Dependence of apparent resistance of four-electrode probes on insertion depth

The apparent resistance of a finite-thickness layer measured with a four-electrode plunge probe depends on the electrode insertion depth, electrode spacing, and layer thickness, as well as the resistivity ratio of an underlying layer. A physical model consisting of air, a saline solution layer, and an agar layer simulates the real situation of resistivity measurement. The saline layer represents the finite-thickness layer whose resistivity is to be measured by a plunge electrode probe, and the agar layer represents an underlying perturbing layer. A micropositioner controls the insertion depth of the four electrodes into the saline solution. With the apparent resistance measured on a semi-infinite-thickness layer of saline solution as standard, measurement results show decreasing apparent resistance and increasing error with increasing electrode insertion depth. This information is important for correct measurement of myocardial resistivity in vivo and in vitro.     

In vitro measurement of myocardial impedivity anisotropy with a miniature rectangular tube

Due to rapid change of fiber orientation, it is difficult to measure myocardial impedivity separately in a longitudinal or transverse fiber direction without mutual influence in the two directions. Previously published values of the longitudinal and the transverse myocardial impedivity were derived indirectly from measurements that mixed the impedivity in all directions. Those values are questionable because the derivations were based on a simplified uniform myocardial fiber model. In this paper, a miniature rectangular tube was devised to facilitate direct measurement of myocardial impedivity in a uniform fiber direction. The average transverse-to-longitudinal ratio of the measured in vitro swine myocardial impedivity was about 1.66 from 1 Hz to 1 kHz and dropped to 1.25 at 1 MHz. The result is important for accurate modeling of the electrical property of myocardium in biomedical research of radio-frequency cardiac catheter ablation.     

Synthesis of Medium Voltage dc-to-dc Converters From Low-Voltage, High-Frequency PWM Switching Converters

Low-voltage dc-to-dc power conversion is a very mature industry which uses high-frequency pulsewidth modulation (PWM) switching techniques. The passive and active components needed to build low-voltage dc-to-dc converters are highly available, affordable and constantly improving. In this paper, a very simple and systematic method of using a large number of low-voltage, high-frequency PWM converters to synthesize highly redundant, medium voltage (4-40 kV) dc-to-dc converters is presented. Theoretical and practical considerations are discussed in necessary detail and test results of an actual 10-kW, 10 kV-to-400 V, converter built from 48 low-voltage forward converters are presented. Real-time and average reduced circuit models are derived to predict the dynamical behavior of the converter and to design the feedback control loop.     

In-vivo measurement of swine myocardial resistivity

We used a four-terminal plunge probe to measure myocardial resistivity in two directions at three sites from the epicardial surface of eight open-chest pigs in-vivo at eight frequencies ranging from 1 Hz to 1 MHz. We calibrated the plunge probe to minimize the error due to stray capacitance between the measured subject and ground. We calibrated the probe in saline solutions contained in a metal cup situated near the heart that had an electrical connection to the pig's heart. The mean of the measured myocardial resistivity was 319 ohm x cm at 1 Hz down to 166 ohm x cm at 1 MHz. Statistical analysis showed the measured myocardial resistivity of two out of eight pigs was significantly different from that of other pigs. The myocardial resistivity measured with the resistivity probe oriented along and across the epicardial fiber direction was significantly different at only one out of the eight frequencies. There was no significant difference in the myocardial resistivity measured at different sites.     

A new catheter design using needle electrode for subendocardial RF ablation of ventricular muscles: finite element analysis and in vitro experiments

Radio-frequency (RF) cardiac ablation has been very successful for treating arrhythmias related with atrioventricular junction and accessory pathways with successful cure rates of more than 90%. Even though ventricular tachycardia (VT) is a more serious problem, it is known to be rather difficult to cure VT using RF ablation. In order to apply RF ablation to VT, we usually need to create a deeper and wider lesion. Conventional RF ablation electrodes often fail to produce such a lesion. We propose a catheter-electrode design including one or more needle electrodes with a diameter of 0.5-1.0 mm and length of 2.0-10 mm to create a lesion large enough to treat VT. One temperature sensor could be placed at the middle of the needle electrode for temperature-controlled RF ablation. From finite element analyses and in vitro experiments, we found that the depth of a lesion is 1-2 mm deeper than the insertion depth of the needle and the width increases as we increase the diameter of the needle and the time duration. We showed that a single needle electrode can produce a lesion with about 10-mm width and any required depth. If a wider lesion is required, more than one needle with suggested structures can be used. Or, repeated RF ablations around a certain area using one needle could produce a cluster of lesions. In some cases, a catheter with both conventional electrode and needle electrode at its tip may be beneficial to take advantage of both types of electrode.     

