Resident hospital doctors' strike.
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Humans↗
Biomedical subjects
W M Smith
Publications and source records attributed to W M Smith.
Defibrillation electrode configurations developed from cardiac mapping that combine biphasic shocks with sequential timing.
Previous canine mapping studies of the transvenous defibrillation lead configuration of right ventricle (RV) to left R2 patch (P) revealed regions of low potential gradient in the left ventricular apex (A) and the right ventricular outflow tract (O). Thus 16 new lead configurations were tested in eight dogs, which incorporated electrodes in A and O to raise the gradient. When used in conjunction with two sequential biphasic shocks, the average defibrillation threshold energy from these configurations was 57% lower than that produced by a single biphasic shock delivered through RV-->P (phase 1 cathode-->anode, p < 0.001). Of the 16 configurations tested, the most effective was RV-->P followed by A-->O. When the shocking order of this configuration was reversed in another eight dogs, no difference in defibrillation efficacy was noted. In individual configurations of RV-->P and A-->O that used a single biphasic shock, defibrillation was not effective. Finally, when two sequential biphasic shocks were delivered to the same two electrodes in seven other dogs, the defibrillation efficacy was low. Thus configurations that use two sequential biphasic shocks can produce low defibrillation thresholds when the shocks are delivered to two different sets of electrodes. The high efficacy may be caused by one shock increasing the potential gradient in regions of low potential gradient that are produced by the other shock.
Animals↗
Adenosine versus verapamil in the treatment of supraventricular tachycardia: a randomized double-crossover trial.
The safety and efficacy of verapamil and adenosine in the acute termination of supraventricular tachycardia were compared in a randomized double-crossover trial. Of 32 eligible patients with either spontaneous or induced narrow complex tachycardia, seven (22%) patients experienced conversion to sinus rhythm with carotid sinus massage. The other 25 patients were randomly assigned to receive either adenosine (n = 14) or verapamil (n = 11). Relative drug efficacies were 100% for adenosine versus 73% for verapamil, p = NS. Adenosine given at less than or equal to 120 micrograms/kg caused conversion in 12 (86%) of 14 patients. The other two patients required 20 mg adenosine for conversion. After conversion the systolic blood pressure increased significantly in the adenosine group but not in the verapamil group. Reinitiation of tachycardia occurred in two (14%) of 14 patients randomized to the adenosine group. Serious adverse hemodynamic effects were observed in one (9%) of 11 patients randomized to verapamil. The incidence of conversion arrhythmias was similar in both treatment groups (adenosine 57%, verapamil 50%, p = NS).
Adenosine↗
Epicardial mapping of ventricular tachycardia induced by a large premature stimulus over nontransmural infarcts.
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Animals↗
High-current stimuli to the spared epicardium of a large infarct induce ventricular tachycardia.
BACKGROUND: Previous studies have demonstrated that both ventricular tachycardia (VT) and ventricular fibrillation (VF) may begin as figure-eight reentry: VT with a longer cycle length from spared tissue adjacent to an infarct by programmed stimulation and VF with a shorter cycle length from noninfarcted tissue by a large premature S2 stimulus. These results suggest that the type of tissue or cycle length of the arrhythmia rather than the mode of induction determines whether the figure eight becomes sustained VT or degenerates into VF. Thus, a protocol similar to that by which a VF threshold is determined may induce VT rather than VF when performed in the spared tissue over an infarct. METHODS AND RESULTS: In 10 dogs, 4 days after occlusion-reperfusion of the left anterior descending coronary artery, 10 S1 stimuli were delivered from a total of 34 right and left ventricular sites outside the infarct. An epicardial S2 stimulus over the infarct was increased in 10-mA steps and introduced in diastole at decreasing cycle lengths of 5 msec until VT or VF was induced. Sustained monomorphic figure-eight VT was induced from 24 S1 sites and VF from nine (p = 0.03). The mean cycle lengths for the initial six arrhythmic cycles was 152 +/- 33 msec for VT and 115 +/- 13 msec for VF (p less than 0.001). Mean transmural infarct extent was 80% in five dogs with only VT, 63% in three dogs with both VT and VF, and 15% in two dogs with only VF. Different morphologies of VT were induced by changing the S1 site, the S2 strength, or the S1S2 coupling interval. In 25 of the 34 arrhythmias, the central part of the initial figure-eight pathway was oriented opposite the S1 activation sequence in that region. CONCLUSIONS: A large S2 stimulus over a nontransmural infarct induces VT if the spared myocardium is thin. This study introduces a useful technique for inducing sustained monomorphic VT in which the location and direction of the figure-eight pathway are known a priori and in which different morphologies of sustained VT can be produced by changing the S1 site.
