms compared with 220 +/- 15 ms on the epicardium (P=0.08). There is centrifugal activation of electrical activity from these rotors, and they give rise to domains that activate at faster rates with evidence of conduction block at the border with slower domains. These rotors frequently localized to border regions of myocardium with bipolar electrogram amplitude of <0.5 mV. The organization of electrical activity during early VF in myopathic human hearts is characterized by wavefronts emanating from a few rotors.     

Moderate hypothermia increases the chance of spiral wave collision in favor of self-termination of ventricular tachycardia/fibrillation

In cardiac arrest due to ventricular fibrillation (VF), moderate hypothermia (MH, 33 degrees C) has been shown to improve defibrillation success compared with normothermia (NR, 37 degrees C) and severe hypothermia (SH, 30 degrees C). The underlying mechanisms remain unclear. We hypothesized that MH might prevent reentrant excitations rotating around functional obstacles (rotors) that are responsible for the genesis of VF. In two-dimensional Langendorff-perfused rabbit hearts prepared by cryoablation (n = 13), action potential signals were recorded by a high-resolution optical mapping system. During basic stimulation (2.5-5.0 Hz), MH and SH caused significant prolongation of action potential duration and significant reduction of conduction velocity. Wavelength was unchanged at MH, whereas it was shortened significantly at SH at higher stimulation frequencies (4.0-5.0 Hz). The duration of direct current stimulation-induced ventricular tachycardia (VT)/VF was reduced dramatically at MH compared with NR and SH. The spiral wave (SW) excitations documented during VT at NR were by and large organized, whereas those during VT/VF at MH and SH were characterized by disorganization with frequent breakup. Phase maps during VT/VF at MH showed a higher incidence of SW collision (mutual annihilation or exit from the anatomical boundaries), which caused a temporal disappearance of phase singularity points (PS-0), compared with that at NR and SH. There was an inverse relation between PS-0 period in the observation area and VT/VF duration. MH data points were located in a longer PS-0 period and a shorter VT/VF duration zone compared with SH. MH causes a modification of SW dynamics, leading to an increase in the chance of SW collision in favor of self-termination of VT/VF.     

Studying semblances of a true killer: experimental model of human ventricular fibrillation

It is unknown whether ventricular fibrillation (VF) studied in experimental models represents in vivo human VF. First, we examined closed chest in vivo VF induced at defibrillation threshold testing (DFT) in four patients with ischemic cardiomyopathy pretransplantation. We examined VF in these same four hearts in an ex vivo human Langendorff posttransplantation. VF from DFT was compared with VF from the electrodes from a similar region in the right ventricular endocardium in the Langendorff using two parameters: the scale distribution width (extracted from continuous wavelet transform) and VF mean cycle length (CL). In a second substudy group where multielectrode phase mapping could be performed, we examined early VF intraoperatively (in vivo open chest condition) in three patients with left ventricular cardiomyopathy. We investigated early VF in the hearts of three patients in an ex vivo Langendorff and compared findings with intraoperative VF using two metrics: dominant frequency (DF) assessed by the Welch periodogram and the number of phase singularities (lasting >480 ms). Wavelet analysis (P = 0.9) and VF CL were similar between the Langendorff and the DFT groups (225 ± 13, 218 ± 24 ms; P = 0.9), indicating that wave characteristics and activation rate of VF was comparable between the two models. Intraoperative DF was slower but comparable with the Langendorff DF over the endocardium (4.6 ± 0.1, 5.0 ± 0.4 Hz; P = 0.9) and the epicardium (4.5 ± 0.2, 5.2 ± 0.4 Hz; P = 0.9). Endocardial phase singularity number (9.6 ± 5, 12.1 ± 1; P = 0.6) was lesser in number but comparable between in vivo and ex vivo VF. VF dynamics in the limited experimental human studies approximates human in vivo VF.     

