detection of the biological signal, by a sensing device which responds to a characteristic signal in the form of electrical, mechanical, electromagnetic, chemical or thermal energy

 transduction, in which the output from the sensor is converted into another form of energy, usually to a continuous electrical signal

 amplification and signal processing to extract and magnify the relevant features of the signal and reduce unwanted noise

 display and storage – the output from the instrument is presented to the operator. Storage for future use may be achieved using mechanical markers, printed copy or computer memory.

 continuous real-time detection, processing and recording of measurements

 increased range of measurements possible

 miniaturization of complicated and powerful instruments

 sophisticated artefact rejection and noise reduction algorithms

 complex on-line mathematical and statistical signal processing in upgradeable software, e.g. Fourier analysis of the EEG

 automated control of the apparatus and the timing and process of measurement, and integration of alarms

 storage in memory, permitting trend analysis, future display and further analysis

 user-friendly audio-visual display of recordings, integrating many simultaneous clinical measurements, and able to be customized by the user

 less maintenance than analogue instruments.

 dependence on electrically powered equipment

 degradation of clinical skills and alternative manual measurements through disuse

 impoverished understanding of the principles of complex measuring equipment and the requirements for correct use

 illusion of the unquestionable accuracy of measurements produced by expensive computer-controlled equipment and presented on an impressive display or typed copy.

The Importance of Repeated Measurements

Differences in repeated clinical measurements arise from three causes:

The anaesthetist must be satisfied with the accuracy and precision of any clinical measurement used in patient management. Repeated measurements which are consistent ensure that the measurement is representative, i.e. precise, but do not ensure accuracy. For example, repeated recordings of invasive arterial pressure may be extremely consistent, but erroneous if the transducer is not calibrated against the correct zero point. Defences against the uncritical acceptance of inaccurate measurements include meticulous care in calibrating instruments and recording of clinical measurements, and reflection on clinical measurements which do not fit the clinical state of the patient or other related measurements. A discrepant result should be rechecked, using a different measurement technique if possible, before it is used to change patient management. This is especially true of complex, operator-dependent techniques such as measurement of cardiac output.

Zero Stability

The ability of a measurement to maintain a zero reading on the display or record when the input signal is zero defines the zero stability. The importance of zero instability depends on the magnitude relative to the signal, e.g. a zero drift of a few millimetres of mercury is much less important for the measurement of arterial pressure than it is for intracranial pressure.

Gain Stability

The majority of biological signals are amplified before reproduction. This ‘gain’ may be fixed or controlled by the user. When set, this should remain constant over the period of recording.

Amplitude Linearity

The degree of amplification of the signal should be constant over the whole range of signal amplitudes. Manufacturers usually specify the degree of linearity of electronic components over a certain amplitude range. The amplitude linearity of a complete clinical system may be confirmed easily in an electronics laboratory by comparing the output to known, standardized test signals.

Hysteresis

Some instruments, such as thermistors and humidity sensors, may display hysteresis. This is a special case of non-linearity, in which the output differs depending on whether the input signal is increasing or decreasing.

Frequency Response

Many biological signals vary in a complicated and rapidly changing pattern. Accurate reproduction of a complex waveform requires that all of the component frequencies which make up the waveform are processed in an identical manner. This requires more than simply equal amplification irrespective of frequency, i.e. no amplitude distortion. It also implies that the relative positions of the various frequency components of the waveform are not shifted, i.e. no phase distortion. In practice, accurate reproduction up to the 10th harmonic of the fundamental frequency is sufficient for clinical purposes, e.g. 30 Hz for an arterial pressure waveform associated with heart rates up to 180 beats min^–1 (3 Hz).

Signal-to-Noise Ratio

Biological signals are obscured to a variable degree by unwanted or extraneous signals which have similar physical characteristics and are described as noise, e.g. heart sounds become difficult to detect in the presence of continuous, noisy breath sounds. The efficiency of isolation of the signal from unwanted biological signals and electronic noise sources in the equipment is defined by the signal-to-noise ratio. The variability of the amplitudes of signal and noise is enormous and the signal-to-noise ratio is described using a logarithmic scale of decibels. Microvolt EEG measurements are particularly susceptible to noise from many sources. Biological noise includes contaminating ECG and EMG potentials, particularly from the scalp muscles, and interference from electrochemical activity at the skin–electrode interface. Electrostatic and electromagnetic linkage between the recording wires and nearby sources of mains electricity generates noise which is predominantly 50 Hz frequency and harmonics. Radiofrequency noise from diathermy or transmitters may also be picked up at this stage. Physical disturbance of the recording wires causes tiny changes in capacitive potentials and may add low-frequency noise, called microphony. Thermal noise is added during amplification, particularly at the input stage when the signal is in the microvolt range. Good amplifier design, electronic filtering of unwanted frequencies and modern techniques of digital signal processing may extract small signals from considerable background noise, but this inevitably introduces some distortion of the signal. Prevention of contamination of the signal by minimizing sources of noise before the signal is amplified is always preferable. The operator is responsible for correctly using measuring instruments to optimize the signal and for applying knowledge of the physical principles of the measurement to minimize contamination by noise.

Mechanical Measuring Instruments

Measuring instruments based on mechanical principles lack the flexibility and automated control of computerized devices, but use ingenious methods for processing and displaying analogue measurements. For example, mechanical spirometers use precision-engineered gears to translate the movement of a piston or vane into the rotation of a calibrated dial.

