Figure 30-13 A to C, Damping and resonance. Pressure measured in an invasive arterial catheter can actually overshoot or amplify the real blood pressure. This phenomenon is referred to as the dynamic frequency response of the fluid-filled arterial line and transducer system. The phenomenon has a physical model that can generate an equation to predict the output pressure response, depending on the frequency of the input pressure and several physical parameters of the system. In the physical model on the right, driving pressure (arterial blood pressure) acts on a mass (the fluid within the arterial pressure tubing) by pushing it up and down against a spring, which stores energy (the compliant pressure tubing), and on a dash pot, which opposes motion in either direction (the resistance of the fluid as it moves to and fro within the pressure tubing). Depending on the input frequency, the output may undergo amplification as it reaches a specific frequency, known as the resonant frequency of the system. On the left side of the figure is a common phenomenon noted when a car drives along a bumpy dirt road. In this situation, the driving force is the bumps in the road, which act on the tire. The car spring is equivalent to compliance to the pressure tubing, and the shock absorber corresponds to the resistance of fluid moving back and forth in the arterial line. The mass of the fluid is analogous to the mass of the front of the car. You may have experienced the phenomenon: when you reach a certain speed as you are driving along a bumpy road, the front of the car starts to oscillate with increasing amplitude. If you speed up or slow down, this phenomenon disappears. The car bounces highest when you have reached the resonant frequency of this harmonic oscillator. See Appendix 4 for a detailed mathematical description of this process.