Search PubMed⌕ Search

Biomedical subjects

R J Sweeney

Publications and source records attributed to R J Sweeney.

At least 19 recordsLinked to original sources
PubMed1999

Relation of atrial refractoriness to upper and lower limits of vulnerability for atrial fibrillation/flutter following implantable ventricular defibrillator shocks.

BACKGROUND: Implantable ventricular cardioverter defibrillator (ICD) shocks can cause atrial fibrillation/flutter (AF). This study investigated the pathogenesis of AF after ICD shocks in a canine model. METHODS AND RESULTS: The study was conducted in 8 dogs. In 5 dogs (group 1), truncated exponential (8 ms, 78% tilt) monophasic and biphasic shocks were delivered through a bipolar epicardial (patch) or endocardial lead. After the last S1 of atrial pacing at a cycle length of 350 ms, shocks of 0.1 to 7.6 A (0.005 to 27.7 J) were delivered, timed to the atrial effective refractory period (AERP). Ventricular defibrillation thresholds were also determined. In 3 dogs (group 2), the effect of the open versus closed chest technique on AF induction was tested in the endocardial biphasic shock configuration. AF was induced in all 8 dogs and in all waveforms and configurations. Mean AF duration was 11.5+/-6 s, with a mean ventricular rate of 184+/-37 bpm. Ventricular shocks could induce AF only if they were timed between an AERP of -60 to 40 ms, -40 to 60 ms, -40 to 60 ms, and -20 to 60 ms in the epicardial monophasic, epicardial biphasic, endocardial monophasic, and endocardial biphasic configurations, respectively. The mean+/-SD of the upper limit of vulnerability (ULV) for AF induction (in J) was 5. 2+/-0.6, 3.5+/-0.4, 5.2+/-1.2, and 2.5+/-0.1 for the epicardial monophasic, epicardial biphasic, endocardial monophasic, and endocardial biphasic configurations, respectively (P<0.05). The lower limit of vulnerability (LLV) was 0.8+/-0.1, 0.8+/-0.1, 0.9+/-0, and 0.6+/-0 for the epicardial monophasic, epicardial biphasic, endocardial monophasic, and endocardial biphasic configurations, respectively (P=NS). The ventricular defibrillation threshold (in J) for all wave forms and configurations was higher than the ULV (P<0. 05). CONCLUSIONS: (1) An atrial LLV and ULV exist for ventricular ICD shock-induced AF; (2) the shock-induced AF is related to both shock intensity and its timing to AERP; and (3) avoiding this atrial window of vulnerability may minimize the risk of post-ICD shock AF.

Animals↗
PubMed1999

Double pulse transthoracic defibrillation in the calf using percent fibrillation cycle length as spacing determinate.

Recent studies have found that when multiple pulses of energy are used for defibrillation with implantable electrodes, the spacing between these pulses is better determined as a percentage of the fibrillation cycle length (CL), rather than as a fixed function of time. Here, this concept is further tested in the transthoracic defibrillation of calves, which are approximately the size of heavy humans. Eleven 90-110 kg calves (101 +/- 6 kg) were used in evaluating the effectiveness in achieving transthoracic ventricular defibrillation of ten double pulse waveforms (two 50 A 4-ms rectangular monopulses) having leading edge-to-edge spacings of 4 ms (a 50 A 8-ms rectangular monopulse) and 50, 60, 70, 80, 90, 100, 110, 120, 130 percent fibrillation CL, respectively. In each of these waveforms, the total time when 50 A current was flowing (on time) was 8 ms. Our results show an unequivocal adverse interaction between the pulses, when the spacing is around 60%-70% fibrillation CL; but that the two pulses combined to defibrillate as effectively as a single 8-ms pulse when the spacing is around 110%-130% fibrillation CL. Electrocardiographic analysis suggests that the adverse interaction is due to a refibrillation phenomenon. This study confirms that double pulses can interact and have a negative effect on defibrillation efficacy. Our data suggests that the mechanism of this interaction involves the second pulse reinitiating fibrillation when the pulse separation is in a critical range of values. Our results are also compatible with the hypothesis that the spacing of multiple pulses is better determined as a percentage of the fibrillation CL than as absolute time, although more study is necessary to fully test this hypothesis.

Animals↗
PubMed1997

Refractory period extension during ventricular pacing at fibrillatory pacing rates.

