We haveinvestigated theeffects ofelectric field stimulation on membranerepolarization inrabbit papillary muscles andassessed theconsequencesoftheseeffects forthedispersion ofintracellular potentials andtheproduction ofa propagation wave front orunidirectional block inrelatively refractory tissue. Thestimuli studied hadelectric field strength of0.25-14 V/cm,duration of2 msec, andfield orientation along oracrossthemyocardial fibers. Thefield strengths toexcite themuscles indiastole were 0.68or 1.23V/cmforstimuli oriented along or acrossthefibers, respectively (p<0.01, along versus across). A2.5-V/cm stimulus given neartheendoftheaction potential (AP)produced either no response or,after increasing thestimulus delay only2-3msec,a full responsewithalmost no APdurations that were intermediate. Forstimulation alongandacross thefibers, respectively, givenat70%oftheAP duration, a 4-V/cmstimulus produced APprolongation (measured at90%repolarization) of209%, and4% (p<0.05), an 8-V/cmstimulus produced AP prolongation of36%and20%(p<O.O5), anda 14-V/cm stimulus produced APprolongation of36%and30%o(p=NS). Foreither orientation, APprolongation by stimuli of8V/cmor14V/cmincreased gradually asthestimulus delay was increased. Thedifferent effects inrelatively refractory tissue ofstimuli of2.5V/cmcompared with8V/cmcan explain thepropagation wavefront andblock that occurwithelectrically induced functional reentry intheheart. After stimulation withfields below acritical strength (-5V/cm), alarge intracellular potential difference may occur among cells that aresufficiently recovered tobecomeexcited bythestimulus andcells that arenotsufficiently recovered tobecome excited, consistent withthereported propagation wave front wherethetwogroupsof cells are closely opposed. Afterstimulation withfields abovethecritical strength, differences in intracellular potentials among cells duringrepolarization may bedecreased, andtheintracellular potentials may beina rangeinwhichsodium current isinactivated, consistent withthereported absence ofapropagation wavefront. Thus, thedifferent effects oflowandhighelectric field strengths can account forthe"critical point"mechanismforunidirectional blockandreentry. (Circulation Research 1992;70:707-715)

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.

In this paper, we document a fabrication process that yields linear arrays of rectangular platinum black electrodes spaced 25 mum apart with edge-to-edge separation of 20 microm. The spatial arrangement is therefore sufficiently fine to insure stimulation and recording within cardiac tissue space constants, as six electrodes with dimensions of either 5 x 100 microm2, 5 x 250 microm2, or 5 x 500 microm2 were positioned in a 130-microm2 span in the arrays. Despite the small electrode sizes and available surface areas, favorable impedance characteristics were identifed. Averages ranged from 111 kOmega to 146 kOmega at 0.5 Hz and from 14 kOmega 39 kOmega at 500 Hz. Differences in impedances between the electrode sizes tested were small. Potential differences (deltaphis) recorded using the two central electrodes during stimulation with combinations at separations of only 75 microm, 100 microm, and 125 microm had low signal noise. As a preliminary test of the use of these arrays for possible application to impedance measurements in cardiac tissue, the deltaphis recorded during stimulation were compared to deltaphis obtained from finite-difference simulations using an isotropic volume conductor model. Anticipated decays in deltaphi with widening electrode separation identified in those simulations matched the decays in the recorded deltaphis closely. These findings are significant because they suggest intracellular and interstitial microimpedance mesurements in heart experiments will be straightforward.

