Scaling approach to study the changes through the gestation of human fetalcardiac and circulatory behaviors

During human gestation, fetal body size increases considerably and importan t transformations occur to hemodynamics of the cardiovascular system of the fetus. Vascular compliances and resistances as well as the cardiac functio n show important changes. In order to investigate these modifications, a ma thematical approach based on scaling techniques was developed. Vascular and cardiac parameters of the human fetus were related by allometric equations to the anatomical dimensions of vessels that, in turn, depend on the fetal body weight and the gestational age. A scaling factor (b) was identified f or each parameter under study: vascular resistances and flow inertances dec rease with gestational age (b = -1 for viscous losses and b = -1.33 for con vective dissipations, b = -0.33 for flow inertances) whereas vascular compl iances remarkably increase (b = 1.33). Scaling factors were also adopted fo r the fetal cardiac parameters, according to experimental data on the devel opment of fetal myocardium. Parameter values calculated for each week of th e last trimester of the fetal gestation, wen tested using a mathematical lu mped parameter model, previously developed for a human fetus near the term of the gestation. The validation of the scaling method adopted for the para meters was performed by comparing the results of the simulations with a gro up of data obtained by Doppler velocimetry at different stages of fetal nor mal gestation. The adopted allometric equations were appropriate in describ ing the development of the human fetal circulatory system. The ductus venos us, the ductus arteriosus, and the foramen ovale, that conclude their funct ion at the birth moment, as well as the lungs and the brain, do not follow the general growth rate and require different scaling factors. (C) 2000 Bio medical Engineering Society.