Refractory period extension (RPE) has been proposed as a basic mechanism for defibrillation but it remains unclear if RPE exists at the fast rates associated with ventricular fibrillation. In 7 pentobarbital anesthetized dogs, we measured refractory periods with and without 8 ms rectangular transcardiac shocks at left ventricular pacing rates of 200-600 beats/min. To achieve these high rates, an incremental rate pacing method was used to produce pacing train timing sequences requiring 4.5-27 seconds. A variably timed premature stimulus followed the last stimulus in each pacing train. To determine refractoriness, a 128 electrode array (4 x 4 cm) was used to detect the presence, or absence of an activation sequence sweeping away from the pacing site. At each rate, a control refractory period (RPc) was measured and refractory periods were also measured for 8 and 12 V/cm shocks with coupling intervals of 60% to 90% of RPc. RPc decreased as the rate increased with a minimum RPc of 94 ms at a rate of 600 beats/min (100 ms cycle length). RPE/RPc versus shock coupling interval was similar at all pacing rates. RPE/RPc increased with increased coupling interval or higher shock intensity. We conclude that during ventricular pacing at fibrillatory rates tissue is nearly always in a refractory state; that RPE exists at fibrillatory activation rates; and that RPE/RPc versus shock coupling interval does not vary strongly with pacing rate. These findings support the hypothesis that RPE contributes to defibrillation.

Animals↗
PubMed1997

Defibrillation efficacy using high-frequency switching to proportion current among simultaneous shock pathways.

INTRODUCTION: Multiple-pathway electrode configurations generally allow improved current distribution over the heart and lower defibrillation thresholds than single-pathway systems. However, current distributions using multiple pathways are largely determined by electrode type and location. We hypothesized that switching the current among multiple pathways at high frequency (HF) could allow the switching duty cycle to control the proportion of time-averaged current flowing in each pathway, thus permitting altered (possibly improved) defibrillation efficacy using the same electrodes and shock waveform. METHODS AND RESULTS: In dogs, we measured the current (I50) for 50% defibrillation success using catheter electrodes in the right ventricular apex (cathode) and superior vena cava (A-pathway anode) and a subcutaneous patch on the left chest wall (B-pathway anode). In group 1 (N = 7), we measured I50s for shocks that used HF to proportion 10% to 90% of the current to the A-pathway. Shocks with 10% to 30% of the current in the A-pathway had significantly lowr I50s than nonproportioned shocks using all three electrodes. However, the resistance differed among single and simultaneous pathways so energy did not necessarily parallel these changes. In group 2 (N = 6), we measured I50s for shocks to the B-pathway alone, for nonproportioned shocks to A and B, and for shocks that proportioned 80% of the current to the B-pathway using either HF, sequential, or amplitude proportioning methods. All proportioning methods had similar I50s that were significantly lower than the I50 for nonproportioned shocks to A and B and that were comparable to shocks to the B-pathway alone. CONCLUSIONS: Shocks with most current proportioned to the B-pathway had lower defibrillation currents than nonproportioned shocks using both pathways. Thus, defibrillation efficacy was changed by HF proportioning without changing the electrodes or shock waveform. These findings suggest that HF proportioning may be a method to improve defibrillation.

Animals↗
PubMed1996

Refractory interval after transcardiac shocks during ventricular fibrillation.

BACKGROUND: Measurements of refractory period extension by shocks during ventricular pacing at fast rates predict that all tissue should be refractory for a brief interval after shocks during fibrillation. This study experimentally determined whether a refractory interval was present just after a shock during fibrillation. METHODS AND RESULTS: In pentobarbital-anesthetized dogs, rectangular monophasic (4-ms) or biphasic (2.5/1.5-ms) shocks were followed with a 2-ms postshock stimulus (PSS) delivered to the defibrillation electrodes. We measured the effect of PSS on the shock current (I50) required for 50% defibrillation success. In group 1 (n = 6), a 1.0-A PSS had no effect on I50 when delivered up to 35 ms after monophasic shocks but greatly increased I50 when delivered at 50 to 90 ms. A 0.5-A PSS had no effect at any timing. In group 2 (n = 6), we compared 1.0-A PSSs after monophasic and biphasic shocks. The effect of PSS after monophasic shocks was similar to group 1. After biphasic shocks, PSS at the same timings had similar effects but caused even greater increases in I50. CONCLUSIONS: We conclude that after both monophasic and biphasic shocks during fibrillation, there is a postshock interval during which the heart is refractory to the refibrillating effect of PSS. The interval is shorter for biphasic than for monophasic shocks with the same duration and defibrillation efficacy. These findings support the refractory period extension hypothesis for defibrillation and suggest that propagating depolarization activity is absent immediately after defibrillation shocks but that it develops again at the end of the refractory interval or later.

