Method for High Accuracy Differential Pressure Measurements Using Fluid-Filled Catheters OREN M. ROTMAN, 1 URI ZARETSKY, 1 AVRAHAM SHITZER, 2 and SHMUEL EINAV 1 1 Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv University, 69978 Tel Aviv, Israel; and 2 Faculty of Mechanical Engineering, Technion, Israel Institute of Technology, 32000 Haifa, Israel (Received 19 March 2014; accepted 2 May 2014) Associate Editor K. A. Athanasiou oversaw the review of this article. Abstract—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 mea- surements 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 measure- ments, and showed very good agreement. Keywords—Pressure drop, Pressure line, Common mode pressure, Common mode error, Double-lumen catheter, Dual-lumen catheter. ABBREVIATIONS CMP Common mode pressure CMP p2p Common mode pressure effect peak- to-peak (of P cmp ) (mmHg) Err cmp Common mode pressure estimation error (mmHg) P cmp Common mode pressure (mmHg) P d True pressure drop (mmHg) P dm Measured pressure drop (mmHg) P g Gauge pressure (mmHg) RMSE Root mean square error (mmHg) W n (t) Percent weight of term n (in Eq.(1)) (%) INTRODUCTION Under a good approximation, the fluid-filled catheter can be characterized as a second-order linear system. 5,6 Gauge pressure measurements using fluid-filled cathe- ters are widely used in medical practice for blood pres- sure measurements and are known to distort pressure signals depending on the catheter system transfer func- tion. 5,6 The output signal can be corrected using an inversed transfer function, 5 or by harmonic analysis. 7 The transfer function is affected by the measurement system components: the catheter diameter, length, material, the fluid viscosity, presence of air bubbles inside the extension tubing, and the pressure transducer characteristics. 3,6 Other applications require simulta- neous pressure measurements from two locations, or a single differential measurement, such as in aortic ste- nosis catheterization 2,4 or in measurement of pressure drop along arterial flow. For example, in clinical prac- tice, pressure drop is used as a surrogate for flow 1 (e.g., across a coronary stenosis), since direct and accurate flow measurement inside arteries is very difficult. In comparison to gauge pressure measurement using a single fluid-filled catheter and a gauge pressure sensor, measurement and restoration of differential pressure using a fluid-filled double-lumen catheter and a differ- ential sensor might be more challenging since the two Address correspondence to Oren M. Rotman, Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv Univer- sity, 69978 Tel Aviv, Israel. Electronic mail: orenrotman1@gmail. com Annals of Biomedical Engineering (Ó 2014) DOI: 10.1007/s10439-014-1026-4 Ó 2014 Biomedical Engineering Society