Artificial Ventilators

When pressure is applied by the ventilator to drive gas into the lungs, energy is expended to overcome airway resistance R to gas flow in the airways, in order to store gas in the alveoli, whose readiness to having their volume increased is represented by the concept of compliance, C. The storage of gas within individual compliances represents potential energy storage. The acceleration of gas and anatomical components within the system represent kinetic energy change, resisted by the inertance, I, of the system. At conventional ventilation frequencies, these kinetic energy changes are negligible compared with the other energy changes taking place. Inertance can be ignored and the system behaves like a flow resistor in series with a compliance. These variables determine the pressure and volume changes that take place within the lung. As ventilation frequency increases into the high range, inertance becomes significant and the frequency response of anatomical structures becomes important, with phase differences between pressure and volume signals occurring [Lin et al. 1989]. Mechanical resistance, R, in the system is largely due to resistance to gas flow down airways and is defined as pressure change per unit flow ΔP/Q, typically 4 cm H2O l−1 s. at 0.5 l s−1. However there is a contribution from viscous resistive forces in the lung and chest wall tissues. High resistance may require long inspiratory times, while expiratory times that are too short may lead to gas trapping in alveoli. Excessive resistance may mean that the power required to ventilate the patient may exceed that available to the ventilator. Compliance, C, is a measure of the capacitative properties of the alveoli and is defined as volume change per unit pressure change ΔV/ΔP. It is not uniform throughout the respiratory cycle and has values in the range 0.05–0.10 L (cmH2O)−1. Dynamic compliance is the value given to this variable throughout the inspiratory period to the end of inspiration, when airway pressure is highest. During the inspiratory pause, airway pressure falls to a plateau during which the static compliance can be measured, which is greater than the dynamic compliance.

Gas Supply and the Anaesthetic Machine

In Europe and other advanced medical communities, medical gases are generally supplied by pipeline, with cylinders available as back up. Large hospitals usually have oxygen supplied and stored in liquid form, since one volume of it provides 840 volumes of gaseous oxygen at 15◦C. It is stored in a secure Vacuum Insulated Evaporator (VIE) on the hospital site. The arrangement is shown in Figure 22.1. The VIE consists of an insulated container, the inner layer of which is made of stainless steel, the outer of which is made of carbon steel. The liquid oxygen is stored in the inner container at about−160◦C (lower than the critical temperature of−118◦C) at a pressure of between 700 and 1200 kPa. There is a vapour withdrawal line at the top of the VIE, from which oxygen vapour can go via a restrictor to a superheater, where the gas is heated towards ambient temperature. Where demand exceeds supply from this route, there is also a liquid withdrawal line from the bottom of the VIE, from which liquid oxygen can be withdrawn; the liquid can be made to join the vapour line downstream of the restrictor and pass either through the superheater or back to the top of the VIE. The liquid can also be made to pass through an evaporator before joining the vapour line. After passing through the superheater, the oxygen vapour is passed through a series of pressure regulators to drop the pressure down to the distribution pipeline pressure of 410 kPa. It should be remembered that no insulation is perfect and there is a pressure relief valve on top of the VIE in case lack of demand and gradual temperature rise results in a pressure build up in the container. There is a filling port and there is usually considerable wastage in filling the VIE; the delivery hose needs to be cooled to below the critical temperature, using the tanker liquid oxygen itself to cool the delivery pipe. The whole VIE device is mounted on a hinged weighing scale and is situated outside the hospital building, protected by a caged enclosure, which also houses two banks of reserve cylinders.

Blood Gas Analysis

A blood gas machine has electrodes to measure pH, pCO2 and pO2 and often measures Hb and some biochemistry as well [King et al. 2000]. Derived values from such a device include O2 saturation, O2 content, bicarbonate, base excess and total CO2. This is the Clarke electrode described in the previous section on gas analysers and is suitable for both respiratory and blood O2 analysis. A pH unit has been defined in Chapter 1 as. In words, this can be described as ‘the negative logarithm, to base ten, of the hydrogen ion concentration’. The physical principle on which the pH electrode is based depends on the fact that when a membrane separates two solutions of different [H+], a potential difference exists across the membrane. In a pH electrode, such a membrane is usually made of glass and the development of a potential difference between the two solutions is thought to be due to the migration of H+ into the glass matrix. If one solution consists of a standard [H+], the pH of the other solution can be estimated by measurement of the potential difference between them. The glass membrane used is selectively permeable to H+. No current flows in this device, which does not wear out, in contrast to the Clark electrode, in which current does flow and that does need periodic replacement. The pH measurement system is shown diagrammatically in Figure 17.1. It consists of two half cells. In one half it has an Ag/AgCl electrode and in the other a Hg/HgCl2 (calomel) electrode. Each electrode maintains a fixed electrical potential. The Ag/AgCl electrode is surrounded by a buffer solution of known pH, surrounded by the pH sensitive glass. Outside the glass membrane is the test solution, usually blood, whose pH is to be measured. It is the potential difference across the glass, between these two solutions, which is variable. The blood or other solution is separated from the calomel electrode by a porous plug and a potassium chloride salt bridge to minimise KCl diffusion. The potential difference across the system is about 60 mV per unit of pH change at 37◦C.

