Form and function of fetal and neonatal pulmonary arterial bifurcations

Bifurcation is a basic form of vascular connection. It is composed of a parent vessel of diameter d₀, and two daughter vessels, d₁ and d₂, where d₀ > d₁ ≥ d₂. Optimal values for the bifurcation area ratio, β = (d₁/² + d₂/²)/d₀/² and the junction exponent, x, in d₀/(x) = d₁/(x) + d₂/(x), are postulated to be universal in nature. However, we have hypothesized that the perinatal pulmonary arterial circulation is an exception. Arterial diameters were measured in pulmonary vascular casts of a fetal lamb (140 days gestation/145 days term) and a neonatal lamb (1 day old). The values for β and x were evaluated in 10,970 fetal and 846 neonatal bifurcations sampled from the proximal and intermediate arterial regions. Mean values and confidence intervals (CI) for the fetus were β = 0.890 (0.886-0.895 CI) and x = 1.75 (1.74-1.76 CI); and for the newborn were β = 0.913 (0.90-0.93 CI) and x = 1.79 (1.75-1.82 CI). These values are significantly different from Murray's law (β > 1, x = 3) or the West-Brown-Enquist law (β = 1, x = 2). Therefore, perinatal pulmonary bifurcation design appears to be distinctive and exceptional. The decreasing cross-sectional area with branching leads to the hemodynamic consequence of shear stress amplification. This structural organization may be important for facilitating vascular development at low flow rates; however, it may be the origin of unstable reactivity if elevated blood flow and pressure occurs.