Stimulus-induced critical point. Mechanism for electrical initiation of reentry in normal canine myocardium.

The hypothesis was tested that the field of a premature (S2) stimulus, interacting with relatively refractory tissue, can create unidirectional block and reentry in the absence of nonuniform dispersion of recovery. Simultaneous recordings from a small region of normal right ventricular (RV) myocardium were made from 117 to 120 transmural or epicardial electrodes in 14 dogs. S1 pacing from a row of electrodes on one side of the mapped area generated parallel activation isochrones followed by uniform parallel isorecovery lines. Cathodal S2 shocks of 25 to 250 V lasting 3 ms were delivered from a mesh electrode along one side of the mapped area to scan the recovery period, creating isogradient electric field lines perpendicular to the isorecovery lines. Circus reentry was created following S2 stimulation; initial conduction was distant from the S2 site and spread towards more refractory tissue. Reentry was clockwise for right S1 (near the septum) with top S2 (near the pulmonary valve) and for left S1 with bottom S2; and counterclockwise for right S1 with bottom S2 and left S1 with top S2. The center of the reentrant circuit for all S2 voltages and coupling intervals occurred at potential gradients of 5.1 +/- 0.6 V/cm (mean +/- standard deviation) and at preshock intervals 1 +/- 3 ms longer than refractory periods determined locally for a 2 mA stimulus. Thus, when S2 field strengths and tissue refractoriness are uniformally dispersed at an angle to each other, circus reentry occurs around a "critical point" where an S2 field of approximately 5 V/cm intersects tissue approximately at the end of its refractory period.

Activation during ventricular defibrillation in open-chest dogs. Evidence of complete cessation and regeneration of ventricular fibrillation after unsuccessful shocks.

To test the hypothesis that a defibrillation shock is unsuccessful because it fails to annihilate activation fronts within a critical mass of myocardium, we recorded epicardial and transmural activation in 11 open-chest dogs during electrically induced ventricular fibrillation (VF). Shocks of 1-30 J were delivered through defibrillation electrodes on the left ventricular apex and right atrium. Simultaneous recordings were made from septal, intramural, and epicardial electrodes in various combinations. Immediately after all 104 unsuccessful and 116 successful defibrillation shocks, an isoelectric interval much longer than that observed during preshock VF occurred. During this time no epicardial, septal, or intramural activations were observed. This isoelectric window averaged 64 +/- 22 ms after unsuccessful defibrillation and 339 +/- 292 ms after successful defibrillation (P less than 0.02). After the isoelectric window of unsuccessful shocks, earliest activation was recorded from the base of the ventricles, which was the area farthest from the apical defibrillation electrode. Activation was synchronized for one or two cycles following unsuccessful shocks, after which VF regenerated. Thus, after both successful and unsuccessful defibrillation with epicardial shocks of greater than or equal to 1 J, an isoelectric window occurs during which no activation fronts are present; the postshock isoelectric window is shorter for unsuccessful than for successful defibrillation; unsuccessful shocks transiently synchronize activation before fibrillation regenerates; activation leading to the regeneration of VF after the isoelectric window for unsuccessful shocks originates in areas away from the defibrillation electrodes. The isoelectric window does not support the hypothesis that defibrillation fails solely because activation fronts are not halted within a critical mass of myocardium. Rather, unsuccessful epicardial shocks of greater than or equal to 1 J halt all activation fronts after which VF regenerates.

An analytical comparison of the information in sorted and non-sorted cosine-tuned spike activity.

Spike sorting is a technologically expensive component of the signal processing chain required to interpret population spike activity acquired in a neuromotor prosthesis. No systematic analysis of the value of spike sorting has been carried out, and little is known about the effects of spike sorting error on the ability of a brain-machine interface (BMI) to decode intended motor commands. We developed a theoretical framework to examine the effects of spike processing on the information available to a BMI decoder. We computed the mutual information in neural activity in a simplified model of directional cosine tuning to compare the effects of pooling activity from up to four neurons to the effects of sorting with varying amounts of spike error. The results showed that information in a small population of cosine-tuned neurons is maximized when the responses are sorted and there is diverse tuning of units, but information was affected little when pooling units with similar preferred directions. Spike error had adverse effects on information, such that non-sorted population activity had 79-92% of the information in its sorted counterpart for reasonable amounts of detection and sorting error and for units with moderate differences in preferred direction. This quantification of information loss associated with pooling units and with spike detection and sorting error will help to guide the engineering decisions in designing a BMI spike processing system.