Randomized comparison of anatomic and electrogram mapping approaches to ablation of the slow pathway of atrioventricular node reentrant tachycardia

OBJECTIVES: The purpose of this study was to prospectively compare in random fashion an anatomic and an electrogram mapping approach for ablation of the slow pathway of atrioventricular (AV) node reentrant tachycardia. BACKGROUND: Ablation of the slow pathway in patients with AV node reentrant tachycardia can be performed by using either an anatomic or an electrogram mapping approach to identify target sites for ablation. These two approaches have never been compared prospectively. METHODS: Fifty consecutive patients with typical AV node reentrant tachycardia were randomly assigned to undergo either an anatomic or an electrogram mapping approach for ablation of the slow AV node pathway. In 25 patients randomly assigned to the anatomic approach, sequential radiofrequency energy applications were delivered along the tricuspid annulus from the level of the coronary sinus ostium to the His bundle position. In 25 patients assigned to the electrogram mapping approach, target sites along the posteromedial tricuspid annulus near the coronary sinus ostium were sought where there was a multicomponent atrial electrogram or evidence of a possible slow pathway potential. If the initial approach was ineffective after 12 radiofrequency energy applications, the alternative approach was then used. RESULTS: The anatomic approach was effective in 21 (84%) of 25 patients, and the electrogram mapping approach was effective in all 25 patients (100%) randomly assigned to this technique (p = 0.1). The four patients with an ineffective anatomic approach had a successful outcome with the electrogram mapping approach. On the basis of intention to treat analysis, there were no significant differences between the electrogram mapping approach and the anatomic approach with respect to the time required for ablation (28 +/- 21 and 31 +/- 31 min, respectively, mean +/- SD, p = 0.7) duration of fluoroscopic exposure (27 +/- 20 and 27 +/- 18 min, respectively, p = 0.9) or mean number of radiofrequency applications delivered (6.3 +/- 3.9 vs. 7.2 +/- 8.0, p = 0.6). With both the anatomic and electrogram mapping approaches, the atrial electrogram duration and number of peaks in the atrial electrogram were significantly greater at successful target sites than at unsuccessful target sites. CONCLUSIONS: The anatomic and electrogram mapping approaches for ablation of the slow AV nodal pathway are comparable in efficacy and duration. If the anatomic approach is initially attempted and fails, the electrogram mapping approach may be successful at sites outside the areas targeted in the anatomic approach. With both the anatomic and electrogram mapping approaches, there are significant differences in the atrial electrogram configuration between successful and unsuccessful target sites.     

Multi-loop control for quasi-resonant converters

A multiloop control scheme for quasi-resonant converters (QRCs) is described. Like current-mode control for pulse width modulation (PWM) converters, this control offers excellent transient response and replaces the voltage-controlled oscillator (VCO) with a simple comparator. In this method, referred to as current-sense frequency modulation (CSFM), a signal proportional to the output-inductor current is compared with an error voltage signal to modulate the switching frequency. The control can be applied to either zero-voltage-switched (ZVS) or zero-current-switched (ZCS) QRCs. Computer simulation is method applied to a ZCS buck QRC. A circuit implementation is presented that allows multiloop control to be used on circuits switching up to 10 MHz. This circuit requires few components and produces clean control waveforms. Experimental results are presented for zero-current flyback and zero-voltage buck QRCs, operating at up to 7 MHz. Good small-signal characteristics have been obtained     

