ABSTRACT
The calculation of vascular resistance is based in part on Poiseuille’s law, which states that flow in a tube is directly related to pressure and cross-sectional area of the tube and inversely to the length of the tube and the viscosity of the fluid flowing in the tube. In the vascular system the length of the tube and the viscosity of the fluid are assumed to be constant so that the pressure gradient across a vascular bed divided by the flow through the bed is equal to the resistance of the bed. Therefore, the formulas for pulmonary and systemic vascular resistance are as follows:
Rp ¼ ðPA mean pressure LA mean pressureÞ Qp
Rs ¼ ðAO mean pressure RA mean pressureÞ Qs
There are two different types of pulmonary resistance: arteriolar resistance is resistance calculated across the vascular
h i and total resistance of the
. In the clinical setting we are usually
only interested in arteriolar resistance. The units for this expression of vascular resistance are
called Wood’s units (named after Paul Wood) and are in mmHg/L/min. To convert this to metric units, the Wood’s unit is multiplied by 80 to yield the units of dyneseccm5. Because of the considerable size range of pediatric patients most cardiologists index the Wood’s units to body surface area (Wood’s unit m2). The normal values for systemic and pulmonary resistance are <20 units for systemic and <3 Wood’s units for pulmonary.