Liver ablation guidance with acoustic radiation force impulse imaging: challenges and opportunities.

Previous studies have established the feasibility of monitoring radiofrequency (RF) ablation procedures with acoustic radiation force impulse (ARFI) imaging. However, questions remained regarding the utility of the technique in clinically realistic scenarios and at scanning depths associated with abdominal imaging in adults. We address several of these issues and detail recent progress towards the clinical relevance of the ARFI technique. Results from in vitro bovine tissues and an in vivo ovine model are presented. Additional experiments were conducted with a tissue-mimicking phantom and parallel receive tracking techniques in order to further support the clinical feasibility of the method. Thermal lesions created during RF ablation are visualized with high contrast in both in vitro and in vivo hepatic tissues, and radial lesion growth can be monitored throughout the duration of the procedure. ARFI imaging is implemented on a diagnostic ultrasonic scanner, and thus may be a convenient option to guide RF ablation procedures, particularly when electrode insertion is also performed with sonographic guidance.

Comparison of defibrillation probability of success curves for an endocardial lead configuration with and without an inactive epicardial patch.

OBJECTIVES: This study sought to assess the effect of passive "bystander" epicardial electrodes on defibrillation efficacy. BACKGROUND: We hypothesized that an inactive epicardial patch placed in an area of low potential gradient from an endocardial electrode shock might affect defibrillation efficacy through its effects on the shock field and the underlying potential gradient. METHODS: We studied the effects of an inactive 18-cm2 titanium mesh patch placed on the anterolateral left ventricular epicardium on the 50% probability of successful defibrillation. A biphasic shock with both phases 6 ms in duration was delivered between superior vena cava and right ventricular catheter electrodes 10 s after the electrical induction of ventricular fibrillation. Six dogs underwent an up/down defibrillation protocol randomized with or without the patch on the heart. RESULTS: Mean 50% (+/-) probability point for energy doubled with the conductive patch on the heart, from 8.0 +/- 3.2 to 16.8 +/- 7.0 J (p < 0.01), and leading-edge voltage increased from 334 +/- 64 to 477 +/- 98 V (p < 0.01). Mean 50% probability points for energy and leading-edge voltage were not significantly changed when the procedure was repeated using a nonconductive patch in another six dogs as a control group. In a saline-saturated foam model, measurements from electrodes placed around and under the patch revealed a 72% mean decrease in the potential gradient in the foam under the conductive patch. CONCLUSIONS: A passive defibrillator patch can markedly increase the energy requirements for defibrillation, probably by decreasing the potential gradient under the patch. These results suggest the use of caution when passive electrodes are present, for example, when a patient receives a nonthoracotomy defibrillator system while epicardial electrodes from a previously implanted system are left in place.

Cardiac mapping at Duke Medical Center.

CLINICAL PERSPECTIVE: Ventricular fibrillation is a major, but little understood, cause of death. Although defibrillation with electric shock has been in use for several decades, and although many advances have been made in the design of defibrillators (including the development of implantable devices for high-risk patients), this procedure has generally been an empiric one. The mechanisms of the procedures are being intensely studied at Duke University and other institutions in the hope of developing more energy efficient and clinically effective defibrillation equipment.

A rationale for the use of sequential coronary artery bypass grafts.