Animals↗
Cardiac potential and potential gradient fields generated by single, combined, and sequential shocks during ventricular defibrillation.
BACKGROUND: Potential gradient field determination may be a helpful means of describing the effects of defibrillation shocks; however, potential gradient field requirements for defibrillation with different electrode configurations have not been established. METHODS AND RESULTS: To evaluate the field requirements for defibrillation, potential fields during defibrillation shocks and the following ventricular activations were recorded with 74 epicardial electrodes in 12 open-chest dogs with the use of a computerized mapping system. Shock electrodes (2.64 cm2) were attached to the lateral right atrium (R), lateral left ventricular base (L), and left ventricular apex (V). Four electrode configurations were tested: single shocks of 14-msec duration given to two single anode-single cathode configurations, R:V and L:V, and to one dual anode-single cathode configuration, (R+L):V; and sequential 7-msec shocks separated by 1 msec given to R:V and L:V (R:V----L:V). Defibrillation threshold (DFT) current was significantly lower for R:V----L:V than for the other configurations and markedly higher for L:V. Despite these differences, the minimum potential gradients measured at DFT were not significantly different (approximately 6-7 V/cm for each electrode configuration). Potential gradient fields generated by the electrode configurations were markedly uneven, with a 15-27-fold change from lowest to highest gradient, with the greatest decrease in gradient occurring near the shock electrodes. Although gradient fields varied with the electrode configuration, all configurations produced weak fields along the right ventricular base. Early sites of epicardial activation after all unsuccessful shocks occurred in areas in which the field was weak; 87% occurred at sites with gradients less than 15 V/cm. Ventricular tachycardia originating in high gradient areas near shock electrodes followed 11 of 67 successful shocks. CONCLUSIONS: These data suggest that 1) defibrillation fields created by small epicardial electrodes are very uneven; 2) achievement of a certain minimum potential gradient over both ventricles is necessary for ventricular defibrillation; 3) the difference in shock strengths required to achieve this minimum gradient over both ventricles may explain the differences in DFTs for various electrode configurations; and 4) high gradient areas in the uneven fields can induce ectopic activation after successful shocks.
Animals↗
Three-dimensional potential gradient fields generated by intracardiac catheter and cutaneous patch electrodes.
BACKGROUND: Defibrillation may be improved if electrode configurations can be found that create a larger and more even voltage gradient field across the heart. This study determined the magnitude of the shock gradient fields generated by four nonthoracotomy electrode configurations for defibrillation. METHODS AND RESULTS: In six dogs, a catheter was inserted containing a right ventricular apical electrode (V) and a right atrial electrode (A). A cutaneous patch electrode (P) was placed on the left lateral thorax. Shock potentials were recorded simultaneously from 128 electrodes in the left ventricular and right ventricular subepicardium and subendocardium, ventricular septum, and atria. With the chest closed, 50-mA shocks were given during diastole via the following lead configurations: V----A (V, cathode; A, anode); V----P; V----A+P; and V+A----P. Potential gradients were calculated at the subepicardium and subendocardium in millivolts per centimeter per volt of shock. In most dogs, the V----A+P configuration produced higher gradients throughout the ventricles than did V----A, V----P, or V+A----P. The maximum potential gradient was smaller for the V+A----P configuration than for V----A, V----P, or V----A+P. The gradient fields for the configurations with the catheter alone or combined with P were uneven. CONCLUSIONS: It is possible to estimate shock gradient fields in three dimensions. Of the four configurations tested, V----A+P produced the highest gradients and V+A----P produced the lowest high gradient. The gradient fields were uneven throughout the ventricles.