Panoramic optical mapping reveals continuous epicardial reentry during ventricular fibrillation in the isolated swine heart

During ventricular fibrillation (VF), activation waves are fragmented and the heart cannot contract synchronously. It has been proposed that VF waves emanate from stable sources ("mother rotors"). Previously, we used new optical mapping technology to image VF wavefronts from nearly the entire epicardial surface of six isolated swine hearts. We found that VF was not driven by epicardial rotors, but could not exclude the presence of stable rotors hidden within the ventricular walls. Here, we use graph theoretic analysis to show that, in all 17 VF episodes we analyzed, it was always possible to trace sequences of wavefronts through series of fragmentation and collision events from the beginning to the end of the episode. The set of wavefronts that were so related (the dominant component) consisted of 92%+/-1% of epicardial wavefronts. Because each such wavefront sequence constitutes a continuous activation front, this finding shows that complete reentrant pathways were always present on the epicardial surface and therefore, that wavefront infusion from nonepicardial sources was not strictly necessary for VF maintenance. These data suggest that VF in this model is not driven by localized sources; thus, new anti-VF treatments designed to target such sources may be less effective than global interventions.     

Enhanced sensitivity of aged fibrotic hearts to angiotensin II- and hypokalemia-induced early afterdepolarization-mediated ventricular arrhythmias

Unlike young hearts, aged hearts are highly susceptible to early afterdepolarization (EAD)-mediated ventricular fibrillation (VF). This differential may result from age-related structural remodeling (fibrosis) or electrical remodeling of ventricular myocytes or both. We used optical mapping and microelectrode recordings in Langendorff-perfused hearts and patch-clamp recordings in isolated ventricular myocytes from aged (24-26 mo) and young (3-4 mo) rats to assess susceptibility to EADs and VF during either oxidative stress with ANG II (2 μM) or ionic stress with hypokalemia (2.7 mM). ANG II caused EAD-mediated VF in 16 of 19 aged hearts (83%) after 32 ± 7 min but in 0 of 9 young hearts (0%). ANG II-mediated VF was suppressed with KN-93 (Ca(2+)/calmodulin-dependent kinase inhibitor) and the reducing agent N-acetylcysteine. Hypokalemia caused EAD-mediated VF in 11 of 11 aged hearts (100%) after 7.4 ± 0.4 min. In 14 young hearts, however, VF did not occur in 6 hearts (43%) or was delayed in onset (31 ± 22 min, P < 0.05) in 8 hearts (57%). In patch-clamped myocytes, ANG II and hypokalemia (n = 6) induced EADs and triggered activity in both age groups (P = not significant) at a cycle length of >0.5 s. When myocytes of either age group were coupled to a virtual fibroblast using the dynamic patch-clamp technique, EADs arose in both groups at a cycle length of <0.5 s. Aged ventricles had significantly greater fibrosis and reduced connexin43 gap junction density compared with young hearts. The lack of differential age-related sensitivity at the single cell level in EAD susceptibility indicates that increased ventricular fibrosis in the aged heart plays a key role in increasing vulnerability to VF induced by oxidative and ionic stress.     

Synchronization of ventricular fibrillation with real-time feedback pacing: implication to low-energy defibrillation

Wavefront synchronization is an important aspect preceding the termination of ventricular fibrillation (VF). We evaluated the defibrillation efficacy of a novel multisite pacing algorithm using optical recording-guided synchronized pacing (SyncP) in the excitable gaps. We compared the effects of SyncP with traditional overdrive pacing (ODP) at 90% of the VF cycle length (VFCL) and high-frequency pacing (HFP; 43-215 Hz) on spontaneous VF termination in isolated rabbit hearts. For SyncP, the pacing current was triggered by the activation of a reference site and was delivered when the optical potential of the pacing site was in an excitable gap. We measured VFCL and the spatial dispersion of VFCL (SDCL) from five points (3 points in the paced area and 2 points in the nonpaced area) and the distribution of phase singularities during the prepacing, pacing, and postpacing periods. The results showed that 1) the VF termination rate of SyncP (16.0%, n = 106) was higher than that of ODP (2.1%, n = 48, P < 0.01) or HFP (1.6%, n = 129, P < 0.0001); 2) energy consumption for SyncP (7.6 +/- 9.3 mJ) was significantly lower than that of ODP (14.0 +/- 14.8 mJ, P < 0.0001); and 3) SyncP, but not ODP or HFP, decreased SDCL in the paced area during the pacing (P < 0.01) and postpacing (P < 0.05) periods compared with the prepacing period. We conclude that SyncP is effective in inducing wavefront synchronization and is more effective at facilitating spontaneous VF termination than non-SyncP.     