The use of sequential coronary artery bypass grafts has been advocated because of improved graft runoff and increased blood flow through the graft with this technique. To examine the influence of runoff, quantitated in terms of coronary vascular resistance, on the velocity of blood flow through coronary artery bypass grafts, we made two sets of simultaneous pressure and flow measurements in 106 single grafts and in 35 double sequential grafts. The first set of measurements was obtained following the aortic anastomosis and the second set of measurements was made following completion of the coronary anastomosis. Resistance of the coronary bed was calculated from the two sets of measurements. The velocity of blood flow through the grafts was calculated from vein graft diameter and the second flow measurement. No significant difference was found between the diameter of single vein grafts (4.0 +/- 0.05 mm.) and sequential vein grafts (4.1 +/- 0.09 nm.). Coronary vascular resistance in the sequential grafts (100.0 +/- 15.6 RU) was lower than that in single grafts (174.6 +/- 14.6 RU, p = 0.001). Velocity of blood flow through the proximal segment of the sequential grafts (11.1 +/- 1.1 cm/sec) was higher than that through single grafts (7.5 +/- 0.6 cm/sec, p = 0.003). The proximal segment of the sequential bypass graft has a higher velocity of blood flow than that seen in a single bypass graft. To obtain maximum hemodynamic advantage with the possibility of improving long-term patency rates, it is advisable to use the smaller coronary artery for the proximal sequential anastomosis.

Does reducing capacitance have potential for further miniaturisation of implantable defibrillators?

OBJECTIVE: To determine whether considerably smaller capacitors could replace 125 microF capacitors as the standard for use in implantable defibrillators. METHODS: Measured energy, impedance, voltage, and current delivered were compared at defibrillation threshold in 10 mongrel dogs for defibrillation using 75 microF and 125 microF capacitors alternated randomly. Defibrillation was attempted with biphasic shocks of comparable tilt between an endocardial lead in the right ventricular apex and a "dummy" active can of an experimental implantable device placed in the subpectoral position. RESULTS: A reduction of capacitor size of 40% was associated with an increase in voltage of 21% and in current of 22%. With a 65% tilt, no significant differences were found between the two capacitances with respect to the impedance or energy required for defibrillation. CONCLUSIONS: Multiple advances in electrode material, electrode configuration, shock morphology, and shock polarity have reduced defibrillation energy requirements. Smaller capacitors could be used in implantable cardioverter/defibrillators without a major decrease in effectiveness.

Is pancreaticoduodenectomy with mesentericoportal venous resection safe and worthwhile?

BACKGROUND: Whether or not superior mesentericoportal venous resection (SM-PVR) associated with pancreaticoduodenectomy (PD) is safe and worthwhile has not been fully confirmed. The aim of the present study was to investigate results of this surgical procedure performed for pancreatic head and periampullary neoplasms. METHODS: As a first analysis, postoperative morbidity and mortality after PD with (n = 31) or without SM-PVR (n = 119) were investigated in 150 patients with pancreatic head and periampullary neoplasms. As a second analysis, rates of margin-negative resection and survival after SM-PVR (n = 21) and without SM-PVR (n = 66) were compared in 87 patients with pancreatic ductal adenocarcinoma of the pancreatic head. In these patients undergoing SM-PVR (n = 21), survival rate was investigated in patients who did (n = 13) and did not (n = 8) undergo a margin-negative resection. RESULTS: In the first analysis, duration of surgery and volume of blood transfused perioperatively were higher in patients undergoing SM-PVR. However, mortality, morbidity rates, and mean hospital stay did not differ between patients who did undergo SM-PVR (31 patients, 3.2%, 48.4%, and 22.2 days, respectively) and who did not (119 patients, 2.5%, 47.1%, 25.9 days, respectively). No postoperative death occurred in the recent part of the present study, since 1994, in patients undergoing SM-PVR. In the second analysis of pancreatic ductal adenocarcinoma, rates of margin-negative resection and 2-year survival did not significantly differ between patients who did and did not undergo SM-PVR (62% and 22%, respectively, versus 73% and 24%). In patients undergoing SM-PVR, survival rate was significantly higher for patients undergoing a margin-negative resection (n = 13) than for patients undergoing a macroscopic or microscopic margin-positive resection (n = 8, 2-year survival = 57.1% versus 0%, P <0.05). CONCLUSION: PD combined with SM-PVR can be performed safely. This surgical procedure is followed by a promising survival rate and can be recommended in order to obtain a margin-negative resection; however, candidates for SM-PVR should be carefully selected.

Estimation of tissue resistivities from multiple-electrode impedance measurements.