Animals↗
PubMed1996

Effects of flecainide, encainide, and clofilium on ventricular refractory period extension by transcardiac shocks.

OBJECTIVE: The mechanisms by which pharmacological agents alter electrical defibrillation are not fully understood. It has been proposed that, in addition to directly stimulating tissue, defibrillation may involve refractory period extension (RPE) produced by the shock. Accordingly, pharmacological agents might modulate defibrillation by altering RPE. This study examined the effect of Class I and Class III antiarrhythmic agents on RPE by transcardiac shocks. METHODS: In four groups of pentobarbital anesthetized dogs, RPE was measured during rapid ventricular pacing before and after administration of either the Class I agents flecainide (n = 7) or encainide (n = 7), the Class III agent clofilium (n = 7), or vehicle (n = 5). Measurements included QRS duration during sinus rhythm and a conduction time, QTC interval and refractory period, and RPE for 4- to 10-V/cm shocks delivered 20-80 ms before the end of the tissue absolute refractory period. For the 6-V/cm shocks, the interval after the shock during which tissue remained refractory (RIAS) was also computed. RESULTS: Drugs affected QRS duration, conduction time, QTC, and refractory period ( without shocks) in accordance with their anticipated Class I and Class III actions. Without drugs, significant RPE was observed in all animals for all shocks delivered 40 ms or less before the end of the refractory period. Clofilium, encainide, and flecainide had a tendency to increase RPE but only clofilium produced a significant increase. For 6-V/cm shocks with different timings, the minimum RIAS was found to be approximately 43 ms, and occurred for shocks given 20-30 ms before the end of the refractory period. CONCLUSIONS: At drug dosages that produced moderate Class III ( approximately equal to 15%) or strong Class I (approximately equal to 35%) effects, only the Class III agent significantly increased RPE and RIAS. Thus, in addition to altering tissue excitability, the effect of antiarrhythmic agents to increase RPE and the minimum RIAS may help explain their influence on defibrillation threshold.

Animals↗
PubMed1996

Defibrillation using a high-frequency series of monophasic rectangular pulses: observations and model predictions.

INTRODUCTION: Capacitor-discharge type waveforms are practical for defibrillation devices but may not be optimum. Discharging a capacitor as a series of high-frequency (HF) pulses may allow effective waveform shaping by modulating the pulses. This approach could lead to improved defibrillation by allowing waveforms that would otherwise be unachievable with a capacitor-discharge approach. However, little is known about defibrillation with HF. METHODS AND RESULTS: In open chest pentobarbital anesthetized dogs, we measured defibrillation thresholds for continuous rectangular waveforms with 5-, 10-, and 20-msec durations and for 10- and 20-msec long series of HF rectangular pulses. HF series had a 50% "on-time" duty cycle at 100 Hz to 20 kHz. At 1 kHz and above, defibrillation with HF required the same time-averaged current but approximately twice the peak current and energy as defibrillation with continuous waveforms having the same envelope duration. At lower frequencies, defibrillation peak current and energy approached values required for the continuous waveforms. While waveforms were not actually filtered, the heart responded as though the HF series were low-pass filtered. A filtered effective waveform model with a 3.7-msec time constant predicts these HF data and makes reasonable predictions for various continuous waveform shapes. CONCLUSION: Defibrillation is possible using HF pulses up to 20 kHz and has a frequency response similar to a low-pass filter. A filtered effective waveform model predicts these HF results and may help explain how waveforms influence defibrillation efficacy. While the unmodulated HF pulsing used in this study increased defibrillation requirements, these findings support the concept that HF pulse modulation can be used to change the effective shape of a waveform, which could permit more efficacious waveform shapes and a net reduction of thresholds.

Analysis of Variance↗
PubMed1995

Characteristics of multiple-shock defibrillation.

