David J. Baker

The Classification and Properties of Toxic Hazards

A wide range of toxic hazards exists, both natural and man-made. A toxic hazard may be defined as any substance which has the ability to cause harm or damage to living organisms. The term ‘toxin’ is often used synonymously with any poison, but should be reserved to mean any toxic chemical which originates from a biological organism. Toxic trauma is the result of acute exposure to hazardous substances that cause life-threatening, seriously disabling acute effects and the intermediate effects that follow. Toxic agents may be classed as toxic industrial chemicals (TIC) or agents of chemical warfare (CW). Some agents such as chlorine and phosgene are both TIC and CW agents. Both TIC and CW agents may be classified in terms of their actions on somatic systems. TIC are also classified and identified using the UN HAZMAT system which assigns each agent into one of nine classes and gives an identification code number. Each toxic agent has four distinct properties, physical form, persistencytoxicity and latency, which determine their action in the body and also the risks of transmission of the hazard to other persons.

The Structure of the Airways and Lungs

This chapter provides an introduction to the anatomy of the airways and lungs and how it is relevant to the management of artificial ventilation. The route through which air enters and leaves the lungs is known as the respiratory pathway or tree. This is conveniently divided in to upper and lower airways. The upper and most of the lower airways are involved only in humidification and conduction of air to the extreme parts of the lung, where oxygen transfer takes place to the blood. At the level of the nose and pharynx the airway is shared with the food pathway to the oesophagus. The larynx plays a key role in protecting the airway from the entrance of food and other foreign bodies. Each level of the respiratory tree is associated with potential problems with the management of airways and the provision of artificial ventilation. An understanding of the structure of the airways and lungs is essential to successful management of the airway and artificial ventilation.

Basic Principles of Mechanical Ventilation

This chapter discusses the gradual adoption of mechanical ventilators in emergency care and provides a basic classification of how they provide artificial ventilation (intermittent positive pressure ventilation or IPPV). This is based on the respiratory cycle which is common to all ventilators whether simple or complex. The type of IPPV produced by a ventilator is termed a ‘mode.’ Many different modes exist and are sometimes given trademarked names by manufacturers. However a basic classification exists which has been used here. The essential function of different modes is to adapt to the degree of ventilatory effort a patient in respiratory failure may still have. In emergency ventilation complete respiratory support is required for a patient who is not breathing. A spectrum of modes exists between total artificial ventilation and a patient who is still breathing spontaneously but with reduced efforts. In the hospital, support is usually provided using pressure support found on ICU ventilators. Recently however a number of transport ventilators have been produced with this function. During IPPV positive end – expiratory pressure (PEEP) can improve the ventilation efficiency in patients who are still being ventilated. The equivalent support in a spontaneously breathing patient is CPAP but it must be noted that this is not a ventilator mode. Before receiving CPAP a patient must be seen to be breathing adequately.

The Management of Respiratory Failure: Airway Management and Manual Methods of Artificial Ventilation

This chapter concerns emergency airway management and manual methods of artificial ventilation using both the basic technique of expired air ventilation, and the bag-valve (BV) device. The former is part of primary cardiopulmonary resuscitation. The latter is widely used in emergency medicine and if used correctly can provide effective artificial ventilation. The device is recommended for use in resuscitation by international resuscitation guidelines which set out the appropriate tidal volumes and rates of ventilation to be used. However, there are recognized dangers in using a BV device. There is increasing evidence to show the considerable variation in both tidal volume and frequency, when it is used in emergency ventilation. In particular, high inflation pressures and large tidal volumes may be delivered which can cause barotraumas and volutrauma, as well as gastric insufflation if the device is being used without a protected airway. Equally, hypoinflation may occur leading to hypoxia

Managing Ventilation During Transport

Previous chapters have considered the provision of artificial ventilation in an emergency to overcome life – threatening hypoxia. Transport ventilation is different from emergency ventilation and concerns the safe movement of a patient who is dependent on a ventilator, but in a stable condition from one location to another. This may range from short distances with in the hospital, for example from the ICU to imaging units or over many thousands of miles during a medical evacuation from one continent to another. There are inherent risks associated with transport ventilation associated with both the environment and the movement of the patient and special vigilance is required.