In order to measure in vivo resistivity of tissues in the thorax, the possibility of combining anatomical data extracted from high-resolution images with multiple-electrode impedance measurements, a priori knowledge of the range of tissue resistivities, and a priori data on the instrumentation noise is assessed in this study. A statistically constrained minimum-mean-square error estimator (MIMSEE) that minimizes the effects of linearization errors and instrumentation noise is developed and compared to the conventional least-squares error estimator (LSEE). The MIMSEE requires a priori signal and noise information. The statistical constraint signal information was obtained from a priori knowledge of the physiologically allowed range of regional resistivities. The noise constraint information was obtained from a priori knowledge of the linearization error and the instrumentation noise. The torso potentials were simulated by employing a three-dimensional canine torso model. The model consists of four different conductivity regions: heart, right lung, left lung, and body. It is demonstrated that the statistically constrained MIMSEE performs significantly better than the LSEE in determining resistivities. The results based on the torso model indicate that regional resistivities can be estimated to within 40% accuracy of their true values by utilizing a statistically constrained MIMSEE, even if the instrumentation noise is comparable to the measured torso potentials. The errors obtained using the LSEE with the same linearized transfer function and level of instrumentation noise were about five times larger than those obtained using the MIMSEE. For larger measurement errors the MIMSEE performs even better when compared to the LSEE.

Real-time three-dimensional intracardiac echocardiography.

Using catheter-mounted 2-D array transducers, we have obtained real-time 3-D intracardiac ultrasound (US) images. We have constructed several transducers with 64 channels inside a 12 French catheter lumen operating at 5 MHz. The transducer configuration may be side-scanning or beveled, with respect to the long axis of the catheter lumen. We have also included six electrodes to acquire simultaneous electrocardiograms. Using an open-chest sheep model, we inserted the catheter into the cardiac chambers to study the utility of in vivo intracardiac 3-D scanning. Images obtained include a cardiac four-chamber view, mitral valve, pulmonic valve, tricuspid valve, interatrial septum, interventricular septum and ventricular volumes. We have also imaged two electrophysiological interventional devices in the right atrium, performed an in vitro ablation study, and viewed the pulmonary veins in vitro.

Application of sonomicrometry and multidimensional scaling to cardiac catheter tracking.

This paper describes a technique for tracking the three-dimensional (3-D) position of a cardiac catheter using sonomicrometry and the mathematical method of multidimensional scaling (MDS). Sonomicrometry is used to measure the distances between ultrasonic transceivers. MDS is then used to calculate the 3-D coordinates of the ultrasonic transceiver locations, including the catheter tip, from the measured distances. Feasibility of catheter tracking was initially studied using simulated data from a geometric model in which the actual coordinates of all transceivers were known. The method was then shown to be feasible in vivo by tracking a catheter-mounted piezoelectric transducer using seven reference crystals sewn to the epicardial surface of a sheep heart. Simulation results indicate that a catheter can be tracked with a root-mean-square (rms) error of 1.51 +/- 0.05 mm and an average-distance error of e = 1.06 +/- 0.27 mm using 12 reference points. In vivo results showed acceptable stress values (G < 0.05) for 95% of the data samples with an average-distance error of e = 0.52 +/- 0.66 mm. These simulation and experimental results show that sonomicrometry and MDS can be used to accurately localize the 3-D position and track the motion of a catheter tip within the heart.

Temperature-controlled and constant-power radio-frequency ablation: what affects lesion growth?

Radio-frequency (RF) catheter ablation is the primary interventional therapy for the treatment of many cardiac tachyarrhythmias. Three-dimensional finite element analysis of constant-power (CPRFA) and temperature-controlled RF ablation (TCRFA) of the endocardium is performed. The objectives are to study: 1) the lesion growth with time and 2) the effect of ground electrode location on lesion dimensions and ablation efficiency. The results indicate that: a) for TCRFA: i) lesion growth was fastest during the first 20 s, subsequently the lesion growth slowed reaching a steady state after 100 s, ii) positioning the ground electrode directly opposite the catheter tip (optimal) produced a larger lesion, and iii) a constant tip temperature maintained a constant maximum tissue temperature; b) for CPRFA: i) the lesion growth was fastest during the first 20 s and then the lesion growth slowed; however, the lesion size did not reach steady state even after 600 s suggesting that longer durations of energy delivery may result in wider and deeper lesions, ii) the temperature-dependent electrical conductivity of the tissue is responsible for this continuous lesion growth, and iii) an optimal ground electrode location resulted in a slightly larger lesion and higher ablation efficiency.

