Determination of Catheter - Manometer System Linearity

In an attempt to evaluate the pressure distortion due to clinical catheter-manometer systems, it becomes necessary to establish their linearity. While others have either discussed or have sought to establish the existence of linearity by investigation, no classification in present clinical systems has been made. Available discussions of the effect of a catheter-manometer system transfer function on pressure pulses start with the assumption that the transfer function is linear. Without linearity tests of the systems employed and with some investigators' results implying nonlinearity, this assumption remains unjustified. The work presented here establishes a linearity classification for systems now in use. A system is considered linear if its transfer function coefficients can be shown to be independent of pressure and time in the applicable zone of pressure and frequency. The coefficients of mass and damping, found with the aid of Womersley's equations, satisfy this criterion. The catheter spring constant, a complex number, and the remaining coefficient then determine system linearity. Different classes of catheters having differing cross-sectional structures lead to linearity in one case but do not necessarily imply linearity in a second case. With this in mind, linearity is then individually established for different catheters. Additionally, the compressibility of the catheter's fluid, previously ignored, is shown to influence the catheter's spring constant in some cases. The adverse effects of air bubbles on system fidelity is often not considered in the clinical application.