Analogue Computers

Analogue computers use hardware comprising electronic circuits and operational amplifiers. Signals are processed in the form of continuously variable electric potentials. Analogue hardware components continuously perform a wide variety of mathematical functions on a rapidly changing input waveform. Integration and differentiation are formidable mathematical tasks for a digital computer, which can be solved simply and cheaply using analogue circuits comprising capacitors and resistors. Integration of the flow signal from a pneumotachograph produces a volume waveform.

Microcomputers and Digital Signal Processing

Digital signal processing offers a powerful alternative to mechanical processing and analogue computation. A fundamental step in this process is the conversion of a continuous analogue electrical signal into a discrete digital form. This analogue-to-digital conversion is achieved by measuring or ‘sampling’ the continuous input signal at regular intervals, to produce a series of discrete measurements over time which are in a suitable format for digital computation. The overwhelming advantage of digital processing is that the manipulation of the digitized signal is performed by a flexible and unlimited series of software calculations which range from mathematical functions to the analysis of statistical properties and trends.

Analogue-to-Digital Conversion

The core processing units of digital computers assume one of two stable states, i.e. a binary, rather than decimal, code. This imposes a limit on the resolving power of the digital processor, although with increasing processing power this limit has become negligible. Earlier 8-bit computing comprised a binary number of eight digits representing 2⁸ integer decimal numbers, from binary 00000000 = decimal 0 to binary 11111111 = decimal 255. In short, an 8-bit converter can resolve an analogue signal with an accuracy of one part in 255, i.e. with an amplitude resolution of 0.4% of full scale. A 12-bit converter is more accurate, with a resolving power of one part in 4095 or 0.02% of full scale. More modern processors are capable of 32-bit computing corresponding to a range of 2³² integer decimal numbers (a resolving power of 0.00000002%), with 64-bit processors now commonly found in domestic computer equipment. Whilst highly accurate, the cost of this improvement in resolution is more expensive hardware to digitize, process and store considerably more digital information.

Amplitude resolution is not the only determinant of the accuracy of analogue-to-digital conversion. Resolution over time, determined by the sampling frequency, is also important. A relatively low sampling frequency may provide a representative sample of values for a slowly changing waveform but it may inadequately represent high-frequency components and introduce an aliasing error, in which different signals become indistinguishable. The Nyquist theorem suggests that the minimum sampling frequency to maintain the integrity of the waveform is at least twice the highest frequency component with significant amplitude in the input signal waveform, e.g. a sampling frequency of 100 Hz would adequately capture the fastest rate of change in a physiological pressure signal.

The immensely powerful and complicated hardware and software programming instructions responsible for performing the tasks of digitizing, processing, storing and displaying the input signal are hidden from view in the commercial ‘black box’.

Analogue Displays

A continuously variable signal, such as pressure or temperature, is represented by an analogue display in terms of the amplitude of a physical quantity on a calibrated scale, dial, electrical meter or printed record. The glass thermometer incorporates a wedge-shaped lens which magnifies the appearance of the mercury column against the calibrated background scale. The height of a water column manometer is a linear, visual scale of pressure. Simple mechanical displays are accurate and easily understood, but are inconvenient to read and most suitable for intermittent discrete measurements.

Mechanical spirometers and flowmeters record flow on a dial driven by gears. Electrical moving coil meters use a coil of wire suspended in a magnetic field which rotates in proportion to the applied current and moves a pointer on a calibrated dial. Alternatively, the amplified and filtered electrical signal could drive a chart recorder which produces a continuous printed record of the amplitude of measurements against time. Limitations common to these mechanical devices include fragile moving parts, and inertia which impairs the frequency response to rapidly changing signals.

The cathode ray oscilloscope is an effective screen-based display for continuous analogue electrical signals. A heated cathode generates a stream of electrons which are focused and accelerated onto a phosphorescent coating which lines the flat surface of the tube to generate a bright spot. The position of the electron beam in both x- and y-axes is controlled by electrostatic plates. The continuously varying input signal is applied to the y-plates so that deflection in the vertical y-axis is proportional to the amplitude of the signal. The absence of mechanical parts results in a high-frequency response. An electronic time-base circuit delivers a saw-tooth voltage to the x-plates which drives the electron beam across the x-axis at a constant rate and returns the beam to the left-hand side at the start of each sweep. This produces a dynamic image of signal amplitude against time. Alternatively, a second input signal may be applied to the x-plates to produce an x-y graphical plot, e.g. pressure–volume loop. Cathode ray oscilloscopes are widely used in electronic engineering and signal processing, but have been replaced in clinical practice by microprocessor-controlled displays.

Microprocessor-Controlled Displays

Digital signal processing has revolutionized clinical measurement. Modern monitors comprise a single system integrating various measurements of physiology, and display information as discrete measurements as well as continuous analogue waveforms (Fig. 16.1). This paradox, the conversion of analogue information into digital and then back to analogue, illustrates the real power of digital signal processing to manipulate and present information in a relevant and user-friendly manner. Data patterns (waveforms, trends, graphs) can be recreated or processed in other ways from the original digital signal to assist the anaesthetist, with the original digital signal being stored in a computer record without degradation of the quality of the signal.

