Biomedical subjects

William M Smith

Publications and source records attributed to William M Smith.

36 records · Page 2Linked to original sources

PubMed2003

Effects of burst stimulation during ventricular fibrillation on cardiac function after defibrillation.

The purpose of defibrillation is to rapidly restore blood flow and tissue perfusion following ventricular fibrillation (VF) and shock delivery. We tested the hypotheses that 1) a series of 1-ms pulses of various amplitudes delivered before the defibrillation shock can improve hemodynamics following the shock, and 2) this hemodynamic improvement is due to stimulation of cardiac or thoracic sympathetic nerves. Ten anesthetized pigs received a burst of either 15 or 30 1-ms pulses (0.1-10 A in strength) during VF, after which defibrillation was performed. ECG, arterial blood pressure, and left ventricular (LV) pressure were recorded. Defibrillation shocks and burst pulses were delivered from a right ventricular coil electrode to superior vena cava coil and left chest wall electrodes. Sympathetic blockade was induced with 1 mg/kg timolol and trials were repeated. The first half of this protocol was repeated in two animals that were pretreated with reserpine. Heart rate (HR) after 1-, 2-, 5-, and 10-A pulses was significantly higher than after control shocks without preceding pulse therapy. Mean and peak LV pressure measurements increased 38 and 72%, respectively, following shocks preceded by 5- and 10-A pulses compared with shocks preceded by no burst pulses. Mean and peak arterial pressures increased 36 and 43%, respectively, following shocks preceded by 5- and 10-A pulses compared with shocks preceded by no burst pulses. After beta-blockade, HR, mean and peak arterial pressures, and mean LV pressure were not significantly different after pulses of any strength compared with control shocks. LV peak pressure following the 10-A pulses was significantly higher than with no burst pulses but was significantly lower than the response to the 10-A pulses delivered without beta-blockade. HR, mean and peak arterial pressures, and mean and peak LV pressure responses after 15 or 30 5- or 10-A pulses were similar to the responses to the same pulses after beta-blockade. We conclude that a burst of 15-30 1-ms pulses delivered during VF can increase HR, arterial pressure, and LV pressure following defibrillation. beta-Blockade or reserpine pretreatment prevents most of this postshock increase in HR, arterial pressure, and LV pressure.

Adrenergic Uptake Inhibitors↗

PubMed2003

Identification of transmural necrosis along a linear catheter ablation lesion during atrial fibrillation and sinus rhythm.

BACKGROUND: Determining whether a linear catheter radio frequency (RF) ablation lesion is transmural may be difficult, especially during atrial fibrillation. We hypothesized that changes in pacing thresholds and electrogram amplitude during atrial fibrillation and sinus rhythm could be used to assess whether a radiofrequency ablation resulted in transmural necrosis. METHODS: A hexapolar, linear, RF ablation catheter was positioned between the caval veins in the right atrium of seven sheep. Pacing thresholds and electrogram amplitudes during atrial fibrillation and sinus rhythm were measured before and after the application of RF energy. Sites along the linear lesion were assessed histologically. RESULTS: The electrogram amplitude in atrial fibrillation decreased significantly more at transmural sites (unipolar recording: 33 +/- 11% transmural vs. 22 +/- 13% non-transmural, p < or = 0.01; bipolar recording: 62 +/- 9% transmural vs. 43 +/- 15% non-transmural, p < or = 0.01). The electrogram amplitude in sinus rhythm decreased significantly more at transmural sites (unipolar recording: 49 +/- 18% transmural vs. 15 +/- 20% non-transmural, p < 0.001; bipolar recording: 63 +/- 17% transmural vs. 42 +/- 19% non-transmural, p = 0.002). The pacing threshold increased significantly more at sites with transmural necrosis (unipolar: increased by 378 +/- 103% transmural vs. 207 +/- 93% non-transmural, p < 0.001; bipolar: 370 +/- 80% transmural vs. 259 +/- 60% non-transmural, p < 0.001). CONCLUSIONS: The amplitude of the atrial electrogram from an ablation catheter can be used to discriminate areas with transmural necrosis from those without transmural necrosis during either atrial fibrillation or sinus rhythm. Termination of atrial fibrillation may not be necessary to estimate the histologic characteristics of an ablation lesion.

