Introduction

Anaesthesia machines (support and therapy)

Anaesthesia may be done by the injection of an agent into the blood and/or by inhaling a substance like nitrous oxide or a volatile vapour like sevoflurane. In addition to gas anaesthesia, anaesthetics are also given directly into the blood stream. Here we will describe the anaesthesia gas / vapour delivery system as a separate machine, although it may be an integral part of a ventilator and/or a patient monitoring system. Its main purpose is to deliver fresh gas to the patient via a breathing system and perhaps a ventilator. It may comprise also the gas scavenging system and the suction device. The suction described later in this chapter is an important additional device, and the breathing system may be directly coupled to the machine or also to a ventilator supplied by the machine. The multigas analyzer may also be on the same trolley.
Fig.25 shows the main components: the supply of medical grade air, N₂O and O₂ gases, the gas mixer with flow monitors, the vaporizer and some safety devices.

Figure 25. Anaesthesia machine

The gas supply may either be from local gas bottles filled to high pressure e.g. 60 bar (N₂O) or 150 bar (O₂), and equipped with manometers, pressure reduction valves and simple gas flowmeters. Or it may be taken from the hospital installed gas pipeline system at medium high pressure (3-7 bar).

The gas mixer has individual gas flow sensors [L/min] for each gas, measured before mixing. Flow setting is adjusted with individual spindle valves. The gas mixer is connected to the vaporizer, where anaesthetic volatile vapors may be added such as: Halothane, Enflurane, Isoflurane, Sevoflurane, Desflurane and ether.

Often the machine comprises a gas scavenging system and suction for clearing airways. The surgeon may have their own suction for use in the wound.

Risk considerations Anaesthesia machine

Loss of oxygen is of course critical, and a special oxygen flush can supply large direct oxygen flow (NB! lung pressure). Functioning of the suction may be critical. Anaesthetic dose is important, and concentration measurements in the breathing system near the patient are very useful.

Spirometers (diagnosis)

A spirometer is an instrument for measuring lung volumes. There are basically two different types: the water sealed and the pneumotachometer models. This is an instrumentation which usually need not be sterile, disinfection procedures are sufficient.

Water sealed models

Fig.26 is an outline of the system. The patient is connected to the respiration tube via a tight mask. With a few breaths the patient respires into a closed volume dominated by the gas drum. The drum rises and sinks with minimal friction following the respiration. The drum is balanced by a counterweight, and the vertical movement is registered by a pen

Figure 26 Spirometer, watersealed

fixed to the connecting wire. The scale is graduated in litre. The respirogram is then drawn on the paper passing under the pen. Because of the inertia of the system the instrument is best adapted to static or slowly changing volumes. Precautions must be taken to avoid problems with temperature changes and humidity with condensation. An important advantage of the instrument is stability and ease of calibration; it is well suited to be a standard reference instrument in a laboratory. The spirometer represents a closed volume of maximum compliance. In the form shown on Fig.26 the spirometer can not determine the residual lung volume. By using a dilution technique with a tracer gas⁷ absolute volumes can be determined.

Risk considerations

The breathing system in its simplest form as shown is closed and without CO₂ absorber, the measuring time is accordingly very limited. It may have a bothersome cleaning and disinfection procedure between each patient.

Electronic models

The electronic models may be very small, wireless and convenient, and they are also well adapted to dynamic measurements e.g. of FEV (Forced Expiratory Volume, often during the first second, FEV₁). The model may be just a mouthpiece with a differential pressure transducer coupled to Pitot tubes or a flow resistance in the form of a narrowing tube or Fleisch flow resistor, se earlier in this chapter. In this form they are also known as pneumotachometers. The signal from the transducer may be transferred wirelessly to a computer system, where signal processing may produce e.g. volumes by integration of flow, peak flow values etc.

Risk considerations Spirometers

The tubes in contact with the mouth need not necessarily be sterile, but disinfected or acquired for single patient use.

whole body plethysmographs (diagnosis)

Determination of absolute lung volumes and airway resistance can be performed in a whole body plethysmograph, Fig.27. The patient is closed into a chamber and breaths into a mouthpiece with a pressure sensor and a flow outlet where the flow can be stopped by a closing actuator. Chamber volume V_c is known, and the chamber pressure P_c and mouthpiece pressure P_lu are measured. At the end of a normal expiration the gas flow is closed, and the patient performs forced ventilation against the closed mouthpiece. The variations in chamber pressure ΔP_c and airway pressure ΔP_lu are measured as static values (sufficient time at inspiration and expiration effort). In this way it is no flow at the sampling moment so that the mouthpiece pressure is equal to the lung pressure. By considering the whole body but the lungs incompressible the forced respiration pressure diminishes the lung volume and increases the chamber volume by the same amount, and reduces the chamber pressure P_c. By using Boyle-Mariottes law it is possible to show that the lung volume (residual volume included) V_lu is given by:
Equation 7 V_lu = - V_c ΔP_c /ΔP_lu

Risk considerations

Some patients have a problem with being in a closed narrow chamber (claustrophobia). Mouthpiece and airway must be disinfected or be of single-use types.

Figure 27 Whole body plethysmograph