Most of the current monitoring systems follow good ergonomic principles, with different variables separated consistently by position on the screen and by colour. This allows the most important information to be placed centrally in large symbols or fonts and in bold colours, with less important data either relegated to small print, or placed in submenus. However, the flexibility of most monitors implies that it is still possible for individuals to change colours and priorities, often making the monitor much less effective. Whenever possible, departments should ensure that all monitors have identical default settings to reduce confusion (these are usually password protected). Unfortunately, the lack of international standards means that confusion may still occur if monitors from multiple sources are used in the same unit.

Despite many attempts to simplify patient data into geometric shapes or bar graphs, data continue to be displayed most often as simple numbers, supported by waveforms, e.g. invasive pressure, and a graphical display of trends over time. Trends are particularly useful when clinical problems may produce gradual change. For example, in neurosurgery, a gradual decrease in end-tidal carbon dioxide concentration is often associated with multiple air emboli.

Input Impedance and Common Mode Rejection

Amplifiers for biological signals require high common mode rejection and high input impedance. The input and electrode impedances act as a potential divider: high electrode impedance and low amplifier input impedance attenuate the electrical signal across the amplifier. The input impedance of modern amplifiers exceeds 5 MΩ to avoid problems, and careful attention must be paid to minimizing electrode impedance, particularly for EEG recordings.

Differential amplification is a powerful method of reducing unwanted noise. The potential difference between two input signals is amplified, but electrical signals common to both are attenuated. This feature is termed ‘common mode rejection’ and very effectively reduces mains interference in all biological signals and electrocardiographic contamination of much smaller electroencephalographic signals. The common mode rejection ratio (CMRR) for a typical differential amplifier exceeds 10 000:1. In other words, a signal applied equally to both input terminals would need to be 10 000 times larger than a signal applied between them for the same change in output.

Frequency Response

The bandwidth of the amplifier must cover the range of frequencies which are important in the signal. In practice, amplifiers require a flat frequency response for ECG from 0.14 to 50 Hz, for EEG from 0.5 to 100 Hz and for EMG from 20 Hz to at least 2 kHz.

Low-frequency interference, largely caused by slow fluctuating potentials generated in the electrodes, produces baseline instability and drift. This is removed by incorporating a network of resistors and capacitors which function as a simple high-pass filter allowing biological signals to pass, but attenuating low-frequency noise. This introduces a compromise in amplifier design between signal trace fidelity and stability of recording. For example, amplifiers designed for diagnostic electrocardiography have long time constants with optimal reproduction of the waveform at the expense of baseline instability, especially to movement. In comparison, continuity of recording is more important when the electrocardiogram is used for monitoring during anaesthesia; high-pass filtering produces a short time constant and good baseline stability at the expense of waveform reproduction. Low-frequency elements of the ECG, such as the T wave, may become differentiated by phase shift in the high-pass filter and appear distorted or biphasic.

Other filters can attenuate particular frequencies. Highly selective band reject filters attenuate 50 Hz interference from the signal. Low-pass filters are used to eliminate higher-frequency artefacts from an EEG signal. The purpose of filtering is to reduce unwanted noise relative to the signal. When the frequency range of signal and noise overlap, some degree of signal degradation is inevitable.

Noise and Interference

Electrical noise arising from the patient, the patient-electrode interface or the surroundings may seriously interfere with accurate recording of biological potentials.

Indirect Methods

An adequate arterial pressure is essential for tissue perfusion; even when perfusion is adequate, hypotension may lead to renal failure. Indirect methods of measuring arterial pressure do not depend on contact between arterial blood and the system for signal recognition and transduction. Arterial pressure may be most rapidly estimated by palpating a pulse, although this method is too unreliable as a single technique. The majority depend on signals generated by the occlusion of a major artery using a cuff, known as the Riva-Rocci method. Systolic pressure can be estimated by the return of a palpable distal pulse; auscultation of the Korotkoff sounds can determine systolic and diastolic pressures. These methods, however, are too time-consuming during anaesthesia and often impossible because of poor access to the patient’s arm.

Oscillometric Measurement of Arterial Pressure

The oscillometric measurement of arterial pressure estimates arterial pressure by analysis of the pressure oscillations which are produced in an occluding cuff by pulsatile blood flow in the underlying artery during deflation of the cuff (Fig. 16.3). The original automatic oscillometers used two cuffs. The upper cuff was inflated to occlude the arterial flow and then gradually deflated. As the blood flow began to pass under the upper cuff, the small changes in volume were detected by the lower cuff with an electromechanical pressure transducer. Modern machines use a single cuff with two tubes for inflation/measurement. During slow deflation, each pulse wave produces a pressure transient in the cuff which may be distinguished from the slowly decreasing ambient pressure in the cuff. Above systolic pressure, the transients are small, but suddenly increase in magnitude when the cuff pressure reaches the systolic point. As the cuff pressure decreases further, the amplitude reaches a peak and then starts to diminish. The mean arterial pressure correlates closely with the lowest cuff pressure at which the maximum amplitude is maintained. As the cuff pressure reaches diastolic pressure, the transients abruptly diminish in amplitude. To avoid high cuff pressures and long deflation times, monitors inflate the cuff to just above a normal systolic pressure and then slowly decrease the pressure until a pulse is detected. Consequently, estimates of diastolic pressure can be unreliable. If a pulse is not detected, the cuff is then inflated to a higher pressure. This process may be repeated several times before a measurement is made.