Action potential duration restitution and ventricular fibrillation due to rapid focal excitation

The focal source hypothesis of ventricular fibrillation (VF) posits that rapid activation from a focal source, rather than action potential duration (APD) restitution properties, is responsible for the maintenance of VF. We injected aconitine (100 microg) into normal isolated perfused swine right ventricles (RVs) stained with 4-[beta-[2-(di-n-butylamino)-6-naphthyl]vinyl]pyridinium (di-4-ANEPPS) for optical mapping studies. Within 97 +/- 163 s, aconitine induced ventricular tachycardia (VT) with a mean cycle length 268 +/- 37 ms, which accelerated before converting to VF. Drugs that flatten the APD restitution slope, including diacetyl monoxime (10-20 mM, n = 6), bretylium (10-20 microg/ml, n = 3), and verapamil (2-4 microg/ml, n = 3), reversibly converted VF to VT in all cases. In two RVs, VF persisted despite of the excision of the aconitine site. Simulations in two-dimensional cardiac tissue showed that once VF was initiated, it remained sustained even after the "aconitine" site was eliminated. In this model of focal source VF, the VT-to-VF transition occurred due to a wave break outside the aconitine site, and drugs that flattened the APD restitution slope converted VF to VT despite continuous activation from aconitine site.     

Electrophysiologic properties and ventricular fibrillation in normal and myopathic hearts.

This study tests the hypothesis that moderate myocardial dysfunction is associated with altered myocardial anisotropic properties and structurally altered ventricular fibrillation (VF). Mongrel dogs were randomized to either a control group or a group that was rapidly paced at 250 beats/min until the left ventricular ejection fraction was < or = 40%. Changes in anisotropic properties and the electrical characteristics of VF associated with the development of moderate myocardial dysfunction were assessed by microminiature epicardial mapping studies. In vivo conduction, refractory periods, and repolarization times were prolonged in both longitudinal and transverse directions in myopathic animals versus controls. VF was different in myopathic versus control animals. There were significantly more conducted deflections during VF in normal hearts compared with myopathic hearts. Propagated deflection-to-deflection intervals during VF were significantly longer in myopathic hearts compared with controls (125.5 +/- 49.06 versus 103.4 +/- 32.9 ms, p = 0.009). There were no abnormalities in cell size, cell shape, or the number of intercellular gap junctions and there was no detectable change in the expression of the gap junction proteins Cx43 and Cx45. Moderate myocardial dysfunction is associated with significant electrophysiological abnormalities in the absence of changes in myocardial cell morphology or intercellular connections, suggesting a functional abnormality in cell-to-cell communication.     

The role of NO in ischemia/reperfusion injury in isolated rat heart

Nitric oxide (NO) is an important regulator of myocardial function and vascular tone under physiological conditions. However, its role in the pathological situations, such as myocardial ischemia is not unequivocal, and both positive and negative effects have been demonstrated in different experimental settings including human pathology. The aim of the study was to investigate the role of NO in the rat hearts adapted and non-adapted to ischemia. Isolated Langendorff-perfused hearts were subjected to test ischemic (TI) challenge induced by 25 min global ischemia followed by 35 min reperfusion. Short-term adaptation to ischemia (ischemic preconditioning, IP) was evoked by 2 cycles of 5 min ischemia and 5 min reperfusion, before TI. Recovery of function at the end of reperfusion and reperfusion-induced arrhythmias served as the end-points of injury. Coronary flow (CF), left ventricular developed pressure (LVDP), and dP/dt(max) (index of contraction) were measured at the end of stabilization and throughout the remainder of the protocol until the end of reperfusion. The role of NO was investigated by subjecting the hearts to 15 min perfusion with NO synthase (NOS) inhibitor L-NAME (100 mmol/l), prior to sustained ischemia. At the end of reperfusion, LVDP in the controls recovered to 29.0 +/- 3.9 % of baseline value, whereas preconditioned hearts showed a significantly increased recovery (LVDP 66.4 +/- 5.7 %, p < 0.05). Recovery of both CF and dP/dt(max) after TI was also significantly higher in the adapted hearts (101.5 +/- 5.8 % and 83.64 +/- 3.92 % ) as compared with the controls (71.9 +/- 6.3 % and 35.7 +/- 4.87 %, respectively, p < 0.05). NOS inhibition improved contractile recovery in the non-adapted group (LVDP 53.8 +/- 3.1 %; dP/dt(max) 67.5 +/- 5.92 %) and increased CF to 82.4 +/- 5.2 %. In contrast, in the adapted group, it abolished the protective effect of IP (LVDP 31.8 +/- 3.1 %; CF 70.3 +/- 3.4 % and dP/dt(max) 43.25 +/- 2.19 %). Control group exhibited 100 % occurrence of ventricular tachycardia (VT), 57 % incidence of ventricular fibrillation (VF) - 21 % of them was sustained VF (SVF); application of L-NAME attenuated reperfusion arrhythmias (VT 70 %, VF 20 %, SVF 0 %). Adaptation by IP also reduced arrhythmias, however, L-NAME in the preconditioned hearts increased the incidence of arrhythmias (VT 100 %, VF 58 %, SVF 17 %). In conclusion: our results indicate that administration of L-NAME might be cardioprotective in the normal hearts exposed to ischemia/reperfusion (I/R) alone, suggesting that NO contributes to low ischemic tolerance in the non-adapted hearts. On the other hand, blockade of cardioprotective effect of IP by L-NAME points out to a dual role of NO in the heart: a negative role in the non-adapted myocardium subjected to I/R, and a positive one, due to its involvement in the mechanisms of protection triggered by short-term cardiac adaptation by preconditioning.     