BACKGROUND Knowledge of the shock potential gradient (W) and postshock activation is limited to internal defibrillation of short-duration ventricular fibrillation (SDVF).OBJECTIVE The purpose of this study was to determine these variables after external defibrillation of long-duration VF (LDVF).METHODS In six pigs, 115-20 plunge needles with three to six electrodes each were inserted to record throughout both ventricles. After the chest was closed, the biphasic defibrillation threshold (DFT) was determined after 20 seconds of SDVF with external defibrillation pads. After 7 minutes of LDVF, defibrillation shocks that were less than or equal to the SDVF DFT strength were given.RESULTS For DFT shocks (1632 +/- 429 V), the maximum minus minimum ventricular voltage (160 +/- 100 V) was 9.8% of the shock voltage. Maximum cardiac del V (28.7 +/- 17 V/cm) was 4.7 +/- 2.0 times the minimum del V (6.2 +/- 3.5 V/cm). Although LDVF did not increase the DFT in five of the six pigs, it significantly lengthened the time to earliest postshock activation following defibrillation (1.6 +/- 2.2 seconds for SDVF and 4.9 +/- 4.3 seconds for LDVF). After LDVF, 1.3 +/- 0.8 episodes of spontaneous refibrillation occurred per animal, but there was no refibrillation after SDVF.CONCLUSION Compared with previous studies of internal defibrillation, during external defibrillation much less of the shock voltage appears across the heart and the shock field is much more even; however, the minimum del V is similar. Compared with external defibrillation of SDVF, the biphasic external DFT for LDVF is not increased; however, time to earliest postshock activation triples. Refibrillation is common after LDVF but not after SDVF in these normal hearts, indicating that LDVF by itself can cause refibrillation without requiring preexisting heart disease.

Background: Interest in combining antiarrhythmic drugs has been prompted by the lack of efficacy of monotherapies and the toxicity resulting from high doses of individual agents. Objectives: We tested the hypothesis that procainamide and sotalol combined have greater beneficial effects on restitution, on the dispersion of refractoriness, and on decreasing the complexity of ventricular fibrillation (VF) than either drug alone. Methods: Six open‐chest pigs received intravenous procainamide (15 mg/kg load and 50 μg/kg/min maintenance) followed by sotalol (1.5 mg/kg). Another six pigs received sotalol first and procainamide second. Before drugs and after each drug, 20‐second episodes of electrically induced VF were recorded from a 21 × 24 unipolar electrode plaque (2 mm spacing) sutured on the lateral posterior left ventricular epicardium. Restitution properties and dispersion of refractoriness were estimated from activation recovery intervals during pacing. Results: The combination of the two drugs reduced the maximum slope of the restitution curve and during VF reduced the number of wavefronts, the activation rate, the percentage of wavefront families exhibiting reentry, and the conduction velocity more than either drug alone. In addition, in the group that received sotalol first, both drugs together reduced the SD and the coefficient of variation of the spatial dispersion of refractoriness compared with baseline. Conclusions: Procainamide and sotalol combined have greater beneficial effects on restitution properties, dispersion of refractoriness, and the complexity of VF than either drug alone compared with baseline.

Background: Acute ischemia causes myriad changes including increased catecholamines. We tested the hypothesis that elevated catecholamines alone are arrhythmogenic. Methods and Results: A 504 electrode sock was placed over both ventricles in six open‐chest pigs. During control infusion of saline through a catheter in the left anterior descending coronary artery (LAD), no sustained arrhythmias occurred, and the refractory period estimated by the activation recovery interval (ARI) was 175 ± 14 ms in the LAD bed below the catheter. After infusion of isoproterenol at 0.1 μg/kg/min through the catheter, the ARI in this bed was significantly reduced to 109 ± 10 ms. A sharp gradient of refractoriness of 43 ± 10 ms was at the border of the perfused bed. Sustained monomorphic ventricular tachycardia occurred after drug infusion in the perfused bed or near its boundary in all animals with a cycle length of 329 ± 26 ms and a focal origin. The maximum slope of the ARI restitution curve at the focal origins of the tachyarrhythmias was always <1 (0.62 ± 0.15). Similar results with a focal arrhythmia origin occurred in two additional pigs in which intramural mapping was performed with 36 plunge needle electrodes in the left ventricular perfused bed. Conclusion: Regional elevation of a catecholamine, which is one of the alterations produced by acute ischemia, can by itself cause tachyarrhythmias. These arrhythmias are closely associated with a shortened refractory period and a large gradient of the spatial distribution of refractoriness but not with a steep restitution curve.

