Transthoracic Impedance: IV. Studies of the Infant During the First Two Hours of Life

Summary The initial functional adaptation has been studied with the transthoracic impedance measuring technique in infants from 15 minutes to two hours of life. The series consists of 20 newborns with early and 20 with late clamping of the umbilical cord. All infants showed a marked fall in basic impedance IZ, I between 15 and 30 minutes of age. From 45 to 75 minutes jZ, I was virtually unchanged, a t a level somewhat lower for the late‐clamped group. During the second hour a rising IZ,/ was noted in the same group. With the aid of a mathematical formula an extrapolation backwards from 15 minutes to the time of birth was made for lZol. The results obtained directly and after the extrapolation are discussed together with previous known facts about the adaptation process in animals and infants. Main factors influencing IZ, I are air filling of the lungs, pulmonary blood volume and pulmonary interstitial fluid volume. The dynam‐ic interplay between these variables during the first hours is discussed. The results suggest a pronounced increase of pulmonary blood volume and pulmonary interstitial fluid volume during the first half hour of life, simultaneously with the establish‐ment of a functional residual capacity of air in the lungs. That infants with late clamping of the umbilical cord fall to a lower level of IZ,/ for the time period from 45 to 75 minutes is inter‐preted as reflecting a larger pulmonary blood volume in combination with a smaller func‐tional residual capacity as compared with the early‐clamped group. A rise in IZ, I for late‐clamped infants during the second hour of life points in the direction of an approximation between the two groups as pulmonary fluid and blood decreases and functional residual capacity increases in the late‐clamped group. No significant changes in transthoracic impedance were found in a control group of newborns past the post‐natal period of early adaptation to extra‐uterine life.