Commercial instruments incorporate mechanisms for improving the reliability of the measurement. For example, at each successive plateau pressure during the controlled deflation, successive pressure fluctuations are compared and accepted only if they are similar. All automatic oscillometric instruments require a regular cardiac cycle with no great differences between successive pulses. Accurate and consistent readings may be impossible in patients with an irregular rhythm, particularly atrial fibrillation.

Clinical studies comparing automatic oscillometric instruments with direct arterial pressure have demonstrated good correlation for systolic pressure with a tendency to overestimate at low pressures and underestimate at high pressures. Mean and diastolic pressures were less reliable. The 95% confidence interval for all three indices exceeded 15 mmHg. The disadvantages of automated oscillometry are shown in Table 16.3.

Direct Measurement

To measure arterial pressure directly, a cannula (usually 20–22G parallel-sided Teflon) must first be inserted into an artery (usually the radial because occlusion of the artery may be compensated for by flow through the ulnar artery). As fluids are incompressible, the pressure in the artery is transmitted directly to a transducer, which converts pressure into an electrical signal which is displayed by the monitor. The cost and complexity of pressure transducers are compensated by convenience, accuracy, continuity of measurement and an electrical output which may be processed, stored and displayed according to the requirements. The transducer should be at the level of the left ventricle and the transducer opened to the atmosphere to provide a zero reading before use. Monitors usually display systolic and diastolic pressures as well as the mean pressure, calculated automatically by integrating the area under the pressure waveform. The waveform provides useful additional information: a rapidly appreciated estimate of pressure, a qualitative assessment of the adequacy of the frequency response and damping, and an assessment of relative hypovolaemia during positive pressure as identified by the variability or ‘swing’ in the waveform.

The advantage of direct measurements is a real-time measure of arterial pressure, which is essential when administering drugs such as vasopressors to critically ill patients (Table 16.4). Such measurement systems also provide a means for obtaining samples for arterial blood gas analysis and other blood tests. The use of arterial cannulae has therefore become standard practice for severely ill patients, both in the operating theatre and in the intensive care unit.

However, errors are common as a result of malpositioning of the transducers and failure to zero the transducer before use. For example, if the operating table is moved upwards while the transducer remains static, the difference in height artificially increases the pressure reading. Further, while modern disposable sets are usually reliable and accurate, they may occasionally malfunction. Unusual readings should therefore be checked against a reading from a non-invasive monitor. Complications relating to arterial cannulae are shown in Table 16.5.

Accuracy of Arterial Pressure Measurements

The accuracy of pressure measurements, particularly using indirect methods, needs to be considered. Invasive direct measurement of arterial pressure is the usual standard for comparison. However, the catheter-transducer system must be carefully set up and tested for optimal performance and this is hard to achieve in clinical practice. Arterial pressure varies throughout the arterial tree and the measured pressure depends on the site of measurement. As the pulse wave travels from the ventricle to peripheral arteries, changes in vessel diameter and elasticity affect the pressure waveform, which becomes narrower with increased amplitude. Differences in arterial pressure between limbs are common, particularly in patients with arterial disease.

Indirect methods using an occluding cuff make intermittent measurements, with the systolic and diastolic readings reflecting the conditions in the artery at two instants at which the end-points are detected. By contrast, direct pressure measurements are the average of a number of cycles, more precisely reflecting mean pressures. Indirect measurements may be compromised by taking a small number of infrequent samples from a variable signal.

 Long catheters inserted via the antecubital fossa are relatively easy and safe to insert but are of small diameter. Catheters inserted via the basilic or cephalic vein are sometimes difficult to advance past the shoulder. It is also difficult to determine if the tip of the catheter is within a central vein without X-ray imaging. Thrombosis of the veins is common if the catheter is left in situ for more than 24 h.

 Femoral venous catheters are inserted just below the inguinal ligament. They are also relatively easy to insert and may be of large gauge to allow rapid transfusion of fluids. This route is often chosen in children. However, the site of insertion is often within a skin fold, making skin sepsis more likely.

 Internal jugular catheters are used most commonly because the vein is superficial, of larger diameter, and easily managed. This is the route which is often most appropriate for use in an emergency. However, the insertion point is adjacent to several vital structures, including the carotid artery, lung, brachial plexus and cervical spine, with the result that direct needle trauma to these structures can occur. Current guidelines recommend the use of an ultrasound probe for insertion of a catheter via the internal jugular route to improve the accuracy of insertion and to minimize complications.

 Subclavian catheters suffer the same problems as those in the internal jugular vein, although the point of insertion under the clavicle may make it easier to anchor the catheter to the skin. However, if accidental arterial puncture occurs, the overlying clavicle obscures bleeding and makes direct compression of the artery impossible. The proximity of the pleura is associated with a risk of accidental lung puncture. The subclavian route should therefore be used only when the internal jugular approach is contraindicated.

Pulmonary Artery Pressure

Although a central venous cannula may be used to estimate venous volume, it measures the filling of the right side of the heart. However, cardiac output and systemic arterial pressure are determined primarily by the filling pressure of the left side of the heart. When introduced, the pulmonary artery flotation catheter (PAFC), with its ability to measure cardiac output and left atrial pressure, appeared to be a major advance. Recently, however, frequent complications, a lack of evidence of improved survival and the introduction of non-invasive techniques to estimate cardiac output have led to a decline in its use. The PAFC is also known as a Swan-Ganz or balloon tip catheter (Fig. 16.7).