Registration of three-dimensional cardiac catheter models to single-plane fluoroscopic images.

Transvenous cardiac procedures require accurate positioning of catheters within the geometrically complex cavities of the heart. Recently, nonfluoroscopic catheter tracking technologies have been developed to quantitate the (degrees-of-freedom) three-dimensional positions of intracardiac catheters. This paper presents a projection-Procrustes method to register an animated three-dimensional (3-D) model of multiple intracardiac catheters with a single-plane fluoroscopic image. Applying the computed transformation to the catheter coordinates enables the animated 3-D model of the catheters to be viewed from the same perspective as the fluoroscopic image. Mathematical simulations show that the computed transformation parameters are sensitive to both the position errors in the 3-D catheter coordinates and to the spatial distribution of the catheter-mounted transducers. Simulations with a realistic geometric model of three catheters with four transducers per catheter showed an angular error of 1.91 degrees +/- 0.27 degree for 3-D catheter position errors of 2.0 mm. An in vitro experiment demonstrated the feasibility of the method using a water tank phantom of three catheters and fluoroscopic images taken over an 80 degrees range. The mean angular error was 0.61 degree +/- 0.48 degree. The results of this study indicate that the projection-Procrustes method is a useful tool for registering 3-D catheter tracking models to single-plane fluoroscopic images.

Locating a catheter transducer in a three-dimensional ultrasound imaging field.

Cardiac procedures rely on fluoroscopy for catheter guidance and visualization. However, fluoroscopy provides poor contrast of myocardial structures and exposes both the patient and health care providers to ionizing radiation. As an alternative to fluoroscopy, real-time three-dimensional (3-D) ultrasound imaging has the potential to provide a safe means for tracking catheter position in 3-D while simultaneously imaging the heart's anatomy. A method is described for locating a catheter-mounted transducer in the 3-D ultrasound imaging field. The distance from the imaging transducer to the catheter transducer is measured by time of flight, while the angular position is determined by a spatial crosscorrelation of the received signals with stored receive profiles. Results from simulations with 20-dB SNR demonstrated a mean accuracy of 0.22 +/- 0.13 mm at a 70-mm range. In vitro testing showed a resolution of 0.23 +/- 0.11 mm at a range of 75 mm and a resolution of 0.47 +/- 0.47 mm at a range of 97 mm. With combined catheter position and imaging, this tracking method has the potential to replace fluoroscopy and enhance interventional procedures.

Calculating endocardial potentials from epicardial potentials measured during external stimulation.

This paper presents a boundary integral method for calculating the potential field generated by external stimulation at locations within the heart using realistic heart geometry and samples of the potential taken from the epicardial surface. This method assumes the heart is homogeneous and isotropic. To test the method we made epicardial and endocardial measurements in dogs during transthoracic pacing stimuli. From the epicardial potential measurements we predicted the endocardial potential values and compared them with the measured data. Despite the seemingly gross assumptions, the mean correlation coefficient between the measured and predicted potentials for three dogs and eleven stimulation electrode configurations was 0.985, and the mean rms error was 17%.

Predicting patterns of epicardial potentials during ventricular fibrillation.

Ventricular fibrillation (VF) is a fatal cardiac arrhythmia, characterized by uncoordinated propagation of activation wavefronts in the ventricular myocardium. Short-term predictions of epicardial potential fields during VF in pigs were attempted using linear techniques, and prediction accuracy was measured at various stages during sustained episodes. VF was induced in five pigs via premature electrical stimulation. Unipolar electrograms were recorded from an epicardial array of 506 electrodes in a 22 x 23 array with 1-mm spacing. Optimal spatial basis functions (modes) and time-varying weighting coefficients were found using the Karhunen-Loeve decomposition. Linear autoregressive (AR) models incorporating the dynamics of only a few spatial modes led to predicted patterns that were qualitatively similar to observed patterns. Predictions were made 0.256 s into the future, based on 0.768 s of past data, over an area of approximately 5 cm2 on the ventricular epicardium. The mean squared error of predictions varied from as much as 1.23 to as little as 0.14, normalized to the variance of the actual data. Inconsistency in long-term forcasts is partly due to the limitations of linear AR models. Changes in predictability, however, were consistent. Predictability varied inversely with spatial complexity, as measured by the mean squared error of a five-mode approximation. Predictability also increased significantly during the first minute of VF.