A simple scheme for unity power-factor rectification for highfrequency AC buses

A simple scheme is proposed for offline unity power factor rectification for high-frequency AC buses (20 kHz). A bandpass filter of the series-resonant type, centered at the line frequency, is inserted between the line and the full-wave rectified load. The Q=Z ₀/R_L formed by the load and the characteristic impedance of the tank circuit determines the power factor, the boundary between continuous and discontinuous conduction modes, the peak stresses, and the transient response of the rectifier. It is shown that for Q>2/π the rectifier operates in continuous conduction mode and the output voltage is independent of the load. Also, it is shown that for Q>2 the line current is nearly sinusoidal with less than 5% third-harmonic distortion and the power factor is essentially unity. An increase in Q causes an increase in the peak voltages of the tank circuit and a slower transient response of the rectifier circuit. The DC, small-signal, and transient analyses of the rectifier circuit are carried out, and the results are in good agreement with simulation and experimental results     

Diagnosing syncope. Part 1: Value of history, physical examination, and electrocardiography. Clinical Efficacy Assessment Project of the American College of Physicians

PURPOSE: To review the literature on diagnostic testing in syncope and provide recommendations for a comprehensive, cost-effective approach to establishing its cause. DATA SOURCES: Studies were identified through a MEDLINE search (1980 to present) and a manual review of bibliographies of identified articles. STUDY SELECTION: Papers were eligible if they addressed diagnostic testing in syncope or near syncope and reported results for at least 10 patients. DATA EXTRACTION: The usefulness of tests was assessed by calculating diagnostic yield: the number of patients with diagnostically positive test results divided by the number of patients tested or, in the case of monitoring studies, the sum of true-positive and true-negative test results divided by the number of patients tested. DATA SYNTHESIS: Despite the absence of a diagnostic gold standard and the paucity of data from randomized trials, several points emerge. First, history, physical examination, and electrocardiography are the core of the syncope workup (combined diagnostic yield, 50%). Second, neurologic testing is rarely helpful unless additional neurologic signs or symptoms are present (diagnostic yield of electroencephalography, computed tomography, and Doppler ultrasonography, 2% to 6%). Third, patients in whom heart disease is known or suspected or those with exertional syncope are at higher risk for adverse outcomes and should have cardiac testing, including echocardiography, stress testing. Holter monitoring, or intracardiac electrophysiologic studies, alone or in combination (diagnostic yields, 5% to 35%). Fourth, syncope in the elderly often results from polypharmacy and abnormal physiologic responses to daily events. Fifth, long-term loop electrocardiography (diagnostic yield, 25% to 35%) and tilt testing (diagnostic yield < or = 60%) are most useful in patients with recurrent syncope in whom heart disease is not suspected. Sixth, psychiatric evaluation can detect mental disorders associated with syncope in up to 25% of cases. Seventh, hospitalization may be indicated for patients at high risk for cardiac syncope (those with an abnormal electrocardiogram, organic heart disease, chest pain, history of arrhythmia, age > 70 years) or with acute neurologic signs. CONCLUSIONS: Many tests for syncope have a low diagnostic yield. A careful history, physical examination, and electrocardiography will provide a diagnosis or determine whether diagnostic testing is necessary in most patients.     

Error analysis of tissue resistivity measurement

We identified the error sources in a system for measuring tissue resistivity at eight frequencies from 1 Hz to 1 MHz using the four-terminal method. We expressed the measured resistivity with an analytical formula containing all error terms. We conducted practical error measurements with in-vivo and bench-top experiments. We averaged errors at all frequencies for all measurements. The standard deviations of error of the quantization error of the 8-bit digital oscilloscope with voltage averaging, the nonideality of the circuit, the in-vivo motion artifact and electrical interference combined to yield an error of +/- 1.19%. The dimension error in measuring the syringe tube for measuring the reference saline resistivity added +/- 1.32% error. The estimation of the working probe constant by interpolating a set of probe constants measured in reference saline solutions added +/- 0.48% error. The difference in the current magnitudes used during the probe calibration and that during the tissue resistivity measurement caused +/- 0.14% error. Variation of the electrode spacing, alignment, and electrode surface property due to the insertion of electrodes into the tissue caused +/- 0.61% error. We combined the above errors to yield an overall standard deviation error of the measured tissue resistivity of +/- 1.96%.