A PAFC is a long catheter with three or four lumens, and a thermistor near the tip. It is inserted into a neck vein through a large cannula. A flexible plastic sheath allows the catheter to be inserted, withdrawn and rotated after insertion without desterilizing it. After insertion into the superior vena cava, saline is injected to inflate a balloon at the tip. The pressure at the tip is measured via a transducer and displayed on a monitor. The catheter is then advanced slowly so that the blood flow directs the catheter toward the pulmonary artery. As the catheter is advanced, a series of changes in pressure is observed, marking the progression through the right atrium and right ventricle into the pulmonary artery (Fig. 16.8). Eventually, the balloon ‘wedges’ into a pulmonary artery. At this point, the tip is isolated from the pulmonary artery and measures the pressure in the pulmonary capillaries, which is taken to reflect left atrial pressure. Although the ability to estimate left atrial pressure is useful, the interpretation of measurements is as problematic as for central venous pressure (see above). For the same reasons, measuring the changes after a fluid challenge is more useful than a single reading.

The ability to measure the filling pressure of the left ventricle as well as the cardiac output was a major advance, and has led to many advances in our understanding of cardiac physiology and the mechanisms and treatments of diseases such as sepsis. However, the process of insertion described above is not always straightforward and prolonged manipulation may be needed to direct the catheter into the pulmonary artery. Arrhythmias are extremely common with catheter insertion and the technique carries all the risks of central venous catheterization noted above in addition to the risks shown in Table 16.7.

Cardiac Output

Cardiac output is closely linked to oxygen delivery; in addition, studies have shown that low cardiac output is linked to increased mortality. However, it must be remembered that cardiac output is intermittent and not continuous and that factors such as the pressure changes caused by ventilation (especially positive pressure ventilation), changes in heart rate and arrhythmias produce complex changes. Therefore, monitors do not produce consistent results even when synchronized with the heartbeat and respiratory cycle.

Dilution techniques have been regarded as the ‘gold standard’ against which other methods are compared.

Oesophageal Stethoscope

This consists of a balloon-tipped catheter which may also carry a temperature probe. It is connected to either an earpiece or stethoscope and inserted into the patient’s oesophagus. The catheter is advanced until the heart sounds are maximal and then taped at the nose. It provides a constant monitor of both heart rate and ventilation and is said to be able to identify the characteristic ‘millwheel murmur’ of an air embolus. It is most often used in children. In addition to being inexpensive, it carries little morbidity and has the great advantage of forcing the anaesthetist to remain beside the patient.

Respiratory Rate

The rate may be timed clinically or more often derived from the capnograph. Many ECG monitors use the leads to pass a very small high-frequency alternating current across the chest; the increase in electrical impedance (resistance to an alternating current) produced by inhalation is measured and the respiratory rate calculated.

Airway Pressure

The lungs are damaged easily and although anaesthetic machines incorporate pressure relief valves, these are designed to protect the machine rather than the patient. Although electronic ventilators usually allow a maximum airway pressure to be set to protect the patient, excessive pressure may still be exerted by manual compression of the reservoir bag. The self-inflating bags used for resuscitation are often capable of exerting extremely high pressures. As even transient peaks of pressure may lead to lung trauma or pneumothorax, a pressure monitor should be used whenever positive-pressure ventilation is used.

As all ventilators now incorporate pressure monitors, separate airway pressure monitors are rarely used. Most ventilators include a pressure transducer in the form of a piezoelectric crystal that converts pressure to an electrical potential, which is then measured by the monitor and displayed.

Although such monitors are both accurate and reliable, it must be remembered that they measure the pressure within the monitor and not the airway pressure. Therefore, the measured pressure may not be reliable if a narrow tracheal tube, long breathing circuit or high-frequency ventilation are used. High lung pressures may be a particular problem in obese patients, those positioned head down and those with bronchospasm.

In devices without electronic components, such as ventilators used for transport, pressure is measured by devices in which the air pressure deforms a bellows or a metal tube. The deformation is linked to a needle with the pressure read from a scale. These are simple devices and are usually reliable, but are susceptible to damage by excess pressure.

Spirometers

Wet spirometers consist of a rigid cylinder suspended over an underwater seal and counterbalanced. Gas entering the bell causes it to rise. This linear displacement is proportional to the volume of gas. Wet spirometers are accurate at steady state and have been used to calibrate other volume-measuring devices. However, they are bulky and inconvenient to use, and the frequency response is damped by friction between the moving parts, causing the instrument to under-read with rapidly changing rates of flow.

Dry spirometers are more convenient for clinical work. Gas displaces a rolling diaphragm or bellows and the expansion is recorded and related to gas volume. The ‘Vitalograph’ is a specialized type of bellows spirometer used for lung function testing. The patient makes a maximal forced exhalation into the spirometer through a wide-bore tube. The expansion of the wedge-shaped bellows is recorded by a stylus on a pressure-sensitive chart. The stylus moves across the x-axis (time) at a constant rate. The resultant plot represents the volume–time plot of the patient’s expiration (Fig. 16.12). The FVC is the maximal volume expired. Understanding of technique and active cooperation of the patient are essential for accurate and precise recordings. The patient must make an airtight seal with the mouthpiece and the nose is occluded with a nose clip. Expiration should be as forcible and rapid as possible. Several attempts are recorded. The highest value measured is recorded because the technique is dependent on voluntary effort, which usually improves with practice.