INTRODUCTION: A new method for defibrillation allows two shocks to be combined to defibrillate with reduced current by adjusting their separation according to the cycle length of the fibrillation event. We investigated various aspects of this new method to better understand its characteristics and applicability to defibrillation. METHODS AND RESULTS: In 48 pentobarbital-anesthetized dogs, we measured the current for 50% defibrillation success using the new method with sequences of rectangular shocks. Group 1 studied the role of shock total duration and found that two-shock sequences followed a strength-duration curve similar to, but below, that for single shocks. Group 2 studied the role of amplitude and duration balance between shocks and found that two-shock sequences with equal shocks performed best. Group 3 studied whether the new method could be used with either biphasic waveforms or sequential shock pathways. Current reduction for the combined methods equaled the product of current reduction by each method, demonstrating that these methods can be effectively combined. Group 4 extended the method to include three-shock and four-shock sequences and found that a fourth shock did not further improve defibrillation. The optimum three-shock sequence required 33% lower current (P < 0.002) and 34% greater energy (P < 0.095 = NS) than a single shock. CONCLUSIONS: The new method allows defibrillation to be distributed over several fibrillatory cycles and has an improved strength-duration relationship. Two- or three-shock sequences using equal shocks permit a substantial reduction of defibrillation current that can be combined with the reduction for biphasic and sequential methods. Thus, the method may have application in low-current defibrillation devices.

Animals↗
PubMed1995

Countershock strength-duration relationship for myocardial refractory period extension.

OBJECTIVES: To determine the strength-duration relationship for refractory period extension (RPE) in order to understand better the influence of shock waveform on RPE. METHODS: In six open-chest pentobarbital-anesthetized dogs, the RPE was measured by rectangular transcardiac shocks that produced 2- to 32-V/cm local voltage gradients at the measurement site. At each intensity, measurements were made for shocks with 2- to 32-msec durations delivered 30 msec before the end of the tissue refractory period. RESULTS: These shocks produced up to 40% RPE. The RPE varied strongly with shock intensity and duration, with more RPE for stronger or longer shocks. At 32 V/cm, early portions of the shock waveform contributed most to RPE. At 8 and 16 V/cm, later portions made relatively larger contributions that were still smaller than those of the early portions. At 4 V/cm, the contributions to total RPE were spread over the entire waveform. At 2 V/cm, shocks failed to produce significant RPE. CONCLUSIONS: For rectangular shock waveforms, the relationship between RPE and duration is approximately linear at low intensity, but at higher intensity greater RPE is produced by earlier, rather than later, portions of the waveform. This may be because RPE by early portions of the waveform changes the effective timing in the refractory period for later portions of the same waveform. These results provide new insight into the possible role of waveform on defibrillation efficacy.

Animals↗
PubMed1994

Double-pulse defibrillation using pulse separation based on the fibrillation cycle length.

INTRODUCTION: We investigated a method of defibrillation in which two shocks were delivered to the same electrodes with a separation based on the cycle length of the fibrillation event (FCL). METHODS AND RESULTS: In pentobarbital anesthetized dogs, a computerized system determined the FCL from the fibrillation event, computed the desired double-pulse (DP) shock separation, and immediately delivered the DP shocks. In group 1, energy for 50% success at defibrillation (E50) was measured using separations from 55% to 95% of the FCL and remeasured after administration of flecainide, clofilium, or vehicle to change the FCL. Both drugs increased FCL by approximately 25%. Plots of E50 versus %FCL aligned before and after drug showed that the optimum pulse separation followed the FCL. In group 2, E50s were measured for 55% to 185% FCL separations in clofilium or vehicle-treated animals. The optimum DP E50 was at 85% FCL and was not significantly different from the single-shock E50. In group 3, no differences were found when comparing the probability of success versus total energy relationships for single and optimum DP shocks. Group 4 compared E50s for single and DP shocks using a single-catheter configuration and three-electrode configurations (catheter-subdermal patch). E50s for single and DP shocks were equal using the single-catheter configuration but DP shocks required approximately 20% more energy in the three-electrode configurations. CONCLUSION: In single-pathway lead configurations, two shocks with 85% FCL separation can be reliably combined to defibrillate using the same total energy as a larger single shock. Since the energy is unchanged but the total duration is doubled, DP shock currents are reduced by 20% to 30%.

Animals↗
PubMed1993

The defibrillation threshold: a comparison of anesthetics and measurement methods.