Animals↗

PubMed2003

Adaptive pacing during ventricular fibrillation.

While it has been shown that pacing during ventricular fibrillation (VF) can capture a portion of the epicardium, little is known about the characteristics of the area captured or about whether adaptively changing the pacing rate during VF will increase the area captured. In six open-chested pigs, pacing during VF was performed from the center of a plaque containing 504 electrodes 2 mm apart in a21 x 24 array on the anterior right ventricle. Simultaneous recordings from the 504 electrodes were used to construct activation maps from which the area of epicardium captured by pacing was determined. Four pacing algorithms were examined: (1) fixed rate pacing at 95% of the median VF activation rate, (2 and 3) adaptive pacing in which the pacing timing and/or rate is reset in real time if capture is not obtained, and (4) pacing at a slowly increasing rate after initial capture. Regional capture, defined as control of the myocardium under at least 10 plaque electrodes, was achieved in 71% (92/129) of pacing episodes. The incidence of capture was not significantly different for pacing algorithms 1-3. The maximum area captured for each pacing episode with algorithms 1-3 was 3.8 +/- 2.0 cm2(mean +/- SD). Within each animal, the pattern of capture was similar among all pacing episodes, no matter which algorithm was use dr = 0.85 +/- 0.25). The region of greatest capture extended away from the pacing site along the long axis of the myocardial fibers. However, the area of captured epicardium toward the right ventricular side of the pacing electrode was 9.7 times greater than toward the left ventricular side. This principal direction toward the right ventricular side of the pacing electrode was the same direction traveled by the majority of VF activation fronts before capture occurred. The absence of recorded activations at the pacing site for 20 consecutive stimuli predicted 83% of the time that regional capture was present. With algorithm 4, the pacing rate could be increased 7.1%+/- 4.3%while maintaining capture; however, the area of capture progressively decreased as the pacing rate increased. While pacing from the anterior right ventricular epicardium during VF, the area of capture is repeatable and is markedly asymmetrical with almost 10 times as much epicardium captured on the side of the pacing electrode closest to the acute margin of the right ventricle as on the opposite side. This marked asymmetry is associated both with myofiber orientation and with the direction of spread of activation and hence the direction of dispersion of refractoriness during VF just before pacing is initiated. It is possible to perform adaptive pacing algorithms in real time during VF; however, the two adaptive algorithms tested did not capture significantly more epicardium than a simple fixed-rate pacing algorithm. Although it is possible to maintain capture while increasing the pacing rate during VF, the area of capture decreases.

Algorithms↗

PubMed2003

Emission ratiometry for simultaneous calcium and action potential measurements with coloaded dyes in rabbit hearts: reduction of motion and drift.

INTRODUCTION: Optical measurements of the cardiac calcium transient (Ca) and transmembrane action potential (AP) may be performed simultaneously with emission ratiometry to lessen motion artifacts and photobleaching effects. We examined changes in emission spectrum in perfused rabbit hearts coloaded with Rh237 and a green-emitting Ca dye (Fluo-4 or Oregon Green BAPTA 1) to determine wavelength bands for emission ratiometry and to test whether ratiometry reduces motion artifacts and drift. METHODS AND RESULTS: A 488-nm laser illuminated hearts while a spectrofluorometer collected fluorescence from 489 to 838 nm at 1 kHz. Ratiometry with the Ca- and AP-insensitive emission band 663 to 685 nm (IS) as denominator and the Ca-sensitive band 510 to 532 nm as numerator lessened motion artifacts, which was quantified as a 1.4-fold increase in relative amplitude of the Ca (P < 0.05). Ratiometry with the AP-sensitive band 772 to 794 nm as denominator and the IS as numerator produced a 1.7-fold increase in relative amplitude of the AP (P < 0.05). The ratiometry decreased photobleaching-dependent drift by a factor of 0.6 (P < 0.05) for Ca and 0.45 (P < 0.05) for AP. CONCLUSION: Simultaneous Ca and AP emission ratiometry reduces motion artifacts and drift in hearts with coloaded dyes.