Gas Meters

Dry gas meters are widely used in the gas industry and are used also in medicine, e.g. in some mechanical ventilators, to measure large volumes of gas. Displacement of bellows controls valves which alternately direct gas flow to fill and empty the bellows, and also drives the pointer on a calibrated recording dial. Irregularities within each cycle disappear when the meter has returned to the same position in the cycle; hence, accuracy of measurements improves with increasing multiples of meter volume.

The Dräger Volumeter

The volume of gas which flows through the Dräger volumeter is related to the rotation of two light, interlocking, dumbbell-shaped rotors. It is a simple meter and accurate when dry, but is affected by moisture.

The Wright Respirometer

This device contains a light mica vane which rotates within a small cylinder (Fig. 16.13). Inflowing air is directed on to the vane by tangential slits. Rotation of the vane drives a gear chain and pointer on a dial. This mechanism is described as inferential because it does not measure either the volume or the flow of all of the gas flowing through the device. It is calibrated for normal tidal volumes and breathing rates by a sine wave pump. However, the meter seriously over-reads at high tidal volumes and under-reads at low tidal volumes because of the inertia of the moving parts.

Integration of the Flow Signal

The flow signal from a rapidly responding flowmeter may be integrated electronically over time to calculate volume. However, the process of integration exaggerates the effect of baseline drift; a small and insignificant change in the baseline of the flow signal may produce substantial and increasing error in the volume signal over time. This effect is minimized by limiting the duration of integration, e.g. to a single tidal volume by resetting the integrator to zero at the end of each inspiration.

Indirect Methods of Measuring Tidal Volume

Methods which depend on measuring inspired or expired gases require a leak-free connection. This is feasible with tracheal intubation. A physiological mouthpiece and connection to the measuring apparatus may change the pattern of breathing and is suitable only for short-term use. Several indirect methods have been developed which enable tidal volume to be derived from measurements of chest wall movement.

Volume–Time Methods

Flow rate may be calculated from spirometric measurements of volume of gas per unit time. These methods are accurate when corrected for temperature and pressure and are used widely to calibrate other flowmeters, but are slow, cumbersome and have limited clinical application.

Most clinical methods are based on the relationship between pressure decrease and flow across a resistance – either a fixed pressure decrease across a variable orifice or a variable pressure change across a fixed orifice.

Variable Orifice (Constant Pressure Change) Flowmeters

The orifice through which gas flows enlarges with the flow rate so that the pressure difference across the orifice remains constant. This is the physical principle of the Rotameter.

Variable Pressure Change (Fixed Orifice) Flowmeters

The resistance is maintained constant so that changes of flow are accompanied by changes in pressure across the resistance element.

Other Devices for Measuring Gas Flow

Measurements other than pressure change across an orifice have been used to measure flow.

 Transit time required for the sample to flow along the sampling catheter usually accounts for the greater part of the total delay. It is minimized by using a narrow and short sampling catheter with a rapid sampling flow rate.

 Response time required for the instrument to react to the change in gas concentration. It consists of the time required to wash out the analysis cell and delays imposed by the sensing mechanism.

Thermal Conductivity

A gas with a high thermal conductivity conducts heat more readily than one with a low conductivity. In the katharometer, gas is passed over a heated wire and the degree of cooling of the wire depends on the temperature of the gas, the rate of gas flow and the thermal conductivity of the gas. The reduction in temperature of the wire reduces its resistance and produces an electrical signal related to the gas concentration. In clinical practice, katharometers are usually used for the measurement of CO₂ and He, and as detectors in gas chromatography systems.

Refractive Index: Interference Refractometers

The speed of light slows through transparent materials to a degree determined by the refractive index of that substance. The delay caused by the passage of light through the gas depends on the number of gas molecules present; hence the refractive index also depends on the pressure and temperature of the gas. This extremely small delay is measured using the phase lag by the principle of interference. When light waves from a common source are passed through two linear slits in an opaque sheet and focused on to a screen, an interference pattern is produced. Bright areas where light from the two sources in phase is reinforced alternate with dark bands where the light paths differ in length by half a wavelength, i.e. they are out of phase and attenuating each other. When a gas is introduced into one light path, it delays transmission of the light waves with a reduction in wavelength and an alteration in the position of the dark bands. If the refractive index of the gas is known, the change in position can be related to the number of gas molecules in the light path and hence to the partial pressure of the gas. Interference refractometers are calibrated using known concentrations of gas or vapour. The response is essentially linear and remains stable after calibration.

This method of analysis is used to calibrate flowmeters and vaporizers accurately. Portable devices are useful for monitoring pollution by anaesthetic gases and vapours.

 Magnetic susceptibility

 Absorption of radiation

 Mass spectrometry

 Gas–liquid chromatography.

Oxygen

Oxygen concentration in a breathing system is measured using either a fuel cell or a paramagnetic analyser.

The fuel cell is the more common device. It contains a lead anode within a small container of electrode gel. When exposed to oxygen, the lead is converted to lead oxide, producing a small voltage which may be measured and amplified. Fuel cells are small, robust and reliable, although they require calibration at regular intervals. After a period of around 6 months, they require replacement because the lead becomes oxidized. Accuracy is better than  ±  1% with a response time of < 10 s.