In 18 open-chested mongrel dogs (18.0 +/- 1.7 kg) we compared three anesthetics and three methods for measuring the defibrillation threshold. Six animals were anesthetized with pentobarbital (30 mg/kg) and maintained with a pentobarbital infusion (4 mg/kg per hour). All other animals were anesthetized with sodium brevital (10 mg/kg) and maintained with either halothane gas (1.5%, N = 6) or isoflurane gas (1.8%, N = 6). In each dog, we measured the energy required for 50% successful defibrillation (E50) with: (A) a 3 reversal--up/down method; (B) a 15 shock--up/down method; and (C) a percent success method. Anesthetics and methods were selected in a balanced random order. Ventricular fibrillation was induced with 50 Hz electrical pacing. After 15 seconds, monophasic truncated exponential shocks were delivered by way of a spring-patch electrode configuration. The animal was rescued (if needed) and fibrillation/defibrillation episodes were repeated at 3 minute intervals. After each determination of the E50, the E50 was delivered in ten successive defibrillation trials to determine its actual success rate. We found no significant difference in E50 among anesthetics; a significant difference (P < 0.05) in E50 between method C (9.7 +/- 2.6 joules) and method B (8.2 +/- 1.6 joules); no significant difference among anesthetics or methods for the actual success rate of the E50 (45 +/- 21% successful); and method A required significantly fewer fibrillation episodes and number of shocks and less cumulative energy than the other methods. We concluded that the anesthetics tested had little effect on E50 but that the method used to determine E50 could have an effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthetics↗
PubMed1991

Characterization of refractory period extension by transcardiac shock.

BACKGROUND: To better understand the refractory period extension (RPE) produced by transcardiac shocks and its possible role in defibrillation, we measured RPE under various experimental conditions. METHODS AND RESULTS: Using ventricular pacing in pentobarbital-anesthetized dogs, we characterized RPE in relation to the anatomic site of pacing, the local voltage gradient (LVG) produced by the shocks at the pacing site, and the pacing rate and pacing current used to make the measurements. We also determined if RPE persisted into the next refractory period after the shock and measured RPE at the end of 30-second episodes of acute ischemia to the pacing site, which were caused by occluding the left anterior descending artery. Each anatomic site tested showed RPE, which increased sharply with increasing LVG at lower levels but less sharply at higher LVG. The RPE versus LVG was approximated with an exponential curve that had an exponential constant of about 5-6 V/cm. At faster pacing rates, RPE occurred earlier in the refractory period but was unchanged when expressed as a percent increase of refractory period. RPE did not vary with the pacing current and was present only in the refractory period during which the shock was delivered. The RPE was not significantly altered by acute ischemia. These results show that transcardiac shocks selectively extend the refractory period of tissue proportional to the LVG and the timing of the shock in the refractory period. They are consistent with the concept that RPE prevents depolarization from tissue directly excited by a shock from propagating to tissue that was refractory to that same shock. CONCLUSIONS: The insensitivity of RPE to short ischemic episodes and the presence of RPE at increased activation rates suggest that RPE might exist under conditions of fibrillation and be a major determinant of the success or failure of defibrillation.

Acute Disease↗
PubMed1990

Ventricular refractory period extension caused by defibrillation shocks.

In pentobarbital-anesthetized dogs, transcardiac shocks of up to 30 J or pacing stimuli were delivered to myocardial tissue at different times in the electrical cycle. When delivered midway or later into electrical systole, shocks, but not pacing stimuli, greatly extended the refractory period as determined by left ventricular pacing. There was a positive correlation between both the shock energy and timing and the amount of delay. A 30-J shock given 10 msec before the end of the refractory period extended the refractory period by 63 +/- 15 msec (p less than 0.001), whereas the same shock given 40 msec earlier produced only 25 +/- 10 msec (p less than 0.001) of extension. By comparison, a 5-J shock given at those times produced 36 +/- 18 (p less than 0.005) and 10 +/- 8 msec (p less than 0.001) of extension, respectively. When delivered early into electrical systole, both a pacing stimulus and a shock had no substantial effect on the tissue refractory period. Because the tissue that is late in electrical systole would otherwise be the first to repolarize if no shock were given, the selective refractory period extension may create a period after the shock during which no tissue is repolarized to a level sufficient for wavefront propagation. Thus, the energy- and time-dependent refractory period extension may help explain the mechanism by which ventricular defibrillation occurs during transcardiac shocks.

Animals↗
PubMedDate not supplied

System designed to improve communication process between patient and technologist.

With the use of more specialized equipment in our radiology departments, we are able to examine more patients with greater efficiency. However, our role as radiologic technologists is only partially fulfilled if our concern over the number of patients and the functional aspects of the equipment restricts us from adequately communicating with the patient during the radiographic examination.

Communication↗