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Biomedical subjects

W M Smith

Publications and source records attributed to W M Smith.

At least 163 records · Page 9Linked to original sources
PubMed1988

Influence of shock strength and timing on induction of ventricular arrhythmias in dogs.

We delivered strong shocks via electrodes on the left ventricular apex and the right atrium in seven dogs during the T wave of atrial pacing while recordings were made from 56 epicardial electrodes. After shocks that induced arrhythmias were given, the earliest activation occurred in the middle of the ventricles for lower-energy shocks and in the base for higher-energy shocks. For shocks late in the vulnerable period, activation was recorded soon after the shock, whereas for shocks early in the vulnerable period activation was not recorded for a mean of 70 ms (+/- 17 ms SD) after the shock. We also gave 1-J shocks during right and left ventricular pacing. For shocks early in the vulnerable period, activation initiating fibrillation arose in a focal pattern from the paced region. For shocks during the midportion of the vulnerable period, fibrillation arose by two leading circle reentrant loops rotating in opposite directions, one on the left and the other on the right ventricle. For shocks at the end of the vulnerable period, the two reentrant loops fused on the side of the heart opposite the pacing site to again form a single focal activation pattern. Thus the initial activation patterns of arrhythmias initiated by shocks, the time from the shock until earliest postshock activation, and the site of earliest postshock activation are strongly influenced by the coupling interval and strength of the shock.

Animals↗
PubMed1988

Epicardial activation after unsuccessful defibrillation shocks in dogs.

To study defibrillation, shocks were given to seven dogs during electrically induced fibrillation, while recordings were made from 56 epicardial electrodes. Shocks were given via electrodes on the left ventricular apex and the right atrium, creating an uneven shock field with much higher potential gradients in the apex than in the base of the ventricles. For unsuccessful 0.01- to 0.05-J shocks, activation occurred soon after the shock at many sites in both the base and the apex. For 0.1- to 0.5-J shocks, the number of early activation sites was greatly decreased, and the latency from the shock until earliest recorded activation was greatly increased at the apex but not at the base. For 1- to 5-J shocks, one to three early sites were present and confined to the base, with a long latency between the shock and the appearance of these early sites. The latency and location of earliest activation were similar to those after 1- to 5-J shocks given to induce fibrillation during normal paced rhythm. Shocks of 10 J successfully defibrillated. These findings suggest that the shock field can have at least three effects. One, a weak field fails to halt the activation fronts of fibrillation. Two, a stronger field halts but then reinduces fibrillation in a manner similar to that of the same strength field during the vulnerable period of normal rhythm. Three, a still higher field halts fibrillation without reinitiating it. Successful defibrillation requires a shock strong enough to create this third field intensity throughout the ventricles.

Animals↗
PubMed1988

Mortality associated with diastolic hypertension and isolated systolic hypertension among men screened for the Multiple Risk Factor Intervention Trial.

The large cohort of white men (317,871) 35 to 57 years old at initial screening for possible enrollment into the Multiple Risk Factor Intervention Trial (MRFIT) was examined with regard to initial blood pressure levels and subsequent coronary heart disease (CHD), stroke, and all-cause mortality. The overall prevalence of isolated systolic hypertension (ISH), defined as systolic blood pressure (SBP) greater than or equal to 160 mm Hg and diastolic blood pressure (DBP) less than 90 mm Hg, was 0.67% among white men screened for MRFIT and increased with age (0.31% among 35- to 39-year-olds to 1.7% among 55- to 57-year-olds). The 6 year CHD and all-cause mortality rates in men over 50 were highest in those with ISH compared with both subjects with diastolic hypertension and those with normal pressure. The relative risk of death from stroke in those with ISH, compared with that in those with SBP less than 160 mm Hg and those with DBP less than 90 mm Hg, was 3.0 (95% confidence interval 1.3 to 6.8). In addition, at any level of DBP, the level of SBP appeared to be the major determinant of all-cause and CHD mortality. The determinants of ISH in individuals under 60 years of age as well as the possible efficacy of its treatment should be evaluated further.