Effects of heart isolation,voltage-sensitive dye,and electromechanical uncoupling agents on ventricular fibrillation

We tested whether the interventions typically required for optical mapping affect activation patterns during ventricular fibrillation (VF). A 21 x 24 unipolar electrode array (1.5 mm spacing) was sutured to the left ventricular epicardium of 16 anesthetized pigs, and four episodes of electrically induced VF (30-s duration) were recorded. The hearts were then rapidly excised and connected to a Langendorff perfusion apparatus. Four of the hearts were controls, in which 24 additional VF episodes were then mapped. In the remaining 12 hearts, four VF episodes were mapped after isolation, four more episodes were mapped after exposure to the voltage-sensitive dye di-4-ANEPPS, and six more episodes were mapped after exposure to the electromechanical uncoupling agents diacetyl monoxime (DAM; 20 mmol/l, n = 6) or cytochalasin D (CytoD; 10 micromol/l, n = 6). VF episodes were separated by 4 min. VF activation patterns were quantified using custom pattern analysis algorithms. From comparisons with time-corrected control data, all interventions significantly changed VF patterns. Most changes were broadly consistent with slowing and regularization due to loss of excitability. Heart isolation had the largest effect on VF patterns, followed by CytoD, DAM, and dye.     

Intracellular calcium dynamics at the core of endocardial stationary spiral waves in Langendorff-perfused rabbit hearts

In vitro models of sustained monomorphic ventricular tachycardia (MVT) are rare and do not usually show spiral reentry on the epicardium. We hypothesized that MVT is associated with the spiral wave in the endocardium and that this stable reentrant propagation is supported by a persistently elevated intracellular calcium (Ca(i)) transient at the core of the spiral wave. We performed dual optical mapping of transmembrane potential (V(m)) and Ca(i) dynamics of the right ventricular (RV) endocardium in Langendorff-perfused rabbit hearts (n = 12). Among 64 induced arrhythmias, 55% were sustained MVT (>10 min). Eighty percent of MVT showed stationary spiral waves (>10 cycles, cycle length: 128 +/- 14.6 ms) in the endocardial mapped region, anchoring to the anatomic discontinuities. No reentry activity was observed in the epicardium. During reentry, the amplitudes of V(m) and Ca(i) signals were higher in the periphery and gradually decreased toward the core. At the core, maximal V(m) and Ca(i) amplitudes were 42.95 +/- 5.89% and 43.95 +/- 9.46%, respectively, of the control (P < 0.001). However, the trough of the V(m) and Ca(i) signals at the core were higher than those in the periphery, indicating persistent V(m) and Ca(i) elevations during reentry. BAPTA-AM, a calcium chelator, significantly reduced the maximal Ca(i) transient amplitude and prevented sustained MVT and spiral wave formation in the mapped region. These findings indicate that endocardial spiral waves often anchor to anatomic discontinuities causing stable MVT in normal rabbit ventricles. The spiral core is characterized by diminished V(m) and Ca(i) amplitudes and persistent V(m) and Ca(i) elevations during reentry.     