The principle of the paramagnetic analyser is that oxygen molecules are attracted weakly to a magnetic field (paramagnetic). Most other anaesthetic gases are repelled by a magnetic field (diamagnetic). In the original analysers, a powerful magnetic field was passed across a chamber which contained two nitrogen-containing spheres suspended on a wire. When oxygen was introduced into the chamber, it tended to displace the spheres, causing them to rotate. The degree of rotation was measured to estimate the oxygen concentration.

A fast differential paramagnetic oxygen sensor has been designed on the pneumatic bridge principle. The sample and reference gas are drawn by a common pump through two tubes surrounded by an electromagnet alternating at 110 Hz. Pressure differences between the two tubes are related to the paramagnetic properties of the sample and reference gases. The phasic changes in pressure are extremely small and measured with a miniature microphone. The output of the device is linear, very stable and has a fast response time of less than 150 ms.

These monitors are accurate, reliable and do not need frequent maintenance. The monitors measure the partial pressure of oxygen, but display oxygen as a percentage. If the pressure within the circuit is increased, for example when a gas-driven ventilator is employed, they can overestimate the oxygen concentration.

Carbon Dioxide and Anaesthetic Gases

Oxygen Tension

Oxygen Content

The total amount of oxygen in blood may be measured directly, but this is technically demanding and rarely used. The van Slyke technique uses a chemical and volumetric or manometric analysis of oxygen content. Oxygen is driven from a small sample by denaturing the haemoglobin with acid. The volume of gas at atmospheric pressure, or the pressure at constant volume, is recorded before and after the chemical absorption of oxygen. The change is related directly to the oxygen content of the fixed volume of blood. Alternative detectors have been used to measure oxygen displaced from haemoglobin, e.g. a galvanic cell.

These time-consuming and operator-dependent laboratory techniques have been replaced by calculation of oxygen content from measurements of the oxygen saturation of haemoglobin, haemoglobin concentration and the tension of oxygen in blood:

Accurate estimates require that the oxygen saturation of haemoglobin is measured directly, and not calculated from oxygen tension and an arbitrary but unmeasured oxyhaemoglobin dissociation curve.

Oximetry: Measurement of Oxygen Saturation

The Isolated Forearm Technique

In this method, a patient is anaesthetized, and then a tourniquet is applied to the upper arm and inflated to above systolic arterial pressure. A muscle relaxant is then administered (if required as part of the appropriate anaesthetic technique). Because the tourniquet prevents the muscle relaxant passing to the arm, the forearm muscles still function and are supplied by nerves passing under the tourniquet. If awareness occurs, the patient is able to signal by moving the forearm. Unfortunately, the technique signals only that the patient is already awake and its duration is limited by the ischaemia caused by the tourniquet.

The electroencephalogram and evoked potentials

The electroencephalogram (EEG) is a small, complex signal which is recorded usually from at least four electrodes fixed to specially prepared sites around the head. It has an amplitude of 50–200 μV and a frequency content which is classified conventionally into four categories:

The spiking, transient depolarization, then repolarization, of action potentials in neurones in the brain is sufficiently asynchronous and transient to be unrecordable from the scalp or surface of the brain. It is believed that the EEG is generated by the summation of synchronous postsynaptic potentials on the dendrites of sheets of large and symmetrically arranged pyramidal cells in cortical layers III and IV. Recording of these microvolt signals with acceptable levels of artefact and interference is difficult. Visual analysis of EEG recordings is subjective and requires experience.

The complexity of the raw EEG signals makes its use in the operating theatre impractical. Cerebral function monitors using fewer electrodes have been developed which average the EEG signal, but have been shown to be unreliable in monitoring depth of anaesthesia. However, the Cerebral Function Analysing Monitor (CFAM) has found a role in neurosurgery, and in other specialities in which monitoring cerebral function is crucial. For example, sedated and paralysed patients are unable to display classical tonic-clonic activity during generalized seizures. CFAM enables clinicians to detect seizure activity in neurosurgical intensive care patients and titrate sedation against burst-suppression of the EEG. Two symmetrical pairs of EEG electrodes are placed on either side of the patient’s head. The monitor then analyses the EEG amplitude and the frequency of the waveforms into delta, theta, alpha and beta waves as well as displaying wave suppression information.

Two further methods make the EEG signal easier to interpret, based on spectral analysis. The principle is that any complex wave may be broken down into a series of sine waves. Fast Fourier analysis produces an output in terms of the proportion of each frequency which contributes to the total signal. Bispectral analysis uses relationships between the phase and power of different frequencies within the original signal.

Initial attempts using spectral analysis showed that general anaesthesia produces a reduction in the mean frequency and spectral edge (frequency below which 95% of activity occurs). Ultimately, increasing doses of anaesthetic drugs produce a suppressed or isoelectric EEG. Unfortunately, these changes have not proved to be reliable as a measure of depth of anaesthesia.

The bispectral index (BIS monitor) is a number from 100 (awake) to 0 (deeply anaesthetized) produced by a commercially available monitor. Although the exact methods used have not been published, it identifies patterns within 30-s periods of EEG recording by first excluding episodes likely to be produced by diathermy or muscle activity, and periods of burst suppression. The remaining activity is subjected to bispectral analysis with the final result adjusted to take account of the proportion of suppressed EEG. It appears to provide reliable measurements in clinical use. However, it is not clear how factors such as different anaesthetic agents, hypoxaemia or epileptic activity affect the results.

