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INTRODUCTION

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A transducer is any device that converts energy from one form to another. A pressure transducer converts a pressure waveform (kinetic and potential energy) into an electrical signal (electrical energy). Invasive arterial blood pressure monitors measure the constant variation of blood pressure through an arterial catheter connected to fluid-filled tubing, which in turn is connected to a pressure transducer. The arterial pulse pressure is transmitted through a pressurized column of saline into a flexible diaphragm causing the shape of the diaphragm to change. The displacement of the diaphragm is measured by a strain gauge. Strain gauges work based on the principle that the electrical resistance of a wire increases as it extends. When several strain gauges are incorporated into a Wheatstone bridge circuit, the movement of the diaphragm stretches or compresses several wires and alters the resistance of the unit. This process results in the generation of a current and electrical signal. The pressure transducer then sends this electrical signal via a cable to a processor where it is filtered and displayed as a waveform.

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RESONANCE AND DAMPING

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The physical display of the blood pressure waveform is influenced by resonance and damping. Resonance refers to the amplification of a signal that can occur when a certain force is applied to a system. Every system has a frequency at which it oscillates freely, called the natural frequency. If a force with a similar frequency to the natural frequency is applied to a system, the system will oscillate at maximum amplitude. This phenomenon is called resonance. Resonance produces excessive amplification that distorts the electrical signal, resulting in greater systolic pressure, lower diastolic pressure, and increased pulse pressure. To prevent resonance, it is important for the invasive arterial blood pressure (IABP) system to have a much higher natural frequency than the frequency of the force applied to the system. The natural frequency of the system can be increased by reducing the length of tubing, reducing the compliance of the tubing, reducing the density of the fluid in the tubing, or by increasing the diameter of the tubing.

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Like resonance, damping can also alter the signal displayed from a transducer (Figure 27-1). Damping refers to the decrease of signal amplitude that accompanies a reduction of energy in an oscillating system. Increased damping will manifest as a decrease in systolic blood pressure and an increase in diastolic blood pressure. In the pressure transducer system, most damping arises from friction between the tubing and fluid in the tubing. Other factors that decrease energy in the system and cause damping include three-way stopcocks, bubbles, clots, arterial vasospasm, large catheter size, and narrow, long, or compliant tubing. By contrast, an underdamped system can also cause signal distortion. In an underdamped system, the tracing can resemble a resonant system with increased systolic amplitude, and decreased diastolic amplitude.

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FIGURE 27-1

Square wave flush test with intraarterial blood pressure ...

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