Respiratory Gas Analysers

The purpose of respiratory gas analysis during anaesthesia is to identify and measure the concentrations, on a breath by breath basis, of the individual gases and vapours in use. It may also be useful as a guide to cardiac function or to identify trace contaminant gases. Different techniques use different physicochemical properties of the gas or vapour. An understanding of the physical principle underlying each method is necessary in order to recognise the value and limitations of each. In terms of the device’s ability to respond on a breath by breath basis, there are two important components: the time taken for the gas to be sampled from the anaesthetic machine or breathing system, the delay time; then there is the time taken for the device to measure the gas concentration, the response time. This is depicted in Figure 16.1. Most of the delay occurs in the delay time or transit time and can be reduced either by analysing the gas sample close to the airway, or by using as short and thin a sampling tube and as high a sampling flow rate to the analyser as possible [Chan et al. 2003]; the sampling flow rate is usually of the order of 100 to 200 ml min−1. If minimal fresh gas flow rates are being used in a circle anaesthetic breathing system and the sampled gas is not returned to the breathing system, then a high gas sampling rate could represent a significant gas leak. Figure 16.1 shows a sigmoid curve of recorded gas concentration change in response to a square wave input change. The response of a gas analyser is often expressed as the time taken to produce a 90–95% response to a step or square wave input change. A square wave change in gas concentration can be produced by moving a gas sampling tube rapidly into and out of a gas stream, by bursting a small balloon within a sampling volume containing a gas sample, or by switching a shutter to a gas sample volume using a solenoid valve. An important part of the use of gas analysers is zeroing and calibration since they are all prone to drift in both zero and gain.

Gas pressure, Volume and Flow Measurement

The physics of pressure, flow and the gas laws have been discussed in Chapter 7 in relation to the behaviour of gas and vapour. This section will focus on the physical principles of the measurement of gas pressure, volume and flow. Unlike a liquid, a gas is compressible and the relationship between pressure, volume and flow depends on the resistance to gas flow (or impedance if there is a frequency dependence between pressure and flow in alternating flow, see Chapter 4 for the electrical analogy of this) in conduits (bronchi, anaesthetic tubing); it also depends on the compliance of structures being filled and emptied (alveoli, reservoir bags, tubing or bellows). Normal breathing occurs by muscular expansion of the thorax, thus lowering the intrathoracic pressure, allowing air or anaesthetic gas to flow towards the alveoli down a pressure gradient from atmospheric pressure. When positive pressure ventilation occurs, gas is ‘pushed’ under pressure into the alveoli. Depending on the exact relationship between the ventilator and the lungs, different relationships exist between airway pressure (rather than alveolar pressure, which cannot easily be measured) and gas flow and volume. Gas pressure measurement devices were traditionally in the form of an aneroid barometer, a hollow metal bellows calibrated for pressure and temperature, which contracts when the external pressure on it increases, and expands when it decreases. The movement is linked to a pointer and indicator dial. It is often more convenient to make the device in the shape of part of a circular section, but the principle is the same. This is what the Bourdon gauge, which commonly measures pressure in gas cylinders, looks like. The detection of movement of the diaphragm of an aneroid barometer can take several forms. The movement can either be linked via a direct mechanical linkage to a pointer, or diaphragm movement can be linked to a capacitative or inductive element in an electrical circuit, such as a Wheatstone bridge. Airway pressure during spontaneous breathing or artificial ventilation is low. The preferred units of measurement are cm H2O and the range of values is between −20 and +20 cmH2O. The aneroid barometer to measure this will therefore be of light construction, using thin copper for the bellows material.