Adult↗
PubMed1988

Mechanism of ventricular vulnerability to single premature stimuli in open-chest dogs.

To determine the mechanism of ventricular vulnerability to electrical stimulation, we simultaneously recorded from 120 transmural electrodes in a 35 X 20 X 5-mm portion of right ventricular infundibulum in seven dogs. Baseline pacing (S1) was performed from outside the mapped region followed by single premature stimulation (S2) of increasing strength at the center of the mapped region. In five of six episodes of ventricular fibrillation and 26 of 30 episodes of repetitive responses, complete reentrant pathways were observed. Earliest activation following S2 was not at the site of S2 stimulation but was at a point between the S1 and S2 sites of stimulation. Activation spread away from the early site toward the opposite side of the mapped region around the sides of an arc of block near the S2 site to form a "figure-of-eight." The activation fronts coalesced to activate the region around the S2 site last and, if the difference in times between activation at the early site and near the S2 site was large, reentered the tissue toward the S1 site. Ventricular refractory periods were determined in four dogs following S1 pacing; the regions with the greatest nonuniformity in the dispersion of refractoriness were not the regions of unidirectional block after S2 stimulation. Thus, 1) ventricular fibrillation and repetitive responses induced electrically with S1 and S2 stimuli at different ventricular sites arise by figure-of-eight reentry, 2) this reentry is caused by the ability of S2 stimulation both to prolong refractoriness near the S2 site and to initiate a propagated response in the region between the S1 and S2 sites, and 3) a nonuniform dispersion of refractoriness is not crucial for the electrical induction of reentry leading to ventricular fibrillation or repetitive responses when S1 and S2 stimuli are given at different locations on the right ventricular outflow tract.

Animals↗
PubMed1988

Transmural activations and stimulus potentials in three-dimensional anisotropic canine myocardium.

Epicardial and endocardial pacing are widely used, yet little is known about the three-dimensional distribution of potentials generated by the pacing stimulus or the spread of activation from these pacing sites. In six open-chest dogs, simultaneous recordings were made from 120 transmural electrodes in 40 plunge electrodes within a 35 X 20 X 5-mm portion of the right ventricular outflow tract during epicardial and endocardial pacing at a strength of twice diastolic threshold and at 1 mA. The magnitude of extracellular potentials generated by the stimulus and the activation times were compared in regions proximal (less than 10-12 mm) and distal to the pacing site. Local fiber orientation was histologically determined at each recording electrode. For endocardial pacing, endocardial potentials were larger than epicardial potentials only in the proximal region (p less than 0.001); while in the distal region, epicardial potentials were larger (p less than 0.001), and endocardial activation occurred earlier than epicardial activation for both regions (p less than 0.001). For epicardial pacing, epicardial potentials were larger than endocardial potentials in both regions (p less than 0.001), and epicardial activation occurred earlier only in the proximal region (p less than 0.02), while endocardial activation occurred before epicardial activation in the distal region (p less than 0.01). In planes of recording electrodes parallel to the epicardium and endocardium, the initial isochrones were elliptical with the major axes of the ellipses along the mean fiber orientation between the pacing site and recording plane rather than along the local fiber orientation in the recording plane. Thus, the ellipses in each plane rotated with respect to each other so that in three dimensions the activation front was helicoid, yet the twist of the helix was less than that of the corresponding transmural rotation of fibers. For pacing from the right ventricular outflow tract, we conclude that beyond 10-12 mm from endocardial and epicardial pacing sites epicardial stimulus potentials in both cases are larger than endocardial potentials because of resistivity differences inside and outside the heart wall and activation in both cases is primarily endocardial to epicardial because of rapid endocardial conduction, and we conclude that the initial spread of activation is helicoid and determined by transmural fiber direction.

