Equipment in Anaesthesia and Critical Care

A complete guide for the FRCA

Daniel Aston, Angus Rivers, Asela Dharmadasa

• Publisher: Scion Publishing
• Year: 2013
• Print ISBN: 9781907904059
• eBook ISBN: 9781907904158
• Topic: Medicine
• Subtopic: Anesthesiology & Pain Management

Highly commended in the Anaesthesia category of the 2014 BMA Medical Books awards! From reviews:
"Altogether, this textbook offers an excellent detailed overview of basic principles, mechanics, and physics of technical equipment used in anaesthesia and intensive care medicine." Critical Care, Feb 2014 "This book is excellent. I have read a number of equipment related books and found most of them stale and filled with minutia. Instead this fresh look at equipment really makes it interesting to read and keeps your attention. With sections that give quick answers to how things work, advantages and disadvantages, in an easy to read format you get exactly what you want to know RIGHT away. In addition the full color photos and well done charts really add to the depth of this book. It is my opinion this may be one of the best books on the market for quick review of anesthesia and critical care equipment. Let's be honest, no one is sitting at home reading equipment books for pleasure so when you NEED the information you likely need it right away. This book does exactly that; easy format, clear and concise information with everything you need to know about a piece of equipment easy to find and within a page or two. I would recommend this to all trainees in anesthesia but also for those providers who like to be prepared in the operating room to deal with any issue that might arise." www.nurse-anesthesia.org, 8 Feb 2014 Equipment in Anaesthesia and Critical Care specifically follows the syllabus published by the Royal College of Anaesthetists and is the perfect guide for candidates studying for their FRCA qualifications. The book is beautifully illustrated and features high quality colour photographs throughout. The text is engaging and comprehensive, relating each piece of equipment back to its basic physics, mechanics and clinical context. The clear, concise and standardised format means that information is easy to access and perfect for revision. Every major piece of equipment is featured alongside colour photos, reproducible line diagrams, and information on its uses, how it works, pros and cons, and safety considerations. The authors, all senior registrars in anaesthesia and critical care, appreciate exactly what candidates need to know to pass the FRCA exam. They have identified and tackled difficult subjects which are often glossed over or omitted in other resources, to produce a book that is comprehensive, engaging and to the point. In addition to FRCA candidates, the book is also suitable for operating department practitioners, critical care staff, and physicians with an interest in anaesthesia or critical care.

Chapter 1

Medical gases

1.1 Vacuum insulated evaporator

1.2 Cylinder manifolds

1.3 Medical gas cylinders

1.4 Compressed air supply

1.5 Oxygen concentrator

1.6 Piped medical gas supply

1.7 Medical vacuum and suction

1.8 Scavenging

1.9 Delivery of supplemental oxygen

1.10 Nasal cannulae

1.11 Variable performance masks

1.12 Venturi mask

1.13 Nasal high flow

Vacuum insulated evaporator

Fig. 1.1.1: The main and backup vacuum insulated evaporators outside a hospital.

Fig. 1.1.2: A schematic diagram of a vacuum insulated evaporator.

Overview

The vacuum insulated evaporator (VIE) is a storage tank for liquid oxygen with a vacuum insulated wall designed to keep the contents below -160°C. The wall consists of an inner stainless steel shell and an outer carbon steel shell. It may rest on a weighing tripod.

Uses

VIEs provide the piped oxygen supply in most hospitals.

How it works

General principles

Liquid oxygen is produced by fractional distillation of air, off-site. It is delivered to the hospital on a regular basis and stored in the VIE. Oxygen has a critical temperature of -119°C, meaning that above this temperature it must exist as a gas; the VIE is therefore kept between -160°C and -180°C.

The VIE is not actively cooled. Instead, as suggested by the name, it relies on insulation and evaporation to maintain the low temperature. Insulation is provided by the vacuum wall, which minimizes conduction and convection of heat into the chamber. The small amount of heat which does enter the VIE causes some of the liquid oxygen to evaporate. Evaporation uses energy in the form of heat (the latent heat of vaporization) and therefore the VIE remains cool.

Low and high use situations

The pressure in the VIE is approximately 700 kPa (7 Bar, the saturated vapour pressure of oxygen at -160°C). If left unvented (say all the oxygen taps in the hospital were turned off), the pressure in the VIE would rise as oxygen slowly evaporated. To prevent an explosion in this situation, a pressure relief valve vents unused oxygen into the atmosphere.

If instead demand is high, the rapid vaporization of large quantities of oxygen causes a drop in temperature, resulting in the reduction of vapour pressure and therefore reduced supply. In this circumstance, a valve is electronically opened, allowing liquid oxygen to enter an evaporator coil exposed to ambient temperature. This pipe is also known as a superheater, though the only heat required is that from the air surrounding it – the large temperature difference causes rapid warming and vaporization.

Fig. 1.1.3: Superheater coils. The pipes leading from the VIE are covered in frost because of the extreme cold.

Oxygen leaving the VIE is extremely cold and exceeds pipeline pressure. Before entering the hospital pipeline, it is therefore passed through another superheater that brings it to ambient temperature, and a pressure regulator that reduces its pressure to 400 kPa (4 Bar).

Measuring the contents

The amount of oxygen remaining in the VIE can be calculated from its mass. Traditionally this is done by weighing it using a tripod weighing scale – the VIE pivots on two legs, with the third resting on the scale. The VIE’s empty (tare) weight is known and subtracted from the measured value to give the weight of oxygen inside.

Alternatively, the oxygen contents may be calculated from the difference between the vapour pressure at the top of the VIE and the pressure at the bottom of the liquid oxygen. Using these pressures, it is possible to calculate the height of the fluid column and, by knowing the VIE’s cross-sectional area, the volume of liquid oxygen remaining.

Advantages

• Storing liquid oxygen is highly efficient in terms of space. It expands to 860 times its volume as it vaporizes to 20°C.
• Compared with a cylinder at room temperature, liquid oxygen is stored at a much lower pressure (700 instead of 13 700 kPa).
• The VIE does not require power to store oxygen in a liquid state.
• Oxygen is therefore cheaper both to deliver and to store as a liquid.

Disadvantages

• Initial equipment costs are much higher than a cylinder manifold.
• A backup cylinder manifold and/or second VIE is required in case of interruption to the oxygen supply.
• If demand is not fairly continuous a significant amount of oxygen will be unused and vented.

Safety

• The VIE must be kept outside the building because of the fire risk.

Cylinder manifolds

Fig. 1.2.1: A cylinder manifold.

Overview

A manifold is a pipe with several openings, in this case connected to cylinders supplying pipeline oxygen, nitrous oxide or Entonox.

Uses

Manifolds are used to supply piped nitrous oxide and Entonox, and they may also be used as a primary oxygen supply in small hospitals, or as a backup supply for larger hospitals.

How it works

The manifold usually connects two groups (occasionally there may be more) of high capacity cylinders (size J or L). Each cylinder is connected to the manifold and then to the...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Preface
• Acknowledgements
• Abbreviations
• 1 Medical gases
• 2 Airway equipment Masks, supraglottic airways and airway adjuncts
• 3 Breathing systems
• 4 Ventilators
• 5 Delivery of anaesthetic agents
• 6 Monitoring equipment
• 7 Filters and humidifiers
• 8 Regional anaesthesia
• 9 Critical care
• 10 Surgical equipment relevant to anaesthetists
• 11 Radiological equipment
• 12 Miscellaneous
• 13 Sample FRCA questions
• Index