Monitoring: Equipment and Computers
Background and Goal of Study: The damping coefficient and natural frequency are standard parameters for anesthesiologists to evaluate the dynamic responses of pressure monitoring lines. But in fact, under the lumped constant model described by the second order ordinary differential equation the dumping coefficient can be derived from amplitude gain at natural frequency. We demonstrated the defectiveness of the dumping coefficient and natural frequency with our high-precision frequency response measurement method (step-response analysis).
Materials and Methods: A blood pressure wave converter (Biotek 601A) connected to a function signal generator, 2 channeled-pressure amplifiers and personal computer were used. After introducing a square wave into the blood pressure monitoring line, proximal (input) and catheter-end (output) pressure waves were recorded with pressure amplifiers. Frequency response curves (amplitude and phase), the dumping coefficient and natural frequency were displayed after analysis with our original computer program.
Results and Discussions: There were discrepancies between dumping coefficients and natural frequencies, and frequency response characteristics.
Two curves (Fig. 1) had the same dumping coefficient (0.20). The natural frequency of the left curve (33.2 Hz) was higher than that of the right one (26.4 Hz), so the dynamic response of the left monitoring line seemed better than that of the right one. But in fact the right frequency response curve had an excellent dynamic response with a wide, flat, low frequency range and without pressure wave distortion.
Conclusion(s): The dumping coefficient and natural frequency cannot elucidate the shape of frequency response curves. Therefore these parameters became less useful for dynamic response evaluation.