This experiment was designed to reveal the developing pattern of the ECG occurring in chick embryos during the period between 11 and 22 somites and to relate these findings to the appearance of blood circulation and the ability of the heart to exhibit atrioventricular block under the influence of digitalis. Using a unipolar electrode placed directly upon hearts of a graded series, the first recorded ECG was a simple V wave at 11 somites (sometimes 10) which increased in amplitude and rate through the 16 somite stage. In 16 or 17 somite embryos two deflections appeared on the descending limb of the V wave. In the light of subsequent changes these were recognized as the first indications of the P wave and QRS complex. Within a n hour of development (17 to 18 somites) three events coincided, i.e., appearance of the primitive P wave and QRS complex; the beginning of blood circulation as evidenced by flow through extraembryonic capillaries; and the ability of the hearts to show a block at the AV junction. This last occurs despite the fact that an obvious PR interval is not present at 18 somites. However, from the 18 to the 21 to 22 somite stage rapid changes in the pattern of the ECG were recorded. The PR interval emerges; the P wave and QRS complex become incisive and are followed by a n obvious T wave.The developmental pattern is the exact duplicate of the changes which can be recorded in reverse, by treating a single 60 hour embryonic heart with eserine (Paff, Boucek and Glander, '66) or apparently any acetylcholinesterase inhibitor (work in progress).

Electrical and mechanical events of the atrium, ventricle and conus of the 72-hour chick embryo heart were recorded by a specially designed instrument. The electrical-mechanical delay and the time of muscular contraction differed among the chambers, the atrium being the most rapid and the conus, the slowest. Muscular relaxation times of the atrium and ventricle were approximately one-half that of the conus. Mechanical performance, gaged by the approximations of work and power, was greatest for the ventricle, but based on the muscle mass (protein), values for the ventricle and conus were similar. Electrical activation of the ventricle followed a consistent pathway, suggesting the existence of a preferential conduction system prior to the development of the His bundle.

While studying the electrocardiograms of developing hearts it was noted that an adult type ECG could not be obtained in a n embryonic heart which could not be blocked with digitalis. The need to know more about heart-block was obvious. Intact embryos of 36 to 120 hours were floated on to glass plates and treated with digoxin-Tyrode solution. Before 36 & hours no dissociation of beat could be produced. The heart just stopped. At 40 -C hours a conal block followed by a midventricular block appeared. At 42 % hours conal and midventricular blocks were followed by the first appearance of AV block. It was intermittent. At 47 -C hours conal block jumped to an incisive AV block. Midventricular block was rare. At 72 hours conal and then AV block occurred. Combinations such as four atrial to two ventricular to one conal beat could be readily produced while blood circulated.Electrocardiograms and cinephotomicrographs were taken of 72 and 96 hour hearts. Most striking was the fact that the typical Wenckebach phenomenon of adult heart-block could be duplicated, i.e., an increase of PR interval preceding AV block followed by a shorter PR interval which again gradually increased preceding AV block. This sequence was repeated over and over. A n explanation of these events is offered.Heart-block, defined as defective transmission of impulses occurring in certain important regions of the conduction system (Wiggers, '51), is a well recognized entity accompanying abnormalities of the post-natal heart. Not well known is the fact that it is possible to block the embryonic heart previous to the development of an histologically recognized conduction system. The classical experiments of Erlanger, '06 in which heart-block was produced in the adult dog by compressing the atrioventricular (AV) bundle find their counterpart in the work of Johnstone, '24 in the embryonic chick heart of two, three and four days. By placing a ligature around the heart at any level he demonstrated that the part of the heart cephalic to the ligature beat at a slower rate than that caudal to it. Later Paff and Johnson, '38 described a technique by which blockage of the isolated 48 hour embryonic chick heart could be produced by ouabain. Also Paff, '40 and Lehman and Paff, '42 used the time of appearance of block in digitalis treated 48 hour embryonic hearts as the basis for assaying cardiac glycosides.Our present interest in heart-block arose during studies of the electrocardiogram (ECG) in the embryonic heart. We had been using digitalis to enhance the action of the heart and noted that a distinct P wave and QRS complex could not be obtained from an embryonic heart unless it had developed the ability to undergo AV block, i.e., dissociation of beat between atrium and ventricle under the influence of digitalis. This ability of digitalis t o affect atrioventricular conduction was used therefore to study block and the development of a functional conduction system in the embryonic heart. TechniqueThe procedures followed in producing heart-block were simple. Be...