Predicting air-balloon and water-filled infant catheter frequency responses

The authors test the validity of the assumption that fluid-filled catheters fit a second-order system. They conclude that air-balloon catheter (A-BC) and water-filled catheter (W-FC) frequency responses do not adequately fit second-order systems. This finding casts doubt on the validity of defining and applying second-order system mathematical models to predict such catheter-manometers' frequency responses. Second-order fitted curve:original recorded curve deviations are significantly larger in W-FCs than in A-BCs, in large FG-sized catheters, and at f/sub A5/ than at f/sub r/. The frequency-response system order of differing fluid-filled catheter-manometers should be verified experimentally prior to predicting their frequency responses using the typical equations applied to square-wave response techniques. The authors recommend that the sine-wave generator, rather than the square-wave response technique, be used to measure in vitro frequency responses of compliant fluid-filled catheters.