BACKGROUND: The accurate recording of intraarterial pressure depends upon an appropriate dynamic response of the monitoring system. Generation of a square wave (SW) at the catheter tip is the engineering and in vitro laboratory gold standard. Fast flush (FF) testing is the clinical test of choice. Results from these two test methods have been assumed equal but have not been empirically confirmed. METHODS: We studied three different 5.1 cm catheter sizes (16 G, 18 G, 20 G Becton Dickinson, Sandy, UT) attached to three different lengths of arterial pressure tubing (36 in, 91.4 cm; 72 in, 182.9 cm; 108 in, 274.3 cm). An arterial recording system was assembled in the standard fashion by attaching a catheter to arterial pressure tubing, which was attached to a transducer (TXX-R, Ohmeda, formerly Viggo-Spectramed, Oxnard, CA) whose signal was recorded by a strip chart recorder (Gould 2400, Rolling Meadows, IL). The system was attached to a pressurized saline flush. The catheter tip was inserted into one port of a pressure generator. With the other port of the pressure generator open to atmosphere, FF tests were performed by activating the flush device of the transducer. Subsequent step response signals from the FF tests were then recorded from which natural frequency (fn) and damping coefficient (zeta) were calculated. Next, square waves were generated by closing the port that was open to atmosphere and attaching a signal generator to a pressure generator. Square waves so generated were recorded as described above and natural frequency and damping coefficients calculated. These procedures were repeated after 0.05 cc of air was introduced in the transducer and repeated again in a system containing a damping device (R.O.S.E., Resonant OverShoot Eliminator, Viggo-Spectramed, Oxnard, CA). RESULTS: There was no significant difference between fn and zeta as calculated from the step response generated from the FF test versus fn and zeta as calculated from the square wave (SW) test in systems without air. However, in systems containing air, fn by FF testing was always less than fn by SW testing for all catheter sizes and extension tubing lengths (p < 0.05). Damping was also always greater by FF testing than by SW testing in systems with air for all catheter sizes and extension tubing lengths (p < 0.05). The R.O.S.E device created marked qualitative differences, although exact fn and zeta could not be quantified. CONCLUSIONS: For the characterization of dynamic response of invasive blood pressure monitoring systems, the FF test and SW test yield identical results. However, under certain conditions-air, R.O.S.E device-dynamic response as measured by FF testing was not equivalent to dynamic response as measured by the gold standard-the SW test. Specifically, small amounts of air in fluid-filled invasive blood pressure monitoring systems cause a slightly worse dynamic response as measured by FF testing versus the laboratory gold standard-the SW test.