Action Potentials↗
PubMed1988

Extracellular field required for excitation in three-dimensional anisotropic canine myocardium.

It is not known how well potential gradient, current density, and energy correlate with excitation by extracellular stimulation in the in situ heart. Additionally, the influence of fiber orientation and stimulus polarity on the extracellular thresholds for stimulation expressed in terms of these factors has not been assessed. To answer these questions for myocardium in electrical diastole, extracellular excitation thresholds were determined from measurements of stimulus potentials and activation patterns recorded from 120 transmural electrodes in a 35 X 20 X 5-mm region of the right ventricular outflow tract in six open-chest dogs. Extracellular potential gradients, current densities, energies, and their components longitudinal and transverse to the local fiber orientation at each recording site were calculated from the stimulus potentials produced by 3-msec constant-current stimuli. The resulting values in regions directly excited by the stimulus field were compared with the values in regions not directly excited but activated by the spread of wavefronts conducting away from the directly excited region. Magnitudes of 3.66 mA/cm2 for current density, 9.7 microJ/cm3 for energy, and 804 mV/cm for potential gradient yielded minimum misclassifications of 8%, 13%, and 17%, respectively, of sites directly and not directly excited. A linear bivariate combination of the longitudinal (l) and transverse (t) components of the potential gradient yielded 7% misclassification (threshold ratio t/l of 2.88), and linear combination of corresponding current density components yielded 8% misclassification (threshold ratio t/l of 1.04). Anodal and cathodal thresholds were not significantly different (p = 0.39). Potential gradient, current density, and energy strength-duration curves were constructed for pulse durations (D) of 0.2-20 msec. The best fit hyperbolic curve for current density magnitude (Jm) was Jm = 3.97/D + 3.15, where Jm is in mA/cm2, and D is in msec. Thus, for stimulation during electrical diastole 1) both current density magnitude and longitudinal and transverse components of the potential gradient are closely correlated with excitation, 2) the extracellular potential gradient along cardiac cells has a lower threshold than across cells, while current density thresholds along and across cells are similar, 3) anodal and cathodal thresholds are approximately equal for stimuli greater than or equal to 5 mA, and 4) the extracellular potential gradient, current density, and energy excitation thresholds can be expressed by strength-duration equations.

Action Potentials↗
PubMed1987

Pharmacodynamics and pharmacokinetics of oral pirmenol.

The efficacy, pharmacokinetics, and pharmacodynamics of pirmenol, a class Ia antiarrhythmic agent, were studied in patients with frequent symptomatic premature ventricular complexes (PVCs). Pirmenol was given every 12 hours to eight patients in a dose-ranging protocol, and median PVC suppression of 94% (range 72% to 100%) was achieved. The median effective pirmenol dose was 300 mg/day (range 200 to 500 mg/day), and mean (+/- SD) trough plasma pirmenol concentration at the effective dose was 0.98 +/- 0.29 micrograms/ml. The mean half-life of elimination was 10.5 +/- 2 hours. There was considerable overlap among patients with respect to plasma pirmenol concentration and times at which PVC frequency returned to 25%, 50%, and 75% of baseline during drug washout trials. Altering pirmenol's dose interval (while maintaining a constant daily dose) from 12 to 6 hours did not improve drug efficacy. Pirmenol was given to seven patients for long-term therapy (24 to 44 months). Median PVC suppression at 24 months was 70%. Pirmenol is safe and well tolerated, and it can be administered twice daily for PVC suppression.

Administration, Oral↗
PubMed1987

A new sock electrode for recording epicardial activation from the human heart: one size fits all.

