Chapter 27

### INTRODUCTION

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.

### RESONANCE AND DAMPING

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.

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.

###### FIGURE 27-1

Square wave flush test with intraarterial blood pressure ...

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

### Create a Free MyAccess Profile

Note: If you have registered for a MyAccess profile on any of the Access sites, you can use the same MyAccess login credentials across all sites.

Passwords must be between 6 and 40 characters long (no whitespace), cannot contain characters #, &, and must contain:
• at least one lowercase letter
• at least one uppercase letter
• at least one digit

### Benefits of a MyAccess Profile:

• Bookmark your favorite content such as chapters, figures, tables, videos, cases and more
• Self-Assessment quizzes saved for quick review
• Custom Curriculum access for both instructors and learners

## Subscription Options

### AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more