1 The Anaesthesia Machine

The Association of Veterinary Anaesthetists (AVA) defined several years ago the recommended requirements that a veterinarian must meet in order to perform general anaesthesia in a dog, cat or horse. Any veterinarian who performs general anaesthesia must be able to,

1. secure the animal's airway (e.g. by endotracheal intubation),

2. administer oxygen,

3. perform manually controlled ventilation (e.g. by using an Artificial Manual Breathing Unit (AMBU bag), an anaesthesia machine or, in case of horses, a demand valve),

4. administer drugs and infusion solutions intravenously, ideally via a venous catheter and

5. perform cardiopulmonary resuscitation (CPR).

In order to fulfil these 5 requirements, the veterinarian must, on the one hand, have the necessary materials and equipment on site and, on the other hand, be proficient in the necessary techniques (e.g. intubation, catheterisation, manual ventilation).

With the help of a modern anaesthesia machine, inhalation anaesthetics and oxygen can be administered to the patient in a safe and dosed manner. Carbon dioxide (CO2) can be removed from the exhaled air. Most anaesthesia machines can act as breathing monitor and be used for manual ventilation.

The anaesthesia machine generally consists of the patient part (= breathing system) and the actual machine part ( ? Fig. 1.1).

Fig. 1.1 Schematic overview of an anaesthesia machine with patient part (shown here as an example with a circle system as breathing system) and the actual machine part; APL valve: adjustable pressure limiting valve.

1.1 Patient part or breathing system

The patient part of the anaesthesia machine includes all parts of the breathing system that are downstream of the fresh gas outlet, i.e. usually the non-rebreathing system or the circle system as a rebreathing system.

1.1.1 Non-rebreathing system or rebreathing system?

The non-rebreathing system (or semi-open system) differs from the rebreathing system (or semi-closed and closed system) in the way it eliminates CO2. In the semi-open system, this works by increasing the fresh gas flow, which flushes out the CO2 with the exhaust air. In the semi-closed and closed system (= circle system, because of the constantly recirculating gas flow), the CO2 is taken out of the system by the absorber lime. It is called semi-closed because the supply of O2 and inhalation anaesthetic exceeds the consumption of the animal, i.e. the exhaust air continuously escapes via the pressure relief valve. A system is closed when the supply of O2 and inhalation anaesthetic exactly covers the patient's needs and (despite the pressure relief valve being open) no more exhaust air escapes from the system.

The open system has a special position among the anaesthesia systems. This is classically the so-called Schimmelbusch mask with which patients were anaesthetised with ether at the beginning of gas anaesthesia. Nowadays, anaesthesia chambers, such as those used for the induction of anaesthesia in small pets/laboratory animals or exotics, are also included ( ? Fig. 1.2). There is no fixed definition, in short: "everyone gets something, including the anaesthetist". Consequently, it should only be used with excellent suction (e.g. under an exhaust hood). To work with an open system, one does not necessarily need an anaesthesia machine. Usually, the semi-open (non-rebreathing) system and the semi-closed (rebreathing) system are used in veterinary medicine because, among other things, the exposure of staff and the environment to hazards from the use of the open system is too high and conditions for safely performing anaesthesia with the closed system are too difficult to fulfil under practice conditions.

Fig. 1.2 Open system for anaesthesia induction in small mammals and exotics.

1.1.2 Non-rebreathing system

There are many different non-rebreathing systems, all with slightly different set ups. One system commonly used is the so-called Bain system ( ? Fig. 1.3), which is also called Mapleson D system. It can be designed as a coaxial system, i.e. the inspiratory tube lies within the expiratory tube, which can be advantageous.

Fig. 1.3 Example for a non-rebreathing system (Bain or Mapleson D system); APL valve: adjustable pressure limiting valve.

Fig. 1.3a Normal system.

Fig. 1.3b Coaxial system (breathing hose within hose).

In non-rebreathing systems, the entire exhaled gas mixture is completely discharged from the system with the exhaust air. For this reason, the fresh gas flow must be at least as large as the respiratory minute volume, which is approx. 200-250 ml/kg/min.

Definition

Minute ventilation

Minute ventilation is calculated from tidal volume (10-15 ml/kg) times respiratory rate (depending on the species, approx. 8-12 /min for small animals, up to 20 /min for small companion animals). The tidal volume is one of the few parameters that is constant in almost all animal species. The standard values for respiratory rate are listed in ( ? Table 2.1 ).

Due to the advantages of the Bain system ( ? Table 1.1 ), its use is recommended for small mammals, cats and dogs under 5-8 kg. It can also be used for larger animals without any problems, but then the waste and thus the costs increase disproportionately.

Advantages

Disadvantages

Low dead space volume

High fresh gas flow (large quantities), inhalation anaesthetic and oxygen are wasted, i.e. discarded unused = more expensive and harmful for the environment

Little respiratory resistance

Loss of heat and moisture from the patient

Rapid changes in concentration (inhalation anaesthetic, oxygen) are possible, the system is very "direct"

Coaxial systems are difficult to clean and unrecognised disconnection of the inner tube may occur

Various sizes available, also coaxial systems where theoretically the exhaled air warms the inspiratory gases

An accidentally closed pressure relief valve can quickly lead to barotrauma

Little weight on the patient (tubes and adapters are lightweight)

In the case of poor/insufficient exhaust, possible contamination of the environment with waste gases

1.1.3 Rebreathing or circle system

The circle system can be described as semi-closed or closed depending on the amount of fresh gas flow. As mentioned above, in the semi-closed system the supply of O2 and inhalation anaesthetic exceeds the consumption by the animal. Consequently, part of the exhaust air must continuously escape from the system through the pressure relief valve (hence "semi-closed"). If the circle system is used in such a way that supply and consumption by the patient are exactly the same, the system "closes". This means that even though the pressure relief valve is open, neither O2 nor inhalation anaesthetic escapes. Under practical conditions, the semi-closed system is much more convenient, because the use and the safety range correspond better to the clinical requirements.

Clinical relevance

Closed system and low-flow anaesthesia

In order to work with a closed system, several conditions must be met:

1. The patient's oxygen requirement must be calculated (4-8 ml O2/kg/min). To be able to administer these small amounts correctly, the flowmeter must have a particularly...