Blood Pressure Measurement

Blood pressure measurement occurs either non-invasively or invasively, and usually refers to systemic arterial pressure measurement, but can also refer to systemic venous or pulmonary arterial pressure measurement. In 1733 the Reverend Stephen Hales was the first person to measure the blood pressure in vivo in unanaesthetised horses by direct cannulation of the carotid and femoral arteries. In doing so he observed the pulsatile nature of flow in the circulation. In 1828 Poiseuille developed the mercury manometer, and used it to measure blood pressure in a dog. The mercury manometer has, of course, become the standard technique against which other techniques are compared. The earliest numerical information on blood pressure measurement came from direct rather than indirect measurement in 1856 by Faivre, using Poiseuille’s device. However, in the last part of the nineteenth century, non-invasive measurement techniques were developed. In 1903, Codman and Cushing introduced the concept of routine intraoperative blood pressure measurement, which at the time was a revolutionary concept. Nowadays it is a fundamental part of minimal monitoring criteria. There are several techniques of non-invasive BP (NIBP) measurement, all of which function by occluding the pulse in a limb with a proximal cuff, then detecting its onset again distally, on lowering the cuff pressure. Detection methods include palpation, auscultation, plethysmography, oscillotonometry and oscillometry. Accuracy of all non-invasive techniques depends on cuff size in relation to the limb concerned, and over which artery the cuff is placed. Such techniques of NIBP measurement are necessarily intermittent. Much discussion has taken place on the accuracy of these devices, and the accuracy of diastolic pressure measurements needs improving, and there are ideas proposed for new non-invasive devices [Tooley and Magee 2009]. In the absence of a stethoscope, this technique is simple and reliable. After inflating the cuff on the upper arm to a pressure of above that of systolic, the cuff is then deflated while palpating the brachial artery and the systolic pressure is measured with a mercury column at first detection of the pulse. A study by van Bergen [1954] showed that BP can be underestimated by this method by up to 25% at 120 mmHg.

Principles and Standards of Anaesthetic Monitoring

The World Federation of Societies of Anaesthesiology (WFSA) adopted standards relating to the safe practice of anaesthesia in 1992 and such standards had already been proposed by a number of countries in order to cut the morbidity due to anaesthesia itself. In the modern era it is easy to forget that historically anaesthesia and surgery did indeed have associated morbidity and mortality and there was very little assistance from technology to monitor patients. The evolution of these standards is based on two main requirements of monitoring. The first is to record anticipated deviations from normal values, which require accurate measurement to ensure patient safety. The second is to warn of unexpected, life-threatening events that, by definition, occur without warning, and could affect the fit, young patient as easily as the old and infirm. All international standards stress the importance of the continual presence of a fully trained and accredited anaesthetic person, and one Australian study demonstrated that many mishaps occur in the absence of such a person [Runciman 1988]. This applies to general and regional anaesthesia, sedation and recovery. Because perceptions of safety and standards vary throughout the world, despite the presence of an International Standards Organisation, debate about the minimum requirements for monitoring continue. Central to the maintenance of these standards is the quality of persons entering the specialty, the quality of training programmes, and the continuing education of specialists throughout a professional lifetime [Sykes 1992]. It is difficult to determine with certainty the effect that additional technological monitoring has on safety. One clear example is the inability of the trained human eye to detect cyanosis, this human failure occurring maximally at 81–85% oxygen saturation. Clearly, the pulse oximeter has improved the quality of cyanosis detection. Numerous studies all over the world have shown that mortality due to anaesthesia itself fell significantly between the 1950s and the 1980s, by which time extensive technological monitoring was being introduced, and training programmes had been very much improved. Utting [1987] reviewed 750 cases of death and cerebral damage reported to the British General Medical Council between 1970 and 1982 that were thought to be the result of errors in technique.

Behaviour of Fluids (Liquids and Gases, Flow and Pressure)

A fluid can be either a liquid or a gas. Fluids exhibit different flow behaviours depending on their physical properties, in particular viscosity and density. Flow characteristics also depend on the geometry of the pipes or channels through which they flow, and on the driving pressure regimes. These principles can be applied to any fluid, and the complexity of the analysis depends on the flow regimes described in this section [Massey 1970]. Fluid flow is generally described as laminar or turbulent. Laminar flow, demonstrated by Osborne Reynolds in 1867, is flow in which laminae or layers of fluid run parallel to each other. In a circular pipe, such as a blood vessel or a bronchus, velocity within the layers nearest the wall of the pipe is least; in the layer immediately adjacent to the wall it is probably actually zero. In fully developed laminar flow, the velocity profile across the pipe is parabolic, as shown in Figure 7.1, and as discussed in Chapter 1. Peak velocity of the fluid occurs in the mid line of the pipe, and is twice the average velocity across the pipe at equilibrium, and layers equidistant from the wall have equal velocity. The importance of laminar flow is that there is minimum energy loss in the flow, i.e. it is an efficient transport mode. This is in contrast to turbulent flow, where eddies and vortices (flow in directions other than the predominant one) mean that energy in fluid transport is wasted in production of heat, additional friction and noise. The result is that the pressure drop required to drive a given flow from one end of the pipe to the other is greater in turbulent than in laminar flow. The shear stress τ, which is the mechanical stress between layers of fluid and between the fluid and the tube wall, is proportional to the velocity gradient across the tube (dv/dr) of the fluid layers. The constant of proportionality between these two variables is the dynamic viscosity, η.