Action Potentials↗

PubMed2003

Discrimination of left atrial and pulmonary vein potentials in patients with paroxysmal atrial fibrillation.

INTRODUCTION: Ablation of muscular fascicles around the ostium of pulmonary veins (PVs) resulting in electrical isolation of the veins may prove to be an effective treatment for atrial fibrillation (AF). Correctly discriminating atrial and PV potentials is necessary to effectively isolate PVs from the left atrium in patients with paroxysmal AF. METHODS AND RESULTS: A training set of 151 electrode recordings obtained from 10 patients with AF was used to develop an algorithm to discriminate atrial and PV potentials. Bipolar electrograms were collected from a multielectrode basket catheter placed sequentially into each PV. Amplitude, slope, and normalized slopes of both bipolar and quadripolar electrograms (difference between adjacent bipoles) were entered into a binary logistic regression model. A receiver operating characteristic curve was used to define a threshold able to effectively discriminate atrial and PV potentials. The normalized slopes of both domains, bipolar and quadripolar, produced a logistic function that discriminated atrial and PV potentials against a threshold (0.38) with 97.8% sensitivity and 94.9% specificity. The algorithm then was evaluated on a test set of 214 electrode recordings from four patients who also had paroxysmal AF. These patient electrograms also were evaluated by two independent electrophysiologists. The algorithm and electrophysiologists matched identification of activation origin in 84% of electrograms. CONCLUSION: Atrial and PV potentials acquired from a multielectrode basket catheter can be discriminated using the normalized slopes of bipolar and quadripolar electrograms. These additional parameters need to be included by physicians determining the preferential ablation site within PVs.

Adult↗

PubMed2003

Development of an optrode for intramural multisite optical recordings of Vm in the heart.

INTRODUCTION: Optical mapping of transmembrane potential (Vm) is an important tool in the investigation of impulse propagation in the heart. It provides valuable information about spatiotemporal changes of Vm that cannot be obtained by other techniques, but it presently is limited to measurements from the heart surfaces. Therefore, the goal of this work was to develop a technique for intramural multisite optical measurements of Vm using fiberoptic technology. METHODS AND RESULTS: An optrode, a bundle of thin optical fibers, was developed for measuring intramural optical signals at multiple sites in the heart. The optrode consisted of seven fibers with diameter of 225 microm arranged in a hexagonal pattern that were used to deliver excitation light to the myocardium, to collect the emitted fluorescence, and to project the light onto a 16 x 16 array of photodiode detectors. Rabbit hearts were stained with the Vm-sensitive dye RH-237. Fluorescence was excited using a 100-W Hg lamp. Intramural action potentials were recorded at multiple sites separated by 2 mm inside the left ventricle. Signal-to-noise (RMS) ratio was 21.2 +/- 12 (n = 7) without averaging or ratiometry and with negligible cross-talk (<1.9%) between the neighboring photodiodes. The size of the recording area for an individual fiber was estimated at approximately 0.8 mm. CONCLUSION: These data demonstrate feasibility of multisite transmural measurements of Vm without signal averaging and ratiometry. This technique might become useful in studies of transmural impulse conduction during arrhythmias and defibrillation.

Action Potentials↗

PubMed2002

Regional differences in ventricular fibrillation in the open-chest porcine left ventricle.

