THE USE OF FLUID-FILLED CATHETERS AS REFERENCE BLOOD PRESSURE MONITORS

The standard ISO 81060–2:2018 allows the use of invasive blood pressure monitoring systems as reference gauges in clinical trials of automated non-invasive automated sphygmomanometers. The reference measuring system is subject to requirements for a maximum permissible error of ±2 mmHg, and the resonant frequency and damping coefficient must also be characterized. The 'catheter-sensor' system used in clinical practice only has defined parameters required by the standard for the chamber with the pressure sensor. The characteristic parameters of the whole measuring system cannot be defined even when the type of catheter used is known, because after every irrigation the system changes the values of its natural frequency (fn) and damping ratio (ζ). These parameters directly define the frequency response of the system, its resonant frequency and the damping coefficient. The characteristic parameters of the 'catheter-sensor' system were defined on the basis of an analysis of a second-order linear model and the measurement of the step response of the real system. Measurements have shown that repeated irrigation of the same 'catheter-sensor' system can change the value of the system’s natural frequency by tens of Hz. In well-irrigated systems, the accuracy required by the standard was met. The following values were determined for this system: fn = 38.8 Hz and ζ = 0.130. In the second case, when the system was probably affected by air bubble compliancy, the measurement accuracy was much lower. The discovered deviation was tens of mmHg. This system had fn = 6.5 Hz a ζ = 0.281