Animals↗
Effect of pulse separation between two sequential biphasic shocks given over different lead configurations on ventricular defibrillation efficacy.
BACKGROUND: Two sequential biphasic shocks delivered over separate lead configurations markedly improve defibrillation efficacy compared with a single shock alone. We investigated the effect of varying the intershock interval between sequential biphasic shocks on defibrillation. METHODS AND RESULTS: Defibrillation thresholds (DFTs) were obtained in six dogs for shock separations ranging from 0.2 to 125 msec. The first shock was given from a catheter electrode in the right ventricular apex to a patch on the left lateral thorax; the second was from a small patch on the left ventricular apex to a catheter electrode in the right ventricular outflow tract. When the interval between shocks was less than or equal to 10 msec or greater than or equal to 75 and less than or equal to 125 msec, the mean DFTs were less than that previously found for the first shock by itself (4.2 versus 7.4 J, p = 0.002). At a separation of 50 msec, however, there was a marked rise in the DFT to 27 J. The mean DFT for the second shock at a delay of 50 msec was not different from the mean DFT previously found for the second shock by itself (7.2 versus 7.0 J). These results were confirmed in another six dogs using defibrillation probability-of-success curves. In 12 other dogs, probability-of-success curves were generated for delays between shocks as a percentage of the activation interval during ventricular fibrillation. Minimum defibrillation energy requirements were at two separations, 0.2 msec and 90% of the activation interval. CONCLUSIONS: The optimal intershock interval between two sequential biphasic shocks is either less than or equal to 10 msec or greater than or equal to 75 and less than or equal to 125 msec. The marked rise in the DFT at a shock separation of 50 msec, requiring more energy than that for the first shock alone, suggests that the second shock at this time delay is likely to reinduce fibrillation after it is halted by the first shock until the second shock is strong enough to defibrillate independently of the first shock.
Animals↗
Effect of field stimulation on cellular repolarization in rabbit myocardium. Implications for reentry induction.
We have investigated the effects of electric field stimulation on membrane repolarization in rabbit papillary muscles and assessed the consequences of these effects for the dispersion of intracellular potentials and the production of a propagation wave front or unidirectional block in relatively refractory tissue. The stimuli studied had electric field strength of 0.25-14 V/cm, duration of 2 msec, and field orientation along or across the myocardial fibers. The field strengths to excite the muscles in diastole were 0.68 or 1.23 V/cm for stimuli oriented along or across the fibers, respectively (p less than 0.01, along versus across). A 2.5-V/cm stimulus given near the end of the action potential (AP) produced either no response or, after increasing the stimulus delay only 2-3 msec, a full response with almost no AP durations that were intermediate. For stimulation along and across the fibers, respectively, given at 70% of the AP duration, a 4-V/cm stimulus produced AP prolongation (measured at 90% repolarization) of 20% and 4% (p less than 0.05), an 8-V/cm stimulus produced AP prolongation of 36% and 20% (p less than 0.05), and a 14-V/cm stimulus produced AP prolongation of 36% and 30% (p = NS). For either orientation, AP prolongation by stimuli of 8 V/cm or 14 V/cm increased gradually as the stimulus delay was increased. The different effects in relatively refractory tissue of stimuli of 2.5 V/cm compared with 8 V/cm can explain the propagation wave front and block that occur with electrically induced functional reentry in the heart.(ABSTRACT TRUNCATED AT 250 WORDS)
Action Potentials↗
Existence of both fast and slow channel activity during the early stages of ventricular fibrillation.