It has been hypothesized that during ventricular fibrillation (VF), the fastest activating region, the dominant domain, contains a stable reentrant circuit called a mother rotor. This hypothesis postulates that the mother rotor spawns wavefronts that propagate to maintain VF elsewhere and implies that the ratio of wavefronts propagating off a region to those propagating onto it (propoff/propon) should be >1 for the dominant domain but <1 elsewhere. To test this prediction in the left ventricular (LV) epicardium of a large animal, most of the LV free wall was mapped with 1008 electrodes in 7 pigs. VF activation rate was faster in the posterior than in the anterior LV (10.0+/-1.3Hz versus 9.3+/-1.3Hz; P<0.001). The anterior LV had a higher fraction of wavefronts that blocked than did the posterior LV and had a propoff/propon ratio <1 (P<0.001). The mean conduction velocity vectors of the VF wavefronts pointed in the direction from the posterior to the anterior LV. Although these findings favor a dominant domain in the posterior LV, the facts that the anterior LV had a higher incidence of reentry than did the posterior LV and that the posterior LV did not have propoff/propon significantly different from 1 do not. Thus, quantitative regional differences are present over the porcine LV epicardium during VF. Although these differences are not totally consistent with the presence of a dominant domain within the LV free wall, the mean conduction velocity vector is consistent with one in the septum.

Animals↗

PubMed2002

The effects of acute and chronic amiodarone on activation patterns and defibrillation threshold during ventricular fibrillation in dogs.

OBJECTIVES: The goal of this study was to evaluate the effects of acute and chronic amiodarone on activation patterns during ventricular fibrillation (VF), ventricular effective refractory period (VERP) and defibrillation threshold (DFT). BACKGROUND: Acute and chronic amiodarone may act through different mechanisms. METHODS: The VERP, VF activation patterns and DFT were determined in 24 dogs. Twelve dogs received acute intravenous amiodarone (10 mg/kg, n = 6) or saline (n = 6), and 12 dogs received chronic oral amiodarone (20 mg/kg/day, n = 6) or placebo (n = 6). Epicardial VF activation patterns were recorded with 504 electrodes. Quantitative descriptors of VF were calculated. RESULTS: The DFT was unchanged by acute or chronic amiodarone. Although chronic amiodarone significantly extended the VERP, acute amiodarone did not. In the mapped region, acute and chronic amiodarone decreased the number of VF wavefronts by 42% and 60%. Acute amiodarone decreased conduction block by 22%, while chronic amiodarone increased block by 41% but decreased wave fractionation by 50%. Both chronic and acute amiodarone increased the size of the core of re-entrant circuits and decreased the incidence of re-entry by 44% and 57%; however, chronic amiodarone increased wavelength, while acute amiodarone did not. CONCLUSIONS: Neither acute nor chronic amiodarone change the DFT. While both acute and chronic amiodarone decrease the number of wavefronts, decrease the incidence of re-entry and increase the size of re-entrant cores in the mapped region during VF, they achieve these antiarrhythmic effects through different electrophysiologic mechanisms. Chronic amiodarone prolonged the VF cycle length and slowed conduction velocity, indicating it increased the wavelength and/or the excitable gap.

Amiodarone↗

PubMed2002

Impact of myocardial ischemia and reperfusion on ventricular defibrillation patterns, energy requirements, and detection of recovery.

BACKGROUND: Shocks that have defibrillated spontaneous ventricular fibrillation (VF) during acute ischemia or reperfusion may seem to have failed if VF recurs before the ECG amplifier recovers after shock. This could explain why the defibrillation threshold (DFT) for spontaneous VF appears markedly higher than for electrically induced VF. METHODS AND RESULTS: The DFT for electrically induced VF (E-DFT) was determined in 15 pigs before ischemia, followed by left anterior ascending or left circumflex artery occlusion. VF was electrically induced 20 minutes after occlusion, followed 5 minutes later by reperfusion. Whether spontaneous or electrically induced, VF during occlusion or reperfusion was treated with up to 3 shocks at 1.5xE-DFT. If all 3 shocks failed, shock strength was increased. Thirty minutes after reperfusion, the other artery was occluded and the protocol was repeated. Defibrillation was considered successful if postshock sinus/idioventricular rhythm was present for > or = 30 seconds. VF recurring within 30 seconds after the shock was considered immediate or delayed if the first postshock activation complex in a rapidly restored ECG recording was VF or sinus/idioventricular rhythm, respectively. Defibrillation efficacy at 1.5xE-DFT was significantly higher for electrically induced ischemic VF (76%) than for spontaneous VF (31%). The incidence of delayed recurrence after electrically induced nonischemic (3%) or ischemic (20%) VF was significantly lower than after spontaneous VF (75%). Mean VF recurrence time after spontaneous VF was 4.6+/-5.3 seconds. CONCLUSIONS: Spontaneous VF can be halted by a shock but then quickly restart before a standard ECG amplifier has recovered from postshock saturation, making it appear that the shock failed.