Paediatric Artificial Ventilation for the Non-specialist

Paediatric artificial ventilation includes ventilation of neonates, infants and children. Although the principles are the same as in adult ventilation there are important differences between adult and paediatric anatomy and physiology. Paediatric ventilation in hospital and during transport is the work of specialists in the field. However, non-specialists should be familiar with the subject and particularly in how to provide artificial ventilation during emergencies, working in remote locations and during disasters. Manual techniques, including rescue ventilation and the use of the paediatric bag valve mask are important. There are a number of portable pneumatic ventilators which have been used in emergency and transport ventilation for a number of years and these are now supplemented by a range of more complex electronic ventilators which are capable of providing more adapted ventilation. However, pressure controlled ventilation with PEEP and the used of CPAP remain the mainstays of paediatric ventilation. This chapter provides a basic overview of the subject.

Portable Mechanical Ventilators

Following over 40 years of development there is now a range of portable mechanical ventilators available which are used both inside and outside the hospital by non – specialists from the medical, paramedical and nursing professions. These devices range in their complexity from simple resuscitation devices which can replace the use of the bag – valve mask through to complex computer – controlled electronic ventilators for use in specialised patient transport and a number of classifications are possible. A basic knowledge of the function, controls and monitoring and alarm systems of a portable mechanical ventilator is essential for its safe use. The alarm systems supplement rather than replace good clinical observation and practice. Before any first use of a new ventilator there must be commissioning checks, preferably using a test lung. Equally, there must be user checks before a ventilator is used in emergency or in transport. The effective use of portable ventilators requires a suitable training programme and is helped by the use of cockpit – style checklists. Although portable ventilators are increasingly widely – used around the world there is a relatively limited number of publications concerning their use and there is a need for more work in this field. Several publications have shown the potential deficiencies of bag – valve compared with mechanical ventilation.

How the Lungs Work: Mechanics and Gas Exchange with the Blood

Breathing and respiration are often used interchangeably but breathing relates to the normal action of the chest and lungs while respiration is defined as external, which concerns the transport of oxygen through the lungs to the alveoli and removal of carbon dioxide and internal, which deals with the way oxygen is used in the cells of the body. Ventilation of the lungs (the flow of air to the alveoli) is determined by compliance (distensibility) of the lungs and resistance to gas flow in the airways. The amount of air in the lungs is measured by spirometry which describes the volumes of air at each stage of breathing. Tidal volume and functional residual capacity are the two most important of these in relation to artificial ventilation. The concentration of oxygen in the alveoli is determined by the alveolar air equation which links the concentration in the inspired air with ventilation and the concentration of carbon dioxide.

Artificial Ventilation in Difficult and Extreme Environments

Artificial ventilation may be required in difficult and extreme environments which are quite different from normal hospital and emergency medical practice. These include following disasters, where the medical infrastructure may be severely overloaded or disrupted in developed and developing nations alike, respiratory epidemics where there is a requirement for mass ventilation, in toxic environments, in high and low pressure conditions and in medical practice in areas of the world where the medical services are very limited. All these circumstances provide challenges for those providing artificial ventilation and for the equipment they are using. These can be overcome by careful planning, training, familiarity with the potential hazards and how the circumstances can affect the performance of the ventilators being used.

A Brief History of Artificial Ventilation

Artificial ventilation (AV) of the lungs to replace failed normal breathing goes back many thousands of years but the development of modern AV started in the seventeenth century, first with positive pressure ventilation, then by negative pressure methods to simulate normal breathing and finally in the twentieth century back to positive pressure ventilation which is now the clinical standard. AV may be delivered by a variety of methods ranging from simple expired air resuscitation through to complex computer- controlled machines now used in the intensive care units of hospitals. Mechanical ventilation over the past 50 years has followed increasingly divergent pathways, with the development of ICU ventilators operated by specialists along with a parallel development of simple portable emergency and transport ventilators which are designed to be used by medical, nursing and paramedical personnel who are not normally involved in ICU ventilation.