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Biomedical subjects

M N Levy

Publications and source records attributed to M N Levy.

At least 109 records · Page 6Linked to original sources
PubMed1978

The effect of changing interpulse intervals on the negative chronotropic response to repetitive bursts of vagal stimuli in the dog.

The chronotropic responses to repetitive bursts of vagal stimulation were determined in open-chest, anesthetized dogs. Either 5 or 10 electrical pulses were included in each stimulus burst, and the interpulse interval (deltat) was varied over the range of 5 to 150 msec. As the frequency of the stimulus bursts was progressively changed, the sinoatrial (SA) nodal pacemaker cells became synchronized with the repetitive bursts of stimuli over a certain range of burst frequencies. The magnitude of this frequency range varied with deltat. For 5 and 10 pulses/burst, the values of deltat that produced the greatest magnitude of this frequency range were 30.2 and 24.3 msec, respectively. Also, over the range of values of deltat from 5 to 50 msec, the magnitude of the negative chronotropic effect of the vagal stimulus burst varied directly with deltat. It is likely that, as the interpulse interval is increased within the range of values, either more acetylcholine is released from the vagal nerve endings per pulse or there is less saturation of the receptors on the pacemaker cell membranes during each burst.

Action Potentials↗
PubMed1977

Two variants of concealed trigeminy.

Long rhythm strips were analyzed from three patients with frequent unifocal ventricular extrasystoles. The predominant rhythm in each patient was concealed trigeminy; i.w., the number of conducted sinus beats, S, between extrasystoles satisfied the equation S = 3n --1, where n is any positive integer. In one of these patients, about one-fourth of the values of S did not satisfy this equation; however, all such exceptional values of S satisfied the equation S = 3n. In the other two patients, very few of the exceptional values of S fit the equation S = 3n, but they did conform to the equation S = 3n --2. It is proposed that in all forms of concealed trigeminy, there is a characteristic 3:1 block in the re-entry loop that is responsible for the extrasystoles. Furthermore, it is postulated that immediately after each manifest extrasystole, the 3:1 block is converted transiently to a 4:1 or 2:1 block in the "3n" and "3n--2" variants, respectively. After the first penetration of the block site after a manifest extrasystole, if the re-entrant impulse is concealed, the 3:1 ratio is resumed in both variants until the next manifest extrasystole appears.

Adult↗
PubMed1977

A subvariant of concealed bigeminy.

A subvariant of the "even variant" of concealed bigeminy was studied in two patients. The typical pattern consisted of two types of sequences of conducted sinus beats between extrasystoles. One pattern was bigeminal, i.e., alternating sinus beats and extrasystoles. Between bigeminal sequences were intervals in which there were more than one conducted sinus beat between extrasystoles. In such longer sequences, the numbers of sinus beats were almost invariably even. In the bigeminal sequences, the coupling intervals progressively diminished for each successive extrasystole in the sequence. The proposed explanation for the subvariant was based on a reentry loop in which there were three sites of block: proximal, intermediate, and distal. Block was postulated to take place at the proximal site after those extrasystoles with the shortest coupling intervals, at the intermediate site after odd-numbered conducted sinus beats in the non-bigeminal sequences, and at the distal site ("concealment") after the even-numbered sinus beats in these longer sequences.

Aged↗
PubMed1977

Correlation of the mechanical responses of the heart with the norepinephrine overflow during cardiac sympathetic neural stimulation in the dog.

The changes in heart rate and right ventricular contractile force were measured in anaesthetised dogs during stimulation of each ansa subclavia, and the responses were correlated with the overflow or norepinephrine (NE) into the coronary sinus blood. The increase in heart rate was considerably greater during stimulation of the right than of the left ansa subclavia. Conversely, left ansa stimulation had a slightly greater effect on right ventricular contractile force than did right-sided stimulation. The changes in norepinephrine overflow into the coronary sinus blood paralleled the alterations in contractile force; during stimulation at 2 and 4 Hz, the rates of norepinephrine overflow were 50 and 34% greater, respectively, with left-sided than with right-sided stimulation. On both the right and left sides, stimulation of the anterior limb of the ansa subclavia produced greater increments in heart rate and contractile force than did stimulation of the posterior limb. Similarly, the rates of norepinephrine overflow during anterior ansal stimulation were 100 and 75% greater than during posterior ansal stimulation for the right and left sides, respectively. Thus, both limbs of the two ansae subclaviae innervate the myocardial regions drained by the coronary sinus, but the greatest innervation arises from the anterior limb on the left side.

