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Patient monitoring and monitoring equipment

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Abstract

General anaesthesia carries an inherent risk for every patient. Checking the equipment before every anaesthetic and selecting an appropriate anaesthetic protocol on the basis of a thorough history and pre-anaesthetic examination of the patient will help to reduce the risk. This chapter considers clinical monitoring, monitoring equipment and guidelines for monitoring.

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Figures

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7.1 Ergonomics in theatre. (a) An ‘anaesthetic window’ allows continuous evaluation of the patient. (b) Strategic positioning of the anaesthetic machine, monitoring equipment, fluids and syringe drivers to facilitate monitoring of the patient and all equipment.
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7.2 Tip of an oesophageal stethoscope.
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7.5 Clinical assessment of anaesthetic depth during maintenance with volatile anaesthetics.
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7.6 Mainstream capnography. The analyser transmits a beam of infrared light through the window of the connection piece, which is placed between the endotracheal tube and the Y-piece of the anaesthetic breathing system.
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7.7 Sidestream capnography. The monitor continuously withdraws gas through a sampling line, which is attached to the connection piece. (a) Normal connector. (b) Low dead space connector between the endotracheal tube and the Y-piece of the anaesthetic breathing system. (c) Low dead space connector. (d) A water trap filters out water vapour from the sample, to allow infrared analysis of a dry gas sample inside the monitor. Clearly visible is the outlet for the sampled gas. (c, Courtesy of Asher Allison, Animal Health Trust, Newmarket, UK)
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7.8 Microstream capnography connector. (Courtesy of Asher Allison, Animal Health Trust, Newmarket, UK)
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7.9 Normal single-breath capnogram. Note that expiration starts before phase II, because initially dead space gas, which does not contain carbon dioxide, is exhaled. The alpha (α) angle is increased with a partial airway obstruction or when positive end-expiratory pressure is applied. The beta (β) angle increases when there is rebreathing of carbon dioxide or when the monitor response time is long relative to the respiratory cycle time, for example, in small patients with rapid respiratory rates and a relatively low gas sampling rate. See the text for further explanation of the different phases. CO = partial pressure of inspired carbon dioxide; ʹCO = partial pressure of end-tidal carbon dioxide.
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7.10 Capnograms. (a) In some obese or pregnant patients, an additional peak in carbon dioxide concentration (phase IV) can be seen at the end of the plateau phase. A similar capnogram may sometimes be seen when there is a leak in the sampling line or water trap. (b) When ventilation continues during cardiac arrest, the expiratory carbon dioxide concentration quickly decreases over a few breaths, because carbon dioxide produced in the tissues is no longer being transported to the lungs. A similar capnogram can also result from major pulmonary embolism or major reduction in cardiac output. (c) The normal plateau is distorted or absent because of dilution of carbon dioxide by ambient air or fresh gas. This may be caused by a leak (underinflated cuff, leak in the sampling line or water trap) or because the sampling site is too close to the area of fresh gas delivery. In this scenario the P´CO is usually underestimated. (d) When respiratory rate is high and tidal volume and sampling rate (in sidestream capnographs) are low, mixing of gases in the sampling line can cause ‘damping’ of the capnogram, eventually leading to a sinusoidal shape. (e) A gradual rise in both inspiratory and expiratory carbon dioxide concentrations is typically seen when carbon dioxide absorbent is exhausted or when the fresh gas flow in non-rebreathing systems is insufficient. (f) ‘Curare cleft’ (arrowed) during intermittent positive pressure ventilation, resulting from spontaneous respiratory effort in a patient recovering from neuromuscular blockade. This may also be seen as a result of manipulation of the thorax or abdomen during surgery. (g) Shallower expiratory upstroke (‘shark fin’ appearance of capnogram) resulting from expiratory resistance to airflow; this can, for example, be seen in cats with asthma. (h) ‘Cardiogenic’ oscillations at the end of the plateau may be seen especially when respiratory rate is low.
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7.11 (a) Transmission pulse oximeters. (b) A relatively thin layer of tissue, such as the toe, is a suitable site for placement of the pulse oximeter.
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7.12 Reflectance pulse oximeter probe. (Courtesy of Dr Carolyn McKune, University of Saskatchewan, Canada)
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7.13 Oxyhaemoglobin dissociation curve. O = arterial oxygen tension; O = arterial haemoglobin saturation with oxygen.
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7.14 Placement of a wet swab between the tongue and the pulse oximeter probe may result in better pulse oximeter readings.