We have developed an eight-channel telemetry system for studying experimental models of chronic cardiovascular disease. The system is an extension of a previous device that has been miniaturized, reduced in power consumption and provided with increased functionality. We added sensors for ventricular dimension, and coronary artery blood flow and arterial blood pressure that are suitable for use with the system. The telemetry system consists of a front end, a backpack and a host PC. The front end is a watertight stainless steel case with all sensor electronics sealed inside; it acquires dimension, flow, pressure and five cardiac electrograms from selected locations on the heart. The backpack includes a control unit, Bluetooth radio, and batteries. The control unit digitizes eight channels of data from the front end and forwards them to the host PC via Bluetooth link. The host PC has a receiving Bluetooth radio and Labview programs to store and display data. The whole system was successfully tested on the bench and in an animal model. This telemetry system will greatly enhance the ability to study events leading to spontaneous sudden cardiac arrest.

Background: Following successful defibrillation of the long duration ventricular fibrillation (LDVF) associated with sudden cardiac arrest (SCA), refibrillation occurs in over half of resuscitation attempts. In experimental animals with normal hearts, refibrillation almost never occurs following short duration VF lasting < 1 min (SDVF). It is not known if refibrillation following LDVF is due to the cardiac disease that caused the initial LDVF of SCA or if the LDVF itself causes abnormalities that can initiate VF. Further, the mechanism of refibrillation initiation is unknown. We tested the hypothesis that refibrillation occurs frequently following LDVF in previously normal hearts and propagates from a common area within the heart. Methods : In 6 pigs, 115–120 plunge needles containing 3– 6 electrodes each were inserted throughout the heart. With the chest closed, external defibrillation pads were placed in a left lateral to right lateral configuration. Following successful defibrillation of 20 s of SDVF, the animal was observed for spontaneous VF for at least 4 min. After termination of 7 min of LDVF, the animal was placed on cardiopulmonary bypass and observed for 15 min for spontaneous VF. Results: Following LDVF, 1.33 ±0.8 episodes of spontaneous VF occurred in each animal. The mean time to VF was 71 s (range 5 – 139 s) following defibrillation of LDVF. Three animals had a second episode of VF within 3 min of termination of LDVF. Earliest sites of origin of spontaneous VF occurred equally in the LV (3), RV (3), and septum (2) and arose equally from the endocardium (3), epicardium (2), and myocardium (3). Refibrillation originated in the posterior half of the ventricles 75% of the time. The first refibrillation cycle appeared focal in 75% of cases and reentrant in 25%. Premature ventricular beats arising out of the posterior RV base immediately preceded 75% of VF episodes. Conclusions: Spontaneous VF is common after defibrillation of LDVF but not SDVF in previously health swine, indicating that refibrillation does not require preexisting cardiac disease but can be caused by the detrimental effects of LDVF. Most episodes of refibrillation begin as focal activity within the posterior half of the ventricles.

BACKGROUND Earliest recorded postshock myocardial activations in pigs originate in the subepicardium of the apex and lateral free wall of the left ventricle (LV) 30-90 ms after the shock.OBJECTIVE The purpose of this study was to determine whether the Purkinje system is a candidate for the source of postshock activations by performing endocardial and transmural postshock activation mapping.METHODS In five pigs, 32 plunge needles with 12 electrodes (1-mm spacing) were inserted into the LV apex and Lateral free wall. Up to 70 plunge needles with six electrodes (2-mm spacing) were spread throughout the remainder of the LV, while 9-12 plunge needles with four electrodes (2-mm spacing) were inserted into the right ventricle. A basket catheter with 32 bipolar recording sites was inserted into the LV. Defibrittation-threshold (DFT)-level shocks were delivered during 10 episodes of electrically induced ventricular fibrillation. Electrograms of postshock activation cycles were analyzed for Purkinje and myocardial activations.RESULTS Purkinje activations were recorded before Local myocardial activation in 9% of basket electrograms and in 15% of plunge needles during the first postshock activation cycle. Purkinje activations were identified during the first and subsequent several postshock activation cycles in at least one basket and one needle electrogram in 96% and 98% of defibrillation episodes, respectively.CONCLUSIONS The Purkinje system is active during the early postshock activation cycles after DFT-level shocks. Further studies are required to determine whether activation initiates in the Purkinje system or whether it is activated by the myocardium or by Purkinje-myocardial junctional cells.