Animals↗
PubMed1977

Profound reflex bradycardia produced by transient hypoxia or hypercapnia in man.

A quadraplegic patient was able to breathe adequately for the first 3 days after his cervical spinal cord was transected by a bullet. However, for several days thereafter spontaneous ventilation became inadequate, most likely due to edema of the cord, and mechanical ventilation was required. When the ventilator was disconnected for required tracheal aspiration, before there was any tracheal stimulation, profound bradycardia, hypotension and syncope occurred within a few seconds. The bradycardia could be diminished by atropine and its onset delayed by prior ventilation with oxygen. Because of the rapid onset of the hemodynamic changes and their correlation with relatively small changes in PaO2 and PaCO2, it is suggested that this vagally mediated bradycardia represents the primary cardiac reflex response to peripheral chemoreceptor stimulation, which, although extensively studied in experimental animals, has not before been described in man. Just as in animals, it seems likely that the inability to hyperventilate permitted the primary cardiac reflex to occur rather than the usual response of tachycardia to chemoreceptor stimulation which is prepotent with spontaneous ventilation. An understanding of this reflex was important in the clinical mangement of this patient.

Adult↗
PubMed1977

Improving the fracture resistance of dental ceramic.

A time-temperature sequence for optimally strengthening dental ceramic by ion exchange was investigated. Stress relaxation occurs at higher temperatures and/or longer times. Improved strenghts are obtained in a reasonable period of time that could be used for clinical application.

Dental Porcelain↗
PubMed1976

Progressive reduction in norepinephrine overflow during cardiac sympathetic nerve stimulation in the anaesthetized dog.

The left ansa subclavia was stimulated at supramaximal intensity at a frequency of Hz in open-chest, anaesthetized dogs for periods of about 10 min. Arterial blood pressure, myocardial contractile force, and coronary sinus blood flow reached peak values after 1 or 2 min of stimulation, and then declined gradually throughout the remainder of stimulation. In hearts that averaged 159 +/- 10 (SEM) g, the norepinephrine (NE) overflow rose to a peak value of 470 +/- 128 ng/min at the end of 1 min of sympathetic stimulation in the control animals. By the 10th min of stimulation, the NE overflow had diminished to 97 +/- 24 ng/min. In animals pretreated with cocaine (5 mg/kg) or with phenoxybenzamine (5 mg/kg), the peak NE overflow rates were 288 +/- 62 and 980 +/- 148 mg/min, respectively, and the rate of NE overflow declined to levels of 104 +/- 39 and 128 +/- 30 ng/min, respectively by the 10th min of stimulation. Since there was a profound, progressive diminution in the rate of NE overflow during sustained cardiac sympathetic stimulation regardless of whether or not NE reuptake was suppressed, it is concluded that the reduction in NE overflow reflects a curtailment of neuronal release of NE rather than an acceleration of reuptake.

Animals↗
PubMed1976

Effect of ischaemia on overdrive suppression in isolated, blood-perfused atrial preparations of dogs.

The magnitude of overdrive suppression of the SA nodal pacemaker activity was markedly influenced by its ischaemia which was produced by occlusion of the perfusion tubing close to the sinus node artery in the in-situ and isolated canine sino-atrial node preparations. In all isolated preparations, postdrive suppression was significantly augmented after occlusion of the perfusion system for at least 120 seconds. It is concluded that total ischaemia depresses sino-atrial node function in the atropinized canine heart and that postdrive suppression is a good index of such impaired function.

Animals↗
PubMed1976

Effect of carotid sinus nerve stimulation pattern on cardiorespiratory responses.

The reflex responses to steady and intermittent stimulation of the carotid sinus nerve (CSN) were compared in anesthetized dogs. Intermittent stimulation was less effective than steady stimulation in reducing the arterial blood pressure, and the disparity was exaggerated after acute sinoaortic denervation. With the sinoaortic nerves intact, at low mean stimulation frequencies the heart rate responses were greater during intermittent than during steady CSN stimulation. At higher mean stimulation frequencies, however, steady CSN stimuli were more effective than were the intermittent type. After sinoaortic denervation, steady stimuli evoked greater heart rate responses than did intermittent stimuli over the entire mean frequency range studied. Reflex changes in respiratory depth and frequency were also greater during steady than during intermittent CSN stimulation. The greater efficacy of steady than of intermittent stimulation in evoking.the observed reflex cardiovascular and respiratory changes is probably ascribable to the pronounced frequency limitation at the first synapse of the baroreceptor reflex in the brain.