Although sodium channels have been reported to be inactive after 5-10 minutes of ventricular fibrillation (VF), their state during early VF is unknown. In 12 open-chest dogs, a floating glass microelectrode was used to record intracellular action potentials from the right ventricle during pacing and during electrically induced VF. Before any drug was administered, an initial episode of VF was continuously recorded for at least 20 seconds followed by defibrillation. Recordings were made during VF episodes after superfusion for 15 minutes around the microelectrode site by low (2.8 x 10(-5) M) and high (10(-4) M) concentrations of tetrodotoxin (TTX) in five dogs, or by low (4 microM) and high (100 microM) concentrations of verapamil in another four dogs. In three dogs, VF was induced without drugs three times to determine if the effects observed in the previous dogs were caused by the drugs or by successive episodes of VF. Ten consecutive action potentials were analyzed at the onset and after 5, 10, 15, and 20 seconds of VF. Action potential amplitude and duration during paced rhythm or VF were not changed by the local perfusion of either TTX or verapamil. In the TTX group, the maximum upstroke rate of depolarization of an action potential (Vmax) during paced rhythm was 104 +/- 14 V/sec for control cycles before any drug was given, 86 +/- 15 V/sec for the low TTX concentration, and 55 +/- 14 V/sec for the high TTX concentration (p less than 0.05 versus other two). Vmax decreased from 55 +/- 32 V/sec at the beginning of VF to 37 +/- 27 V/sec after 20 seconds of VF for predrug VF, from 39 +/- 20 V/sec to 18 +/- 11 V/sec for low-dose TTX VF, and from 18 +/- 13 V/sec to 12 +/- 7 V/sec for high-dose TTX VF (p less than 0.05 among the three groups). In the dogs receiving verapamil, VF was still inducible with Vmax not significantly different from predrug VF at the onset and after 5 or 20 seconds of VF but with Vmax smaller (p less than 0.05) for verapamil than for predrug VF after 10 or 15 seconds of VF. In three dogs, Vmax was not significantly different during three successive episodes of VF when no drug was given between the episodes.(ABSTRACT TRUNCATED AT 400 WORDS)
Action Potentials↗
Visualization of bioelectric phenomena.
Biomedical investigators are currently able to acquire and analyze physiological and anatomical data from three-dimensional structures in the body. Often, multiple kinds of data can be recorded simultaneously. The usefulness of this information, either for exploratory viewing or for presentation to others, is limited by the lack of techniques to display it in intuitive, accessible formats. Unfortunately, the complexity of scientific visualization techniques and the inflexibility of commercial packages deter investigators from using sophisticated visualization methods that could provide them added insight into the mechanisms of the phenomena under study. Also, the sheer volume of such data is a problem. High-performance computing resources are often required for storage and processing, in addition to visualization. This chapter describes a novel, language-based interface that allows scientists with basic programming skills to classify and render multivariate volumetric data with a modest investment in software training. The interface facilitates data exploration by enabling experimentation with various algorithms to compute opacity and color from volumetric data. The value of the system is demonstrated using data from cardiac mapping studies, in which multiple electrodes are placed in an on the heart to measure the cardiac electrical activity intrinsic to the heart and its response to external stimulation.
Algorithms↗
Surgical correction of the Wolff-Parkinson-White syndrome.
The results of corrective surgery in 75 consecutive patients with Wolff-Parkinson-White (WPW) syndrome are reported. There were 47 male and 28 female patients with a median age of 27 years. Intraoperative mapping disclosed 88 accessory pathways, of which 83 were successfully divided at the primary operation without mortality. Two patients underwent successful reoperation at three days and two years respectively. Patient success rates were 93% and 96% for first and total operations. Complications, usually minor, occurred in 20 patients, including permanent pacemaker implantation in two. Surgical correction of WPW syndrome is recommended as a safe alternative to lifelong medical treatment.
Adolescent↗
The use of benazepril in hypertensive patients age 55 and over.
