See Also: Biography of Barouh Berkovits
The Wedensky Effect in the Human Heart
Artificial electrical stimulation has proved to be of great value in understanding electrophysiological events occurring in the human heart. Progress in the clinical arrhythmias had been made either by interpolation from animal experiments or by careful analysis of certain tracings in which the various parameters could not be controlled at will. Several authors have shown the presence of a supernormal phase and of a vulnerable phase under controlled conditions in man. One mechanism that has been implicated in the origin of arrhythmias is the Wedensky effect, first described in neuromuscular studies as a prolonged lowered threshold of excitability induced by strong stimulus; that under certain conditions one impulse could trigger a second impulse without invoking vulnerability or supernormality. And that such a phenomenon, and not a re-entry mechanism, could adequately explain the origin of extrasystoles occurring late in the cardiac cycle. Extrasystoles were due primarily to a disturbance of cardiac excitability, not of conductivity. 
FIGURE 3.
Exposure of the Wedensky effect in the human heart during intermittent paired electrical stimulation. Numbers indicate interspike (S†-S†) intervals in hundredths of a second. The first step in this process consists in identifying the supernormal phase. The driving rate was around 60 per minute. Numbers in the top strips indicate the distance between driving and testing stimulus artifacts. The latter did not stimulate the ventricles when occurring at intervals shorter than 0.425 sec or longer than 0.615 sec. Hence, testing stimuli produced a propagated response only during the supernormal phase, that is, towards the end of the T wave and slightly afterwards. In a second step, the intensity of the testing stimuli was lowered below the level required to detect super-normality. Note that when the intensity of the driving stimuli was increased from twice to 15 times above diastolic threshold, the second (previously subthreshold) impulse is now able to produce a propagated response. This is a classic Wedensky effect.
Castellanos A, Lemberg L, Johnson D, Berkovits BV. The Wedensky Effect in the Human Heart. Brit Heart J 1966:28:276-283 Back to Top
Atrial Synchronized Pacemaker Arrhythmias: Revisited
Synchronized pacemakers have been used successfully in the treatment of Stokes-Adams attacks. Their major advantage lies in the correction of the A-V block by artificially restoring physiological atrioventricular conduction. Several complex arrhythmias have appeared after its implantation, during its normal behavior as well as during its malfunction. In some cases, the ectopic rhythms were iatrogenically created with the purpose of studying the response of the pacemaker. Four possible mechanisms by which synchronized stimulus artefact could appear during an antecedent T wave were presented. Differences between true and false escapes were stressed. One tracing showed a rare form of pacemaker capture by a high A-V nodal rhythm. In another patient, the synchronized pacer was captured by a continuous unit. An increase in rate of the latter led to a previously undescribed arrhythmia, hereby labeled pacemaker-to-pacemaker block. In this case, there were two different centers competing to capture the synchronized pacemaker. 
FIGURE 6.
Three different mechanisms by which the stimulus artefacts can follow the extrasystolic R waves: (a) V-P synchronization (Ex.1); (b) A-P synchronization by a sinoatrial contraction preceding or merging with the ectopic ventricular contraction (Ex.2); (c) A-P synchronization by the retrograde atrial activation produced by an ectopic ventricular contraction (Ex. 3). The numbers below the extrasystoles indicate their order of appearance in the strip. "A" represents atrial contractions of sinoatrial origin except for A-, which is produced by the retrograde conduction of one of the extrasystoles.
Castellanos A, Lemberg L, Rodriguez-Tocker L, Berkovits BV. Atrial Synchronized Pacemaker Arrhythmias: Revisited. Amer Heart J 1968; 76:199-208 Back to Top
Pacemaker Vectorcardiography
The spatial vectorcardiograms of 20 patients with implanted pacemakers were studied. Ten had left ventricular and 10 right ventricular units. The orientation of the stimulus artifacts did not prove to be an important parameter in the distinction between each type of instrument. The spikes appeared as a sharp deflection which lasted more than the accepted value of 2.5 msec. Initial delays, attributed to conduction through ordinary cardiac muscle, was seen in all patients with left ventricular and in 8 of the patients with right ventricular pacemakers. The morphology of the loops elicited by left ventricular stimulation resembled vectorcardiograms considered diagnostic of right bundle branch block with or without right ventricular hypertrophy. Beats originating in the right ventricle yielded vectorcardiograms similar to those seen in cases of left bundle branch block. 
FIGURE 3.
Right ventricular endocardial pacemaker. In the vectorcardiogram the stimulus artefacts are oriented to the left, superiorly and slightly anteriorly. The QRS loop shows an initial delay associated with a spatial (two plane) mid delay or plateau. Similar types of loops have been described in patients with W-P-W coexisting with left bundle branch block. There is left axis deviation in the electrocardiogram.
Castellanos A, Lemberg L, Salhanick L, Berkovits BV. Pacemaker Vectorcardiography. Amer Heart J 1968;75:6-18. Back to Top
Munk-Gaskell Phenomenon
Induction of repetitive responses originating in an area other than the one subjected to artificial stimulation was reported by Munk in 1878. This author observed that after placement of the second ligature of Stannius in the heart of the frog, one single mechanical stimulus applied to the upper border of the ventricles produced a series of contractions which initially were rapid, but later became slower. In 1900 Gaskell showed that the secondary responses originated in the A-V junction.
The Munk-Gaskell phenomenon can be produced easily in the amphibian and reptile heart. Figure 1 was recorded after the ventricles had stopped following application of the second ligature of Stannius. The small, upwards-directed atrial deflections are seen most clearly at the beginning of the strip. Thereafter, five mechanically induced ventricular extrasystoles are followed by five spontaneous extrasystoles originating in another center. Note that the rate of the latter rhythm diminishes gradually.
Castellanos A, Johnson D, Berkovits BV. Significance of multiple responses produced by electrical depolarization of the heart. Acta Cardiologica 1966;21:157-166.
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