Our hypothesis was that the relationship between the internal aortic diamet
er and the Doppler flow velocity across the aortic isthmus could be modeled
by applying the principle of conservation of mass flow. The aortic diamete
r decreased at the isthmus by a mean of 18% (t = 11.02, p < 0.0001), while
the flow velocity increased by a mean of 44% (t = 10.09, p < 0.0001). The m
ean peak mass flow rate was 34.5 ml/s preisthmus and 32.9 ml/s at the isthm
us with excellent correlation (r = 0.830). We conclude that the increase in
Doppler velocity observed in the descending aorta can be explained by the
normal narrowing observed at the aortic isthmus and application of the cont
inuity equation for conservation of mass flow.