Electrocardiogram in rabbit fetuses.

• Although indirect fetal electrocardiograph}' in human pregnancies is becoming common,' electrocardiograms of mammalian fetuses in utero by the direct approach are not. During a general electrophysiological study of rabbit fetuses, we found that electrocardiograms could be obtained easily with needle electrodes. We expanded our observations by making recordings of cardiac potentials via needle electrodes in two ways: 1. with the needle driven through the uterine wall and held against the fetus, and 2. with the fetus extracted and the electrode inserted into it. Rabbits have a gestation period of 32 days. Transition from the embryonic to the fetal stage occurs at about 10 to 11 post-coital days. Until about the 11th or 12th day, the fetuses are tiny masses of gelatinous tissue about five mm long. Thereafter, they rapidly assume characteristic mammalian form and by the 1.6th day have a crown to rump length of 13 to 18 mm. Earlier studies in this laboratory have shown that the first visible movements occur at 17 days and the earliest electromyographic potentials occur at about the same day. The earl iest human fetal electrocardiograms were recorded through surface electrodes on the abdomen and back of the mother at 11 or 12 weeks. This age corresponds roughly to the rather advanced age of 21 days in the fetal rabbit. At 22 weeks, electrocardiograms are easy to obtain in human pregnancies. Although correlation of the stages of human and rabbit development is not completely reliable, we are confident that even at mid-term (16 days) the rabbit fetus is relatively much "younger" than the human fetus at 12 weeks.