Simultaneous recording of epicardial activation from multiple sites during open heart surgery is essential for studying unstable ventricular arrhythmias. A previously described sock electrode array for this purpose requires custom-woven nylon sock material and expensive machined button electrodes. The limited compliance and elasticity of nylon requires that a new sock be individually fitted for each heart. Despite careful fitting, 17-20% of electrodes do not make satisfactory epicardial contact in dogs. Further, electrodes frequently dislodge from the sock and wires break at the button electrode solder joint. Recognizing these limitations, we formed a new sock from Xspan tubular dressing material and devised electrodes that attach securely to the sock. In six dogs, 90% +/- 3% of electrodes made satisfactory contact using the same Xspan sock, significantly (p less than .01) more than with the nylon sock despite far less labor. The same size X span sock with 60 snap electrodes was used to record from 27 human hearts of widely different dimensions. Satisfactory epicardial contact was obtained in 90% +/- 14% of electrodes in the 18 patients with Wolff-Parkinson-White syndrome (WPW) and 75% +/- 15% of electrodes in the nine patients with coronary artery disease. In no case did an accessory pathway fail to conduct following sock placement. The hemodynamic effect of the Xspan sock was evaluated in four dogs and was found to be minimal. Both the Xspan sock and the snap electrodes are easily made from inexpensive, readily available materials. The same Xspan sock accommodates a wide range of heart sizes, and the electrodes supported by the Xspan sock record significantly better and with less dislodgement and wire breakage than previous socks.

Animals↗
PubMed1987

Simultaneous multichannel cardiac mapping systems.

There is much current interest in simultaneous multichannel cardiac mapping. In this paper we give recommendations for the construction of a cardiac mapping system. Because the field of cardiac mapping is relatively young, optimum mapping techniques and all possible applications have not yet been developed. Therefore, the mapping system should be flexible and it should have many capabilities. The system should be digital; if variable gains are used, the amplifiers should be programmable and controlled by a microprocessor. It should be possible to analyze previous recordings and acquire additional recordings simultaneously. The mapping system should be able to record continuously for at least tens of minutes and preferably for hours. The recorded data stream should be a self-contained unit, holding all important electrophysiologic information as well as the recorded electrode signals. The programs should be written in C under a UNIX operating system. A minimum of 64 channels should be used for epicardial or endocardial mapping and a minimum of 128 channels for three-dimensional intramural mapping. The leakage current requirements for multichannel mapping systems are too stringent and should be re-evaluated. The major limitation to progress in cardiac mapping is neither the hardware nor the software; it is the electrode: its construction, its placement, its fixation, and the interpretation of its recordings.

Cardiac Pacing, Artificial↗
PubMed1986

Computer-assisted intraoperative mapping of the entire ventricular epicardium in the Wolff-Parkinson-White syndrome.

Intraoperative mapping with a hand-held, roving electrode requires a sustained rhythm lasting 5 to 10 minutes. To overcome this limitation, a computerized mapping system that records from 60 epicardial electrodes simultaneously was used to study 16 patients with Wolff-Parkinson-White syndrome. A sock containing 6 rows of electrodes arranged concentrically from base to apex was place over the ventricles. The total time from placing the sock to analyzing the most basal row of electrode recordings was 5 minutes. A 39 X 44-mm plaque containing 56 electrodes was than placed across the atrioventricular (AV) groove for detailed simultaneous mapping of the ventricle and atrium in the preexcited region identified from the most basal row of sock electrodes. During plaque placement and recording, the remaining sock recordings were analyzed and a complete isochronal epicardial map was drawn. The plaque recordings were then analyzed. This technique rapidly detects early activation at the AV groove as do other computer systems using only a band of electrodes around the AV groove. Also, complete epicardial mapping supplied important additional information. One patient with a posterior paraseptal accessory pathway had ventricular epicardial breakthrough below the strip recorded by the AV band. When more than 1 early activation site was present along the AV groove, complete maps allowed multiple pathways to be differentiated from normal activation fronts ascending from the bundle branches. Complete epicardial maps allowed the study of rapidly changing or short-lived electrical events including isolated premature impulses, initiation and termination of reciprocating tachycardia by pacing, entrainment and changing degrees of fusion created by pacing during reciprocating tachycardia, and ventricular responses during atrial fibrillation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1986

Long-term tolerance of amiodarone treatment for cardiac arrhythmias.