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7.15 Plethysmogram (yellow trace). Note the similarity to an arterial pressure trace and the synchrony with the electrocardiogram (green trace).
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7.18 (a) Continuous flush system used to prevent clot formation and damping of the pressure waveform during invasive arterial blood pressure monitoring. (b) Strain gauge transducer used for blood pressure monitoring.
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7.19 Damped arterial blood pressure trace.
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7.20 Arterial blood pressure waveform. DAP = diastolic arterial pressure; SAP = systolic arterial pressure.
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7.21 Doppler monitoring of arterial blood pressure.
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7.22 Central venous pressure (CVP) monitoring. The three-way stopcock is positioned such that fluid administration is discontinued and the open-ended tube (the ‘fluid column’) is connected to the central venous catheter. The red line represents a pressure of 0 cmHO, at the level of the right atrium. The red arrow indicates the height of the fluid column and hence the CVP (here, 11.2 cmHO).
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7.23 Normal ECG.
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7.24 Alternative placement of a three-lead ECG (base–apex lead).
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7.25 Probes for transoesophageal electrocardiography in cats or small dogs (A) and medium to large dogs (B). White arrows indicate the cable connectors for normal ECG leads; red arrows indicate the electrodes, which need to be positioned close to the heart. If the probe is inserted too deeply into the oesophagus, the QRS waveform will be inverted. Probe B can also be used as an oesophageal stethoscope and allows measurement of body temperature using an intra-oesophageal thermistor. (Courtesy of Tanya Duke-Novakovski, Western College of Veterinary Medicine, University of Saskatchewan, Canada)
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7.26 (a) An oxygen sensor. (b) Oxygen sensor placed in an anaesthetic breathing system.
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7.27 Wright’s respirometer. (Courtesy of Asher Allison, Animal Health Trust, Newmarket, UK)
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7.28 Dräger Volumeter.
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7.29 Pitot tube for sidestream spirometry placed between the Y-piece of the anaesthetic breathing system and the endotracheal tube.
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7.30 Bourdon gauge.
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7.31 Spirometry loops during (a) spontaneous breathing, (b) intermittent positive pressure ventilation (IPPV) in a controlled mode and (c) IPPV in an assisted mode. For each type of breathing, (1) pressure (P)–volume (V) and (2) flow(V)–volume (V) loops are shown. During spontaneous breathing, the pressure is negative during inspiration and positive during expiration. During IPPV in a controlled mode, the pressure increases during inspiration and decreases to zero again during expiration. In an assisted mode, the patient generates a negative pressure, triggering the ventilator to deliver a breath, which leads to a positive pressure during the remainder of inspiration. During expiration, the pressure returns to zero.
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7.32 Variations in normal spirometry loops. For each type of breathing, (1) pressure (P)–volume (V) and (2) flow (V)–volume (V) loops are shown. (a) Continuous positive airway pressure with spontaneous breathing, deliberately or because, for example, the adjustable pressure-limiting valve is inadvertently closed. The loop starts and ends at a pressure above zero. Note the more rectangular shape of the pressure–volume loop and the decreased flow during expiration. (b) Intermittent positive pressure ventilation combined with positive end-expiratory pressure. The loop starts and ends at a pressure above zero. Note the somewhat decreased flow during expiration. (c) Increased work of breathing (solid line) compared to the normal curve (dashed line), e.g. in an obese patient. A higher pressure is needed to deliver the same tidal volume. (d) Reduced compliance of the respiratory system (solid line) compared with the normal curve (dashed line) causes the pressure–volume loop to be less steep (closer to the X-axis).
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7.33 Abnormal spirometry loops during intermittent positive pressure ventilation. For each type of breathing, (1) pressure (P)–volume (V) and (2) flow (V)–volume (V) loops are shown. (a) In the presence of a leak between the sampling site and the patient’s lungs the expired volume is lower than the inspired volume. (b) The expired volume is higher than the inspired volume. This artefact is often seen during mechanical ventilation, possibly because of compression of gases by the ventilator during inspiration. (c) Spontaneous respiratory efforts by the patient during intermittent positive pressure ventilation. In this case the inspiratory phase of the loops is distorted. (d) Airway secretions can cause ‘ripples’ on the spirometry loops.
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7.34 A thermometer carefully placed in a nostril of a dog. (Courtesy of Marieke de Vries, Davies Veterinary Specialists, Higham Gobion, UK)

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