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Biomedical subjects

D M Mirvis

Publications and source records attributed to D M Mirvis.

At least 127 records · Page 7Linked to original sources
PubMed1981

Isoarea distributions during ectopic stimulation of isolated rabbit hearts.

These studies were designed to explore the effects of ectopic left ventricular stimulation on the area distributions recorded during the QRS, ST-T and QRST intervals. Twenty rabbit hearts were suspended in a spherical volume conductor, perfused with an electrolyte solution and stimulated from an anterior and/or a posterior subepicardial left ventricular site. ECG signals were sensed from 32 surface electrodes and were processed to compute areas under the three intervals, to construct area maps and to fit the computed areas to a single moving dipole equivalent cardiac generator model. Results demonstrated that control distributions were overtly nondipolar in 62.5% of cases. Second, ectopic pacing increased the dipolarity of the QRS and ST-T intervals significantly by both dipole ranging and isoarea methods. All area distributions became dipolar. Last, the QRST distribution response to ectopic stimulation varied widely, with major as well as minor changes in patterns occurring with alteration in excitation sequence. Thus, (1) ectopic stimulation changed electrical field properties of mean cardiac forces, and (2) QRST area distributions were dependent upon activation sequence. Each of these observations has direct bearing upon the ventricular gradient concept.

Animals↗
PubMed1981

Body surface distributions of repolarization potentials after acute myocardial infarction. III. Dipole ranging in normal subjects and in patients with acute myocardial infarction.

Dipole ranging is a method to determine the location, strength and orientation of the heart vector from body surface measurements. This technique was applied to electrocardiographic data recorded from forty-five normal subjects and from twenty patients suffering an acute myocardial infarction. Moments and locations during the QRST interval were determined by the potential integration formulae of Gabor and Nelson. Results in the normal population showed stability of both variables during cardiac recovery. In subjects with acute infarction, dipole loci were stable during much of the ST-segments of loci compatible with the anatomic positions of the lesions. Thus, dipole ranging methods can provide physiologically relevant information when applied to human electrocardiographic problems.

Electrocardiography↗
PubMed1981

Body surface distributions of repolarization potentials after acute myocardial infarction. II. Relationship between isopotential mapping and ST-segment potential summation methods.

Isopotential body surface distribution derived from 14 subjects with acute anterior and 14 patients with acute inferior myocardial infarction were studied to examine three possible difficulties with ST-segment summation methods as used for evaluation of infarction severity. First, recorded sums of potentials recorded from 150 electrodes placed on anterior and posterior thoracic surfaces and from five subsets of 42 electrodes located only on the left anterior precordium were compared. Lead subsets detected only one-half of total thoracic positivity in acute anterior infarction and the sums of potentials in these sets were significantly altered by small (approximately 1-1.5-inch) deviations in grid location. Second, differing isopotential distributions could yield nearly identical sums of potentials in left precordial electrode grids. Third, the effects of the wide variations in the normal ST-segment isopotential distribution were evaluated by studying 45 normal subjects and application of difference map and departure map and departure map techniques. The results illustrate the significant variation in potentials considered to be generated by the ischemic lesion caused by differences in control patterns. Thus, the recording of potential sums over limited torso areas, the attention to potential magnitudes rather than to distributions and the assumption that the normal ST segment is isoelectric represent significant problems in applying ECG methods to the quantitative evaluation of myocardial infarction.

Acute Disease↗
PubMed1980

Body surface distribution of exercise-induced QRS changes in normal subjects.

Previously reported clinical studies have suggested an important diagnostic role for exercise-induced QRS changes. To explore the spatial features of such alterations, anterior thoracic isopotential distributions were examined at rest and at 85 percent of maximal heart rate in 30 normal male volunteers. Results demonstrated that exercise caused a reduction in positive forces and an augmentation of negative voltages at most anterior thoracic sites. However, the locations of map extrema were not significantly altered. Difference maps, computed by subtracting time-aligned distributions at rest and during exercise, suggested that exercise generated an electrical field characterized by a central anterior minimum bordered superiorly by positive potentials. The minimum reached a peak magnitude in the middle portion of the QRS pattern, then declined in intensity. Although difference patterns during inscription of the QRS complex were similar to those computed during the P-R and S-T segments, time-intensity curves were discontinuous with marked and abrupt early increases and late decreases in strength in the QRS complex. These results suggest that two commonly considered mechanisms (effects of changes in intracavitary blood mass and of atrial repolarization) cannot be solely responsible for the observed effects.

Adult↗
PubMed1980

Comparison of isopotential surface mapping and dipole ranging methods for assessing equivalent cardiac generator properties.

