Form and function of fetal and neonatal pulmonary arterial bifurcations

Bifurcation is a basic form of vascular connection. It is composed of a par ent vessel of diameter d(0), and two daughter vessels, d(1) and d(2), where d(0) > d(1) greater than or equal to d(2). Optimal values for the bifurcat ion area ratio, beta = (d(1)(2) + d(2)(2))/d(0)(2), and the junction expone nt, x, in d(0)(x) = d(1)(x) + d(2)(x), are postulated to be universal in na ture. However, we have hypothesized that the perinatal pulmonary arterial c irculation is an exception. Arterial diameters were measured in pulmonary v ascular casts of a fetal lamb (140 days gestation/ 145 days term) and a neo natal lamb (1 day old). The values for beta and x were evaluated in 10,970 fetal and 846 neonatal bifurcations sampled from the proximal and intermedi ate arterial regions. Mean values and confidence intervals (CI) for the fet us were beta = 0.890 (0.886-0.895 CI) and x = 1.75 (1.74-1.76 CI); and for the newborn were beta = 0.913 (0.90-0.93 CI) and x = 1.79 (1.75-1.82 CI). T hese values are significantly different from Murray's law (beta >1, x = 3) or the West-Brown-Enquist law (beta = 1, x = 2). Therefore, perinatal pulmo nary bifurcation design appears to be distinctive and exceptional. The decr easing cross-sectional area with branching leads to the hemodynamic consequ ence of shear stress amplification. This structural organization may be imp ortant for facilitating vascular development at low flow rates; however, it may be the origin of unstable reactivity if elevated blood flow and pressu re occurs.