Biomedical subjects

M N Levy

Publications and source records attributed to M N Levy.

At least 127 records · Page 7Linked to original sources

PubMed1975

Chronotropic response to acetylcholine injected into the sinus node artery of the isolated atrium of the dog.

Injection of a single bolus of acetylcholine into the sinus node artery of the isolated atrium of the dog produces a triphasic response similar to that previously observed after a single stimulus to the vagus nerves. There is an initial, brief but pronounced, cardiac deceleration followed by a brief, slight acceleration, which is then followed by a more prolonged deceleration which is of lesser magnitude than the initial deceleration. The most likely cause of the intermediate phase of cardiac acceleration is a direct effect of acetylcholine on the pacemaker cells, as it is also with the primary and tertiary phases of cardiac deceleration.

Acetylcholine↗

PubMed1975

Three variants of concealed bigeminy.

Long electrocardiographic strips were analyzed from five patients who exhibited periods of typical "concealed bigeminy," i. e., recurrent unifocal extrasystoles which were separated from one another by odd numbers of normally conducted sinus beats. However, in each of these patients, there were periods in which one of three different variants of concealed bigeminy was observed. Three patients displayed an "even number" variant; i. e., there were large numbers of consecutive extrasystoles which were separated exclusively or preponderantly by even rather than by odd numbers of sinus beats. One other patient exhibited an "interpolated extrasystole" variant: those interectopic intervals which were initiated by an interpolated extrasystole contained an even number of sinus beats, whereas all other interectopic intervals contained an odd number. In the fifth patient, the distribution of the numbers of sinus beats separating extrasystoles was such as to suggest a periodic fluctuation between the classical forms of concealed bigeminy and concealed trigeminy; i. e., a "combined bigeminy and trigeminy" variant.

Aged↗

PubMed1975

The postponed compensatory pause as a manifestation of positive feedback in atrioventricular conduction.

A patient with hypertensive cardiovascular disease was found to have unusual varieties of premature atrial and ventricular contractions. If the premature atrial contraction resulted in a greatly prolonged P-R interval, such that the increment in P-R exceeded the decrement in the preceding R-P, the next sinus P wave, occurring after a normal P-P interval, was found to be blocked. Also, numerous interpolated ventricular extrasystoles were observed in which the postextrasystolic P-R intervals were markedly prolonged and in which the compensatory pauses were postponed for one or two beats. We also present data from one dog in which a premature atrial activation produced a chain reaction such that complete A-V block occurred three beats later. We propose that the chain reaction which evoked the delayed block in the dog and the postponed compensatory pauses in the patient reflects the operation of a positive feedback mechanism in A-V conduction. Positive feedback is initiated by an extremely long P-R, which results in a very short R-P before the next cycle. This then leads to a still longer P-R, which then elicits a still shorter R-P. Block ultimately supervenes when the atrial activation wave arrives at the A-V junction during its effective refractory period.

Aged↗

PubMed1975

Ventricular quadrigeminy as a manifestation of concealed bigeminy.

Long rhythm strips were analyzed from five patients with frequent ventricular extrasystoles. The predominant pattern was quadrigeminal; i.e., three sinus beats between extrasystoles. However, about 20% of the interectopic intervals contained numbers of sinus beats (S) greater than three. Analysis of the distribution of such values of S greater than 3 revealed that there were many more odd than even values (P less than 0.001). Also, carotid sinus pressure yielded only odd values of S greater than 3. This predominance of odd values strongly suggested the existence of concealed extrasystoles. Therefore, all odd values of S greater than 3 were analyzed to determine whether they satisfied the criterion for concealed bigeminy (S = 2n - 1) or for concealed quadrigeminy (S = 4n - 1). The distribution was found to satisfy the criterion for concealed bigeminy, suggesting that the quadrigeminal pattern was a manifestation of a 2:1 rather than a 4:1 block in a re-entry loop. Stable quadrigeminy occurs often in concealed bigeminy, because the re-entrant impulse finds the myocardium excitable after a normal R-R interval but refractory after a compensatory pause.

Adult↗

PubMed1974