Measurement errors in estimating aortic blood velocity by pressure gradient

¹ Section on Clinical Biophysics, Cardiology Branch, National Institutes of Health, Bethesda, Maryland

The computed pressure-gradient technique is important in measuring instantaneous blood flow and velocity in man. Estimation of the instantaneous pressure gradient for this purpose presents many special problems. This study was designed to explore these special measurement problems and to establish certain minimum pressure-recording standards for application of the computed pressure-gradient technique to man. The nature of the functions to be measured was first explored by determining average values and harmonic content of typical pressure, flow, and pressure-gradient curves. In contrast to pressure and flow curves, the harmonic content of pressure-gradient curves was found to exceed the recording capabilities of currently available systems for measuring differential pressure. However, when velocity was computed from pressure gradient, the importance of errors related to higher frequencies is greatly diminished. To meet minimum requirements differential-pressure catheter-manometer systems should have a dynamic imbalance of less than 5% through frequencies of 15 cycles/sec, a single-ended dynamic response uniform (±5%) through 15 cycles/sec, and a static imbalance not greater than ±0.1% over the range 0–300 cm H₂O. Although an optimum pressure tap separation could not be precisely defined, visual examination suggested that a tap separation of 8 cm should not be exceeded.