The ability of two methods of processing surface electrical potentials to detect nondipolar cardiac activity were compared. These were: 1) visual inspection of surface isopotential contours; and 2) quantitative determination of the cardiac equivalent generator properties of the sensed field using a single moving dipole construct. Data were derived from 26 isolated rabbit heart preparations contained within a spherical test chamber. Results demonstrated that: 1) overtly dipolar isopotential surface patterns were frequently associated with poor quantitative fit of surface potentials by the SMD, with up to 33.3% of recorded summed square (SSQ) potential remaining as nondipolar residual; 2) nondipolar surface patterns existed with as little as 6% of SSQ potential being unattributable to the SMD; and 3) the relationship between isopotential patterns and quantitated dipolarity varied significantly from animal to animal. Thus, these data suggest significant discrepancies between the methods that must be carefully considered when they are utilised to describe electrical field characteristcis.

Animals↗
PubMed1980

Body surface distribution of electrical potential during atrial depolarization and repolarization.

Limited information is available documenting body surface isopotential distributions during atrial excitation and recovery. To expand the current data base, body surface isopotential maps from 40 normal subjects were examined. Data were acquired at a gain of 10,000 and isopotential distributions constructed at 2-msec intervals from the onset of the P wave to the onset of the QRS complex. During the initial half of the P wave, a left midprecordial maximum dominated the distribution. Negative potentials existed over the upper back. Subsequently, the maximum migrated to the left; negative potentials moved into precordial areas. Near the end of the P wave, the maximum shifted to the left back as a minimum evolved over the midprecordium. This minimum increased in intensity and remained stationary throughout the PR segment. These patterns are consistent with previously reported epicardial records from canine preparations documenting initial right and then left atrial activation, and repolarization beginning before the end of excitation and enveloping much of the posterior atrial epicardium with low-level positive potential. All distributions had but a single maximum and/or minimum, consistent with a single dipole equivalent cardiac generator.

Action Potentials↗
PubMed1980

Body surface distributions of repolarization forces during acute myocardial infarction. I. Isopotential and isoarea mapping.

Although ST-segment abnormalities during acute myocardial infarction are clinically important, the total thoracic distribution of these repolarization potentials has not been reported. To provide this information, 24 patients with acute myocardial infarction were studied. Isopotential body surface maps were constructed from potentials sensed by 150 anterior and posterior electrodes. Patterns from 12 patients with anterior lesions demonstrated the appearance of repolarization potentials 21.3 +/- 4.6 msec before the end of the QRS complex. During the ST segment, potential distributions were characterized by a single anterior maximum that remained fixed in location but increased in intensity as repolarization progressed. Distributions in the remaining subjects with inferior lesions were analogously characterized by (1) the onset of repolarization 34.6 +/- 12.4 msec before termination of the QRS complex and (2) a single anterior minimum located on the left anterior superior thorax, with positive potentials distributed around the lower thoracic margins. These data suggest that electrocardiographic changes after acute myocardial infarction include (1) marked overlap between activation and recovery patterns and (2) isopotential surface patterns with relatively simple topographic configurations, such as expected of a single-dipole equivalent cardiac generator.

Electrocardiography↗
PubMed1979

Experimental evaluation of the mean electrical equivalent generator of the isolated rabbit heart.

Previous data have documented that the instantaneous cardiac equivalent generator is inadequately modeled as a single dipole. The properties of the mean equivalent generator during the QRS, ST-T and QRST intervals were determined in 22 isolated rabbit heart preparations suspended within a spherical volume conductor and perfused with an electrolyte solution. ECG potentials recorded from 32 surface electrodes were processed to compute instantaneous equivalent generator properties at 0.4 to 2.0 msec intervals during QRS and ST-T. QRS and ST-T areas of each electrode waveform were determined and processed to compute the parameters of the mean generator during QRS, ST-T and QRST intervals. A centric dipole fit 77.96, 71.73 and 67.48 percent of the recorded summed square (SSQ) area during these three intervals. Corresponding values for a single moving dipole model were 86.14%, 87.17% and 86.11%. A four element centric multipole series, in contrast, fit over 98% of SSQ area during each interval. Isoarea maps demonstrated multiple maxima and/or minima in 48% of the studied cases. Thus, mean cardiac forces during activation and recovery, as well as the instantaneous counterparts, are inadequately represented as single dipolar generator sources.

Action Potentials↗
PubMed1979

Electrocardiographic QRS changes induced by acute coronary ligation in the isolated rabbit heart.

