Echo Guided Life Support (EGLS)

Jean-François Lanctôt, Maxime Valois

• Publisher: Sonoscope Inc.
• Year: 2016
• eBook ISBN: 9781777549305
• Topic: Medicine
• Subtopic: Emergency Medicine & Critical Care

The EGLS algorithm is a simple and effective framework designed to guide clinicians in the management of shockpatients.

Over the last decade, bedside ultrasound has gained widespread acceptance and is now considered standard of care in many clinical situations. With appropriate use, ultrasound can decrease the time to diagnosis and narrow the list of differential diagnosis, two factors essential in the treatment of trauma or hemodynamic instability.

An ultrasound assessment is particularly useful for managing a patient in undifferentiated shock. This book describes how and why an ultrasound assessment called the EGLS algorithm can be an essential tool for managing this time sensitive morbid condition.

Section 1 (Physics and Instrumentation) of this book addresses basic concepts such as the physics of ultrasound and the use of sonographic equipment.

Section 2 (Image Acquisition and Interpretation) provides essential information regarding image acquisition and interpretation in the context of hemodynamic instability. A systematic, step by step approach to image acquisition and interpretation is described for lung, cardiac, and IVC ultrasound with a summary of common tips and pitfalls.

Section 3 (Putting It All Together) combines lung, cardiac, and IVC ultrasound with the ‘sonographic physiology 101’ of shock. This section describes how ultrasound can help determine a patient’s physiology and estimate their fluid responsiveness. The book closes with a collection of common clinical scenarios to summarize how shock can be efficiently managed with the EGLS algorithm.

SECTION 1:

PHYSICS AND INSTRUMENTATION

CHAPTER 1

1. IMAGE GENERATION

➢ 1.1 The Piezo-electric effect

➢ 1.2 Attenuation, reflection, impedance, and duration

➢ 1.3 Frequency

➢ 1.4 Ultrasound modes

An understanding of the basic concepts of ultrasound helps to accurately interpret the ultrasound images and distinguish real structures from artifacts. This chapter offers an overview of the basic principles of the physics of ultrasound^1-3 and their role in image generation.

1.1. The Piezo-electric effect

Sound waves are produced by small oscillating changes in air pressure. The number of oscillations per second determines the frequency of the sound wave. A high number of oscillations equates with a high frequency. Ultrasound is a very high frequency sound wave that oscillates with a frequency greater than 20000 cycles per second. An ultrasound probe is capable of producing high frequency sound waves by vibrating quartz crystals located on the distal surface of the probe. Briefly, the ultrasound machine applies short electrical pulses to the crystals, the crystals vibrate at very high frequencies, and generate high frequency sound waves or ultrasound. These high frequency sound waves penetrate through tissues and organs when the ultrasound probe is placed on the patient’s body. When the sound waves encounter a reflective surface within the body, they return to the probe and cause the crystals to vibrate once again. The crystals transform the vibrations back into electrical pulses. This reversible process of conversion of electrical energy to mechanical energy and back to electrical energy is called the Piezo-electric effect. The process allows the probe to function as both a transmitter and receiver of ultrasound.

The ultrasound machine converts the returning electrical pulses into an ultrasound image. The characteristics of the ultrasound image depend on the amount of reflection of the sound wave, and the length of time it takes for the sound wave to reach the tissue and return to the probe.

Figure 1.1 Basis for generating an ultrasound image.

1.2. Attenuation, reflection, impedance, and duration

As the sound wave leaves the probe and travels through tissue, it begins to lose energy gradually, a phenomenon known as attenuation. Some energy is lost as it is transformed into heat. The remaining energy is either transmitted to deeper tissues or reflected back to the probe. The amount of reflection depends on the sound wave’s ability to travel through tissues and, more importantly, through the interface between different tissues.

The sound wave’s ability to travel through tissues is determined by the acoustic impedance (or acoustic resistance) of the tissue. Each tissue has its own acoustic impedance. Sound waves travel easily through tissue with low acoustic impedance (fluid), moderately through tissue with medium acoustic impedance (solid organs), and not at all through tissue with high impedance (bone).

Figure 1.2 Ultrasound is attenuated and reflected as it travels through tissue.

A. Attenuation is caused by energy loss (as heat) while reflection is caused by the difference in acoustic impedance of tissues at an interface.

B. Different tissues and their corresponding acoustic impedances.

An interface that separates tissues with very different impedances will cause a large proportion of the sound wave to be reflected back to the probe. For example, the difference in acoustic impedance between soft tissue and bone is very large and therefore most of the sound wave is reflected back to the probe. The difference in acoustic impedance between two solid organs is smaller and thus less of the sound wave is reflected back to the probe.

Sound waves that penetrate fluids undergo minimal attenuation and reflection. The energy in the sound wave is transported through the fluid without returning back to the probe.

The amount of reflection dictates the intensity of the tissue on the ultrasound image. As the amount of reflection varies, the intensity varies and this is visualized as different shades of grey in the ultrasound image.

Figure 1.3 The intensity of reflection determines the shade of grey on the ultrasound image.

A. Ultrasound cannot penetrate bone and thus 100% of the sound wave is reflected back to the probe. Bone appears white on the ultrasound image and casts an acoustic shadow (black).

B. Ultrasound travels through fluid with little attenuation and no reflection back to the probe. Fluid appears black on the ultrasound image.

A tissue’...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• About the Authors
• Dedication
• Contents
• Section 1: Physics and Instrumentation
• Section 2: Image Acquisition and Interpretation
• Section 3: Putting It All Together
• Conclusion
• Index
• Notes