The fast-flush device test—based on the square-wave principle—has been used in clinical practice to test the accuracy of fluid-filled pressure-monitoring systems. One assumption with the square-wave test is that the system is a sec ond-order approximation. To elucidate the problem, the authors compared, in vitro, a reference test method (frequency response test), valid for second-order systems, with a pure square-wave test and a fast-flush device test. They showed that the two tested systems did not have any relation to the reference system, which suggests that the second-order approximation is not valid. Therefore, the fast-flush device test cannot be used reliably in testing the total chain of cathe ter, tubing, transducer, and monitor for invasive pressure measurement.
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