The electrical fields associated with augmented R- and decreased S-wave amplitudes during acute, severe myocardial ischemia were studied in fifteen isolated rabbit heart preparations. Hearts were suspended in a spherical tank and perfused with oxygenated electrolyte solution. Electrocardiographic signals were recorded from electrodes on the tank's surface and processed by computerized methods. Fifteen minutes after suture ligation of the left anterior descending coronary artery, records from electrodes overlying the lesion demonstrated increased R-wave amplitude and ST-segment elevation. Fitting a single moving dipole to pre- and post-ligation potentials demonstrated that the ligation increased the dipolarity of the field and shifted the terminal QRS dipole to a position topographically related to the location of the ischemic lesion. The effects of injury were further assessed by study of fields computed by subtracting control from post-ligation data. This generated a dipolar field at the instant of maximal dipole moment whose strength directly correlated (r=0.74) with the area of non-perfused epicardium, as determined by post-ligation methylene blue infusion. These results suggest that the electrical fields generated during mid to late dipolarization by coronary ligation are qualitatively similar to those generated during repolarization, and may therefore be of similar clinical and/or experimental value.

Action Potentials↗
PubMed1979

A microprocessor controlled timing device for cardiac electrical stimulation.

A microcomputer controlled device for delivering precisely timed stimuli to cardiac chambers is described. Stimulation patterns are determined by software, rather than hardware design, resulting in enhanced flexibility. Additionally, by providing timing inputs to conventional laboratory stimulators, this instrument can convert these limited use devices into ones capable of generating complex cardiac stimulation patterns at moderate cost.

Cardiac Pacing, Artificial↗
PubMed1979

A system oriented electrocardiographic amplifier.

A low noise, programmable gain amplifier has been developed for acquiring ECG data from a variety of recording environments. Features include: low noise (4muV peak-to-peak) differential inputs; digitally programmable gains (range 100 to 16000) and output DC offsets (range -3.5 to +4 V) controlled via a serial interface; externally selectable low frequency response (0.04 or 0.08 Hz); and an on-board output monitor multiplexer controlled via a second serial interface.

Amplifiers, Electronic↗
PubMed1978

Atrioventricular nodal gap conduction as a manifestation of dual nodal pathways.

Electrophysiologic studies were performed in a 76 year old patient for evaluation of sinus bradycardia. Atrial extrastimuli were induced during sinus rhythm at progressively decreasing coupling (A1-A2) intervals. At an A1-A2 interval of 420 msec, right bundle branch block developed, and at 370 msec conduction failed below the His bundle. When the interval was reduced to 320 msec, conduction resumed with a normal QRS pattern with an abrupt increase in A-H intervals from 165 to 305 msec. These findings are interpreted as type I or atrioventricular (A-V) nodal gap conduction physiologically related to conversion from a rapid to a slow A-V nodal conduction mode.

Aged↗
PubMed1978

Ventricular pre-excitation and prolonged Q-T interval syndromes in a patient with mitral valve prolapse.

A 34-year-old woman with mitral valve prolapse, frequent ventricular dysrhythmias, and chest pain was studied. Surface electrocardiograms demonstrated short PR and prolonged Q-T intervals. A-V nodal conduction times during atrial pacing were characteristic of A-V nodal bypass tract function. The case is thus interpreted to be one of overlap between three syndromes known to predispose to cardiac dysrhythmias, i.e., mitral valve prolapse, Lown-Ganong-Levine syndrome, and prolonged Q-T interval syndrome.

Adult↗
PubMed1978

Harmonic analysis of the left ventricular pressure waveform of the primate.

Most attempts to quantitate myocardial function rely on morphologic features of complex pressure waveforms to reflect the functional properties of the ventricular myocardium. Relationships between waveform components and the function of the organ generating them were examined in 38 rhesus monkeys by harmonic analysis of left ventricular pressure waveforms. In the basal state, harmonic content was closely correlated (r=0.98) with hemodynamic state, as quantitated by heart rate, systolic and diastolic pressures, left ventricular end-diastolic pressure and maximum contractile element velocity. Hemodynamic indices were expressed as significant linear functions of harmonic terms (r=0.71--0.83), in patterns consistent with the principle of superposition. In 20 animals, infusions of dextran, methoxamine, propranolol or ouabain were used to further assess this relationship. Results demonstrated (1) significant correlation between changes in hemodynamic and harmonic parameter (4= 0.99), (2) correlations between each harmonic term and the set of hemodynamic indices such that specific terms varied directly with contractility but not with loading, whereas others correlated significantly only with loading, and (3) that the patterns in these correlations were of such specificity as to permit construction of significant discriminant functions (p less than 0.0001) that accurately characterized the pharmacologically induced hemodynamic change in 85% (56/67) of trials.

Animals↗