Animals↗
PubMed1976

Effect of vagal stimulation on the overflow of norepinephrine into the coronary sinus during cardiac sympathetic nerve stimulation in the dog.

In anesthetized dogs with the chest open, supramaximal stimulation of the left cardiac sympathetic nerves at 2 and 4 Hz produced an increase of 40-50% in ventricular contractile force (CF) and of 40-65% in coronary sinus blood flow. At these frequencies of stimulation, norepinephrine (NE) overflow into the coronary sinus was 29.8 +/- 5.1 (SE) and 54.9 +/- 13.2 ng/min, respectively. Concurrent, supramaximal vagal stimulation, at a frequency of 15 Hz, had no significant effect on coronary sinus blood flow, but caused a 25% reduction in CF and a 30% decrease in NE overflow. The changes in CF and NE overflow evoked by vagal stimulation were prevented by atropine. These results are consistent with the hypothesis that there are muscarinic receptors on the postganglionic sympathetic terminals in the walls of the ventricles. Acetylcholine released during vagal stimulation combines with these receptors, causes a reduction in the liberation of NE, and thereby attenuates the positive inotropic response.

Animals↗
PubMed1976

The effect of the pattern of cardiac sympathetic activity on myocardial contractile force and norepinephrine overflow in the dog heart.

The left of right cardiac sympathetic nerves in open-chest, anesthetized dogs were stimulated at mean frequencies of 2 or 4 Hz. The stimuli were applied intermittently, in patterns with repetition rates or either 60/min or 15/min, to simulate the spontaneous patterns of sympathetic neural activity that occur synchronously with the cardiac or respiratory cycles, respectively. With either repetition rate, intermittent stimulation of the left sympathetic nerves was about 10-20% less effective in enhancing myocardial contractile force (CF) and about 10% less effective in increasing coronary sinus blood flow than was steady stimulation at the same mean frequency. With right-sided stimulation, there was no appreciable difference between steady and intermettent stimulation patterns with respect to the effect on heart rate. With either left-or right-sided stimulation, the rate of norepinephrine (NE) overflow into the coronary sinus blood was 20-40% less with intermittent than with steady stimulation. Cocaine administration did not materially affect this difference in NE overflow. It was concluded that the higher instantaneous frequencies that prevail during intermittent stimulation result in a reduction in the rate of NE release at the sympathetic postganglionic nerve endings in the heart.

Animals↗
PubMed1976

Effects of temperature on norepinephrine-induced sinus acceleration and overdrive suppression in the isolated dog atrium.

Using an isolated, blood-perfused atrium preparation, the effects of temperature on SA nodal pacemaker activity were investigated in 9 preparations. The PP interval decreased as temperature was raised. Regular sinus rhythm and atrial contraction were maintained above approximately 26 degrees C. Below 26 degrees C, sinus depolarization still showed a regular rate, although atrial contractions had ceased. At about 24 degrees C, atrial rhythm became irregular. Below 20 degrees C, atrial depolarization disappeared. Chronotropic responses to norepinephrine were suppressed at decreased temperatures, not only with respect to maximum PP shortening but also to the threshold dose for inducing sinus acceleration. Overdrive suppression was not influenced significantly by decreasing temperature. These results indicate that a temperature decrease causes suppression of SA nodal pacemaker activity, although the SA node continues to function regularly until about 25 degrees C.

Animals↗
PubMed1976

Chronotropic responses to experimental ischemia of the canine sino auricular node.

(1) Using isolated, blood-perfused atrium preparation of dogs, the effect of ischemia on sinus rate was studied in ten preparations. Cessation of atrial perfusion usually induced gradual deceleration of the sinus rate which was not blocked by atropine. Occasionally, brief and slight sinus acceleration was initially observed in three of ten atrium preparations. This positive chronotropic effect was not blocked by a beta-adrenoceptor blocking agent, propranolol. (2) In every preparation, just after release of occlusion, there was an initial profound sinus deceleration, occasionally followed by oscillatory changes in sinus rate. (3) The chronotropic response pattern induced by temporary occlusion and release of the sinus node artery was not influenced by propranolol, phenoxybenzamine or atropine treatment. (4) These results suggest that ischemia exerts its principal effect directly on the sino auricular node pacemaker cells, rather than on neighboring nerve endings.

Acetylcholine↗