Bradley M. Patten

The development of the electrocardiogram of the embryonic heart

Abstract With the aid of three-stage amplification, electrocardiographic records were made from chick embryos during the early stages of heart development. The youngest embryo from which records were obtained successfully was a fifteen-somite chick. At this stage, which is reached on the average with about 33 to 36 hours of incubation, the nearly straight tubular heart consists almost entirely of ventricle. The electrical record obtained from it shows none of the deflections characteristic of the adult electrocardiogram, but takes the form of a curve which first drops below, and then rises above, the isoelectric line. This configuration is consistent with the posteroanterior direction of the progress of contraction shown by superimposed tracings of successive frames from micromoving pictures of the heart action at this stage. Slightly older embryos (sixteen to seventeen somites, average incubation age 37 to 40 hours) yield a record in which there appears a sharp downward deflection, followed by a rapid return to, or above, the isoelectric line. Because of its characteristic configuration and because morphologic studies indicate that the embryonic heart at this stage is practically all ventricle, we interpret this as representing the QRS complex. In the next three or four hours of development, fusion of the cardiac primordia progresses caudally, so that the atrial region becomes definitely differentiated and the sinus venosus begins to take shape posterior to the atrium. Records from embryos in this age range show the appearance of a downward deflection coming about two twenty-fifths of a second ahead of the QRS complex. This we interpret as an inverted P-wave. During the next day of development the ventricular loop is bent backward so that it comes to be in its adult position caudal to the sinoatrial part of the heart. With this shift in relative positions the P-wave appears above the isoelectric line. Thus by the fourth day of development the electrocardiogram has assumed practically its adult configuration. It is to be emphasized that not even wandering neuroblasts reach the heart until considerably later in development than the age range covered in these experiments. Furthermore, “conduction tissue” is not at these ages histologically distinguishable from the remainder of the cardiac muscle. Thus we can trace the appearance of all the major features of the adult electrocardiographic pattern in embryonic hearts so young that they completely lack either a nerve supply or a specialized sinoventricular conduction system.

The changes in circulation following birth

Abstract On the basis of the evidence at present available, the changes in circulation which take place following birth may be summed up as follows: 1. 1. Current conceptions as to the occurrence at the moment of birth of an abrupt rerouting of the blood in the heart and great vessels rest on insufficient evidence. 2. 2. There is an increasing reason to believe that the volume of blood circulating through the lungs at the close of fetal life is adequate to care for the respiratory demands of the newborn infant as soon as the lungs are properly ventilated, and that no radical circulatory upheaval occurs at the time of birth. 3. 3. In the neonatal period, with the assumption of respiratory function, the volume and power of the pulmonary circuit gradually increase, bringing about, by the close of the first month, a new functional balance in intracardiac pressure. 4. 4. It is probable that under normal conditions the progressive closure of the ductus arteriosus accelerates the enlargement of the pulmonary vessels following birth, but there are many cases on record where a normal pulmonary circulation has been established without closure of the ductus. 5. 5. With the establishment of the new interatrial equilibrium brought about by increase in pulmonary circulation the foramen ovale falls gradually into disuse. 6. 6. Anatomical obliteration of the foramen ovale follows slowly in the wake of its functional abandonment. Provided the valvula foraminis ovalis competently covers the foramen ovale, failure of the valve completely to fuse to the septum is no functional handicap as long as there is not an intercurrent disturbance elsewhere in the cardiovascular mechanism or in the lungs. This fusion is incomplete, leaving what might be termed a “probe-patency” in about 20 per cent of all adults. Functional incompetence of the valvula by reason of defective development, which should be clearly distinguished from “probe-patency”, is as uncommon as other cardiac anomalies. 7. 7. Failure of the ductus arteriosus to close within the normal time range is to be regarded as a symptom rather than a cause of circulatory disfunction. Instances of the ductus remaining unclosed in the absence of abnormal conditions which force the maintenance of a blood current through it are exceedingly rare. 8. 8. Following the increase in pulmonary circulation and the closure of the fetal blood passages there is a gradual increase in the left ventricular musculature to equal the right at about three to five months, acquire a definite preponderance by the second year, and its full adult degree of preponderance by about the seventh year.