Statistics

This chapter will provide background to enable the reader to understand basic statistics and be able then to follow more complex statistical ideas. Although statistics is more than the mere analysis of data, it is a subject largely about data, so this will be discussed first. Data can be categorical or numerical, and in these two classifications there are various different types of data. This is the allocation of the individual to one of two categories. Often these relate to the presence or absence of some attribute. These data also have many other names such as binary, dichotomous and attribute data. Examples of such categorisations for patients include: ◆ Male/Female ◆ Smoker/Non-smoker ◆ Anaesthetist/Surgeon ◆ Married/Single. Each of these can be only be one or the other – they could be coded ‘1’ or ‘0’ to be binary (or on, off). For example male = 0, female = 1, or vice versa. Many classifications require more than two categories, such as: blood group, type of doctor, country of birth. Also the two categories, such as described previously, might be expanded into several categories. For example the married/single could be expanded to: married/single/divorced/separated/ widowed. This sort of data is called nominal data where there are several categories, but with no logical order. When there is a natural order (such as in seniority), the data are then called ordinal data. For example, anaesthetists could be divided into: ‘Foundation year 1’, ‘Foundation year 2’, ‘speciality doctor’, consultants’, ‘senior consultants’ and ‘clinical directors’. Ordinal data can be reduced to two categories, with possibly a considerable loss of information (e.g. ‘senior doctors’, ‘junior doctors’). Discrete numerical data are where the observation takes exact numerical values. Counts or events are discrete values. For example: number of children, number of ectopic beats in a time period and so on. Continuous (or analogue) data are usually obtained by some form of measurement. Examples are body temperature, blood pressure, height and weight. These values have an infinite number of possibilities, depending on the measurement interval, and variation. Although there are infinite possibilities, measurement systems usually round the continuous data up, or down, to discrete values. Blood pressure is often rounded up to the nearest 5 mmHg, for example.

Airway Management Devices

The most important interface between the breathing system and the patient’s lungs is an airway management device (AMD). Post-operatively it can be considered to be a means of delivering oxygen enriched air to the patient. Intraoperatively it is intended to secure the patient’s airway, which might otherwise obstruct due to deep anaesthesia, to provide a reasonably gas tight seal to ensure accurate delivery of anaesthetic gases and, if necessary, to protect the lungs against aspiration of gastric contents. Postoperatively, the AMD can be nasal prongs or a variable performance mask, whose efficiencies may not be predictable [Wagstaff et al. 2007]. Intraoperatively it might be an artificial airway with a facemask, a supraglottic airway of one of the many types now available or an endotracheal tube (ETT). A supraglottic airway is one that sits in the pharynx or larynx above the vocal cords and these days is usually a laryngeal mask airway (LMA) of the numerous types now available, a cuffed oropharyngeal airway (COPA), or a Combitube. The LMA types available consist of: the classical LMA; the flexible (reinforced) LMA with a flexible tube to the breathing system; the ‘Proseal’, which has a gastric drainage tube as well as a gas transport tube; the intubating LMA, a device with a rigid right angled tube that acts as a ventilation conduit in the usual way, but through which an endotracheal tube may also be blindly introduced into the trachea; the ‘I-gel’ which has a gastric and a respiratory port as does the Proseal, but is less bulky, and whose bowl does not require inflation with air, but is filled with a gel that expands with body heat to form a seal. These days, almost all devices are made of material that excludes latex, but care should be taken to ensure this is indeed the case when there is a latex sensitive patient. Depending on the exact surgical and anaesthetic circumstances, the anaesthetist’s experience and equipment availability, a choice is made between these devices to secure the airway for a given operation. Additionally, there are other devices available to assist in securing the airway, such as the laryngoscope, the fibre optic bronchoscope and the cricothyrotomy tube.