Which of the following reflects the relationship between flow, resistance, and pressure with regards to shunts in both the fetal and adult circulations?
a) flow = pressure gradient x resistance
b) pressure gradient = flow x resistance
c) resistance = flow / pressure gradient
d) resistance = (pressure gradient)2 / flow
e) Qp = Qs x resistance
Answer b.
This equation, which may be thought of as an extrapolation of Ohm’s law (Voltage = Current x Resistance), represents the determinants of flow direction and quantity through a shunt. Rearranged as: Flow = Pressure Gradient / Resistance, it becomes clear that for circulation in parallel (as with shunted intracardiac flow), decreasing resistance in one limb of the circuit (i.e., pulmonary vascular resistance) will increase flow to that limb and decrease flow to the other.