Activation in unipolar cardiac electrograms: a frequency analysis.

We have developed and tested several detectors of local activations in unipolar cardiac electrograms; the detectors are based on the frequency content of the waveforms. For this study, myocardial regions with no local electrical activity were created with cryoablation in canine ventricles, so that the characteristics of electrograms reflecting local activation could be compared with those with only distant electrical activity. For each electrogram, representations of the original signal were created using the output of bandpass filters; for each representation, the value of the maximum amplitude was taken as a measurement of the frequency content of the electrogram in that frequency band. The content of each frequency band and the first derivative of the signal were tested as discriminators between local and distant electrical activity. Combinations of frequency bands were also tested using a logistic regression technique; certain combinations provided better detection than any of the individual frequencies or the first derivative. The inclusion of frequencies between 500 and 1000 Hz improved the detection performance, suggesting that sampling rates of 1000 samples per second or less may not be adequate for optimal discrimination. A detector based on multivariate analysis of different frequency components of a signal may be more effective than single-band filtering in discriminating between local and distant electrical activity in the heart, especially when those components have very different magnitudes.

In vitro temperature map of cardiac ablation demonstrates the effect of flow on lesion development.

This paper presents an in vitro temperature mapping study of bovine cardiac tissue during radiofrequency ablation. The objectives were to: (i) develop a technique for measuring the spatial and temporal temperature distribution in the tissue and in the blood during ablation, and (ii) use the temperature measurements to characterize the effects of fluid flow on lesion dimensions, ablation efficiency, and temperature distributions. In vitro ablation (20 W, 60 s) of bovine cardiac tissue was performed. The tissue was placed in a saline-dextrose solution maintained at 37+/- 0.5 degrees C. The solution also irrigated the tissue surface and simulated blood flow velocities of (i) 30, (ii) 55, and (iii) 85 mm/s. Thermocouple measurements were recorded from 25 and 2 locations in the tissue and in the fluid, respectively. The lowest flow resulted in the largest lesion, the maximum tissue, fluid, and electrode temperature increases, and the highest ablation efficiency. The lesions were 5.8 +/- 0.81, 4.8 +/- 0.84, and 4.4 +/- 1.25 mm deep, and 9.3 +/- 1.07, 7.9 +/- 1.48, and 7.8 +/- 1.27 mm wide for flows (i)-(iii), respectively. The blood and tissue temperature distributions were asymmetric around the ablating electrode axis with higher temperatures on the outflow than on the inflow side. The experimental measurements were used to validate a numeric model of ablation in an accompanying paper.

A three-dimensional finite element model of radiofrequency ablation with blood flow and its experimental validation.

A novel three-dimensional finite element model for the study of radiofrequency ablation is presented. The model was used to perform an analysis of the temperature distribution in a tissue block heated by RF energy and cooled by blood (fluid) flow. This work extends earlier models by including true flow in place of a convective boundary condition to simulate realistic experimental conditions and to improve the prediction of blood temperatures. The effect of fluid flow on the temperature distribution, the lesion dimensions, and the ablation efficiency was studied. Three flow velocities were simulated: (i) 30, (ii) 55, and (iii) 85 mm/s. The modeling results were validated qualitatively and quantitatively with in vitro data. The correlation coefficients between the modeling and the experimental temperature measurements were 0.98, 0.97, and 0.95 for flows (i)-(iii), respectively. The slopes were 0.89, 0.95, and 1.06, and the mean root mean square differences between modeling and experimental temperature measurements were 17.3% +/- 11.6%, 15.8% +/- 13.4%, and 18.8% +/- 14.9% for flows (i)-(iii), respectively. A comparison of temperature distribution obtained with a convective boundary versus inclusion of fluid motion showed that the convective boundary resulted in a similar tissue temperature distribution, but overestimated fluid temperatures and lacked the flow asymmetry seen in the true flow model.