Animals↗

PubMed2002

Biphasic waveform external defibrillation thresholds for spontaneous ventricular fibrillation secondary to acute ischemia.

OBJECTIVES: The goal of this study was to determine if the defibrillation threshold (DFT) after spontaneous ventricular fibrillation (VF) secondary to acute ischemia differs from the DFT for electrically induced VF in the absence of ischemia in anesthetized, closed-chest dogs and pigs. BACKGROUND: The efficacy of external defibrillators has been tested mainly in animals and humans using E-VF, yet external defibrillators are often used in patients to halt S-VF. METHODS: Protocol 1: biphasic truncated exponential (BTE) waveform shocks were delivered through electrodes placed in an anterior-anterior (A-A) position (left and right lateral thorax) in nine dogs. After measuring the E-VF DFT, acute ischemia was induced with an angioplasty balloon in either the left anterior descending or left circumflex coronary artery, and the S-VF DFT was determined. Protocol 2: in a group of 12 pigs, the E-VF DFT and S-VF DFT were determined for electrodes in the A-A position and in the anterior-posterior position (A-P). Protocol 3: the E-VF DFT was determined in seven pigs. Then up to three shocks 1.5x the E-VF DFT were delivered to S-VF. If defibrillation did not occur, a step-up protocol was used until defibrillation occurred. RESULTS: Protocol 1: the DFT for E-VF was 65 +/- 28 J (mean +/- SD) compared with 226 +/- 97 J for S-VF, p < 0.05. Protocol 2: the DFT was 152 +/- 58 J for E-VF and 315 +/- 123 J for S-VF for A-A electrodes. The DFT was 100 +/- 43 J for E-VF and 206 +/- 114 J for S-VF for A-P electrodes. Protocol 3: 11/37 shocks of strength 1.5x E-VF DFT (182 +/- 40 J) stopped the arrhythmia. The episodes of S-VF not halted by these shocks required energy levels of up to 400 J for defibrillation. CONCLUSIONS: External defibrillation of S-VF induced by acute ischemia requires significantly more energy than VF induced by 60-Hz current in the absence of ischemia. A safety margin >1.5x the DFT for electrically induced VF may be necessary in BTE external defibrillators to defibrillate S-VF.

Animals↗

PubMed2002

Endocardial wave front organization during ventricular fibrillation in humans.

OBJECTIVES: This study was designed to characterize the organization of ventricular fibrillation (VF) on the endocardium of humans. BACKGROUND: Most proposed mechanisms for the maintenance of VF postulate the propagation of a number of activation wave fronts that reenter to maintain the arrhythmia. We tested the hypothesis that, in patients undergoing internal cardioverter-defibrillator implantation, VF consists primarily of a few large wave fronts on the endocardium. METHODS: Electrograms were recorded from a 36-electrode catheter in the left ventricle of 16 patients during VF. Activation times were chosen for a 2-s epoch for each fibrillation episode, and a two-dimensional Kolmogorov-Smirnov test was performed to determine if activation occurred randomly along the catheter over that time interval. The maximum cross-correlation was found for all possible pairs of electrodes on the catheter, and these values were plotted relative to the distance between the two electrodes. An exponential curve was then fit to the data, and a length constant was determined. Activation times were grouped into wave fronts along the catheter, and the lengths of the wave fronts were estimated. RESULTS: The Kolmogorov-Smirnov test showed that activation was not random along the catheter in any of the patients studied. The correlation length determined was 9 +/- 2 cm. The number of wave fronts recorded by the catheter was 9.2 +/- 2.9 wave fronts/s. The length of the pathway of each wave front along the catheter was 6.5 +/- 4.5 cm. CONCLUSIONS: Ventricular fibrillation is well organized on the endocardial surface of humans, consisting primarily of a few large wave fronts on the order of 6 to 9 cm.