Benazepril, a newer angiotensin-converting enzyme inhibitor, has been evaluated for the treatment of mild to moderate hypertension in patients 55 years of age and older. The results of the clinical trials conducted to date indicate that benazepril provides effective antihypertensive therapy in this population, with efficacy comparable to that demonstrated in younger patients. Benazepril does not produce precipitous decreases in diastolic blood pressure following the initial dose, and is well tolerated by the elderly. It has a safety profile similar to that of placebo and generally better than that of hydrochlorothiazide.
Aged↗
Prevalence of postural hypotension at baseline in the Systolic Hypertension in the Elderly Program (SHEP) cohort.
OBJECTIVE: The objective of this study was to examine the prevalence and correlates of postural hypotension (defined as a drop in systolic blood pressure of greater than or equal to 20 mm Hg) in a cohort of elderly persons with isolated systolic hypertension (ISH). DESIGN: Baseline cross-sectional analysis of the 4,736 persons randomized in the Systolic Hypertension in the Elderly Program (SHEP). SETTING: A randomized multi-center double-blind outpatient clinical trial of the impact of treating ISH. PARTICIPANTS: Men and women age greater than or equal to 60 years with the systolic blood pressure (SBP) greater than or equal to 160 mm Hg and diastolic blood pressure (DBP) less than 90 mm Hg. MEASURES: Medical histories were obtained using interviewer-administered, standardized clinical history forms. At entry into the study, seated and standing BP was measured by certified BP technicians using a random zero sphygmomanometer. Postural hypotension (PH) was assessed at 1 and 3 minutes after the participant arose from a seated position. MAIN RESULTS: PH was found in 10.4% of participants at 1 minute and in 12.0% of participants at 3 minutes. 5.3% of participants demonstrated PH at both time intervals while 17.3% demonstrated PH at either or both of the time intervals. Factors significantly (P less than 0.05) associated with the presence of PH were higher mean SBP and a lower mean body mass index. CONCLUSIONS: Somewhat different persons were defined as having PH based upon the 1 minute and 3 minute standing measures of BP, and prevalence estimates of PH can vary depending on whether one or more intervals of measurement are used. Cross-sectional data analysis indicated that PH, in healthy community-dwelling older persons with ISH, may not be associated with a history of disorders or problems usually thought to be related to PH. However, prospective data are needed to determine the prognostic significance of PH, and whether one or multiple measurements carry more significance.
Aged↗
Dispersion of repolarization induced by a nonuniform shock field.
Dispersion of repolarization may contribute to arrhythmias. To determine whether an electrical field stimulus (S2) with a nonuniform potential gradient can induce a dispersion of repolarization, we applied 5 ms rectangular S2 that had a nonuniform or uniform potential gradient during the action potential (AP) of bathed frog ventricular strips. One group had a partitioned bath to produce a nonuniform S2 of 39 +/- 11 V/cm (mean +/- SD) in one half of the 1 x 6 mm strip (H) and 0.3 +/- 0.2 V/cm in the other half (L), and simultaneous intracellular AP recordings in H and L with glass microelectrodes positioned 1.4 +/- 0.4 mm apart. Another group had uniform S2 and an AP recorded near the center of the strip. S1 pacing at 0.5 Hz was performed at one end of the strip and conduction along the strip was monitored. In each experiment, the S2 trials had an S1-S2 interval of 300 ms so that S2 was given during an AP (shocked AP). In both H and L, nonuniform S2 produced cumulative shortening of paced APs and lengthening of each shock AP compared with the paced AP preceding it. Uniform S2 of 1 V/cm did not shorten the paced APs or lengthen the shocked APs indicating that the AP changes in L were not due to the small potential gradient in L. Before beginning nonuniform S2 trials, the AP duration determined at the maximum repolarization rate was 601 +/- 72 ms in H and 602 +/- 71 ms in L (P = ns). During 13-20 nonuniform S2 trials over a 60-80 minute period, paced APs were shortened to 490 +/- 51 ms in H and 515 +/- 39 ms in L while each shocked AP was lengthened, compared with the paced AP preceding it, to 636 +/- 40 ms in H and 561 +/- 21 ms in L (P less than 0.05). Therefore, paced APs after shocks repolarized 25 ms earlier in H than in L and shocked APs repolarized 75 ms later in H than in L. The results show that during the shortened AP in H, the AP in L is shortened, which is consistent with intracellular current from L to H during repolarization. During the prolonged AP in H, the AP in L is prolonged compared with the paced AP preceding it, consistent with intracellular current from H to L during repolarization. Thus, nonuniform shocks can induce a dispersion of repolarization and may induce cell-to-cell interactions during repolarization.