Background: For well over 50 years, it has been assumed that ventricular fibrillation (VF) is maintained solely by reentry in the working myocardium. This hypothesis has never been tested by recording VF with electrodes spaced sufficiently close together to map activation sequences in 3-dimensions. Methods and Results: We recorded for 10 min during VF from the region of insertion of the anterior papillary muscle in the left ventricle of 4 open-chest pigs. A 3-D transmural unipolar electrode array consisting of a 9×9 array of needles with 2-mm spacing and 6 electrodes 2 mm apart on each needle was used to record from the working myocardium. The numbers of wavefronts, foci and reentrant circuits per sec were counted. A focus was identified when a wavefront appeared de novo within the central portion of the mapped region, insuring that it did not arise from another wavefront nor propagate into the mapped region from outside it. While intramural reentry was present early but not late during VF in the mapped region, foci were numerous and increased as VF continued (Figure ). Conclusion: Intramural foci are present during VF in pigs and, as VF continues, increase in incidence while the incidence of reentry in working myocardium decreases. These results suggest that, particularly after the first 2 min of VF, mechanisms other than reentry in the working myocardium maintain VF. It remains to be determined if these mechanisms consist of abnormal automaticity, afterdepolarizations, or reentry in which Purkinje fibers form all or part of the circuit.

During ventricular fibrillation (VF) only 39% of the variation in action potential duration (APD) is accounted for by the previous diastolic interval [DI((n-1))], i.e., restitution, and the previous APD [APD((n-1))], i.e., memory. We tested the hypothesis that a characteristic of the AP upstroke, the maximum rate of depolarization (V(max)), also helps account for its APD. A floating microelectrode was used to make transmembrane recordings at 16,000 samples/s from the anterior left ventricular wall during four 20-s episodes of VF in each of six pigs. V(max), time from V(max) to 60% repolarization (APD(60)), and DI were calculated throughout all episodes. Stepwise linear regression was used to determine how well each APD(60) (APD(60n)) was predicted by V(max) of that AP, the four previous DIs (n-1, n - 2, n - 3, n - 4), and the three previous APD(60)s (n-1, n - 2, n - 3). V(max) entered in the regression equation significantly more often (86% of VF episodes) than either APD((n-1)) (47% of episodes) or DI((n-1)) (58% of episodes). When these three variables entered first or second, their coefficients were almost always positive, consistent with a longer APD associated with 1) a larger V(max), 2) a longer APD((n-1)), and 3) a longer DI((n-1)). R(2) of the regression for all entered variables was 0.51 +/- 0.01 (mean +/- SD). During the first 20 s of VF in swine, V(max) is a more important determinant of APD than the previous DI (restitution) or the previous APD (memory). All variables together account for only one-half of APD variation during VF.

Background— The roles of Purkinje fibers (PFs) and focal wave fronts, if any, in the maintenance of ventricular fibrillation (VF) are unknown. If PFs are involved in VF maintenance, it should be possible to map wave fronts propagating from PFs into the working ventricular myocardium during VF. If wave fronts ever arise focally during VF, it should be possible to map them appearing de novo. Methods and Results— Six canine hearts were isolated, and the left main coronary artery was cannulated and perfused. The left ventricular cavity was exposed, which allowed direct endocardial mapping of the anterior papillary muscle insertion. Nonperfused VF was induced, and 6 segments of data, each 5 seconds long, were analyzed during 10 minutes of VF. During 36 segments of data that were analyzed, 1018 PF or focal wave fronts of activation were identified. In 534 wave fronts, activation was mapped propagating from working ventricular myocardium to PF. In 142 wave fronts, activation was mapped propagating from PF to working ventricular myocardium. In 342 wave fronts, activation was mapped arising focally. More than 1 of these 3 patterns could occur in the same wave front. Conclusions— PFs are highly active throughout the first 10 minutes of VF. In addition to retrograde propagation from the working ventricular myocardium to PFs, antegrade propagation occurs from PFs to working ventricular myocardium, which suggests PFs are important in VF maintenance. Prior plunge needle recordings in dogs indicate activation propagates from the endocardium toward the epicardium after 1 minute of VF, which suggests that focal sites on the endocardium may represent foci and not breakthrough. If so, in addition to reentry, abnormal automaticity or triggered activity may also occur during VF.