Adult↗

PubMed2002

Defibrillation threshold and cardiac responses using an external biphasic defibrillator with pediatric and adult adhesive patches in pediatric-sized piglets.

Before recommendations for using an automatic external defibrillator on pediatric patients can be made, a protocol for the energy of a biphasic waveform energy dosing needs to be determined that will allow ventricular defibrillation of 8 year olds while causing only a minimal amount of cardiac damage to infants. Pediatric- and adult-sized electrode patches were alternately applied to 10 isoflurane-anesthetized piglets weighing 3.8-20.1 kg to approximate the body weights of newborns to children < 8 years old. The defibrillation threshold (DFT) was determined for biphasic truncated exponential waveform shocks. Additional shocks, varying from the DFT to 360 Joules (J), were delivered during sinus rhythm or following 30 s of ventricular fibrillation (VF). The DFT was 2.4+/-0.81 and 2.1+/-0.65 J/kg for pediatric and adult patches, respectively (P = N.S.). The change in left ventricular (LV) dP/dt from baseline as a function of shock strength was significantly different at 1 and 10 s after shocks of increasing energy that were delivered in sinus rhythm, and 1, 10, 20, and 30 s after defibrillation shocks. There was no significant difference in LV dP/dt with increasing shock energy at 60 s with either patch size. The time to return of sinus rhythm, ST-segment deviation, and cardiac output were also not significantly different from baseline 60 s following shocks of up to 360 J delivered during sinus rhythm or VF with either patch. The same amount of energy delivered with a biphasic external defibrillator successfully defibrillated VF whether adult or pediatric patches were used. Cardiac rhythm and hemodynamic variables were unaltered at 60 s after shocks delivered at energies of up to 360 J. These data suggest that there is a substantial safety margin above a DFT strength shock for this biphasic waveform in piglets.

Age Factors↗

PubMed2002

Effect of electrode location in great cardiac vein on the ventricular defibrillation threshold.

This study tested the hypothesis that the DFT could be lowered by delivering a weak auxiliary shock in conjunction with a stronger primary shock with the auxiliary shock electrode near the cardiac region where the primary shock electric field is weakest. This hypothesis was tested by determining the DFTs with the auxiliary shock delivered from different locations within the great cardiac vein (GCV). In 15 dogs, catheters with defibrillation electrodes were placed transvenously in the RV apex, the SVC, and the GCV. An active can electrode and the SVC electrodes were electrically coupled to serve as a return electrode for the RV and GCV electrodes. DFTs were determined for a primary shock through the RV electrode with and without a subsequent auxiliary shock of lower amplitude through the GCV electrode. The leading edge voltage and current at DFT were significantly lowered by addition of the auxiliary shock (17% and 19% decreased, respectively), but energy was not changed. The animals were divided into three groups according to the location of the GCV electrode. The leading edge voltage, current, and total delivered energy at the DFT were significantly lower in animals with the GCV electrode near the apex (22%, 24%, and 13% reduction, respectively) compared with those where the GCV electrode was positioned away from apex (8%, 10% reduction and 18% increase, respectively, P < 0.001). Application of an auxiliary shock to the apical region, near the region where previous studies have indicated that the RV primary shock has its weakest effects, caused the greatest decrease in DFT.

Animals↗

PubMed2002

Reduction of the internal atrial defibrillation threshold with balanced orthogonal sequential shocks.

