Method for High Accuracy Differential Pressure Measurements Using Fluid-Filled Catheters
The advantage of measuring differential pressure using fluid-filled catheters is that the system is relatively inexpensive, but the readings are not accurate and affected by the common mode pressure (CMP) distortion. High accuracy differential pressure measurements are required in various biomedical applications, such as in fluid-dynamic test rigs, or in the cath-lab, from cardiac valves efficacy to functional assessment of arterial stenoses. We have designed and built a unique system in which the pressure difference was measured along the fluid flow inside a rigid circular tube using a fluid-filled double-lumen catheter. The differential pressure measurements were taken across two side-holes near the catheter distal tip, spaced apart by 3 cm. The goal was to overcome the CMP error, which significantly distorted the output differential pressure signal and to formulate a restoration factor. A restoration formula was developed based on simultaneous gauge pressure measurements, and was tested in several different cases. Several representative cases are presented and show that the common mode artifact was reduced by factors of 12–27. The restored pressure gradient signal was validated using direct pressure drop measurements, and showed very good agreement.
Dynamic response of liquid-filled catheter systems for measurement of blood pressure: precision of measurements and reliability of the Pressure Recording Analytical Method with different disposable systems.
PURPOSE
We aimed to compare the effects of a blood pressure transducer system specifically manufactured to limit underdamping artifacts with those of a standard system on hemodynamic parameter estimation and accuracy.


MATERIALS AND METHODS
Forty-three consecutive patients undergoing vascular surgery at the University of Florence, Italy, were included. Arterial blood pressure signal was simultaneously registered with 2 MostCare monitors, connected to the artery either by a standard transducer or a specific transducer manufactured to avoid underdamping artifacts (Resonance Over-Shoot Eliminator [R.O.S.E.]; Becton Dickinson, Becton Drive, NJ). Patients were divided into 2 groups: absence (C group) or presence (R group) of underdamping/resonance artifacts of blood pressure signal. Systolic blood pressure, cardiac index, maximal pressure/time ratio (dP/dt(MAX)), and cardiac cycle efficiency were recorded every 30 seconds for 30 minutes. A total of 2675 measurements were performed with 34.9% incidence of underdamping/resonance artifacts.


RESULTS
All hemodynamic parameters showed clinically acceptable differences in the C group; in contrast, the results differed greatly in the R group between standard and R.O.S.E. transducer (systolic blood pressure bias, 16.7 mm Hg; cardiac index bias, 0.24 L min(-1) m(-2); dP/dt(MAX) bias, 0.92 mm Hg/ms; cardiac cycle efficiency bias, 0.018 units).


CONCLUSIONS
Underdamping/resonance artifacts frequently affect blood pressure measurement in operating rooms and intensive care units and cause severe overestimation of systolic blood pressure and incorrect estimation of hemodynamic parameters when the pulse contour method is used.
Limitation of Correction of Frequency Dependent Artefact in Pressure Recordings Using Harmonic Analysis
Frequency dependent phase and amplitude distortion may theoretically be minimized using correction formulae derived from a model which assumes that water-filled catheter-manometer systems are analogous to an oscillating mass attached to a spring and damping pot. This study defines the practical limits of this correction technique using harmonic analysis of simultaneously recorded water-filled and catheter-tip manometer systems.Some improvement was obtained in tracings recorded with underdamped, high frequency response systems. In no case were exact duplications of the catheter-tip tracings obtained. Divergence of measured vitro frequency response from actual in vivo frequency response was one source of these discrepancies. Loose coupling of the components of the system also changed the behavior from that of the assumed simple mechanical model and at times prevented adequate correction.Catheter-tip manometers, in spite of their expense and relative fragility, still appear to be the best means at present of obtaining accurate pressure tracings, particularly at high heart rates or where time derivatives are to be computed.