Transmural Activation Sequence in Ventricular Fibrillation. Background: Humans are more similar in transmural Purkinje and cardiac ion channel distributions to dogs than pigs. The Purkinje network in pigs is transmural but confined to the endocardium in dogs. Little is known about intramural activation during long-duration ventricular fibrillation (LDVF) given these differences. We tested the hypothesis that the transmural activation sequence is similar in sinus rhythm (SR) and LDVF in dogs as well as pigs, but different between species.Methods and Results: In six pigs and seven dogs, 50-60 plunge needles (six electrodes, 2-mm spacing) were placed throughout the left ventricle. Unipolar recordings were made for >10 minutes of LDVF. SR and LDVF activation times were grouped into waves by linking activations along each needle. Origin (earliest activation) and propagation direction were determined for each wave. The mean wave origin was significantly more endocardial in dogs than pigs for SR and 1 through 10 minutes of LDVF. Predominant propagation direction in LDVF and SR was endocardial to epicardial in dogs, but the opposite or equal in both directions in pigs. Fastest activation rate was epicardial in pigs, but endocardial in dogs with an increasing endocardial-to-epicardial activation rate gradient as LDVF progressed in dogs but not pigs.Conclusions: The transmural activation sequence in SR and LDVF is markedly different between pigs and dogs. These differences may be related to differences in Purkinje fiber and ion channel distributions and suggest that dogs are a better model for investigating activation sequences during LDVF, given the similarities with humans.

Direct recording of Purkinje fiber activity may lead to a better understanding of the role of the specialized conduction system in pathological cardiac conditions. Two studies were conducted in pigs to determine guidelines for effective plunge needle recording techniques. In the first experiment, Purkinje fiber activations were recorded at 16 KHz with 3 bipolar electrodes (2 mm spacing) on epoxy plunge needles, and were later lowpass filtered and downsampled to determine the rate required for effective identification of Purkinje activation. Purkinje spikes were identifiable at sampling rates of 4 KHz and greater, but were not easily distinguished at sampling rates of 2 KHz or less. In the second experiment, 4 plunge needles with 15 electrodes (1 mm spacing) were inserted 8 times into different locations around the left ventricle. Unipolar (15 per needle) and bipolar (14 per needle) signals were recorded simultaneously at a sampling rate of 8 KHz. Purkinje activations were identified in 13/32 plunge needle sites. Of the 13 sites with identified Purkinje activations, 10 were within 2 mm of the endocardium. Bipolar recordings demonstrated Purkinje potentials that were 13% of the amplitude of the following myocardial activation, while unipolar recordings from the same electrodes recorded Purkinje potentials that were only 5% of the amplitude of the following myocardial activations. Three guidelines were developed for effective Purkinje fiber recording: 1) use a minimum sampling rate of 4 KHz., 2) record near the endocardium, and 3) use bipolar rather than unipolar recording electrodes

Plunge needle recording techniques have provided valuable insights into transmural activation in cardiac tissue. Construction of plunge needles has been a costly and time intensive endeavor. Plunge needles constructed with standard printed circuit board (PCB) technology and methods are outlined. PCB plunge needles are less expensive in terms of raw materials and time required for construction than hypodermic stock or epoxy plunge needles. Tested PCB plunge needles recorded signals comparable to signals recorded by other plunge needles. PCB plunge needles provide an economical and rapid alternative to previously published techniques for plunge needle design

The prospect of biological attacks is a growing strategic threat. Covert aerosol attacks inside a building are of particular concern. In the summer of 2005, the Center for Biosecurity of the University of Pittsburgh Medical Center convened a Working Group to determine what steps could be taken to reduce the risk of exposure of building occupants after an aerosol release of a biological weapon. The Working Group was composed of subject matter experts in air filtration, building ventilation and pressurization, air conditioning and air distribution, biosecurity, building design and operation, building decontamination and restoration, economics, medicine, public health, and public policy. The group focused on functions of the heating, ventilation, and air conditioning systems in commercial or public buildings that could reduce the risk of exposure to deleterious aerosols following biological attacks. The Working Group's recommendations for building owners are based on the use of currently available, off-the-shelf technologies. These recommendations are modest in expense and could be implemented immediately. It is also the Working Group's judgment that the commitment and stewardship of a lead government agency is essential to secure the necessary financial and human resources and to plan and build a comprehensive, effective program to reduce exposure to aerosolized infectious agents in buildings.