INTRODUCTION: The aim of this study was to determine the atrial defibrillation threshold (ADFT) of a first shock across the standard right atrium (RA) to distal coronary sinus (dCS) configuration followed by a second shock along the atrial septum with a standard sequential waveform (the second shock leading edge equaled the first shock trailing edge) and a balanced sequential waveform (the leading edges of both shocks were equal). METHODS AND RESULTS: In nine sheep atrial fibrillation was induced with acetyl-beta-methylcholine and burst pacing. A catheter was placed with electrodes in the dCS, proximal coronary sinus (pCS), and RA. A J-shaped catheter was positioned with an electrode at Bachmann's bundle (BB) while another catheter was positioned with an electrode in the superior vena cava (SVC). The ADFTs of six single- and dual-pathway configurations were determined with single, standard sequential, or balanced sequential shocks. The ADFT of the RA-->dCS configuration (0.86 +/- 0.27 J, 159 +/- 29 V, 2.42 +/- 0.36 A) was significantly reduced when followed by an SVC-->pCS (0.58 +/- 0.17 J, 112 +/- 20 V, 1.64 +/- 0.39 A) or a BB-->pCS shock (0.64 +/- 0.16 J, 119 +/- 18 V, 1.81 +/- 0.38 A) with standard sequential shocks. With balanced sequential shocks, the peak voltage and current ADFTs were further significantly reduced (85 +/- 11 V and 1.24 +/- 0.21 A for second shock SVC-->pCS, and 93 +/- 13 V and 1.38 +/- 0.27 A for second shock BB-->pCS). CONCLUSION: The ADFT of the standard RA-->dCS shock is significantly reduced when followed by a second shock along the atrial septum delivered between electrodes in the pCS and either SVC or BB and ADFT is further reduced with balanced sequential shocks.

Animals↗

PubMed2002

Estimated global epicardial distribution of activation rate and conduction block during porcine ventricular fibrillation.

INTRODUCTION: A proposed mechanism of the maintenance of ventricular fibrillation (VF) determined by studying small hearts or segments of large hearts is that a single stable rotor exists at the site of maximal activation rate, which gives rise to activation fronts that propagate into slower activating regions where they frequently block. We wished to determine if two predictions of this hypothesized mechanism are true during VF in large hearts: (1) there is a single maximum in the distribution of activation rates with the activation rate decreasing with distance away from this maximum; and (2) the incidence of block is greater outside than inside the fastest activating region. METHODS AND RESULTS: Six 25-second episodes of VF from each of six pigs were recorded from 504 electrodes over the entire ventricular epicardium. The electrodes were divided into four zones: left ventricular base and apex (LVB and LVA) and right ventricular base and apex (RVB and RVA). A fast Fourier transform was performed on each electrogram, and the mean activation rate was estimated from the dominant (peak) frequency (DF) and block was estimated to be present during those time intervals when double peaks (DPs) were present in the power spectrum. The zones had statistically significant distributions of DF (LVB>LVA>RVA>RVB) and DP incidence (RVA>RVB>LVA>LVB). CONCLUSION: During VF, the LV base has the highest estimated activation rate and the lowest estimated block incidence, and the RV has the slowest rate but the highest block incidence. This is consistent with the concept of VF being maintained by activation fronts originating from the LV base.

Animals↗

PubMed2002

Intelligent multichannel stimulator for the study of cardiac arrhythmias.

An intelligent multichannel stimulator (IMS) has been designed and built for use in a cardiac research environment. The device is capable of measuring and responding to cardiac electrophysiological phenomena in real time with carefully timed and placed electrical stimuli. The system consists of 16 channels of sense/stimulation electronics controlled by a digital signal processor (DSP) data acquisition card and a host computer and can be expanded to include more channels. The DSP allows for powerful and flexible algorithms to be implemented for real-time interaction with the cardiac tissue. Although a number of possible uses can be conceived for such a device, the initial motivation was to improve upon attempts to terminate fibrillation by pacing. The IMS was tested in an open-chest animal model, both in sinus rhythm and during fibrillation. It was shown to be an effective research tool by demonstrating the ability to measure and respond to cardiac activations in real time using complex numerical algorithms and appropriately timed stimuli.