For the 5-year period 1979 to 1983, 242 patients were followed up who had received amiodarone treatment, 156 for supraventricular tachycardia (SVT) and 86 for ventricular arrhythmias. Five patients were lost to follow-up overseas; the rest were followed to cessation of therapy, death or recent review for a mean of 24 +/- 15 months. Male/female incidence was 1.8:1 and mean age was 58 years (range 4 to 88). Half the group had impaired left ventricular function. Adverse effects were recorded in 59% of the patients and led to drug withdrawal in 26% of the total group. In contrast, unsuccessful treatment was the cause of drug withdrawal in only 5% of the patients, although an additional 25% stopped taking the drug for various reasons. Actuarial survival for the whole group, and subgroups with SVT and ventricular tachycardia/fibrillation were 66%, 74% and 52%, respectively, at 50 months. For the whole group, the actuarial probability of being alive and continuing with amiodarone therapy was only 19% at 50 months. Thus, although amiodarone was effective, few patients tolerated the drug on a long-term basis. Although amiodarone remains a valuable treatment for patients with ventricular tachycardia, its long-term effectiveness for patients with SVT is less certain.

Adolescent↗
PubMed1986

Teaching community-oriented primary care (COPC): a practical approach.

To improve education in community-oriented primary care (COPC) and to promote its practice in the community, the University of California's School of Public Health in Berkeley and School of Medicine in San Francisco are collaborating in an innovative program in cooperation with several federally-funded community clinics in the San Francisco Bay Area. The School of Public Health designed a COPC track for graduate public health students from various departments of the school who wished to work in community health care. The track includes a seminar given in the spring of the students' first year in which COPC theory is taught and teams of students working with a faculty advisor and a clinic preceptor design COPC projects for the primary care sites. These projects are then implemented in the summer and fall by students who elect to use this experience to satisfy their fieldwork requirement. This paper is a report of the first year's experience with this collaborative effort.

California↗
PubMed1986

Amiodarone-induced torsades de pointes.

Five cases of amiodarone-induced syncope due to torsades de pointes or ventricular fibrillation are described. Amiodarone was used for recurrent supraventricular tachycardia in four cases and frequent ventricular extra systoles complicating congenital QT prolongation in the remaining case. Each was associated with a marked prolongation in the QTc interval following amiodarone. Three cases had had a previous history of life-threatening ventricular arrhythmias secondary to anti-arrhythmic drugs. Hypokalemia may have been a contributory factor in two. The clinical features, predisposing factors, and treatment are discussed.

Adult↗
PubMed1986

Construction of a multipolar electrode system referenced and anchored to endocardium for study of arrhythmias.

We developed a Teflon plunge electrode system (Teflon plunge) with important advantages over currently used electrodes. The Teflon plunge consists of an anchor-introducer (anchor) attached to the tip of an 0.8-mm diam epoxy-filled Teflon tube supporting six bipolar recording sites. The plunge is inserted through the myocardium into the ventricular cavity perpendicular to the epicardium. Once in the left ventricular cavity the anchor at the tip of the plunge pivots perpendicular to the long axis of the plunge and seats on the endocardium. It is maintained in position with a 4-0 nylon line (line) that extends from its attachment to the anchor through the body of the plunge to the epicardial end where it is secured with a Ligaclip. Thus the electrode contacts are placed a predetermined distance from the endocardium. When the Ligaclip is released, the plunge is removed from the heart leaving the line in the track of the electrode attached to the anchor as a marker for histological studies. The Teflon plunge will facilitate the evaluation of the role of the endocardium in ventricular arrhythmias by locating recording sites a stable, known distance from the endocardium and by marking the electrode track for histological studies.

Animals↗