Auditory Evoked Potentials

Auditory evoked potential monitors measure the slowing of auditory information processing produced by anaesthetic agents. The patient wears headphones which repeatedly play soft ‘clicks’. Each click produces neuronal activity in the auditory cortex. While the signal from a single click is masked by other brain activity, the signal from repeated clicks can be averaged to isolate the auditory signal. The activity in the auditory cortex may therefore be displayed as the auditory evoked potential, which has a characteristic waveform (Fig. 16.21). The time delay for some waves can then be measured and the result converted into a value typically from zero to 100, reflecting depth of anaesthesia. The equipment cannot be used in patients who have impaired hearing and may require several seconds to generate enough data to process. Monitors using this principle are commercially available, but have yet to be used widely. Equipment using either visual or somatosensory stimulation has also been developed.

Other Techniques

Respiratory sinus arrhythmia is the normal variation in heart rate related to the respiratory cycle. This variability is reduced by anaesthesia and has formed the basis of a commercially available monitor. However, it has not achieved widespread use.

Monitors based on physiological measures such as frontalis muscle myography and lower oesophageal contractility have not proved reliable enough for clinical use.

Liquid Expansion Thermometers

Liquid expansion thermometers are simple, reliable instruments. A glass bulb is filled with a liquid (generally alcohol or mercury) and connected to an evacuated, closed capillary tube. The temperature is recorded by the position of the meniscus in the capillary tube against a calibrated scale. If the cross-sectional area of the capillary tube is constant, movement of the meniscus with changing temperature is linear.

Several simple and elegant design features improve usability. A large bulb and very narrow capillary increase the sensitivity. Visibility of the narrow capillary is improved by shaping the thermometer so that the glass forms a lens and by incorporating a strip of white glass behind the capillary. A constriction in the capillary tube permits the mercury to expand but hinders its return to the bulb so that the reading is preserved until the mercury is shaken down. However, glass thermometers are fragile. A large thermal capacity results in a slow response. The instrument is handheld, awkward to read and reset, and cannot be used for remote measurement or recording. Traditional mercury-in-glass thermometers are no longer used because of the risk of breakage and mercury contamination.

Chemical Thermometers

Thermometers based on temperature-dependent changes in chemical mixtures have been marketed. Reversible chemical thermometers contain several cells filled with liquid crystals, each of slightly different composition. At a critical temperature, the optical properties change because of realignment of the molecules, causing reflection instead of absorption of the incident light. An alternative technique consists of rows of cells filled with chemical mixtures which melt at specific temperatures releasing a dye; this single-use, disposable design prevents cross-infection but is relatively expensive.

Resistance-Wire Thermometers

Resistance-wire thermometers are based on the principle that the resistance of some metal wires increases as their temperature increases. Platinum resistance thermometers have a large temperature coefficient of resistance and are very sensitive to small changes in temperature, but are fragile and slow to respond. Single-use probes which incorporate a tiny copper element have been marketed with an acceptable clinical accuracy and response time.

Thermistor Thermometers

Thermistors are semiconductors made from the fused oxides of heavy metals such as cobalt, manganese and nickel. They demonstrate marked and non-linear variation in resistance with temperature, which is usually compensated by electronic processing. Disadvantages include inconsistent variation between individual thermistors, change in resistance over time, and hysteresis during rapid heating and cooling. However, the large temperature coefficient permits the detection of small temperature changes and the tiny ‘pin-head’ size results in a rapid response. They are used widely in invasive temperature monitoring, e.g. in pulmonary artery catheters.

Thermocouple Thermometers

If two dissimilar metals are joined to create an electrical circuit and the junctions are at different temperatures, current flows from one metal to the other. The potential difference which is generated is a function of the temperature difference between the two junctions. All junctions made from the same metals have identical properties. The reference junction must be kept at a constant temperature, or incorporate temperature compensation into the measurement. Common combinations of metals include copper-constantan or platinum-rhodium. The output is small (about 40 mV per °C temperature difference between the junctions), but this is sufficient to be sensed by a galvanometer.

Temperature probes using these properties can continuously measure peripheral and core temperatures. Peripheral temperature is measured by attaching a probe to either a finger or toe. Core temperature probes may be inserted into the nasopharynx, oesophagus or rectum, and although complications are rare, it is possible to cause mucosal trauma, bleeding and even penetration of the pharynx or rectum. When used for prolonged periods, mucosal damage is possible as a result of pressure-related ischaemia. Core temperature probes may also form part of an intravascular catheter, for example within a PAFC. Many monitors accept two probes and automatically display the temperature difference. Care must be taken to avoid the peripheral probe being exposed to warmth from heating blankets.

Dial Thermometers

Dial thermometers exploit the increase in pressure caused by the temperature-dependent expansion of a liquid or gas in an enclosed cavity. This is sensed by a Bourdon pressure gauge and recorded on a dial. Although cheap and robust, they are relatively inaccurate, slow to respond and are only suitable for large temperature changes in heated equipment, e.g. autoclaves.

Bimetallic Strip Thermometers

If strips of two metals with different coefficients of expansion are fastened together throughout their length, the combined strip will bend when heated. Sensitivity is improved by using a long strip which is usually bent in a spiral or coil, with one end fixed and the other connected to a recording pointer. This technique is used for temperature compensation in some anaesthetic vaporizers, and in cheap mechanical thermometers for measuring air temperature.