Algorithms↗

PubMed2002

Ninth cranial nerve stimulation for epilepsy control. Part 1: efficacy in an animal model.

OBJECTIVE: To characterize the effects of stimulation of the nerve of Hering (HN; cranial nerve nine) in controlling seizure activity using a canine model. METHODS: Using penicillin applied topically to a region of the cerebral cortex, 16 seizure-type continuous epileptiform discharges were generated. Ten specimens of HN (five left-sided and five right-sided) were dissected from the cervical region in five dogs and stimulated at varying parameters to determine the effects in controlling epileptiform activity. Electroencephalography (using a multielectrode array), electrocardiography and other vital signs were continuously monitored for side effects. RESULTS: Resolution of continuous epileptiform activity following stimulation was found in 12 of 16 trials (75%); no spontaneous resolution was noted in the absence of stimulation, and stimulation significantly shortened seizure duration (p < 0.05). Mean epileptiform activity duration was 139 s prior to stimulation (range 1-432 s), with a mean poststimulation delay of 17 s until resolution and a mean interictal time of 399 s. Two specimens became free of seizure activity for the duration of our study (p < 0.001). No significant side effects (such as the potentially life-threatening cardiac problems seen with right-sided cervical vagal nerve stimulation) were found with stimulation of either left- or right-sided HN. Stimulation of other regional nerves (e.g. twelfth cranial nerve, nerves of the cervical plexus) failed to yield similar control of epileptiform activity. CONCLUSIONS: The results of this pilot study suggest that stimulation of the HN can successfully control focal seizure activity in the majority of cases. Pending further study, stimulation of the HN may have a role in the management of patients suffering from medically and otherwise surgically refractory epilepsy.

Animals↗

PubMed2002

Discernment of adipose versus nervous tissue: a novel adjunct solution in lipomyelomeningocele surgery.

OBJECTIVE: To determine a solution capable of discerning adipose versus nervous tissue, to aid in surgical separation of the adipose tissue which appears to be visually indistinguishable from nervous tissue in lipomyelomeningoceles (LMMs). METHODS: The following solutes (in normal saline) were investigated, both at 25 and 37 degrees C: beta-carotene, vitamin D, vitamin E, lecithin, hydrogen peroxide, lipase, protease, hyaluronidase, partially purified collagenase, purified collagenase, trypsin, trypsin plus purified collagenase and non-solute-containing saline (control). Each solution was applied to a pediatric lipoma to determine gross effects over a period of approximately 30 min. If a solution appeared to affect the adipose tissue grossly, studies of functional in vivo sensory evoked and spontaneous potentials using that particular solution were conducted upon sheep spinal cord, nerve roots, dura and peripheral nerve. Additionally, histological studies were conducted to determine the effect of that solution upon adipose tissue, spinal cord, myelin, dura and nerve roots. RESULTS: Of all solutions investigated, partially purified collagenase type 1 (T1C; Lot M0M4322, Code CLS-1, Worthington Biochemical Corporation, Lakewood, N.J., USA) at 37 degrees C was the most successful in grossly altering the consistency and appearance of adipose tissue. This change was more apparent over 20-30 min following application of the solution to the adipose tissue. Solutions not containing T1C did not show appreciable results; purified collagenase plus trypsin did not appear comparable or superior to T1C. No significant histological or functional change was noted when comparing the spinal cord, nerve rootlets, myelin, dura or peripheral nerve from the T1C-treated group versus normal (untreated) control groups. CONCLUSION: T1C appears to be a potentially effective solution for application during LMM surgery in the acute setting, and such use of an adjunct solution may significantly aid in the safe surgical resection of LMMs. Pending further research, this technique may be applied for other indications which require discernment or alteration of adipose versus nervous tissue.

Adipose Tissue↗