The fluid-filled plumbing system can modify the pressure signal in what is known as a dynamic response distortion ( G.ardD.ei.1981; G.ardner.and H.,o.l.l.ings.wo.Ë.h 1986).
Most pressure-monitoring systems are underdamped (G.ardD.e.Ll981). Dynamic distortion is caused by modification of the frequency characteristics of the applied pressure waveform as it passes through the plumbing system. The dynamic response of the plumbing system can be tested using a fast-flush test. This is accomplished by first opening the valve of the continuous-flush device and then quickly closing it. For underdamped systems, the rapid closure produces a square wave from which the natural frequency and damping coefficient of the plumbing system can be determined. The natural frequency is determined by measuring the period of a full cycle of oscillation and taking the reciprocal. For example, if the display rate (either oscilloscope or chart recorder) is 25 mm/s and one full cycle of oscillation takes 2 mm, the natural frequency is
The determination of damping coefficient requires the measurement of the ratio of the amplitude of two succeeding cycles of the resulting oscillating signal following a fast-flush test. The amplitude ratio is then plotted on the non-linear amplitude ratio scale on the far right-hand side of Fig. 2. The damping coefficient z is then determined from the linear damping coefficient scale shown in Fig:,?.
Fig. 2 Dynamic response testing methodology for the pressure-monitoring system. See text for an explanation of how to determine the adequacy of the dynamic response. (Adapted from Gardner „(1,981].)
Several factors lead to an inadequate dynamic response, including air bubbles in the plumbing system and pressure tubing that is too long or too elastic ( Gardner...
1981). The best way to enhance the dynamic response of the plumbing system is to maximize the natural frequency. It is clear from Fig 2. that the higher the natural frequency, the greater the range of damping coefficient can be while still reliably reproducing the pressure waveform. Figure...2 also shows that inadequate dynamic response causes two general types of error. If a system is underdamped, the systolic pressure will tend to be overstated and the diastolic pressure understated. However, if the system is overdamped, the systolic pressure will be understated and diastolic pressure overstated. The pulmonary artery catheter is subjected to
'whip' as it moves in the right ventricle, and this artifact tends to cause major overshoot during the systolic phase of the pressure waveform.
Many clinicians use the shape of the pressure waveform to estimate qualitative information about the heart ( Table 1). Since dynamic waveform distortions caused by the plumbing system can mimic many of these conditions, having an adequate dynamic response is essential.
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Table 1 Relation between waveform and heart condition
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