Action Potentials↗
Defibrillation with low voltage using a left ventricular catheter and four cutaneous patch electrodes in dogs.
The purpose of this study was to determine a lower limit of defibrillation thresholds (DFTs) that could be used to evaluate nonthoracotomy lead configurations for implantable defibrillators. A lead configuration that consisted of a left ventricular catheter and four circumferential cutaneous patches was tested because it was hypothesized to create a relatively uniform electric field for defibrillation. In eight anesthetized dogs, three 8F defibrillating catheters with 6 cm platinum clad titanium tips were inserted into the right ventricle (R), right ventricular outflow tract (O), and left ventricle (L). Four cutaneous patch electrodes (4P), each with a surface area of 42 cm2, were placed on the left lateral, right lateral, anterior and posterior thorax. DFTs for ten lead configurations, consisting of different combinations of these electrodes, were evaluated. DFTs were determined by using a modified Purdue technique and applying a single capacitor biphasic shock with both phases 6 ms in duration after 15 sec of electrically induced fibrillation. The L(-)----4P+ configuration produced a lower DFT than R(-)----4P+ (3.2 +/- 1.6 J vs 8.0 +/- 4.2 J, P less than 0.001) with reduced current (2.6 +/- 0.7 A vs 4.1 +/- 1.2 A, P less than 0.001). Lowering the impedance by a mean of 40%, configurations that used four patches produced lower DFTs than those that used a single left lateral patch. The use of an O catheter produced lower DFTs only when used in conjunction with an R catheter.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Current concepts for selecting the location, size and shape of defibrillation electrodes.
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Electric Countershock↗
Response of relatively refractory canine myocardium to monophasic and biphasic shocks.
BACKGROUND: Certain biphasic waveforms defibrillate at lower energies than monophasic waveforms, although the mechanism is unknown. METHODS AND RESULTS: The relative ability of monophasic and biphasic shocks to stimulate partially refractory myocardium was compared because defibrillation is thought to involve stimulating relatively refractory myocardial tissue. Shocks of 25-125 V were given during regularly paced rhythm in 11 open-chest dogs. Computerized recordings of shock potentials, and of activations before and after the shocks, were made at 117 epicardial sites. To quantify the shock field strength, the shock potential gradients were calculated at the electrode sites. Monophasic action potential (MAP) electrode recordings, obtained in five dogs, confirmed direct myocardial excitation by the shock, that is, activations beginning during the shock. Tissue was directly excited up to 4 cm from the shocking electrode, and the area directly excited increased as the shock was made stronger or given less prematurely. In six dogs, strength-interval curves for direct excitation were determined from plots of potential gradient versus refractoriness at each electrode site. The biphasic curves were located to the right of the monophasic curves by 8 +/- 4 msec, indicating a lesser ability to excite refractory myocardium. When the gradient at the directly excited border was at least 3.8 +/- 1 V/cm, conduction failed to propagate away from the directly excited zone after the shock, and MAP recordings made near the border showed a shock-induced graded response. This graded response, which prolonged repolarization, may have been responsible for the failure of conduction from the directly excited zone. Although better for defibrillating, the biphasic waveform was thus less effective than the monophasic one in exciting relatively refractory myocardium. CONCLUSIONS: These results indicated that waveform selection for defibrillation should not be guided solely by the ability of the waveform to stimulate tissue, as these two properties can be discordant.
Action Potentials↗