Nicotine increases ventricular vulnerability to fibrillation in hearts with healed myocardial infarction

The vulnerability of the infarcted hearts to ventricular fibrillation (VF) was tested in in situ canine hearts during nicotine infusion. The activation pattern was mapped with 477 bipolar electrodes in open-chest anesthetized dogs (n = 8) 5-6 wk after permanent occlusion of the left anterior descending coronary artery. Nicotine (129 +/- 76 ng/ml) lengthened (P < 0.01) the pacing cycle length at which VF was induced from 171 +/- 8.9 to 210 +/- 14. 7 ms. Nicotine selectively amplified the magnitude of conduction time and monophasic action potential (MAP) amplitude and duration (MAPA and MAPD, respectively) alternans in the epicardial border zone (EBZ) but not in the normal zone. With critical reduction of the MAPA and MAPD in the EBZ, conduction block occurred across the long axis of the EBZ cells. Block led immediately to reentry formation in the EBZ with a mean period of 105 +/- 10 ms, which, after one to two rotations, degenerated to VF. Nicotine widened the range of diastolic intervals over which the dynamic MAPD restitution curve had a slope >1. We conclude that nicotine facilitates conduction block, reentry, and VF in hearts with healed myocardial infarction by increasing the magnitude of depolarization and repolarization alternans consistent with the restitution hypothesis of vulnerability to VF.     

Measurement of aortic input impedance in mice: effects of age on aortic stiffness

Mice are used with increasing frequency as models of human cardiovascular diseases, but significant gaps exist in our knowledge of vascular function in the aging mouse. We determined aortic input impedance spectra, pulse wave velocity, and augmentation index in adult (8-mo-old) and old (29-mo-old) mice to determine whether arterial stiffening occurred with age in mice as it does in humans. Pressure and blood velocity signals measured simultaneously from the same location in the ascending aorta were used to determine input impedance spectra (0-10 harmonics). The first minimum of the impedance modulus occurred at the second harmonic in adult mice but shifted to the fourth harmonic in old mice. Characteristic impedance (average of 2nd-10th harmonic) was 57% higher in old mice: 471 +/- 62 vs. 299 +/- 10 (SE) dyn.s.cm-3 (P < 0.05). Pulse pressure and augmentation index, determined from the aortic pressure signals, were also higher in old mice: 42 +/- 2.2 vs. 29 +/- 4.9 mmHg (P < 0.05) and 37 +/- 5 vs. 14 +/- 2% (P < 0.005). Aortic pulse wave velocity measured from the timing of upstrokes of the Doppler velocity signals was 45% higher in old mice: 416 +/- 22 vs. 286 +/- 14 cm/s (n = 3, P < 0.01). These results reproduce age-related findings reported in humans and confirm that mice may be used as models of age-related vascular stiffening.     

The Role of Photon Scattering in Voltage-Calcium Fluorescent Recordings of Ventricular Fibrillation

Recent optical mapping studies of cardiac tissue suggest that membrane voltage (V_m) and intracellular calcium concentrations (Ca) become dissociated during ventricular fibrillation (VF), generating a proarrhythmic substrate. However, experimental methods used in these studies may accentuate measured dissociation due to differences in fluorescent emission wavelengths of optical voltage/calcium (V_opt/Ca_opt) signals. Here, we simulate dual voltage-calcium optical mapping experiments using a monodomain-Luo-Rudy ventricular-tissue model coupled to a photon-diffusion model. Dissociation of both electrical, V_m/Ca, and optical, V_opt/Ca_opt, signals is quantified by calculating mutual information (MI) for VF and rapid pacing protocols. We find that photon scattering decreases MI of V_opt/Ca_opt signals by 23% compared to unscattered V_m/Ca signals during VF. Scattering effects are amplified by increasing wavelength separation between fluorescent voltage/calcium signals and respective measurement-location misalignment. In contrast, photon scattering does not affect MI during rapid pacing, but high calcium dye affinity can decrease MI by attenuating alternans in Ca_opt but not in V_opt. We conclude that some dissociation exists between voltage and calcium at the cellular level during VF, but MI differences are amplified by current optical mapping methods.